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British regional geology: Bristol and Gloucester region

2017-09-28T20:52:58Z

BobMcIntosh: /* Upper Carboniferous (Westphalian and Stephanian) */

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The British regional geology: Bristol and Gloucester region has been converted to a series of articles for this wiki. The book is available for purchase at the [https://shop.bgs.ac.uk/Bookshop/product.cfm?p_id=BRG16 BGS Online Shop] Its full reference is:

'''Green, G W. 1992. British regional geology: Bristol and Gloucester region (Third edition). (London: HMSO for the British Geological Survey.)'''

[[File:P894510.jpg|200px|right|British Regional Geology: Central England. Cover image. P894510.]]

== Contents ==

=== [[Bristol and Gloucester region - an introduction|Introduction]] ===

:: [[Bristol and Gloucester region - an introduction#Physiography|Physiography]]

:: [[Bristol and Gloucester region - an introduction#Scenery|Scenery]]

:: [[Bristol and Gloucester region - an introduction#Early geological works|Early geological works]]

=== [[Cambrian, Bristol and Gloucester region|Cambrian]] ===

:: [[Cambrian, Bristol and Gloucester region#Breadstone Shales|Breadstone Shales]]

:: [[Cambrian, Bristol and Gloucester region#Micklewood Beds|Micklewood Beds]]

=== [[Silurian, Bristol and Gloucester region|Silurian]] ===

:: [[Silurian, Bristol and Gloucester region#Tortworth|Tortworth]]

:: [[Silurian, Bristol and Gloucester region#Sharpness|Sharpness]]

:: [[Silurian, Bristol and Gloucester region#Newnham|Newnham]]

:: [[Silurian, Bristol and Gloucester region#Wickwar|Wickwar]]

:: [[Silurian, Bristol and Gloucester region#Mendips|Mendips]]

:: [[Silurian, Bristol and Gloucester region#Batsford (Lower Lemington) Borehole|Batsford (Lower Lemington) Borehole]]

=== [[Old Red Sandstone (Silurian—Carboniferous), an introduction|Old Red Sandstone (Silurian–Carboniferous)]] ===

:: Stratigraphy

::: [[Lower Old Red Sandstone, Bristol and Gloucester region|Lower Old Red Sandstone]]

::: [[Upper Old Red Sandstone, Bristol and Gloucester region|Upper Old Red Sandstone]]

:: [[Upper Old Red Sandstone, Bristol and Gloucester region#West of River Severn|West of River Severn]]

:: [[Upper Old Red Sandstone, Bristol and Gloucester region#Sharpness–Thornbury|Sharpness–Thornbury]]

:: [[Upper Old Red Sandstone, Bristol and Gloucester region#Bristol|Bristol]]

:: [[Upper Old Red Sandstone, Bristol and Gloucester region#Bath|Bath]]

:: [[Upper Old Red Sandstone, Bristol and Gloucester region#Mendip Hills|Mendip Hills]]

=== [[Lower Carboniferous (Dinantian), its classification and sedimentation, Bristol and Gloucester region|Lower Carboniferous (Dinantian)]] ===

:: [[Lower Carboniferous (Dinantian), its classification and sedimentation, Bristol and Gloucester region#Classification|Classification]]

:: [[Lower Carboniferous (Dinantian), its classification and sedimentation, Bristol and Gloucester region#Sedimentation|Sedimentation]]

:: Stratigraphy

::: [[Lower Limestone Shale, Lower Carboniferous, Bristol and Gloucester region|Lower Limestone Shale]]

::: [[Black Rock Limestone, Lower Carboniferous, Bristol and Gloucester region|Black Rock Limestone]]

::: [[Clifton Down Group and Hotwells Group (excluding the arenaceous facies), Lower Carboniferous, Bristol and Gloucester region|Clifton Down Group and Hotwells Group]]

::: [[Arenaceous facies in the Clifton Down Group and Hotwells Group, Lower Carboniferous, Bristol and Gloucester region|Arenaceous facies in the Clifton Down Group and Hotwells Group]]

:: [[Cannington Park, Lower Carboniferous, Bristol and Gloucester region|Cannington Park]]

:: [[Igneous rocks, Lower Carboniferous, Bristol and Gloucester region|Igneous rocks]]

=== [[Upper Carboniferous (Namurian), Bristol and Gloucester region|Upper Carboniferous (Namurian)]] ===

:: [[Upper Carboniferous (Namurian), Bristol and Gloucester region#Bristol area|Bristol area]]

:: [[Upper Carboniferous (Namurian), Bristol and Gloucester region#Southern areas|Southern areas]]

:: [[Upper Carboniferous (Namurian), Bristol and Gloucester region#Western areas|Western areas]]

=== [[Upper Carboniferous (Westphalian and Stephanian) and its classification, Bristol and Gloucester region|Upper Carboniferous (Westphalian and Stephanian)]] ===

:: [[Upper Carboniferous (Westphalian and Stephanian) and its classification, Bristol and Gloucester region#Classification|Classification]]

:: [[Conditions of formation of the Carboniferous Coal Measures and their fauna and flora, Bristol and Gloucester region|Conditions of formation of the Coal Measures]]

:: [[Conditions of formation of the Carboniferous Coal Measures and their fauna and flora, Bristol and Gloucester region#Plants of the coal measures|Plants of the Coal Measures]]

:: [[Conditions of formation of the Carboniferous Coal Measures and their fauna and flora, Bristol and Gloucester region#Fauna of the coal measures|Fauna of the Coal Measures]]

:: [[The Carboniferous coal basins, Bristol and Gloucester region|The coal basins]]

:: [[The Carboniferous coal basins, Bristol and Gloucester region#Other coal measures occurrences|Other Coal Measures occurrences]]

=== [[Intra-Palaeozoic earth movements]] ===

:: [[The Malvern Axis and Pre- and Intra-Carboniferous earth movements, Bristol and Gloucester region#The Malvern 'Axis'|The Malvern 'Axis']]

:: [[The Malvern Axis and Pre- and Intra-Carboniferous earth movements, Bristol and Gloucester region#Pre-Carboniferous movements|Pre-Carboniferous movements]]

:: [[The Malvern Axis and Pre- and Intra-Carboniferous earth movements, Bristol and Gloucester region#Intra-Carboniferous movements|Intra-Carboniferous movements]]

:: [[Permo-Carboniferous earth movements, Bristol and Gloucester region|Permo-Carboniferous movements]]

=== [[Permo-Triassic, its classification and conditions of deposition, Bristol and Gloucester region|Permo-Triassic]] ===

:: [[Permo-Triassic, its classification and conditions of deposition, Bristol and Gloucester region#Classification|Classification]]

:: [[Permo-Triassic, its classification and conditions of deposition, Bristol and Gloucester region#Conditions of deposition|Conditions of deposition]]

:: [[Permo-Triassic Sandstones, Bristol and Gloucester region|Permo-Triassic Sandstones]]

:: [[Mercia Mudstone Group, Permo-Triassic, Bristol and Gloucester region|Mercia Mudstone Group]]

:: [[Penarth Group, Permo-Triassic, Bristol and Gloucester region|Penarth Group]]

=== [[Lower Jurassic and its classification, Bristol and Gloucester region|Lower Jurassic]] ===

:: [[Lower Jurassic and its classification, Bristol and Gloucester region#Classification|Classification]]

:: [[Lower Lias, Jurassic, Bristol and Gloucester region|Lower Lias]]

:: [[Middle Lias, Jurassic, Bristol and Gloucester region|Middle Lias]]

:: [[Upper Lias, Jurassic, Bristol and Gloucester region|Upper Lias]]

=== [[Middle Jurassic (Inferior Oolite Group) its classification and depositional pattern, Bristol and Gloucester region|Middle Jurassic (Inferior Oolite Group)]] ===

:: [[Middle Jurassic (Inferior Oolite Group) its classification and depositional pattern, Bristol and Gloucester region#Classification|Classification]]

:: [[Middle Jurassic (Inferior Oolite Group) its classification and depositional pattern, Bristol and Gloucester region#Depositional pattern|Depositional pattern]]

:: [[Lower and Middle Inferior Oolite, Middle Jurassic, Bristol and Gloucester region|Lower and Middle Inferior Oolite]]

:: [[Upper Inferior Oolite, Middle Jurassic, Bristol and Gloucester region|Upper Inferior Oolite]]

=== [[Middle Jurassic (Great Oolite Group) its classification and depositional pattern, Bristol and Gloucester region|Middle Jurassic (Great Oolite Group)]] ===

:: [[Middle Jurassic (Great Oolite Group) its classification and depositional pattern, Bristol and Gloucester region#Classification|Classification]]

:: [[Middle Jurassic (Great Oolite Group) its classification and depositional pattern, Bristol and Gloucester region#Depositional pattern|Depositional pattern]]

:: Stratigraphy

:: [[Great Oolite Group, Middle Jurassic, Dorset—Somerset Province|Dorset—Somerset Province]]

:: [[Great Oolite Group, Middle Jurassic, Bath, Bath—Cotswolds Province|Bath—Cotswolds Province]]

::: [[Great Oolite Group, Middle Jurassic, Tormarton—Nailsworth, Bath—Cotswolds Province|Tormarton—Nailsworth]]

::: [[Great Oolite Group, Middle Jurassic, Minchinhampton, Bath—Cotswolds Province|Minchinhampton]]

::: [[Great Oolite Group, Middle Jurassic, Cirencester—North Cotswolds, Bath—Cotswolds Province|Cirencester—North Cotswolds]]

::: [[Cornbrash, Middle Jurassic, Bath—Cotswolds Province|Cornbrash]]

=== [[Middle/Upper Jurassic (Kellaways Beds and Oxford Clay), Bristol and Gloucester region|Middle/Upper Jurassic (Kellaways Beds and Oxford Clay)]] ===

=== [[Cretaceous, Bristol and Gloucester region|Cretaceous]] ===

=== [[Post-Variscan structure and sedimentation, Bristol and Gloucester region|Post-Variscan structure and sedimentation]] ===

:: [[Post-Variscan structure and sedimentation, Bristol and Gloucester region#Main structural elements|Main structural elements]]

:: [[Post-Variscan structure and sedimentation, Bristol and Gloucester region#Post-Bathonian structural history|Post-Bathonian structural history]]

=== [[Quaternary, Bristol and Gloucester region|Quaternary]] ===

:: [[Quaternary events in the Lower Severn and Avon valleys|Events in the Lower Severn and Avon valleys]]

:: [[Glacial deposits, Quaternary, Bristol and Gloucester region|Glacial deposits]]

:: [[Terrace gravels, Quaternary, Bristol and Gloucester region|Terrace gravels]]

:: [[Head, including fan gravel, Quaternary, Bristol and Gloucester region|Head, including Fan Gravel]]

:: [[Alluvium, Quaternary, Bristol and Gloucester region|Alluvium]]

:: [[Raised beach deposits, Quaternary, Bristol and Gloucester region|Raised beach deposits]]

:: [[Cave and fissure deposits, Quaternary, Bristol and Gloucester region|Cave and fissure deposits]]

=== [[Geology and man, introduction, Bristol and Gloucester region|Economic geology]] ===

:: [[Coal and coal mining, Bristol and Gloucester region|Coal]]

:: [[Economic minerals other than coal, Bristol and Gloucester region|Roadstone, aggregate and lime]]

:: [[Economic minerals other than coal, Bristol and Gloucester region#Iron|Iron]]

:: [[Economic minerals other than coal, Bristol and Gloucester region#Lead and zinc ores|Lead and zinc ores]]

:: [[Economic minerals other than coal, Bristol and Gloucester region#Manganese|Manganese]]

:: [[Economic minerals other than coal, Bristol and Gloucester region#Brick, tile and pottery clays|Brick, tile and pottery clays]]

:: [[Economic minerals other than coal, Bristol and Gloucester region#Oil shales|Oil shales]]

:: [[Water supply and hot springs, Bristol and Gloucester region|Water supply]]

:: [[Water supply and hot springs, Bristol and Gloucester region#Bath–Bristol hot springs|Bath–Bristol hot springs]]

== Foreword to the third edition ==

The second edition of the Bristol and Gloucester regional guide was produced in 1948, but since that time much new information has been collected and there was a need to produce this third edition to incorporate the information derived from mapping carried out by the British Geological Survey and exploration undertaken by the coal, petroleum and water industries.

The Bristol and Gloucester district is one of the geologically most varied parts of Britain. This in turn provides the scenic variety for which the area is renowned, including the deep gorges of Cheddar Gorge and the Wye Valley, prominent escarpments such as those of the Cotswolds and the Forest of Dean, and the wide valleys flanking the Thames and the Severn. The area is also famous for its caves, such as Wookey Hole. It is the combination of a congenial environment and mineral resources which has attracted people to the Bristol and Gloucester district since the days of early man.

The geology of the district, as outlined in this volume, provides a fascinating insight into the history and evolution of the area, extending back more than 500 million years. The region contains some of the oldest rocks exposed in southern England whilst at the other end of the geological timescale, because the region was located near the edge of the massive ice sheets which at times covered much of Britain, it provides us with a dramatic picture of changing environments over the last two million years.

Elucidating the geological history is essential if we are to understand the controls on the distribution of mineral resources in the region. These resources include lime and aggregate, which are extracted in large quantities, and resources which are not presently mined, such as coal, evaporites, ironstone, clay, lead, (worked from Roman times), zinc and oil shales. Groundwater is one of the most important and widely used resources in the regionthe geological investigations undertaken by the BGS in the region will help to conserve this important resource for present and future generations. Bath and other thermal waters have been the subject of recent investigations which have shown that their composition (and temperature) are closely related to their underlying geology.

The first two editions of this regional guide were very popular and I am confident that the third edition will be equally popular with geologists, planners, environmentalists, explorers, tourists and all those interested in the geological development of this beautiful area and its conservation.

::: British Geological Survey
::: Keyworth
::: Nottingham NG12 5GG
::: 1 March 1992
::: Peter J Cook, DSc
::: Director

== Acknowledgements ==

The author wishes to acknowledge the great help he has received from numerous colleagues on the Survey in writing this account in particular to Dr P M Allen and Mr B J Williams for editing the text, and to members of the palaeontological and petrographical departments for evaluating appropriate sections. Special mention must also be made of Mr R J Wyatt, not only for checking the entire work and seeing it through the press but also for supplying valuable help with Chapter 12 (Great Oolite Group). The figures were prepared by Mr J W Arbon. Thanks are due to the Council of the Geological Society of London for permission to use and adapt Figure 15 (from the Journal of the Geological Society, 1983), and to Messrs Blackie for Figures 32 and 33.

[[Category:Bristol and Gloucester region | 01]]

Hydrogeology and water supply, geology and man, Northern England

2016-04-09T12:51:12Z

BobMcIntosh:

Geological hazards, geology and man, Northern England

2016-04-09T12:49:26Z

BobMcIntosh:

Building stone, geology and man, Northern England

2016-04-09T12:44:21Z

BobMcIntosh:

Metalliferous and associated minerals, geology and man, Northern England

2016-04-09T12:41:04Z

BobMcIntosh:

Industrial minerals, geology and man, Northern England

2016-04-09T12:37:17Z

BobMcIntosh: /* Limestone and dolomite-rock */

Quaternary—Holocene sea-level changes, Northern England

2016-04-09T12:34:07Z

BobMcIntosh:

Holocene, Northern England

2016-04-09T12:32:00Z

BobMcIntosh:

Late glacial period, Quaternary, Northern England

2016-04-09T12:30:21Z

BobMcIntosh:

Fuel and energy, geology and man, Northern England

2016-03-24T20:04:31Z

BobMcIntosh:

Main Late Devensian glaciation of north-east England

2016-03-24T20:00:59Z

BobMcIntosh:

Main Late Devensian glaciation of north-west England

2016-03-24T19:57:31Z

BobMcIntosh:

Main Late Devensian glaciation of the Isle of Man

2016-03-24T19:52:20Z

BobMcIntosh:

Main Late Devensian glaciation, Quaternary, Northern England

2016-03-24T19:50:03Z

BobMcIntosh:

Early and Mid Quaternary, Northern England

2016-03-24T19:48:28Z

BobMcIntosh:

Lithostratigraphy, Quaternary, Northern England

2016-03-24T19:47:16Z

BobMcIntosh:

Record of climate change, Northern England

2016-03-24T19:46:12Z

BobMcIntosh:

Landscape evolution, Northern England

2016-03-24T19:44:42Z

BobMcIntosh:

Basin-margin mineralisation, Cumbrian coast to Northumberland

2016-03-24T19:42:43Z

BobMcIntosh:

Northern Pennine Orefield

2016-03-24T19:41:36Z

BobMcIntosh:

Mineralization in the Isle of Man

2016-03-24T19:39:04Z

BobMcIntosh:

Haematite deposits of Cumbria

2016-03-24T19:38:10Z

BobMcIntosh:

Mineralization in the Lake District

2016-03-24T19:36:29Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
== Introduction ==
[[File:P916088.jpg|thumbnail|Principal metalliferous mining sites of Cumbria and the Lake District. Named localities are those mentioned in the text. P916088.]]
[[File:P649471.jpg|thumbnail|Nodules of graphite set in altered dioritic rock, from the Seathwaite graphite deposit at Borrowdale, Cumbria. (P649471).]]
[[File:P220211.jpg|thumbnail|Tungsten mineralisation in the Harding Vein, at Carrock Fell Mine, Mosedale, Cumbria. Large, black, blade-like crystals of wolframite are set in a quartz gangue. In places, for example just above the hammer, wolframite is partly replaced by pale brown scheelite (arrowed). The hammer head is 17 cm long. (P220211).]]

Epigenetic mineralisation within the Lower Palaeozoic rocks of the Lake District inlier is largely confined to the Skiddaw, Eycott Volcanic and Borrowdale Volcanic groups and the major igneous intrusions [[Media:P916088.jpg|(P916088)]]; mineralisation is rare within the rocks of the Windermere Supergroup. Many of the metalliferous veins of the Lake District appear to exhibit a close structural, and perhaps genetic, relationship to the form of the largely concealed Lake District granitic batholith. Veins are typically concentrated above, or close to, ridges in the roof region of the batholith or above its north and south walls. The virtual absence of veins from the Windermere Supergroup is consistent with their close association with the batholith, which does not extend beneath that division.

The mineralogical composition, chemical characteristics and structural relationships of the deposits give evidence for several mineralising episodes, in some cases within the same vein. Deposits range in age and composition to include: Late Ordovician copper-rich assemblages; Early Devonian tungsten-bearing veins; Carboniferous or early Permian lead–zinc mineralisation, locally accompanied by abundant baryte; widespread post-Permian haematite mineralisation; super-gene assemblages, possibly formed in Jurassic or later times. The earliest phases of mineralisation may be the result of hydrothermal activity during the closing stages of Ordovician magmatism, with subsequent episodes during the tectonic evolution of the region. The Lower Palaeozoic rocks of the Lake District, including the granitic batholith, and the Carboniferous and Permo-Triassic rocks of the adjoining basins appear to have been source rocks for the introduced minerals.

Two belts of extensive metasomatic alteration, which affect Skiddaw Group rocks, may give important clues to some of the mineralising processes. In the northern part of Black Combe and in the Crummock Water aureole are zones of intense metasomatic alteration and bleaching of the rocks which exhibit substantial net additions of such elements as As, B, K and Rb and locally Ca, F and Si, with dehydration and depletions of Cl, Ni, S, Zn, C and in places Cu, Fe, Li and Mn. The inferred presence of an elongate granitic body along the northern margin of the Lake District Batholith and the Early Devonian date for the Crummock Water metasomatism provide strong evidence that Skiddaw Group rocks may have been a source of ore metals for some of the Lake District veins.

No clear uniformity of vein orientation is apparent within the Lake District, though it has been observed that copper-bearing veins are commonly orientated east–west with a southerly dip, whereas mainly lead–zinc veins typically trend within 45° of north and dip towards the east. However, there are many exceptions to this pattern. Although evidence exists for vertical zonation of constituent minerals in a few instances, most of the Lake District deposits show no obvious lateral or vertical zonation of constituent minerals. In this they differ from the commonly zoned veins of the nearby northern Pennines.

The following accounts review the deposits in order of their approximate age, with the oldest first.

== Graphite deposits ==
The Seathwaite graphite deposit in Borrowdale (NY 232 125) is unique in the British Isles and has very few parallels elsewhere in the world. Mining here dates back to at least the 16th century and continued intermittently until 1891. The graphite is closely associated with a dioritic intrusion into the lowest part of the Borrowdale Volcanic Group. It occurs in veins and in a series of at least eight individual, steeply inclined, pipe-like bodies developed at the intersection of faults. The ‘pipes’ are each up to 1 m by 3 m across and from 2 m to 100 m in vertical extent. Within these, the graphite forms discrete nodules from 1 mm up to over a metre across, though nodules up to a few centimetres across were probably commonest [[Media:P649471.jpg|(P649471)]]. The nodules occur within a buff-coloured matrix of intensely altered dioritic rock. Graphite locally appears to replace the host. Within the nodules the graphite is compact, fine grained and remarkable for its purity. It is accompanied by small amounts of quartz, chlorite, pyrite and chalcopyrite.

The petrographical relationships and the inferred high formation temperature (c. 500°C) suggest a close genetic relationship between mineralisation and the intrusion. As all mafic intrusions in the Lake District are of Caradoc age (with the exception of the Early Devonian lamprophyres), the graphite mineralisation at Seathwaite is also likely to be of Caradoc age. The carbon was derived from organic-rich sediments within the underlying Skiddaw Group and was probably deposited from CO2- and CH4-bearing aqueous fluids.

== Copper veins ==
Veins in which copper-bearing minerals are the main metallic constituents comprise an important suite of deposits that were formerly of considerable economic importance. The greatest concentrations of this type of mineralisation occur at Coniston (SD 280 970), Ulpha (SD 186 923) and Haweswater (NY 482 133), with examples more widely distributed at Black Combe, the Vale of Newlands, and in parts of the Caldbeck Fells [[Media:P916088.jpg|(P916088)]]. The peak years for Lake District copper production were firstly during Elizabethan times and then again during the 19th century. Very small amounts of copper ores continued to be raised at Coniston into the early years of the 20th century.

In these deposits, chalcopyrite is the most widespread primary copper sulphide mineral, though tennantite, chalcocite and bornite are locally abundant, the last in situations which suggest some secondary enrichment. Associated ore minerals commonly include abundant arsenopyrite, pyrite and pyrrhotite, with very much smaller amounts of native bismuth, bismuthinite, bismuth sulphoselenides and sulphotellurides, cobalt and nickel minerals, galena and sphalerite. Traces of gold have been identified in a few localities. Gangue minerals mainly comprise quartz, chlorite, dolomite, and locally, stilpnomelane. Magnetite, most of which appears to replace original haematite, is plentiful in a few veins, notably the Bonsor Vein at Coniston [[Media:P246074.jpg|(P246074)]], where mine records suggest the proportion of this mineral increases with depth. Copper-bearing veins in the Consiton area appear to have been widest, up to several metres across, and were most productive where they cut silicic ignimbrites.

Within the Bonsor Vein, temperatures of 350–400°C have been suggested for the deposition of early arsenopyrite and replacement of early haematite by magnetite. Quartz, chlorite, stilpnomelane, calcite, dolomite, pyrrhotite, chalcopyrite, sphalerite and later arsenopyrite were probably deposited at temperatures of around 240°C, with later minerals including pyrite, native bismuth, bismuthinite and galena likely to have been deposited at temperatures as low as 200°C. Limited fluid inclusion studies on quartz from veins regarded as part of this suite in the Vale of Newlands suggest that the mineralising fluids were moderately saline brines (about 5–10 equiv.wt per cent NaCl). The Borrowdale Volcanic Group rocks have been proposed as the source of the metallic elements whereas the Skiddaw Group is considered the most likely source of the sulphur. These veins predate the regional Acadian cleavage and are therefore likely to have formed during or shortly after the final phases of caldera volcanism.

== Tungsten veins ==
A small group of veins, formerly worked at the Carrock Fell Mine (NY 323 329) in Mosedale [[Media:P916088.jpg|(P916088)]], comprise the only known occurrence of tungsten mineralisation in Britain outside of south-west England that has ever attracted commercial interest. Mining for tungsten appears to have begun here in the mid 19th century but proved unsuccessful. Subsequent attempts to resume working all failed, the latest ending in 1981.

The tungsten ores wolframite and scheelite are accompanied at Carrock Fell Mine [[Media:P220211.jpg|(P220211)]] by arsenopyrite, pyrrhotite and pyrite in quartz–muscovite–apatite veins which strike approximately north–south through the Grainsgill cupola of the Skiddaw Pluton, the Carrock Fell Centre, and other rocks associated with them. Minor consitituents of the veins include native bismuth, bismuthinite, bismuth sulphotellurides, molybdenite (with a Re-Os age for mineralisation of about 392 Ma), iron-rich sphalerite and traces of gold. A few specimens of cassiterite have been reported. Significant tungsten mineralisation appears to have been confined to strike lengths of only 1 km centred around the mine, though panned concentrates from surrounding streams suggest that tungsten minerals may be more widely distributed.

The Carrock Fell veins are genetically associated with the Early Devonian Skiddaw Granite, much of which is here metasomatised to greisen. The mineralising fluids are interpreted as moderately saline solutions that were periodically charged with CO2 and enriched in tungsten. Fluid inclusion studies suggest temperatures of 240–295°C for the formation of the greisen, and 265–295°C for wolframite mineralisation. The tungsten-bearing veins are cut by later quartz and galena-bearing veins and there is abundant evidence of a complex series of mineralising events. Supergene alteration has produced a great variety of unusual mineral species.

Elsewhere in the Lake District, small crystals of scheelite have been found, together with some molybdenite, in drusy cavities in the Early Devonian Shap Granite; the molybdenite has given a Re-Os age of about 405 Ma for mineralisation. Scheelite also occurs, accompanied by rare wolframite, in a narrow quartz vein in the Ordovician Broad Oak Granodiorite at Buckbarrow Beck on Corney Fell (SD 135 910). This locality is of particular interest for the abundance of rare supergene species including cuprotungstite, russellite and bismutoferrite. Although relatively high concentrations of tungsten are known in greisen associated with the Eskdale Granite near Devoke Water, no tungsten minerals have been recorded there.

== Apatite–chlorite veins with cobalt ==
A north-north-east-trending vein which cuts bleached Skiddaw Group mudstones within the Crummock Water aureole at Scar Crag on Causey Pike (NY 206 207) is unique amongst exposed Lake District veins in carrying, as its main constituents, quartz, apatite, chlorite and arsenopyrite with traces of the cobalt minerals alloclasite, cobaltite, glaucodot and skutterudite. Unsuccessful attempts were made in the 19th century to work the vein for cobalt ores. Similar quartz–apatite–chlorite veins, though without sulphides, occur at Brown How (NY 115 158) and Crag Fell (NY 095 148) in Ennerdale. The mineralisation is likely to be an Acadian phenomenon genetically associated with formation of the Crummock Water aureole above a concealed granite.

== Tourmaline veins ==
Small amounts of tourmaline are associated with most of the larger granitic intrusions. However, numerous tourmaline-rich quartz veins up to 2 m wide are especially common within parts of the Crummock Water metasomatic aureole.

== Antimony veins ==
In addition to their presence as minor components in other veins, or as minute inclusions within galena in the lead–zinc veins, antimony minerals comprise the main ore minerals in a small, but distinctive suite of veins within the Lake District. The best known occurrence of antimony mineralisation, and the only one known to have been of economic interest, is that at Robin Hood (NY 228 328), near Bassenthwaite, where a few tonnes of stibnite were mined during the 19th century from an antimony-bearing quartz vein in Skiddaw Group rocks. Other veins dominated by antimony ore minerals include a stibnite–berthierite–zinkenite-rich vein in Skiddaw Group rocks at Wet Swine Gill (NY 314 322), Caldbeck Fells, berthierite and jamesonite-bearing veins in Borrowdale Volcanic Group rocks at Hogget Gill (NY 389 112), near Brothers Water, and stibnite veins reported from St Sunday Crag (NY 360 130), also in the Borrowdale Volcanic Group. The Wet Swine Gill Vein may be associated with the nearby Carrock Fell Mine suite of tungsten-bearing veins. A boulder of pure stibnite weighing up to 50 kg, found in boulder clay near Troutbeck Station (NY 390 270), suggests that further antimony-rich veins could be concealed beneath glacial deposits in the northern Lake District. Few investigations have been undertaken on Lake District antimony mineralisation, the origins and age of which remain uncertain.

== Lead–zinc veins ==
Veins in which lead and zinc minerals are the dominant ores form, like the copper-bearing veins, a very important suite of deposits in the Lake District, where they were formerly of considerable economic importance. The most significant concentrations occur in the Vale of Newlands, at Thornthwaite (NY 223 258), Brandlehow (NY 250 196), Helvellyn (NY 325 148), Hartsop (NY 410 125), Eagle Crag (NY 358 142), Greenside (NY 365 174) and Force Crag (NY 200 215), and around Threlkeld and in the Caldbeck Fells. The Lake District has had a long history as a lead producer: the area’s last major lead mine, Greenside, closed in 1962 though attempts at small-scale production of zinc ore with associated baryte continued at Force Crag until 1990.

Galena and sphalerite are the main primary ore minerals in these veins, accompanied locally by minor amounts of chalcopyrite and tetrahedrite. Minute inclusions of native antimony and antimony sulphosalts are common within the galena. Silver is almost invariably present within the galena, probably within minute inclusions of lead, copper and antimony sulphosalts. Assay values of up to 30 ozs of silver per ton of lead (838 ppm) have been recorded from the Caldbeck Fells and high concentrations of silver are known to be present in tetrahedrite at Eagle Crag and elsewhere. Small specimens of native silver have been reported from supergene assemblages within the near-surface parts of veins at Force Crag and Red Gill on the Caldbeck Fells. The Force Crag Vein contains significant concentrations of manganese oxide minerals in its upper, supergene zone: a few tonnes of manganese ore are understood to have been mined here. Gangue minerals in these veins include abundant quartz, baryte, calcite, dolomite and locally siderite. Baryte is especially common in the upper parts of the Force Crag Vein, which was worked partly for this mineral. Fluorite is a minor constituent of several veins but is present in substantial amounts in veins at Brandlehow and Whitecombe Beck, Black Combe. Quartz pseudomorphs and epimorphs after baryte are common, suggesting that baryte was formerly a major constituent of these deposits.

Depositional temperatures in the range 110–130°C have been suggested. Metals may have been derived from rocks of the Skiddaw Group, the granitic batholith or from Carboniferous sediments in the adjoining Solway–Northumberland Trough by a process of convective leaching involving Carboniferous sea water. Sulphur isotope studies support derivation of sulphur from Carboniferous evaporites present at depth in north Cumbria. The Lake District lead–zinc veins are known to cut and thus postdate the copper-bearing veins, and a limited number of K-Ar determinations of altered vein wallrock has given ages of between 360 and 330 Ma. However, many aspects of the veins invite close comparison with those of the Northern Pennine Orefield and it is possible that the lead–zinc veins of the Lake District may be members of the same suite of veins as their Northern Pennine counterparts, but exposed at a deeper structural level within Lower Palaeozoic wall rocks. If so, a Late Carboniferous or Early Permian age of emplacement seems probable since this has been well established for the northern Pennines area. Evidence from boreholes in the Sellafield area suggests that lead–zinc mineralisation may be similar in age to the main phase of haematite mineralisation in west Cumbria, which is likely to be Permo-Triassic or later as discussed below.

== Baryte veins ==
Baryte is a common gangue mineral in several of the lead–zinc veins but is present in unusually great abundance in several veins in the Caldbeck Fells, notably at Potts Gill, where it was an important commercial mineral. There, the baryte is accompanied only by quartz and manganese oxides with traces of sulphide minerals. These veins, and perhaps the baryte mineralisation at Force Crag, may represent a separate episode of baryte mineralisation, perhaps postdating the lead–zinc mineralisation.

== Haematite veins ==
Although haematite is locally a constituent of a variety of veins within the Lake District, it is the dominant mineral within a distinctive suite of veins within the Lower Palaeozoic rocks. Notable formerly economic concentrations of haematite veins include those within Skiddaw Group rocks at the Knockmurton and Kelton Fell mines near Loweswater and within the Eskdale Granite in mines near Boot. Other less economically significant veins include those within Borrowdale Volcanic Group rocks at Ore Gap (NY 241 072), Grasmere (NY 341 098) and Deepdale (NY 390 150), and in the Ennerdale intrusion. Vein widths of up to 7 m were encountered at Kelton Fell. All of these veins are typically filled with haematite with very small amounts of quartz, dolomite or calcite comprising virtually the only gangue minerals. Haematite is usually present as the mammillated fibrous crystalline variety known as ‘kidney ore’, though compact, massive and crystalline forms (specular ore) are found locally. The characteristic form of the haematite and the almost monomineralic nature of the serves to link them genetically with the very large replacement bodies of haematite within the Carboniferous limestones of west and south Cumbria. Their origin and Permo-Triassic (or younger) age are considered below.

== Gold ==
Although claimed from a number of locations within the Lake District, many of these reports must now be regarded as unreliable. Nevertheless, gold has been reliably recorded from a variety of deposits of different ages including the Carrock Fell tungsten deposit, from a gossan in the Black Combe area, in copper veins in the Coniston area, at Dale Head in the Vale of Newlands and in panned concentrates from various streams in the Cockermouth area. The name ‘Goldscope Mine’ (NY 226 185) in the Vale of Newlands is almost certainly a corruption of an old German name and appears to have no connection with the presence of gold. There is no evidence for any gold ever being recovered commercially from the Lake District.

== Supergene mineralisation ==
Parts of the Lake District, particularly the Caldbeck Fells, are famous for the abundance, variety and beauty of supergene minerals within the near-surface parts of the veins. Whereas many of these are the products of weathering related to modern water-table levels, the presence of such minerals at depths well below the present oxidation zone may be the result of earlier supergene processes, perhaps as early as the Jurassic.

== Bibliography ==
Bouch, J E, Naden, J, Shepherd, T J, McKervey, J A, Young, B, Benham, A J, and Sloane, H J. 2006. Direct evidence of fluid mixing in the formation of stratabound Pb-Zn-Ba-F mineralisation in the Alston Block, North Pennine Orefield (England). ''Mineralium Deposita'', Vol. 41, 821–835.

Cooper, M P, and Stanley, C J. 1990. ''Minerals of the English Lake District – Caldbeck Fells. ''(London: British Museum, Natural History.)

Crowley, S F, Bottrell, S H, McCarthy, M D B, Ward, J, and Young, B. 1997 δ34S of Lower Carboniferous anhydrite, Cumbria and its implications for barite mineralization in the northern Pennines. ''Journal of the Geological Society of London'', Vol. 154, 597–600.

Fortey, N J, Ingham, J D, Skilton, B R H, Young, B, and Shepherd, T J. 1984. Antimony mineralisation at Wet Swine Gill, Caldbeck Fells, Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 45, 59–65.

Ixer, R A, Stanley, C J, and Vaughan, D J. 1979. Cobalt-, nickel-, and iron-bearing sulpharsenides from the North of England. ''Mineralogical Magazine'', Vol. 43, 389–395.

Ixer, R A, Young, B, and Stanley, C J. 1996. Bismuth-bearing assemblages from the Northern Pennine Orefield. ''Mineralogical Magazine'', Vol. 60, 317–324.

Lowry, D, Boyce, A J, Pattrick, R A D, Fallick, A E, and Stanley, C J. 1991. A sulphur isotopic investigation of the potential sulphur sources for Palaeozoic-hosted vein mineralization in the English Lake District. ''Journal of the Geological Society of London'', Vol. 148, 993–1004. 272

Millward, D, Beddoe-Stephens, B, and Young, B. 1999. Pre-Acadian copper mineralisation in the English Lake District. ''Geological Magazine'', Vol. 136, 159–176.

Milodowski, A E, Gillespie, M R, Naden, J, Fortey, N J, Shepherd, T J, Pearce, J M, and Metcalfe, R. 1998. The petrology and paragenesis of fracture mineralization in the Sellafield area, west Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 52, 215–241.

Shepherd, T J, and Goldring, D C. 1993. Cumbrian hematite deposits, northwest England. 419–443 in ''Mineralization in the British Isles''. Patrick, R A D, and Polya, D (editors). (London: Chapman and Hall.)

Stanley, C J, and Vaughan, D J. 1982. Copper, lead, zinc and cobalt mineralization in the English Lake District: classification, conditions of formation and genesis. ''Journal of the Geological Society of London'', Vol. 139, 569–579.

Vaughan, D J, and Ixer, R A. 1980. Studies of the sulphide mineralogy of north Pennine ores, and their contributions to genetic models. ''Transactions of the Institution of Mining and Metallurgy'', Vol. 89, B99–100.

Young, B. 1985. The distribution of barytocalcite and alstonite in the Northern Pennine Orefield. ''Proceedings of the Yorkshire Geological Society'', Vol. 45, 199–206.

Young, B. 1987. ''Glossary of the minerals of the Lake District and adjoining areas''. (Newcastle upon Tyne: British Geological Survey.)

Young, B, Styles, M P, and Berridge, N G. 1985. Niccolite-magnetite mineralization from Upper Teesdale, North Pennines. ''Mineralogical Magazine'', Vol. 49, 555–559.

[[Category: Northern England]]

Mineralization in Northern England

2016-03-24T19:33:43Z

BobMcIntosh:

Late Mesozoic and Cenozoic tectonics and magmatism, Northern England

2016-03-24T19:30:31Z

BobMcIntosh:

Triassic and Jurassic environment and lithostratigraphy, Northern England

2016-03-24T19:28:13Z

BobMcIntosh:

Late Permian environment and lithostratigraphy, Northern England

2016-03-24T19:23:52Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
== Introduction ==
[[File:P916085.jpg|thumbnail|Distribution of depositional environments across northern England during Permian and Triassic times (after Cope et al., 1992. Atlas of Palaeography and Lithofacies, Geological Society of London Memoir, No. 13). P916085.]]
[[File:P916086.jpg|thumbnail|Cartoons illustrating the changes in depositional regime through Permian and Triassic times (after Akhurst et al., 1997. The geology of the west Cumbria district. BGS Memoir). P916086.]]
[[File:P916115.jpg|thumbnail|Lithostratigraphical subdivision of the Permian, Triassic and Jurassic rocks of north-west England. P916115.]]
[[File:P916116.jpg|thumbnail|Lithostratigraphical subdivision of the Permian and Triassic rocks of north-east England. P916116.]]
[[File:P221601.jpg|thumbnail|Basal Permian unconformity near the bottom of the face in the disused (and now backfilled) East Thickly Quarry [NZ 2407 2565], County Durham. Thinly bedded dolostone of the Raisby Formation overlies the Marl Slate and Yellow Sands formations, the latter resting on thicker, cross-bedded sandstones of the Pennine Lower Coal Measures Formation. (P221601).]]
[[File:P552051.jpg|thumbnail|Fossil fish remains. (Palaeoniscus) from the Marl Slate Formation in the now-backfilled Down Hill Quarry, West Boldon, Sunderland [NZ 349 601]. (P552051).]]
[[File:P693034.jpg|thumbnail|Collapse-brecciated limestone of the Roker Formation produced following dissolution of the underlying Hartlepool Anhydrite. Trow Point, Sunderland [NZ 385 666]. (P693034). ]]

Following deposition of the dominantly aeolian strata of early Permian times, a profound environmental change occurred over much of northern England [[Media:P916085.jpg|(P916085)]]b. The principal factors driving this change were repeated temporary marine incursions into large areas of the region, and the effects of the resultant higher water table on sedimentation in the surrounding areas. The high ground of the Pennines, Lake District, Isle of Man and the Southern Uplands of Scotland suffered continued erosion and, by the end of the Permian Period, were reduced to areas of very low relief or in some areas had become sites of deposition.

==== West of the Pennines: Cumbrian Coast Group ====
To the west of the Pennines, the Bakevellia Sea flooded into low-lying areas. Marine conditions were established in the main depositional centres of the Solway Firth and East Irish Sea basins, whilst carbonate platforms developed in many marginal marine areas such as south and west Cumbria. Extensive evaporitic sabkhas surrounded the marine areas and, at times of marine regression, extended into the basin centres. Adjacent to high ground and sources of sediment supply, siliciclastic deposition locally overwhelmed the evaporitic sabkhas. As a consequence of this range of palaeoenvironoments, the strata of late Permian age in north-west England are highly variable.

In areas close to sources of sediment supply, evaporite beds are thin and separated by significant thicknesses of reddish-brown siltstones interbedded with fine-grained sandstone and with some conglomerate and breccia. These strata form the Eden Shales Formation and crop out in northern Cumbria, around the margins of the Carlisle and Vale of Eden basins. The outcrop between the two basins is contiguous, indicating that, unlike the situation in the early Permian, late Permian sedimentation was more widespread and not restricted to isolated basin areas. The siltstones and sandstones of the formation are both regularly and irregularly bedded and are locally structureless. The irregularly laminated rocks were deposited by accretion of wind-blown sand and silt; the more evenly laminated strata were deposited by muddy sheet floods. Towards the base of the formation thin beds of gypsum and anhydrite occur, suggesting the periodic establishment of sabkhas and ephemeral shallow lakes that dried out to leave desiccation cracks in siltstone and to produce dried mud flakes that were incorporated into overlying beds of sandstone and conglomerate. East of Appleby, the basal strata comprise laminated sandstone interbedded with calcareous siltstone containing abundant carbonaceous plant debris. These strata are collectively known as the Hilton Plant Beds and probably accumulated as sheet-flood deposits peripheral to a desert lake. The plant beds are well exposed in Hilton Beck (NY 7196 2058). Generally in the Vale of Eden sequence, four distinct evaporite beds exist and are known locally as ‘A’ (the lowest) to ‘D’(the highest) Beds. The ‘A’ bed anomalously contains halite in addition to gypsum and anhydrite. It is likely that the halite was deposited subaerially within the silty sediment of a sabkha environment, possibly causing volume increase of the sediment and so elevating it to a subaerial position where desiccation produced mud cracks.

In basinal areas, distant from the sources of sediment supply, dolostone, anhydrite and halite, with subordinate mudstone, built up to form the St Bees Evaporite Formation [[Media:P916086.jpg|(P916086)]]b, which is the lateral equivalent of the lower part of the Eden Shales [[Media:P916115.jpg|(P916115)]]. The formation is best known from cored boreholes and mine workings in Cumbria where anhydrite is dominant but with gypsum as a secondary replacement. At a shallow depth the evaporite strata may be removed from the sequence by percolating ground waters leading to collapse of the overlying beds. Offshore the formation is less well known but does contain beds of halite, locally in excess of 100 m thick, in addition to anhydrite. Onshore, in south and west Cumbria, carbonate rocks form a major part of the formation and these, together with the evaporite rocks, appear to have formed in response to cyclic sea-level changes. The carbonate rocks contain locally abundant bivalves, stromatolites and ooids that suggest marine deposition in dominantly shallow water [[Media:P916086.jpg|(P916086)]]b. Coeval with the marine deposition of the carbonate rocks, gypsum–anhydrite beds formed in sabkhas and shallow saline lakes peripheral to the marine basins. At times of relatively low sea level the evaporitic facies encroached into the central parts of the basins.

Onshore, in west Cumbria, the St Bees Evaporite Formation is overlain by the St Bees Shale Formation [[Media:P916086.jpg|(P916086)]]c; offshore it is overlain by the Barrowmouth Mudstone Formation [[Media:P916115.jpg|(P916115)]]. Both of these overlying formations are the lateral equivalents of the upper part of the Eden Shales Formation and share many of its lithological characteristics.

==== East of the Pennines: the Zechstein Group ====
To the east of the Pennines, transgression of the Zechstein Sea ended the early Permian continental phase and initiated a prolonged period of marine conditions. The County Durham area lay on the western margin of the Zechstein Sea with the inferred shoreline only slightly to the west of the current Permian outcrop [[Media:P916085.jpg|(P916085)]]b. Reefs developed just offshore and subparallel to the coast, generating a large, shallow lagoonal environment in which significant evaporation could take place. Periodic marine flooding and evaporation led to cyclic sedimentation of carbonates and evaporites. Traditionally, carbonate–evaporite English Zechstein Cycles have been identified, each consisting of sequentially deposited clastic rocks, carbonates and sulphates and culminating in the precipitation of halite and highly soluble potassium and magnesium salts. Five such cycles characterise the marine Permian deposits of north-eastern England, although the cycles are rarely complete and are usually dominated by carbonate deposition. A slightly different picture now emerges from the application of a modern, sequence-stratigraphical approach where sequences are defined as relatively conformable successions of genetically-related strata bounded by unconformities. The unconformities are the result of relative sea-level fall and hence the initial strata of a sequence are those deposited at times of lowstand, or by the ensuing transgression. In north-east England, this alternative approach identifies seven Zechstein Sequences (ZS) consisting of evaporite– carbonate (rather than the traditional carbonate–evaporite) successions. Correlation between Zechstein Sequences and the lithostratigraphical nomenclature of north-eastern England is given in [[Media:P916116.jpg|(P916116)]].

The first sequence (ZS1) began with an initial rapid transgression of the Zechstein Sea that flooded wide areas of early Permian desert. The oldest marine Permian strata preserved in north-east England represent this initial transgression and are the laminated, silty, dolomitic mudstones of the Marl Slate Formation [[Media:P221601.jpg|(P221601)]]. The unusual lithology of the formation has prompted alternative interpretations of its origin, either as a shallow-water lagoonal deposit or as a deeper-water basin deposit. The currently prevailing view is that deposition took place in a barren basin with stagnant waters roughly 200 to 300 m deep, though water depths would have varied significantly, particularly in eastern County Durham, where the flooded draa of the Yellow Sands Formation gave rise to significant variations in sea-floor topography.

The Marl Slate Formation can be traced throughout the entire marine Permian outcrop of north-east England. At outcrop it is yellowish brown in colour but when unweathered it can be seen to consist of alternating bands of dark grey and black sediment that are finely laminated and often bituminous. Locally, the clastic units are interbedded with thin beds of dolostone and dolomitic limestone. Thin layers of aeolian sand are present in many places, particularly near the base of the formation, and are interpreted as Yellow Sands material reworked into the Marl Slate succession by rapid marine transgression. Pyrite, galena and sphalerite coat bedding planes and joints throughout the formation and are thought to have a syngenetic origin, although some degree of diagenetic redistribution is likely. The Marl Slate is well known for its fossil fauna and flora. Plant remains are abundant and well-preserved fossil fish have been collected from a number of localities [[Media:P552051.jpg|(P552051)]]; their preservation may well have been assisted by syngenetic mineralisation.

The top of the Marl Slate Formation is typically marked by a sharp contact with the overlying deposits although in some places, particularly the south-west of County Durham, the contact appears more transitional with thin beds of Marl Slate lithology interbedded with the lowest beds of the overlying formation, known traditionally as the Magnesian Limestone and divided into lower, middle and upper divisions [[Media:P916116.jpg|(P916116)]]. These equate formally to five formations: in ascending order, the Raisby Formation (traditionally the Lower Magnesian Limestone), the Ford Formation (traditionally the Middle Magnesian Limestone) and three formations, the Roker Dolostone, Seaham Residue and Fordon Evaporite (Edlington), and Seaham formations spanning the traditional Upper Magnesian Limestone.

Deposition of the Marl Slate partly filled the early Permian sea-floor topography and gave rise to an undulating, eastwards inclined, marine slope on which the overlying Raisby Formation was deposited. Likely water depths in the depositional basin ranged from approximately 100 m (in present-day coastal areas) to 200 to 300 m in the central parts (now offshore). The Raisby Formation has only a narrow outcrop in north-east England although it does extend beneath younger strata to the east. It is composed of yellow or cream-coloured dolostone and pure, grey limestone (although the latter is rare), which are exposed along an escarpment 30 to 60 m high between Hetton-le-Hole and Ferryhill, County Durham. The formation can be divided into three lithological units distinguished by colour, bedding thickness, texture and compositional variations. The lowermost unit comprises rocks of dolomitic composition through to almost pure limestone in regular beds 15 to 30 cm thick. Laminated argillaceous layers with galena, pyrite and sphalerite are common towards the base of the unit, particularly where there is a transition from the Marl Slate. The middle unit of the Raisby Formation is the most commonly seen in outcrop and consists predominantly of dolostone although, in places, calcitic dolostone is common. These rocks are grey to buff coloured, finely crystalline and thinly bedded in layers 5 to 10 cm thick. The bedding is planar on the large scale, but may be very uneven and nodular in fine detail; abundant stylolitic bedding laminae bear thin films of argillaceous residue. Widespread stratified but brecciated levels exist towards the top of the middle unit, often interbedded with gypsum. The upper unit of the Raisby Formation comprises buff to brown dolostone in irregular lenticular beds up to 45 cm thick. Stylolites and brecciated patches are less common than in the underlying middle unit.

Deposition of the Raisby Formation all but eliminated the sea-floor topography leaving a generally smooth, eastward-dipping surface that was steep enough to cause intermittent instability of the partially lithified sediment accumulation. Contorted and chaotic rock structures produced by minor submarine avalanches and slumping can be seen locally, particularly in the coastal cliffs to the south of South Shields. Such structures are most prevalent towards the top of the formation where the effects of depositional slope angle were probably accentuated by a relative fall in sea level of several metres.

Overlying the Raisby Formation is a thin sequence of fine-grained, clastic dolostone that forms the lowermost strata of the Ford Formation and represents initial lowstand deposition of Zechstein Sequence 2 (ZS2). These beds are known informally as the transitional beds as the junction between them and the underlying dolostone of the upper Raisby Formation is usually indistinct. The earliest transitional beds are barren but the abundance of a fossil shelly fauna increases upwards. The transitional beds are interpreted as marine deposits lain down during a gradual and irregular dilution of the highly saline waters in which the upper parts of the Raisby Formation accumulated. The uppermost transitional beds were probably deposited in near-normal marine conditions, with most of the Ford Formation deposited in the subsequent, shallow-water shelf environment. Shells accumulated in sufficient numbers to form an elongate bank on which a reef-forming fauna could gain a foothold. The formation of the shell bank and subsequent shelf-edge reef had a profound influence on the environment and led to contemporaneous but locally highly varied depositional settings.

The reef itself is perhaps the best-known feature of the upper Permian succession of north-east England. During the early stages of reef formation, growth was predominantly upwards, but there was some lateral expansion subsequently. The lower parts are composed of massive dolomitic limestone and dolostone containing a prolific fauna of brachiopods, bivalves and polyzoa. As the reef built upwards to a maximum height of about 60 m, shallower water encouraged more rapid growth of calcareous algae, whilst increasing salinity led to the gradual extinction of much of the earlier fauna. Consequently, the uppermost parts of the reef are largely of algal origin and contain a wide variety of stromatolitic growth forms, many similar to those found today in the intertidal zone of hypersaline lagoons such as those seen at Shark Bay, Western Australia.

On the western (landward) side of the reef, within a shallow and protected lagoonal environment, a varied sequence of granular ooidal and pisolitic carbonates accumulated with the rate of deposition keeping pace with the upward growth of the reef. These rocks form most of the Ford Formation outcrop and are almost universally dolomitised. The recrystallised platy dolostone crystals, up to 5 mm across, give the rock a characteristic texture known locally as ‘felted’, which is mostly confined to the lagoonal beds of the Ford Formation. The uppermost lagoonal strata interdigitate with the reef deposits.

On the eastern (basinal) side of the reef, deposition was comparatively slow except in the immediate vicinity of the reef edge where wedge-shaped, fore-reef talus aprons developed from detrital material eroded from the front of the reef. The earliest talus aprons were buried by continuing reef growth but have been proved underground in sections at Easington Colliery and in boreholes. Later aprons are exposed within County Durham at Blackhall Rocks [[Media:P221294.jpg|(P221294)]] and Crimdon Beck.

The various reef facies of the Ford Formation crop out in a sinuous belt extending southsouth-east from Down Hill (near Sunderland) towards Hartlepool. The reef lithologies are often more resistant to erosion than the surrounding strata and in places form distinct topographical features such as Beacon Hill near Easington, County Durham. The extent of the reef southwards from Hartlepool is uncertain, although indirect evidence from gravity surveys and boreholes suggest that it may well continue as far south as Seaton Carew. South and west of Seaton Carew, semi-open shelf conditions prevailed during deposition of the Ford Formation, rather than the lagoonal environment present to the north, and its strata are difficult to distinguish in boreholes from those of the underlying Raisby Formation.

Towards the end of Ford Formation times, construction of the reef was terminated by a fall in sea level of several metres. The uppermost member of the Formation is the Hartlepool Anhydrite, which was deposited in shallow water as the lowstand wedge of Zechstein Sequence 3. It consists of dense aggregates of very finely crystalline laths that form nodular masses of almost pure anhydrite. At outcrop in the Tynemouth area, the anhydrite has been largely removed by solution and all that remains is a solution residue of grey-brown, argillaceous and sandy dolostone.

With continuing deposition on the lagoonal and basinal sides of the reef, the Zechstein Sea became increasingly shallow and saline. By the end of Ford Formation times, the reef was largely buried and exercised only minimal control on sedimentation. The Zechstein Sea was by then a very shallow-water, hypersaline and inhospitable shelf-edge environment.

Deposition of the Roker Formation on the shelf and adjacent basin slope marks the onset of the next major marine transgression. The slope facies is one of the most highly varied and spectacular carbonate units of the Permian succession in northern England. At the base of the formation, the Concretionary Limestone Member comprises thinly bedded granular dolostone that is locally recrystallised and contains calcite concretions [[Media:P693033.jpg|(P693033)]]. In the Sunderland area, these concretions are spectacularly well developed and the unit is known informally as ‘cannonball rock’. In the coastal exposures of County Durham, the concretionary limestone can be divided into a lower unit up to 15 m thick containing abundant concretions and an upper unit, up to 20 m thick, in which they are absent. However, overall distribution of concretions is laterally variable. Where concretions are not present, the rock is largely a soft granular dolostone with traces of small-scale cross-bedding and ripple marks. Slightly below the middle of the member is a thinly laminated impure dolostone that is creamy-grey in outcrop but grey and bituminous at depth. It can be split into paper thin, flexible sheets and is know informally as the Flexible Limestone. This unit has yielded fish remains in the Sunderland area whilst plant remains are common throughout its outcrop in northern England. In County Durham and the Tynemouth area, the Concretionary Limestone has been brecciated by collapse following solution of the underlying Hartlepool Anhydrite, and now consists of angular and rounded fragments of grey-brown crystalline limestone in a matrix of brown dolomitic limestone [[Media:P693034.jpg|(P693034)]].

The shelf deposits of Roker Formation consist mostly of the Hartlepool and Roker Dolostones, which are composed almost entirely of soft, granular and ooidal, cross- and ripple-bedded dolostone with little significant variation in lithology. The dolostone units are mainly exposed in coastal cliffs around Roker and Seaham and to the north of Hartlepool, where many have foundered as a result of solution of the underlying Hartlepool Anhydrite. The lithology of the dolostones is consistent with deposition in a shallow-water shelf environment, but the presence of rip-up clasts and minor erosion surfaces may indicate the subaerial emergence of sediment deposited partly within the intertidal zone.

After deposition of the Roker Formation, marked sea-level oscillations have been inferred, the evidence drawn from a curious evaporite that contains sedimentary features characteristic of both shallow and deepwater conditions. This is the Seaham Residue and Fordon Evaporite Formation. Within it, Fordon Evaporites are present today only in the subsurface offshore where they reach a thickness of up to 90 m and consist primarily of gypsum, halite and anhydrite with some dolostone. They formerly extended westwards at least as far as the present coastline, but there they have been largely removed by solution, leaving only the Seaham Residue and resulting in significant foundering of the overlying rocks. The Seaham Residue is up to 9 m thick at its type locality of the coastal cliffs at Seaham Harbour and consists primarily of limestone and a dolomitic clay residue. The formation is believed to represent the lowstand facies marking the initiation of Zechstein Sequence 4, with parasequences within it indicating subordinate sea-level oscillations. In the Stockton area, south of the Seaton Carew Fault, the lateral equivalent of the Seaham Residue and Fordon Evaporites [[Media:P916116.jpg|(P916116)]] is the Edlington Formation (traditionally the Middle Permian Marls).

Deposition of the Fordon Evaporite largely completed the filling of the Zechstein Basin. Sedimentation thereafter took place in a relatively shallow water environment, perhaps less than 20 m deep, below the tidal zone and with little or no basin floor relief. Under these conditions the Seaham Formation accumulated. It is relatively uniform in lithology and comprises the transgressive strata of Zechstein Sequence 4. It consists mostly of mudstone (some calcareous), limestone and dolostone and is exposed in the coastal cliffs around Seaham. The whole formation has been severely disrupted and locally brecciated by foundering arising from solution of the underlying Fordon Evaporite, now represented only by the Seaham Residue. Foundering is generally less severe and less extensive than that affecting the stratigraphically lower Concretionary Limestone. The Seaham Formation carries a distinctive assemblage of bivalves and algae with abundant, small tubular remains of the probable alga Calcinema permiana. Algal laminates and sedimentary features such as cut-and-fill structures, low angle cross-lamination, and low amplitude–long wavelength ripples, all increase in abundance upwards through the formation and may indicate shallowing to a high subtidal environment by the end of its deposition.

The limited diversity, but great abundance, of fossil remains within the Seaham Formation indicates unique environmental controls on the plant and animal population, most probably indicating hypersalinity of the water. Coupled with decreasing water depth, this led to the development by the end of Seaham times of sabkha conditions under which the youngest Permian strata of northern England were deposited. The Billingham Anhydrite Formation is now present only in the offshore area where it is 3 to 6 m thick and composed of gypsum and anhydrite with some dolostone. Offshore to the east of Billingham, the anhydrite is overlain by the much thicker Boulby Halite Formation. The Billingham and Boulby formations contain deposits assigned to Zechstein Sequence 5.

The disappearance of permanent open water and the re-establishment of arid conditions over north-east England are indicated by the nature of the succeeding Rotten Marl Formation. It consists of dull, dark red-brown, silty mudstone commonly with scattered halite crystals and cut by a network of veins containing fibrous halite and gypsum. This formation demonstrates the first real clastic input into the basin but is preserved only in the south-east of the region, south of the West Hartlepool Fault, and offshore. Although the Rotten Marl can be distinguished from the overlying Roxby Formation in offshore boreholes, the same distinction is not possible at outcrop in the Durham area where the boundary between the two units is transitional, with the Rotten Marl commonly represented only by a thin residual layer. This is incorporated stratigraphically at the base of the Roxby Formation which, in such a situation, directly overlies the Boulby Halite Formation. In its lower part, the Roxby Formation is a dominantly mudstone sequence, but the frequency and thickness of siltstone and sandstone interbeds increase upwards until the Roxby Formation passes gradually upwards into the Early Triassic Sherwood Sandstone Group.

The sequence stratigraphy of these upper Zechstein strata is somewhat subjective due mainly to the limited data available. The Rotten Marl Formation was deposited at a time of low sea level, on coastal flood plains, in salt pans and in lagoons. It is more proximal than the Boulby Halite Formation, which implies a sequence boundary between the two with the Rotten Marl representing the intiation of Zechstein Sequence 6. The Roxby Formation was deposited at a time of higher sea level and may represent a further sequence, Zechstein Sequence 7, with related shallow water deposits out towards the basin centre, or it may represent the uppermost strata of Sequence 6.
== Bibliography ==
Barnes, R P, Ambrose, K, Holliday, D W, and Jones, N S. 1994. Lithostratigraphical subdivision of the Triassic Sherwood Sandstone Group in west Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 51–60.

Benton, M J, Cook, E, and Turner, P. 2002. Permian and Triassic red beds and the Penarth Group of Great Britain. ''Geological Conservation Review'', No. 24. (Peterborough: Joint Nature Conservation Committee.)

Brookfield, M E. 2004. The enigma of fine-grained alluvial basin fills: the Permo-Triassic (Cumbrian Coastal and Sherwood Sandstone Groups) of the Solway Basin, NW England and SW Scotland. ''International Journal of Earth Sciences'', Vol. 93, 282–296.

Holliday, D W, Holloway, S, McMillan, A A, Jones, N S, Warrington, G, and Akhurst, M C. 2004. The evolution of the Carlisle Basin, NW England and SW Scotland. ''Proceedings of the Yorkshire Geological Society'', Vol. 55, 1–19.

Holliday, D W, Jones, N J, and McMillan, A A. 2008. Lithostratigraphical subdivision of the Sherwood Sandstone Group (Triassic) of the north-eastern part of the Carlisle Basin, Cumbria and Dumfries and Galloway, UK. ''Scottish Journal of Geology'', Vol. 44, 97–110.

Ivimey-Cook, H C, Warrington, G, Worley, N E, Holloway, S, and Young, B. 1995. Rocks of Late Triassic and Early Jurassic age in the Carlisle basin Cumbria (north-west England). ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 305–316.

Jackson, D I, and Johnson, H. 1996. ''Lithostratigraphic nomenclature of the Triassic, Permian and Carboniferous of the UK offshore East Irish Sea Basin''. (Nottingham: British Geological Survey for the United Kingdom Offshore Operators Association.)

Jones, N S, and Ambrose, K. 1994. Triassic sandy braidplain and aeolian sedimentation in the Sherwood Sandstone Group of the Sellafield area, west Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 61–76.

Smith, D B. 1989. The late Permian palaeogeography of north-east England. ''Proceedings of the Yorkshire Geological Society'', Vol. 47, 285–312.

Smith, D B. 1995. Marine Permian of England. ''Geological Conservation Review'', No. 8. (Peterborough: Joint Nature Conservation Committee.)

Tucker, M E. 1991. Sequence stratigraphy of carbonate–evaporite basins; models and applications to the Upper Permian (Zechstein) of northeast England and adjoining North Sea. ''Journal of the Geological Society of London'', Vol. 148, 1019–1036.

Worley, N E. 2005. The occurrence of halite in the Permian A Bed Evaporite, Kirkby Thore, Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 55, 199–203.

[[Category: Northern England]]

Early Permian environment and lithostratigraphy, Northern England

2016-03-23T21:08:22Z

BobMcIntosh:

Permian, Triassic and Jurassic: deserts, rivers and shallow seas, introduction, Northern England

2016-03-23T21:04:35Z

BobMcIntosh:

Early Permian magmatism, Northern England

2016-03-23T21:03:31Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
==Introduction ==
[[File:P916081.jpg|thumbnail|Outcrop of the early Permian, tholeiitic Whin Sill-swarm and associated dyke subswarms. Key boreholes proving dolerite: Cr Crook; Et Ettersgill; Ha Harton; Lo Longhorsely; Lc Longcleugh; Ro Rookhope; Th Throckley; WB Whitley Bay; Wo Woodland. P916081.]]
[[File:P643594.jpg|thumbnail|Columnar jointed dolerite in the Alnwick Sill at Cullernose Point on the Northumberland coast. In the foreshore is a ‘whaleback’ anticline in the Four Fathom (Sandbanks) Limestone. (P643594).]]
[[File:P916082.jpg|thumbnail|Isopachytes, in metres, on the Whin Sill-swarm. Where more than one leaf of dolerite is present, the total thickness is given. P916082.]]
[[File:P916073.jpg|thumbnail|Representative sections and correlations for the Alston Formation, Yoredale Group,showing correlation with the Eskett Formation of west Cumbria. P916073.]]
[[File:P637452.jpg|thumbnail|Ropy flow-structure on the lower, inner surface of a large, flattened vesicle at the top of the Farne Islands Dolerite Sill at Harkess Rocks, Bamburgh, Northumberland coast [NU 177 358]. The hammer head is 17 cm long. (P637452).]]
[[File:P916083.jpg|thumbnail|Sketch of a south-to-north section through the centre of Castle Hill, Holy Island, showing the alternating dyke-like and sill-like segments of the Holy Island Dyke (after Goulty et al., 2000). Castle Hill and the bench feature are illustrated on the front cover of this book. The bench feature is interpreted as part of a step-and-stair transgression of bedding during dyke intrusion.) P916083.]]
[[File:P916084.jpg|thumbnail|Diagrams illustrating the mechanism of intrusion of the Whin Sill-swarm. a dykes are intruded to within 1 kilometre of the surface; b lateral intrusion of magma leads to gravitational flow down-dip and ponding of the magma in the centre of the basin; c to achieve hydrostatic equilibrium, magma advances up-dip on the other side of the basin with interfingering at the leading edge. Broken lines denote variation introduced where there are multiple dyke sources (after Francis, 1982). P916084.]]

[[File:P916033.jpg|thumbnail|Series of palaeogeographical reconstructions showing continental movements from the Ordovician to the Palaeogene (after Woodcock and Strachan, 2000). P916033.]]

The tholeiitic basaltic intrusions emplaced in northern England during earliest Permian times comprise the Whin Sill-swarm and east-north-east-trending dykes of the Northern England Tholeiitic Dyke-swarm. These intrusions have played an important role in the early development of geological science in Great Britain. In northern England, the word ‘sill’ was used to describe a flat-lying layer of rock, and ‘whin’ meant hard. Hence, the term ‘Whin Sill’ may have been in use long before the origin of the rock was known and is most likely the first geological use of the word ‘sill’. The Great Whin Sill was recognised to be of igneous origin early in the nineteenth century, but there was debate as to whether it was an intrusive sheet or a lava flow. Its intrusive origin was finally established through investigations in Northumberland, and the Great Whin Sill became regarded as the type example of a sill.

== Whin Sill-swarm ==
The Whin Sill-swarm underlies about 4500 km2 of northern England, extending from the southernmost outcrops in Lunedale, west as far as the Pennine escarpment, and north to Holy Island [[Media:P916081.jpg|(P916081)]]. Four separate sills have been identified from the outcrop distribution, magma-flow directions, and differences in petrology and palaeomagnetic signatures. These are the Farne Islands and Alnwick sills in the north, the Great Whin Sill, which crops out in Teesdale, the Pennine escarpment and along the Roman Wall, and the Little Whin Sill in Weardale. The volume of magma intruded was at least 215 km3 and possibly much more, as the sills appear to thicken and extend eastwards for some distance under the North Sea.

The sill-swarm is composed of quartz-dolerite and this tough rock typically forms picturesque crags and scarps that dominate the landscape of parts of north-east England. In Northumberland, the Farne Islands are almost entirely made of dolerite. North of the Tyne valley, the Romans built an important segment of Hadrian’s Wall along the impressive dolerite scarp that runs for a distance of 25 km. In upper Teesdale, the Great Whin Sill forms waterfalls on the River Tees at High Force (NY 880 284) and at Cauldron Snout (NY 814 286), where the river cascades spectacularly over columnar-jointed crags. Columnar cooling joints are well developed in many other places, for example at Sewingshields Crags (NY 800 700), Longhoughton Quarry (NU 231 153), and Cullernose Point (NU 259 221) [[Media:P643594.jpg|(P643594)]]; Low Force (NY 903 277) is a series of rapids where the river flows over columnar-jointed dolerite. Both sills and dykes provide solid foundations for castles at Dunstanburgh, Bamburgh and Holy Island.

The Farne Islands and Alnwick sills were emplaced within Carboniferous rocks overlying the Cheviot Block. From the Farne Islands and Budle Bay to the Kyloe Hills, the Farne Islands Sill is up to about 30 m thick and was intruded into five horizons that range from the upper part of the Fell Sandstone Formation, in the north of the outcrop, to the Eelwell Limestone within the Alston Formation on the Farne Islands. To the south of this, in Beadnell Bay, the horizon of the Alnwick Sill is some 120 m higher in the stratigraphy within the lower part of the Stainmore Formation. The sill’s horizon thence falls stratigraphically southwards to lie at a level within the Tyne Limestone Formation at its most southerly outcrop near Newton Burn (NU 145 068). The Alnwick Sill varies from about 6 to 21 m thick.

The Great Whin Sill is on average about 30 m thick. The thickest single leaf has been recorded in borings in West Allendale (81 m) and Weardale (90 m, but this was near a small horizon change and may be anomalous). The sill tends to thin towards its northern, western and southern margins [[Media:P916082.jpg|(P916082)]]. Along the Roman Wall, the thickness of the sill varies from 20 to 50 m. Multiple sheets occur at outcrop only east of Thockrington (NY 970 800), though in boreholes more than one sheet has been recorded at depth: for example, three dolerite sheets recorded in the Harton Borehole (NZ 3966 6563) total 90 m. In upper Teesdale, the Great Whin Sill intrudes its lowest stratigraphical level within the Melmerby Scar Limestone (for lithostratigraphy see [[Media:P916073.jpg|(P916073)]]. From here, it thins and rises in stratigraphical level in every direction, forming a ‘saucer-shaped’ intrusion. This form is also demonstrated along the sill’s northern outcrop where its level changes systematically from the Oxford Limestone north-eastwards into lowest Namurian strata and westward to its highest stratigraphical level within the Westphalian B (Duckmantian) Coal Measures in the Midgeholme coalfield. The presence of the sill in Coal Measures strata here suggests that before intrusion these rocks had been juxtaposed against Dinantian strata by displacement on the Stublick Fault. The changes in stratigraphical level occur in transgressive steps, separated by significant distances where the intrusion maintains a constant level. In places, the sill transgressions are fault controlled, for example between Steel Rigg and Sewingshields Crags (NY 751 676 to 813 704), but elsewhere the sill rises either gradually or by short steps.

At outcrop near Stanhope in Weardale, the Little Whin Sill is a flat-lying, columnar-jointed dolerite sheet, up to 13 m thick, intruded into the Three Yard Limestone of the Alston Formation. The sill thins westwards and dies out near Ludwell. To the north, in the Rookhope Borehole (NY 937 427), some 6 km north-west of Weardale, the Little Whin Sill is about 2 m thick and it has been encountered north of the River Wear in mines at Stotfield Burn (NY 943 424) and Stanhope Burn (NY 987 413). In the Rookhope Borehole, the Little Whin Sill lies stratigraphically about 120 m above the Great Whin Sill. In the Woodland Borehole (NZ 091 277), some 15 km to the south-east of Stanhope, the Little Whin Sill is encountered 20 m above the Three Yard Limestone, but it is not present at outcrop in upper Teesdale or in boreholes drilled to this level around Crook to the east.

Sill contacts are sharp though irregular in places, implying that the host rocks were lithified prior to intrusion. Locally, for example at Barrasford Quarry (NY 910 742) in Northumberland, narrow dyke-like masses protrude from the upper surface of the Great Whin Sill into the overlying strata. More widely in the Northumberland section of the Great Whin Sill, blocks of the underlying sedimentary rock have been levered up into the sill during intrusion and now form xenoliths close to its base. Similarly detached rafts of host strata are seen within the body of the Alnwick Sill, for example near Cullernose and at Longhoughton (NU 231 153), but, by contrast, xenoliths are rare in the north Pennines part of the Great Whin Sill intrusion. Between Budle Point and Harkess Rocks, the relationship between the Farne Islands Sill and the sedimentary country rock is extraordinarily complex, with numerous fragments and blocks of sandstone occurring within the sill. Here, the sedimentary rocks were probably disrupted immediately prior to intrusion by precursory hydroclastic activity.

Vesicles are present in the marginal zones of the Great Whin Sill along its northern outcrop, though none have been recorded from Teesdale. Of particular interest, however, are flattened amygdales up to 1.5 m long and 60 cm wide, close to the upper contact of the Farne Islands Sill, best exposed at Harkess Rocks (NU 177 356). The inner surface of the gas bubbles is revealed where the amygdale has been removed by later erosion. The lower surface of these vesicles has a tachylitic margin up to 2 mm thick and this displays parabaloid ropy flow-structures, similar in miniature to the surface of pahoehoe lava [[Media:P637452.jpg|(P637452)]]. The linings of the large vesicles must have remained plastic for long enough to allow flattening, elongation and the development of flow structures by the still molten magma moving through the intrusion. These are thought to be rare features of sills. Below this zone are further zones of abundant smaller vesicles, along with narrow zones of peperitic breccia.

The maximum thermal metamorphic effect on the country rock is seen in upper Teesdale, where limestones are recrystallised for over 30 m from the contact and mudstones are spotted for almost 40 m. The metamorphic effect of the three sheets of dolerite in the Harton Borehole can be detected in rocks at distances of 425 m above and 180 m below the sills. Generally, the rank of coal increases dramatically towards an intrusion: vitrinite reflectance increases, the texture changes and ultimately the coal becomes a natural coke. Relatively pure limestones, such as the Melmerby Scar Limestone, are recrystallised with a saccharoidal texture, and known from the characteristic weathering as ‘sugar limestone’. This rock produces distinctive soils that support a relict arctic alpine flora, including the spring gentian, Gentiana verna, on Cronkley and Widdybank fells, for which the area is renowned.

Impure limestone and calcareous mudstone have been metamorphosed into calc-silicate rocks containing such minerals as garnet, vesuvianite, diopside, feldspar, chlorite, epidote and rare wollastonite. Close to the contact, typically dark mudstones become light-coloured, hard porcellanous rocks known as ‘whetstones’; farther away they develop spots, mostly of chlorite, quartz and illite, but sporadically also with andalusite and cordierite. In several places, layers of pyrite nodules within country rocks close to the margins of sills and dykes have been altered to pyrrhotite, for example at Barrasford Quarry and Wynch Bridge. The presence in the contact zone and in rafts of sedimentary rock of wollastonite and vesuvianite in particular, indicate temperatures estimated to be as high as 720°C. Multiple episodes of metamorphism around upper Weardale have been cited as evidence for emplacement of the Little and Great Whin Sill magmas at separate times.

In the Alston Block, mineral veins of the Northern Pennine Orefield cut both the Whin sills and their associated dykes. The dolerite acts as a competent wallrock, like limestone and massive sandstone, and hence is a favourable host for mineralisation (see Chapter 10). An unusual occurrence of magnetite and niccolite-bearing skarn mineralisation at Lady’s Rake Mine in upper Teesdale has been cited as evidence of the interaction between north Pennine mineralising fluids and Whin Sill contact rocks whilst the latter were still hot.

== Northern England Tholeiitic Dyke-swarm ==
Basalt and dolerite dykes associated with the Whin Sill-swarm are typically 3–10 m wide and have north-east to east-north-east trends similar to the structural grain in the underlying Lower Palaeozoic basement. The dykes produce pronounced magnetic anomalies. occur in four, widely separated subswarms, three of which could be regarded essentially as discontinuous single dykes with en echelon offsets [[Media:P916081.jpg|(P916081)]]. Some authors have used the term ‘echelon’ rather than ‘subswarm’.

The dextrally offset northern dykes, belonging to the Holy Island Subswarm, lie at the northern margin of the Cheviot Block. The subswarm has similar palaeomagnetic characteristics to the Farne Islands Sill. South of the Cheviot Hills, the High Green Subswarm can be traced for over 80 km, converging slightly on the Holy Island Subswarm to cross the coastline at Boulmer. Its segments are offset sinistrally and some are up to 65 m wide. This subswarm was emplaced within the Swindon and Cragend–Chartners faults, a zone of east-north-east-trending structures that mark the southern margin of the Cheviot Block. Offsets on the St Oswald’s Chapel Subswarm are broadly sinistral and the subswarm includes the Haltwhistle, Erring Burn, Bavington and Causey Park dykes, the last of which can be traced for some distance offshore from Druridge Bay. Near Hexham, this subswarm swings to a more north-easterly trend, converging on the High Green Subswarm. The High Green and St Oswald’s Chapel dyke subswarms have similar palaeomagnetic signatures to the Alnwick Sill. Finally, near the southern limit of the Great Whin Sill exposures, the Hett Subswarm comprises several dykes to the south of Durham, including the Ludworth Dyke; palaeomagnetic evidence links this subswarm with the Great Whin Sill. On Holy Island (NU 123 416 to 149 419), the Holy Island Dyke is a complex mass which has several short sill-like sectors. Exposures of these exhibit gently dipping planar chilled upper surfaces. Parallel to these are zones of large vesicles in which the skin exhibits ropy flow-structures very similar to those described towards the top of the Farne Islands Sill at Harkess Rocks. A detailed magnetic survey suggests that the intrusion steps upwards through the stratigraphy both to the north and to the east [[Media:P916083.jpg|(P916083)]]. Examples such as this strengthen the thesis that the dykes are feeders to the Whin Sill-swarm.

== Petrology ==
The quartz-dolerite of the sills and dykes is composed of labradorite, subophitic augite and iron-titanium oxides with an intersertal, and commonly micropegmatitic, intergrowth of quartz and alkali feldspar. Minor constituents include hypersthene or pigeonite, hornblende, biotite, apatite and pyrite. Fresh olivine has been found only in the Little Whin Sill in the Rookhope Borehole and at Turn Wheel Linn, but pseudomorphs after this mineral have been found in at least some of the other sills and dykes. The chilled margins and many dykes contain some intersertal, pale brown, microlitic glass locally with skeletal ilmenite, but Ca-poor pyroxene is absent.

Grain size in the sills typically increases from the tachylitic or very fine-grained margins to medium grained in the centre. Where the sill is more than about 50 m thick, a pegmatitic zone may be developed about one third of the way down from the top, for example in upper Teesdale and in the Rookhope Borehole. Thus, this facies is rare in Northumberland where the sills are generally thinner than 50 m, though it is exhibited in Keepershield Quarry (NY 896 727), where the sill is 36 m thick. Pegmatitic patches and veins are characterised by clusters of long, feathery augite crystals and intergrown quartz and alkali feldspar. Ca-poor pyroxenes are absent from the pegmatitic areas and iron-titanium oxides are rare, but biotite and hornblende are important minor constituents. Patches and veins of pink aplitic, fine-grained, quartzo-feldspathic aggregates with almost square phenocrysts of sodic plagioclase are also common in Northumberland, for example in Barrasford Quarry, at Cullernose Point and Dunstanburgh. Segregations such as these are typical final products of differentiation seen in the upper parts of many thick sills. Veins and irregular masses of fine-grained basalt, presumably from later pulses of magma, have been recorded locally in both sills and dykes, for example at Swinburne Quarry (NY 947 765). The percentage of microphenocrysts increases towards the centre of the Great Whin Sill and differences in trace element geochemistry between the chilled margin and the sill interior imply that there was more than one pulse of magma.

The petrographical affinities are dominantly tholeiitic, but geochemically, the sills and dykes are transitional between alkaline and tholeiitic. The Great Whin Sill shows a very slight trend towards iron enrichment, in contrast to the Little Whin Sill which is geochemically homogeneous. It has been suggested from geochemical and mineralogical evidence that the Little Whin Sill may be an early differentiate from the Whin magma. However, the iron-rich nature of the Little Whin Sill and the presence of some resorbed calcic plagioclase crystals suggest that it had already undergone some differentiation prior to its emplacement.

Close to contacts, fault-planes, mineral veins or coal seams, dolerite and basalt may be altered to a pale cream or yellowish brown rock, referred to as ‘white whin’. This is composed of quartz, illite, kaolinite, muscovite, rutile, anatase and carbonates and was formed by the interaction between dolerite and hydrothermal solutions, probably of juvenile origin. Where white whin occurs close to the sill contact, adjacent mudstones show a marked increase in Na2O and the development of abundant albite, suggesting that sodametasomatism has occurred.

In addition to the zones of white whin, a suite of late-stage hydrothermal minerals has commonly developed in joints and vesicles during the final stages of cooling. Quartz-calcitechlorite veins are abundant locally throughout the Whin Sill-swarm, with smaller amounts of chlorite, bowlingite, sericite, stevensite, albite, anatase and titanite also present. Joint-surface veneers and late-stage veins contain abundant pectolite along with analcime, apophyllite, chabazite, prehnite, stilbite and rare datolite.

== Emplacement mechanism ==
In the Midgeholme Coalfield, the Great Whin Sill is intruded into Coal Measures and in the Durham Coalfield, dykes of the Hett Subswarm emplaced within the Coal Measures do not pass up into Lower Permian strata. Near Appleby in the Vale of Eden, pebbles of dolerite are known from Lower Permian breccia (‘brockram’). Thus, the intrusions were emplaced and exposed to erosion during the time interval represented by the unconformity between the upper Carboniferous (Duckmantian–Bolsovian) and lower Permian strata of the region. The age is reinforced by radiometric dates including a K-Ar whole-rock age of 301 ± 6 Ma, a U-Pb baddelyite (ZrO2) date of 297.4 ± 0.4 Ma, both determined from the Great Whin Sill, and an Ar-Ar plagioclase date of 294 ± 2 Ma from the Holy Island Dyke. The Ar-Ar age is younger than the U-Pb date and may support the view that the Farne Islands Sill and Holy Island Dyke Subswarm may have been emplaced later than the Great Whin Sill. Recent palaeomagnetic studies of all the sills and dykes revealed virtual geomagnetic poles that are consistent with the magmatism having occurred in earliest Permian times when the ancient geomagnetic field was reversed.

The dyke subswarms tend to occur at the margins of the main intrusions within the sill-swarm and it has long been suggested that the sills were fed by the dykes. Similarities in geochemistry and palaeomagnetic signature support this relationship, but a direct connection between them has not been proved in the field. Several examples of basaltic dykes cutting the Great Whin Sill have been cited as evidence that the dyke swarms may represent a slightly later event but, even so, the relationship of the dykes to the sills has been explained in a single emplacement model.

To explain the dykes and sills as the products of a single intrusive phase [[Media:P916084.jpg|(P916084)]], it is envisaged that basaltic magma rose along the dykes at the outer margins of the sedimentary basins until it reached hydrostatic equilibrium. The magma then gravitated downwards into the lower, central parts of the basin succession where it accumulated to form an overall saucer-shape intrusion. On the opposite side of the basin, the magma then advanced up dip under the head of pressure, so that here the outer parts of the intrusion tend to be thin and steeper than bedding, pinching out as they approach the surface. This process should be reflected by magma-flow directions in the dykes and sills, determined from features such as fingers and tongues extending from contacts and from the large vesicles in the Farne Islands Sill and Holy Island Dyke. A study of the alignment of magnetic grains in the rock indicates a more complex pattern of magma flow than is suggested by the model, though the flow direction is predominantly north away from the Hett and Holy Island dykes, and southwards at the southern limit of the Farne Islands Sill at Harkess Rocks.

The large vesicles seen in the Farne Islands Sill and Holy Island Subswarm may have formed by repeated localised pressure falls caused by fluid-induced fracturing of the country rock as the tip of the intrusion advanced. Alternatively, they may have formed by rapid decompression during injection of the magma into the sedimentary pile close to the land surface of the time; substantial uplift and erosion in this region before emplacement is implied by this second mechanism. It has been estimated that the Great Whin Sill took about 60 years to crystallise completely, whilst the Little Whin Sill may have taken only one and a half to two years.

== Permian, Triassic and Jurassic: deserts, rivers and shallow seas ==
The major continental collision that drove the Variscan Orogeny during late Carboniferous and early Permian times created the Pangaean supercontinent. Within this landmass, Britain lay in a tropical latitude, approximately 10° north of the equator, and drifted slowly northwards to subtropical latitudes of approximately 30° north by early Triassic times [[Media:P916033.jpg|(P916033)]]d. The depositional environments included widespread deserts, tropical and evaporitic seas, fluvial outwash plains, ephemeral lakes and mudflats.

Erosion of the folded and uplifted Carboniferous strata had generated mature, gently rolling plains across which spread an early Permian desert. By late Permian times, continental extension had opened seaways, flooding low ground across large inland drainage basins. On the western edge of northern England, the Bakevellia Sea developed, covering approximately the area of the present-day Irish Sea and its marginal areas. To the east, the Zechstein Sea covered approximately the area of the present-day North Sea and extended as far to the east as Lithuania and Poland. Early Triassic times saw continental deposition restored over northern England. Large river systems transported sands from the south and aeolian dune fields developed. Further marine transgression in mid to late Triassic times, the result of expansion of the Tethys Ocean from southern Europe, then brought coastal conditions to northern England; open marine conditions followed in Jurassic times.

Today, Permian strata are preserved in northern England to the north-west of the Pennines, around Carlisle, in the Vale of Eden and in west Cumbria, and to the east of the Pennines, over much of County Durham. Permian strata also occur at the north-east end of the Isle of Man but are there entirely obscured by thick superficial deposits. Triassic strata have a similar though more restricted onshore outcrop. In northern England, Jurassic strata are only known from the vicinity of Carlisle; those that succeed the Triassic succession near Middlesbrough fall into another regional district — that of East Yorkshire and Lincolnshire. It is probable that Permian and Triassic rocks once partially covered the Carboniferous strata of the north Pennines, northern Cumbria and Northumberland, and that Jurassic rocks also had a much wider original distribution.

== Bibliography ==
De Paola, N, Holdsworth, R E, McCaffrey, K J W, and Barchi, M R. 2005. Partitioned transtension: an alternative to basin inversion models. ''Journal of Structural Geology'', Vol. 27, 607–625.

Francis, E H. 1982. Magma and sediment – I: Emplacement mechanism of late Carboniferous tholeiite sills in northern Britain. ''Journal of the Geological Society of London'', Vol. 139, 1–20.

Goulty, N R, Peirce, C, Flatman, T D, Home, M, and Richardson, J H. 2000. Magnetic survey of the Holy Island Dyke on Holy Island, Northumberland. ''Proceedings of the Yorkshire Geological Society'', Vol. 53, 111–118.

Liss, D, Hutton, D H W, and Owens, W H. 2002. Ropy flow structures: a neglected indicator of magma-flow direction in sills and dykes. ''Geology'', Vol. 30, 715–718.

Liss, D, Owens, W H, and Hutton, D H W. 2004. New palaeomagnetic results from the Whin Sill complex: evidence for a multiple intrusion event and revised virtual geomagnetic poles for the late Carboniferous of the British Isles. ''Journal of the Geological Society of London'', Vol. 161, 927–938.

Shiells, K A G. 1964. The geological structures of north-east Northumberland. ''Transactions of the Royal Society of Edinburgh'', Vol. 65, 449–481. 270

Stephenson, D, Loughlin, S C, Millward, D, Waters, C N, and Williamson, I. 2003. Carboniferous and Permian Igneous Rocks of Great Britain, North of the Variscan Front. ''Geological Conservation Review'', No. 27. (Peterborough: Joint Nature Conservation Committee.)

Woodcock, N H, and Rickards, B. 2003. Transpressive duplex and flower structure: Dent Fault System, NW England. ''Journal of Structural Geology'', Vol. 25, 1981–1992.
[[Category: Northern England]]

Late Carboniferous structures, Northern England

2016-03-23T20:57:53Z

BobMcIntosh:

Late Carboniferous to early Permian deformation

2016-03-23T20:55:50Z

BobMcIntosh:

Warwickshire Group, Carboniferous, Northern England

2016-03-23T20:03:05Z

BobMcIntosh:

Westphalian Coal Measures, Carboniferous, Northern England

2016-03-23T20:01:04Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
== Introduction ==
[[File:P916077.jpg|thumbnail|Representative sections and correlations for the Pennine Coal Measures and Warwickshire groups. P916077.]]
[[File:P916075.jpg|thumbnail|Outcrop of Westphalian strata and the location of the principal coalfields in northern England, showing variation in rank across the Northumberland and Durham Coalfield. P916075.]]

Stratigraphical summaries for the main coalfields, and the correlations between them, are shown in [[Media:P916077.jpg|(P916077)]].

== Northumberland and Durham ==
The Westphalian Coal Measures succession of the Northumberland and Durham Coalfield accumulated along the northern margin of the Pennine Basin. It has more proximal characteristics, with a smaller marine influence, than are seen to the south in the classic coalfield successions of Lancashire and Yorkshire. Accordingly, fewer marine bands occur in Northumberland and Durham than in coalfields farther south, whilst those that are present are less well developed. In general terms, the Langsettian sequence of Northumberland and Durham has most in common with an upper delta-plain model, and the Duckmantian sequence with an alluvial-plain model.

The base of the Pennine Coal Measures Group in north-east England south of the River Tyne is placed at the Quarterburn Marine Band the inferred correlative of the Subcrenatum Marine Band the base of which defines the base of the Westphalian regionally [[Media:P916114.jpg|(P916114)]]. However, no definite occurrence of ''Gastrioceras subcrenatum'' has been found, nor has the Quarterburn Marine Band been proved north of the river Tyne. There, because the early Westphalian fauna is generally sparse, the base of the Pennine Coal Measures Group has to be taken immediately above the highest occurrence of marine, costate brachiopods. Similar problems exist with regard to the precise position of other boundaries — in Northumberland the Vanderbeckei Marine Band, the base of which marks the base of the Duckmantian (and also the base of the Pennine Middle Coal Measures Group), is indicated only by the presence of a marginal marine ''Lingula'' fauna, whereas a wholly marine fauna (though still lacking the diagnostic Vanderbeckei fauna) occurs at the same level farther south in Durham.

=== Pennine Lower Coal Measures Formation ===
In the Northumberland and Durham Coalfield, the base of the Pennine Coal Measures was traditionally defined by the lowest workable coal, either the Ganister Clay Coal or locally in Northumberland the Brockwell Coal. These coals are now known to lie some distance above the Quarterburn Marine Band, with the intervening strata mainly consisting of sandstone with only thin and impersistent mudstone or coal interbeds. This is essentially a continuation of the lithofacies seen in the underlying Stainmore Formation of the Yoredale Group and it has, in the past, been referred to as ‘Millstone Grit’. Above the Ganister Clay Coal, the Pennine Lower Coal Measures contain a higher proportion of mudstone and coal, with the thicker, more productive seams in the higher part of the succession, commencing with the Brockwell Coal. Below this level the sandstones are coarse grained with some sufficiently siliceous to be termed ganisters. Above the Brockwell Coal the sandstone beds are thinner and finer grained but the increasingly argillaceous Pennine Lower Coal Measures sequence still comprises at least 50 per cent sandstone; five or six significant coal seams are also present. The Pennine Lower Coal Measures here comprise about 220 m of strata.

=== Pennine Middle Coal Measures Formation ===
The Duckmantian to Bolsovian, Pennine Middle Coal Measures comprise up to about 500 m of strata in the Northumberland and Durham Coalfield. The base is defined by the base of the Harvey Marine Band, the local correlative of the Vanderbeckei Marine Band. The Middle Coal Measures contain most of the workable coals, particularly in the lower section of about 180mup tothe High Main Coal. This is the thickest and most widely worked seam over much of the coalfield; it contains excellent quality coal, is locally over 2.5 m in thickness, and formed the original basis for large scale exploitation of the Northumberland and Durham Coalfield. Several marine bands also occur, but like the Pennine Lower Coal Measures, more than 50 per cent of the succession is made up of sandstone. Above the High Main Coal some of these sandstones are coarse grained and massive: examples include the High Main Post (overlying the eponymous coal) and the Seventy Fathom Post, which was worked for grindstones.

=== Pennine Upper Coal Measures Formation ===
The base of the Pennine Upper Coal Measures is taken at the base of the Down Hill Marine Band, the local correlative of the more widely recognised Cambriense Marine Band. It is preserved in the Sunderland area, where it is overlain by about 150 m of poorly exposed strata in the much-faulted Boldon Syncline. The main features of this sequence are the predominance of grey argillaceous strata, the presence of only a few thin coals and the general sparseness of the nonmarine bivalve faunas. Nearby, the presence of Pennine Upper Coal Measures strata concealed beneath Permian to the east and north of Sunderland has been inferred on structural grounds, but no stratigraphical information is available. North of the River Tyne, near Killingworth, on the northern (downthrow) side of the Ninety Fathom Fault, the presence of some 155 m of Upper Coal Measures is again inferred entirely on structural grounds by analogy with the Sunderland district.

== The Tyne valley outliers ==
A series of small faulted outliers of Westphalian strata occur along the Tyne valley, forming the Midgeholme, Plenmeller and Stublick coalfields. The outliers are mostly elongated east–west and consist of southward-dipping Coal Measures; they are terminated abruptly to the south against the Stublick–Ninety Fathom fault system. Opencast coal investigations since the 1980s have improved the stratigraphical correlations with nearby areas of the Northumberland and Durham Coalfield, based largely on lithology, augmented by sparse palaeontological data [[Media:P916077.jpg|(P916077)]].

A cumulative thickness of about 200 m is represented through the three inliers, mostly from the Pennine Lower Coal Measures, but extending upwards into the lowermost Middle Coal Measures. Most of the stratigraphically useful horizons lie within the Lower Coal Measures [[Media:P916077.jpg|(P916077)]]. The Low Main Sandstone and its equivalents can be traced throughout the outliers, and provides a useful lithostratigraphical marker. The presence of the Victoria Shell Bed above the Slag Coal helps to fix the position of the coal and the underlying sandstone-dominated interval. A marine fauna from the Plenmeller West opencast site is believed to be from the equivalent of the Gubeon (Listeri) Marine Band, which places the horizon some 100 m below the Harvey (Vanderbeckei) Marine Band, the boundary between the Pennine Lower and Middle Coal Measures.

== Stainmore outlier ==
Over 300 m of Westphalian Coal Measures strata are preserved in two small areas in the Stainmore outlier, a narrow fault-block located at the intersection of the Dent and Pennine faults, south-east of Brough. They conformably succeed Namurian strata, and dip steeply eastward to be terminated against faults. The succession spans the Pennine Lower and Middle Coal Measures and is closely comparable, both in thickness and lithology, with the equivalent strata seen in the Durham Coalfield, 25 km to the north-east [[Media:P916077.jpg|(P916077)]]. A correlative of the Subcrenatum Marine Band marks the base of the Pennine Lower Coal Measures sequence, whereas the Vanderbeckei Marine Band is replaced by a nonmarine shell bed. Towards the top of the succession there is a well-constrained correlation with the Bensham coal seam of the Pennine Middle Coal Measures in Durham.

== Cumbria ==
The Cumbrian Coalfield [[Media:P916075.jpg|(P916075)]], with its numerous productive coal seams, crops out in a broad arc around the west and north of the Lake District from Egremont in the southwest to Caldbeck in the east. Concealed deposits occur below a cover of Permian and Triassic rocks in the Vale of Eden and north of the Maryport Fault, but have not proved to be economically exploitable. Farther north, the Canonbie Coalfield is situated at the northern margin of the north-north-east trending Solway Syncline and seismic reflection data confirm that the Pennine Coal Measures of Cumbria and Canonbie meet beneath this major structure. Westphalian strata also extend offshore under the Irish Sea and have been encountered in boreholes north of the Isle of Man. Some of the most productive seams were worked from sub-sea collieries with pit-head installations located on the coast at Whitehaven and Harrington.

Working of the Pennine Coal Measures in the Cumbrian Coalfield has always been hampered by a closely spaced set of north-west-trending post-Carboniferous faults. These faults are usually steeply inclined and rapidly switch throw along strike. Also present are a set of north-east-trending, large-throw, low-angled faults which are more widely spaced (5– 10 km); they may represent a postdepositional reactivation of an original set of basin-margin growth faults. The Pennine Lower Coal Measures have a relatively uniform thickness across the coalfield, but in west Cumbria, the Pennine Middle Coal Measures show an overall thickening from south-east to north-west, suggesting that by then the Lake District Block was a positive, though not necessarily emergent, area.

Unlike the situation in Northumberland, the position of the Subcrenatum Marine Band, and hence the base of the Westphalian, is well established from boreholes throughout the Cumbrian Coalfield. In contrast, the Vanderbeckei Marine Band is poorly developed in the Cumbrian Coalfield and its position can only be inferred from the presence of shells or fish remains. The Aegiranum Marine Band and the slightly lower Haughton Marine Band are recorded from the Whitehaven coastal collieries and to the north of Distington. However, over most of the coalfield the Aegiranum and higher marine bands appear to be cut out below the unconformable base of the Whitehaven Sandstone, which itself is no older than the Cambriense Marine Band.

=== Pennine Lower Coal Measures Formation ===
In the Cumbrian Coalfield, the Pennine Lower Coal Measures comprise three to four coarsening-upward cycles, each culminating in a thick bed of fluvial sandstone. Coals, though few in number, are laterally persistent. They may occur singly or in groups of two or three clustered immediately below the base of a major sandstone unit.

The lowest 30 m of the Pennine Middle Coal Measures succession are fine grained, with a number of locally prominent ''Lingula'' bands. The lowest coal seam, the Harrington Four Foot, is recognised across the whole area. It is located a few metres above the Subcrenatum Marine Band, and immediately below the Harrington Four Foot Rock. This is a major sandstone unit, often the only stratigraphical marker for the base of the Coal Measures in old borehole records. To the north, the Harrington Four Foot Rock grades to siltstone and a set of thin coals known collectively as the Albrighton coals appear. More generally, the succession above the Harrington Rock includes coal seams of variable thickness and quality: the Upper and Lower Threequarters seams, the Wythemoor ‘Parrot’ Seam, and the Micklam Fireclay Seam.

Higher in the succession, the next cycle contains the Sixquarters Seam and, immediately overlying it, the Sixquarters Rock sandstone. This seam is one of the prime economic coals of west Cumbria, both onshore and offshore, whilst the sandstone is a regionally prominent channel sand body that has been worked as a building stone. Above the sandstone a mainly argillaceous succession contains, in upward sequence, the Lickbank, the Eighteen Inch and the Little Main seams. The Little Main has been a popular target for mining; the lower two coals are of lesser interest but increase in thickness and quality offshore.

=== Pennine Middle Coal Measures Formation ===
The Vanderbeckei Marine Band, usually represented by a ''Lingula'' shell band in west Cumbria, appears 15 metres above the Little Main seam, but does not mark a change in general lithofacies. In the lower part of the formation, mussel bands are developed locally and a number of coals are present. The coals are mainly of indifferent quality, except for the Yard Seam (locally known as the Metal Band), which was worked throughout the coalfield. Above the Yard Seam, the Pennine Middle Coal Measures display a more pronounced cyclicity, with quite thin units of mudstone with coal separating thicker (25–35 m) sandstone-prone intervals. This section contains the two thickest and most widely exploited coal seams of the area: the Main (Cumbria) Band and the Bannock Band. The two associated sandstone units, the Main Band Rock and the Bannock Band Rock, are relatively thick and have erosive bases so that, in places, the underlying coal seam is cut out.

Continuing upwards through the Pennine Middle Coal Measures, the next 50–70 m are mainly argillaceous but contain three major coal seams: the Tenquarters, Slaty and White Metal. In the north-west of the coalfield the lowest seam has an overlying sandstone unit, the Tenquarters Rock, that forms a prominent cliff along the coast, north of Parton. Above the White Metal seam a major sandstone unit is present called the Countess Sandstone from the disused pit of that name. This sandstone forms a 20-m cliff on the coast from Parton south to Whitehaven and is encountered in boreholes throughout the region.

The Countess Sandstone marks the upper limit of major workable seams in the Whitehaven area. Between the top of this sandstone and the base of the Whitehaven Sandstone Formation (discussed below) is an interval subject to marked lateral variation. A number of thin coal seams are present, and the Black Metal and Brassy seams, are identified. Several, however, have no local name, or obvious regional correlative. In the Whitehaven collieries and from Workington northwards, this interval contains a series of mudstone/sandstone cycles, with thin coals, and the presence of the Aegiranum Marine Band indicates the upward passage from Duckmantian into Bolsovian strata.

=== Pennine Upper Coal Measures Formation ===
The St Helens Marine Band, recorded from the northern part of the coalfield, is correlated with the Cambriense Marine Band and so some 70 m of overlying strata are classed as Pennine Upper Coal Measures. The rocks are largely sandstone and mudstone, mostly reddened to some degree, with a few thin coals and claystone seatearths.

== Canonbie ==
The Canonbie Coalfield is situated at the northern margin of the north-north-east-trending Solway Syncline, beneath which it links with the Cumbrian Coalfield. It is likely that the individual coal seams of Cumbria and Canonbie persist across this structure and although correlation has not been proved by deep boreholes, it is supported by seismic reflection results. The limited area of exposed Coal Measures strata around Canonbie lies mainly to the north of the border in Scotland [[Media:P916075.jpg|(P916075)]]. Interpretation based on boreholes and seismic surveys carried out in the 1980s indicated that the concealed coalfield to the south was larger and had more potential than was previously envisaged. In common with the other coalfields of the region, the Subcrenatum Marine Band has not been found in the Canonbie Coalfield. However, the tentative correlation of a marine band near the base of the Becklees Borehole (NY 3517 7158) with the Templeman’s (Langley) Marine Band of the Cumbrian Coalfield has enabled a generalised stratigraphical succession to be established. The Pennine Lower Coal Measures succession is about 120 m in thickness, the Middle Coal Measures is about 230 m and Upper Coal Measures is about 170 m thick. Coals are present in the Lower Coal Measures but are commonly unnamed and are difficult to correlate. Borehole information indicates that they are generally less than 0.8 m in thickness. The main seams are from the Pennine Middle Coal Measures, principally seven of Duckmantian age [[Media:P916077.jpg|(P916077)]]. There are only a few thin coals present in the Upper Coal Measures, most of them being located close to the base. The exception is the aptly named High Coal, which occurs about 170 m above the base of the Pennine Upper Coal Measures and has been proposed as a convenient marker for the base of the overlying Warwickshire Group.

== Bibliography ==

Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). 1989. The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. ''Occasional Publication of the Yorkshire Geological Society'', No. 6.

Barclay, W J, Riley, N J, and Strong, G E. 1994. The Dinantian rocks of the Sellafield area, West Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 37–49.

Bott, M H P, Swinburne, P M, and Long, R E. 1984. Deep structure and origin of the Northumberland and Stainmore troughs. ''Proceedings of the Yorkshire Geological Society'', Vol. 44, 479–495.

Burgess, I C. 1986. Lower Carboniferous sections in the Sedbergh district, Cumbria. ''Transactions of the Leeds Geological Association'', Vol. 11, 1–23.

Calver, M A. 1968. Distribution of Westphalian marine faunas in Northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society'', Vol. 37, 1–72.

Cleal, C J, and Thomas, B A. 1996. British Upper Carboniferous Stratigraphy. ''Geological Conservation Review Series'', No. 11. (Peterborough: Joint Nature Conservation Committee.)

Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A and Wright, V P. 2004. British Lower Carboniferous Stratigraphy. ''Geological Conservation Review Series'', No. 29. (Peterborough: Joint Nature Conservation Committee.)

Dickson, J A D, Ford, T D, and Swift, A. 1987. The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man. ''Proceedings of the Yorkshire Geological Society'', Vol. 46, 203–229. 268

Fairbairn, R A. 2001. The stratigraphy of the Namurian Great/Main Limestone on the Alston Block, Stainmore Trough and Askrigg Block of northern England. ''Proceedings of the Yorkshire Geological Society'', Vol. 53, 265–274.

Fielding, C R. 1984. A coal depositional model for the Durham Coal Measures of North East England. ''Journal of the Geological Society of London'', Vol. 141, 917–931.

Fraser, A J, and Gawthorpe, R L. 2003. An Atlas of Carboniferous Basin Evolution in Northern England. ''Geological Society of London, Memoir'', No. 28.

Garwood, E J. 1913. The Lower Carboniferous succession in the north-west of England. ''Journal of the Geological Society of London'', Vol. 68 (for 1912), 449–586.

Guion, P D, Fulton, I M, and Jones, N S. 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant High. 45–78 ''in ''European Coal Geology. Whateley, M K G, and Spears, D A (editors). ''Geological Society of London Special Publication'', No. 82.

Heckel P H, and Clayton, G. 2006. Use of the new official names for the Subsystems, Series and Stages of the Carboniferous System in international Journals. Correspondence. ''Proceedings of the Geologists’ Association'', Vol. 117, 393–396.

Johnson, G A L. 1984. Subsidence and sedimentation in the Northumberland Trough. ''Proceedings of the Yorkshire Geological Society'', Vol. 45, 71–83.

Johnson, G A L, and Dunham, K C. 1963. The geology of Moor House. ''Nature Conservancy Monograph'', No. 2. (London: HMSO.)

Johnson, G A L, and Nudds, J R. 1996. Carboniferous biostratigraphy of the Rookhope Borehole, Co. Durham. ''Transactions of the Royal Society of Edinburgh: Earth Sciences'', Vol. 86, 181–226.

Jones, N S, and Holliday, D W. 2006. The stratigraphy and sedimentology of Upper Carboniferous Warwickshire Group red-bed facies in the Canonbie area of SW Scotland. ''British Geological Survey Internal Report'', IR/06/043.

O’Mara, P T, and Turner, B R. 1999. Sequence stratigraphy of coastal alluvial plain Westphalian B Coal Measures in Northumberland and the southern North Sea. ''International Journal of Coal Geology'', Vol. 42, 33–62.

Owens, B, and Burgess, I C. 1965. The stratigraphy and palynology of the Upper Carboniferous outlier of Stainmore, Westmorland. ''Bulletin of the Geological Survey of Great Britain'', Vol. 23, 17–44.

Reynolds, A D. 1992. Storm, wave and tide-dominated sedimentation in the Dinantian Middle Limestone Group, Northumbrian Basin. ''Proceedings of the Yorkshire Geological Society'', Vol. 49, 135–148.

Rippon, J H. 1996. Sand body orientation, palaeoslope analysis, and basin fill implications in the Westphalian A–C of Great Britain. ''Journal of the Geological Society of London'', Vol. 153, 881–900.

Rippon, J H. 1998. The identification of syndepositionally active structures in the coalbearing Upper Carboniferous of Great Britain. ''Proceedings of the Yorkshire Geological Society'', Vol. 52, 73–93.

Rowley, C R. 1969. The stratigraphy of the Carboniferous Middle Limestone Group of West Edenside, Westmorland. ''Proceedings of the Yorkshire Geological Society'', Vol. 37, 329–350.

Smith, T E. 1968. The Upper Old Red Sandstone–Carboniferous junction at Burnmouth, Berwickshire. ''Scottish Journal of Geology'', Vol. 4, 349–354.

Tucker, M E, Gallagher, J, Lemon, K, and Leng, M. 2003. The Yoredale Cycles of Northumbria: High-Frequency Clastic-Carbonate Sequences of the Mid-Carboniferous Icehouse World. ''Open University Geological Society Journal'', Vol. 24, 5–10.

Turner, B R, Younger, P L, and Fordham, C E. 1993. Fell Sandstone Group lithostratigraphy south-west of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone. ''Proceedings of the Yorkshire Geological Society'', Vol. 49, 269–281.

Ward, J. 1997. Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore, Cumbria, England. 277–296 ''in ''Petroleum Geology of the Irish Sea and Adjacent Areas. Meadows, N S, and others (editors). ''Geological Society of London Special Publication'', No. 124.

Waters, C N, Browne, M A E, Dean, M T, and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). ''British Geological Survey Research Report'', RR/07/01.

[[Category: Northern England]]

Pennine Coal Measures Group, Carboniferous, Northern England

2016-03-23T19:58:31Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
== Introduction ==
[[File:P916112.jpg|thumbnail|Stratigraphical classification of the Carboniferous rocks of northern England. Note 1 Although the Carboniferous Subcommission of the International Commission on Stratigraphy has recommended that the terms ‘Dinantian’ and ‘Silesian’ should no longer be used, they are such fundamental units in the description of British Carboniferous rocks that they are likely to be encountered throughout the currency of this guide. P916112.]]
[[File:P916114.jpg|thumbnail|Stratigraphical classification of the Westphalian strata in northern England. Marine Band names refer to the immediately subjacent line. P916114.]]
[[File:P916075.jpg|thumbnail|Outcrop of Westphalian strata and the location of the principal coalfields in northern England, showing variation in rank across the Northumberland and Durham Coalfield. P916075.]]
[[File:P220607.jpg|thumbnail|Strata of the Pennine Coal Measures exposed in the West Chevington opencast site [NZ 2440 9660], Northumberland. (P220607).]]
[[File:P916070.jpg|thumbnail|Distribution of Carboniferous lithofacies across northern England and the surrounding regions (after Cope et al., 1992. Atlas of Palaeogeography and Lithofacies, Geological Society of London Memoir, No. 13). P916070.]]
[[File:P916076.jpg|thumbnail|Schematic illustration of the facies variation within the upper delta plain depositional environment of the Westphalian, Pennine Coal Measures Group (after Guion et al., 1995). P916076.]]
[[File:P916071.jpg|thumbnail|Illustrative logs and interpretations for some types of high-frequency clastic sequences within the Yoredale and Pennine Coal Measures groups of northern England (after Tucker et al., 2003). P916071.]]
[[File:P222338.jpg|thumbnail|Selection of Carboniferous macrofossils: a Crinoid, Woodocrinus? from shale at the base of the Stainmore Formation, immediately above the Great Limestone Member, at Mootlaw Quarry, Northumberland (DL4741, P589464); b Goniatite, ''Cravenoceras'' cf. ''lineolatum'' from shale at the base of the Stainmore Formation, immediately above the Great Limestone Member, at Mootlaw Quarry, Northumberland. (P587665), c Brachiopod, ''Brachythyris'' sp. from shale at the base of the Stainmore Formation, immediately above the Great Limestone Member, at Mootlaw Quarry, Northumberland (DL4353, P587666); d Brachiopods, large Gigantoproductids in the Sugar Sands Limestone, Stainmore Formation, at Sugar Sands Bay, Alnwick, Northumberland. (P643515); e Corals, a polished surface of ‘Frosterley Marble’ from the Great Limestone Member of the Alston Formation, showing detail of the abundant coral ''Dibunophyllum bipartitum'', Weardale, County Durham. (P524822) x0.5; f Trace fossils, ''Teichichnus''-type animal burrows preserved in sandstone from the upper part of the Appletree Limestone cycle of the Tyne Limestone Formation, Hindleysteel Quarry, Henshaw Common, Northumberland [NY 7496 7291]. (P222338).]]

Across most of northern England sedimentary deposition continued unbroken from the Namurian into the Westphalian. From the later part of the Namurian onwards, significant marine influence was progressively lost over the entire region and deposition was increasingly dominated by sand, silt and mud, carried into the region via large river deltas. These initially drained from a land area to the north or north-east but other provenances were active at different times and by the end of the Westphalian sediment input from the south and south-east was important. Subsidence balanced sedimentation so that a stable delta-top environment was maintained and the resulting Westphalian sequence is essentially continuous over large parts of the region. The Westphalian regional stage (formerly a series) is divided into four substages, originally identified as A to D, the three lowest having since been formalised as Langsettian, Duckmantian and Bolsovian; Westphalian D survives [[Media:P916112.jpg|(P916112)]] and [[Media:P916114.jpg|(P916114)]].

The lithostratigraphical units of Westphalian age present in northern England comprise the Pennine Coal Measures Group and the Warwickshire Group. The Pennine Coal Measures are predominantly grey in colour, nonmarine and characterised by vertically stacked, coarsening-upward cycles commonly up to 15 m thick. Each cycle is composed, in upward sequence, of mudstone, siltstone and sandstone and is capped by a seatearth and coal, though coal forms only a minor part of the sequence. Clay ironstone occurs within some of the mudstones. The Warwickshire Group consists of interbedded mudstone, siltstone and sandstone similar to those of the Coal Measures but distinguished by the presence of primary red beds; the overall colour range is from red-brown to grey, and coal is rare. Secondary reddening of the Coal Measures and older Carboniferous strata is common where these rocks lie close below, or have been exhumed from below the basal Permian unconformity. The zone of reddening can extend 100 m or more below the unconformity and is accompanied by a general oxidation of the rock mineralogy. Farther south in the Pennine Basin, the Warwickshire Group ranges up into the Stephanian.

Westphalian strata were laid down over most (probably all) of the region, but the Variscan tectonics of the late Carboniferous and early Permian deformed them into gentle folds and produced extensive faulting. Uplifted areas were soon eroded, and Westphalian beds were preserved only in the downwarped areas that make up the present coalfields [[Media:P916075.jpg|(P916075)]]. The separation of the coalfields is not an original feature, therefore, but a product of Variscan deformation. Subsequent, post-Variscan sedimentation in the region commenced in Permian times and produced an extensive, unconformable cover of Permo-Triassic strata, much of which has since been stripped away by further erosion. ‘Exposed’ coalfields are those where the Westphalian strata crop out at surface; ‘concealed’ coalfields are hidden beneath Permo-Triassic strata.

Westphalian Coal Measures crop out, principally, in the west of the region in the Cumbrian Coalfield, and in the east in the Northumberland and Durham Coalfield which, in Northumberland has some of the best coastal exposures of Westphalian Coal Measures anywhere in Britain. Smaller outliers of Westphalian strata occur around Canonbie on the north side of the Solway Basin and spanning the Anglo-Scottish border, in the Tyne valley along the line of the Stublick Fault, and in the Stainmore Trough [[Media:P916075.jpg|(P916075)]]. Between the Cumbrian and Canonbie coalfields the Coal Measures are concealed beneath the Solway syncline, whence the subcrop continues from north Cumbria into the Vale of Eden.

At more than 1600 m, the thickness of Westphalian strata in the Solway Syncline is the maximum seen in northern England as described in this account. There, the Carboniferous rocks are commonly reddened to a depth of more than 100 m beneath the unconformable cover of Permo-Triassic desert sandstone. In the Northumberland and Durham Coalfield, about 850 m of Coal Measures strata are present in the axial zone of the Boldon Syncline, north-west of Sunderland, whilst offshore from Tynemouth the thickness is locally up to 830 m. The Coal Measures in the east and south-east of the Northumberland and Durham Coalfield are also concealed by Permian and Triassic rocks, but in general the zone of sub-Permian reddening is less well-developed east of the Pennines.

Deep mining in the Westphalian strata of the region has ceased since publication in 1971 of the previous edition of this British Regional Geology guide to Northern England; much of the final working was from collieries extending beneath the sea. Since then, new information has been acquired during development of sites for opencast coal extraction [[Media:P220607.jpg|(P220607)]], and much of the region has been geologically resurveyed to modern standards. This has enabled major advances in our understanding of the geological development of the coalfields. In addition, the application of sequence stratigraphy has brought a new approach to interpretations of the Westphalian succession onshore and improved its correlation with the offshore sequences, particularly that beneath the North Sea.

== The depositional framework ==
The Westphalian coal-bearing strata of central and northern England were deposited in the Pennine Basin, by then a single province continuous with the north-west European paralic belt, from which it became separated by later, Variscan folding. The Pennine Basin was bordered by the Southern Uplands and associated smaller-scale landmasses such as the Cheviot Block to the north, and by the Wales–Brabant Massif to the south [[Media:P916070.jpg|(P916070)]]. Deposition occurred in a widespread fluviolacustrine environment that developed on a low-lying and largely waterlogged plain subjected both to intervals of emergence and to intermittent marine transgressions that became less frequent with time. The succession throughout most of the Pennine Basin is complete.

The lowest Westphalian strata were deposited in a gently subsiding, lower delta-plain environment under the dominant influence of mainly fluvial delta systems. Southward progradation of this depositional system through time resulted in middle Westphalian strata being deposited in a more proximal, upper delta-plain environment dominated by river distributary channels and lacustrine deltas. Later in the Westphalian, a waning fluvial influence caused re-establishment of lower delta-plain conditions. The basin occupied an equatorial position and experienced a humid, tropical climate characterised by high precipitation rates. This combination of factors provided ideal conditions for the development of a high water table, poorly drained palaeosols, peat swamps and the eventual formation of coal seams. Overall, subsidence and sedimentation were able to keep pace with one another throughout deposition. Periodic emergence allowed development of the coal swamps but these were in turn buried by more river-borne sand and silt carried into the northern England sector of the Pennine Basin from the north and east.

During discrete marine incursions at periods of high global sea level, distinctive thin layers of sediment were deposited over wide areas. These marine bands contain characteristic fossil assemblages and have been the traditional means of stratigraphical correlation. Their modern integration with palynological data remains the primary means of correlation within and between coalfields in the Pennine Basin, and thence, from basin to basin, as far as eastern Europe. The marine bands have also allowed preliminary correlation of Westphalian strata onshore with those offshore beneath the North Sea. They represent geographically widespread, short-lived, marine flooding events and are fundamental to a proper understanding of the succession since they reflect the most significant changes in base level (relative sea level) across the entire alluvial plain. The main marine bands — Subcrenatum, Vanderbeckei and Aegiranum — are used to define the substages of the Westphalian [[Media:P916114.jpg|(P916114)]].

An alternative means of long-distance correlation is provided by the beds of volcanic ash — now altered and known as tonsteins — that fell over the coal swamps. Tonstein beds are widely used in Europe for intra- and inter-basinal correlations, and their radiometric dating forms the basis of the modern geochronology for the late Carboniferous.

[[Media:P916076.jpg|(P916076)]] illustrates the varied depositional environments within which the Pennine Coal Measures Group of northern England accumulated. There, and in most of the other coalfields throughout the Pennine Basin, the Coal Measures show a broad, threefold subdivision of lithofacies relative to stratigraphy:

* coal-poor with numerous marine bands in the early Langsettian
* coal-rich with relatively few marine bands in the later Langsettian to mid Duckmantian
* coal-poor with more numerous marine bands from the mid Duckmantian to mid Bolsovian.

The better-developed coals in the middle part of the succession, up to the Vanderbeckei Marine Band, probably reflect changes in relative sea level that occurred at an optimum pace for the initiation and long-term maintenance of coal swamps. The change from poorly developed to well-developed coals is relatively sharp in the Langsettian sequences preserved across the Pennine Basin coalfields of northern England. The subsequent Duckmantian change back to a coal-poor succession is less well defined.

== Clastic lithologies ==
The cyclothemic nature of the Coal Measures has long been recognised and studied. At least 40 such cycles are known from the Pennine Coal Measures Group of County Durham with the main lithologies following one another in each cyclothem, in an ascending order of mudstone at the base (overlying the coal at the top of the underlying cycle), siltstone, sandstone, seatearth, coal [[Media:P916071.jpg|(P916071)]]. The nature and origin of the cyclothems have been much discussed, and it appears that no single explanation will suffice. Such cyclicity is a natural reflection of the interplay of sedimentary processes, and the only external mechanism needed to produce them is continuous subsidence. However, it is generally accepted that the periodic changes in sea level leading to marine flooding events are related to global glacial events, in this case the late Carboniferous glaciation of southern Gondwana which at that time lay over the South Pole. Some activity along the Stublick–Ninety Fathom fault system continued during the Westphalian but, in contrast to circumstances earlier in the Carboniferous, contemporaneous fault activity is not considered to have been a major influence on sedimentary patterns. Only local fault-controlled effects have been proposed and these include deposition of the Langsettian of Cumbria, and the stacking of channel sandstones in the Durham Coal Measures.

The intercoal sequences were deposited during the gradual infilling of shallow interdistributary bays and lakes by shallow-water, crevasse-splay delta complexes [[Media:P916076.jpg|(P916076)]]. They are interbedded with a number of prominent sandstone bodies that were deposited by the low-sinuosity, distributary channels feeding the crevasse-splay systems. These sandstone bodies can be stacked, one above another, to produce significant thicknesses; for example, in west Cumbria, a substantial sandstone thickness is developed where the Bannock Band Rock directly overlies the Main Band Rock.

Mudstones are generally well bedded and grey, but darker and more fissile when carbonaceous. Siderite commonly occurs in thin layers or as flattened nodules. Many of the mudstones contain a shelly fauna and this is usually indicative of a brackish-water depositional environment; marine mudstones are rare. Siltstones are also grey, commonly laminated and with a range of bioturbation structures. They grade imperceptibly into both mudstone and sandstone, the latter being mostly fine grained and quartzofeldspathic.

Seatearths, as preserved palaeosols, may be developed from any of the clastic lithologies and are gradational into the underlying facies. In contrast, the contact with any overlying coal seam is sharp. The seatearths contain abundant carbonaceous plant material (most commonly Stigmaria rootlets), and bright coaly stringers with disseminated pyrite. The rootlets increase in abundance upwards through the seatearth and disrupt any original lamination that might have been present. The seatearths typically break along irregular fracture surfaces that may be either covered with slickensides, or curved and polished. Some sandstone seatearths have been leached of feldspar and silicified to produce the distinctive hard beds called ganisters.

== Coal ==
Across the low-lying, Langsettian to Duckmantian alluvial plain, coal swamps formed as mires developed on abandoned lacustrine crevasse-splay delta systems. Individual abandoned delta systems are thought to have been up to 10 km wide and 20 km long. The associated mires developed over prolonged periods of time and were not necessarily contemporaneous from one delta system to another. Lateral continuity and synchroneity would have been particularly unlikely along the more active marginal parts of the basin. Thus, variations in coal seam thickness across the alluvial plains are to be expected, although it should be stressed that most of the main coal seams, both in Cumbria and in Northumberland and Durham, can be widely correlated.

Mires develop under waterlogged conditions of rising base level and are able to maintain themselves for thousands of years in water depths of about 1 m, probably the optimum water depth for swamp growth. A lowered water table leads to oxidation and destruction of organic matter as the swamp environment gives way to better-drained conditions, no coals form, only overthickened palaeosols. A rise in water level allows the mire to be buried by clastic sediment. Peat-forming environments in the Westphalian of northern England ranged from low-lying and brackish mires to seasonally flooded forest swamps. The resulting coal seams range up to about 2 m in thickness and so a peat:coal compaction ratio in the order of 10:1 would indicate that peats were originally up to 20 m thick; autocompaction during peat growth would reduce that decompacted thickness.

The character and quality of a coal is determined by the conditions in the swamp in which it was formed and by subsequent burial history. Alluvial plain coals typically have low ash and sulphur contents, the latter indicative of acid conditions. The effects of temperature and pressure over long periods of geological time tend to expel water and volatile constituents from coal. Thus, a coal that has been subjected to elevated temperatures typically exhibits a low volatile content, a high carbon content and high calorific value; it is said to have a high ‘rank’. Conversely, coals that remain high in volatiles and have a relatively low carbon content and calorific value are said to be of low ‘rank’. A notable feature of the Northumberland and Durham Coalfield is the geographical variation in the rank of the coals [[Media:P916075.jpg|(P916075)]], with the highest rank found in west Durham but thence decreasing into other parts of the coalfield. No variation is apparent in the Cumbrian Coalfield’s broadly middle-ranking coals. The difference between the two coalfields arises from their contrasting histories of burial and geothermal heat-flow, the latter influenced by distance from the underlying, high heat-production Acadian granites of the North Pennine Batholith.

== Fauna and flora ==
The Pennine Coal Measures Group of northern England contains an abundant and varied fossil fauna that includes both nonmarine and marine species. Nonmarine invertebrates include worms, gastropods, bivalves, eurypterids, crustaceans, insects and fish, whilst the marine faunas include foraminifera, worms, brachiopods, goniatites and conodonts [[Media:P222338.jpg|(P222338)]]. The nonmarine bivalves are of particular importance for biostratigraphical purposes and are the basis of a widely used biozonal scheme [[Media:P916114.jpg|(P916114)]]. Nonmarine fossils are concentrated in the few metres of argillaceous strata that form the basal zone of the various cyclothems, but many cyclothems have no preserved fauna or contain only a small range of invertebrate fossils. Apart from fish remains, a small range of vertebrate fossils has also been recorded, with amphibian bones recovered from mudstone above the Northumberland Low Main Seam at Newsham in Northumberland. Marine fossils are naturally restricted to the marine bands. However, some marine incursions in the southern part of the Pennine Basin failed to reach the more proximal Northumberland and Durham Coalfield, and most of those that did penetrate to the far north of the basin affected that area for only a relatively brief interval. Accordingly, marine bands in northern England [[Media:P916114.jpg|(P916114)]] tend to feature a less varied and abundant fauna than those farther south.

Remains of the coal swamp vegetation are common fossils in the Coal Measures. Masses of compressed plant material make up the coal seams, roots are found in situ in seatearths, drifted leaf fronds and plant stems occur in mudstone and siltstone beds, and chaotic ‘log-jams’ of broken tree trunks and branches are a feature of some of the thicker sandstones. Study of Westphalian floras in northern Britain indicates that the flora of the floodplains was dominated by pteridosperms with some ferns, sphenopsids and lycopods, and that of the peat-forming swamps by lycopods. Today, the lycopod group is only represented by low-growing plants, but during the Westphalian some lycopods were tree-sized, the most familiar being ''Lepidodendron'', with its distinctive bark pattern of rhomboidal scales, and its ''Stigmaria'' root system. ''Calamites'', a giant relative of the present-day horsetail ''Equisetum'' was also common and grew around lakes and on point bars, whilst a range of pteridosperms grew on the levées alongside meandering rivers.

Following significant studies in the first half of the 20th century, a set of plant biozones was established for the Westphalian. More recently palynology, the study of plant spores of various kinds, has become a standard biostratigraphical technique. It has the great advantage of requiring only a small rock sample as a source of many fossils, and been applied extensively in the correlation of borehole sequences of Westphalian strata from the North Sea and between the offshore and onshore basins. This has been an important development because, whilst correlations of the major Westphalian surfaces and marine bands are not in doubt, more precise local and regional correlations are hindered by the impersistent nature of Coal Measure facies which results in the lateral equivalence of major sandstone bodies and coals.
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[[Category: Northern England]]

Southern ‘Dinantian’ successions, Carboniferous, Northern England

2016-03-23T19:50:45Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
== Introduction ==
[[File:P916067.jpg|thumbnail|Summary chart of lithofacies for the Carboniferous successions in northern England (after Waters et al., 2007. BGS Research Report RR/07/01). P916067.]]
[[File:P916072.jpg|thumbnail|Representative sections and correlations for theTournaisian to middle Visean (Asbian) sequences. P916072.]]
[[File:P006879.jpg|thumbnail|‘Basement Beds’ conglomerate from the Marsett Formation, exposed in Ardale Beck [NY 6610 3490], Cumbria, on the Pennine escarpment to the west of Cross Fell. (P006879).]]
[[File:P689698.jpg|thumbnail|Great Scar Limestone Group exposed in the western escarpment of Holmepark Fell, Cumbria [SD 540 795], viewed from Farleton Knott. (P689698).]]
[[File:P916068.jpg|thumbnail|Correlation chart for the traditional district-based Carboniferous lithostratigraphies (named on the figure) and the regional group lithostratigraphy adopted in this account (identified by colour). P916068.]]
[[File:P703191.jpg|thumbnail|Interbedded hemipelagic limestone and mudstone of the Scarlett Point Member,Bowland Shale Formation, exposed in the sea cliff at Scarlett Point [SC 257 661], Isle of Man (GS1039), (P703191).]]
[[File:P703196.jpg|thumbnail|Pillow lavas of the Scarlett Volcanic Member, Bowland Shale Formation, seen at Close ny Chollagh [SC 245 670], Isle of Man (GS1042).(P703196).]]

A different lithostratigraphy from that described above from the Solway–Northumberland Trough is applied to the rock sequences deposited during Dinantian rifting to the south of the Maryport–Stublick–Ninety Fathom fault system, [[Media:P916067.jpg|(P916067)]] and [[Media:P916072.jpg|(P916072)]]. The southern successions described in this section are peripheral to the Lake District and Isle of Man blocks, spread across the Alston Block, or form the lower part of the Stainmore Trough stratigraphy.

== Ravenstonedale Group ==
The oldest strata of the Ravenstonedale Group, the Pinskey Gill Formation, crop out south of the River Lune between Tebay and Kirkby Stephen. There, 50 m of mudstone, fine-grained sandstone, and limestone-dolostone rest on a smooth marine planation surface cut across Silurian rocks and contain an impoverished Courceyan marine fauna. Deposition of the formation was apparently in a shallow embayment with restricted marine circulation at the western limit of the Stainmore Trough, where the beds were probably laid down in somewhat shallower-water conditions than pertained across the main part of the trough. The Pinskey Gill Formation is unique to this district, but shows similarity in stratigraphical position and lithology to the subsurface Raydale Dolostone Formation, which is known only from boreholes located farther to the east in the Stainmore Trough.

In scattered outcrops across Ravenstonedale, the Courceyan marine beds are overstepped by quartz-pebble conglomerate, lithic sandstone and mudstone, gypsiferous in places, which comprise what were formerly called the Basal or Basement Beds [[Media:P006879.jpg|(P006879)]]. All of these occurrences are now brought together as the Marsett Formation in the north of England, and the equivalent Langness Conglomerate Formation in the Isle of Man. These formations encompass the first regionally extensive Carboniferous deposits, which originated as an accumulation of contemporaneous regolith, alluvial fan, river and marginal marine sediment. The two formations vary in thickness across the region. In some places only 10 to 30 m of strata are seen, which may be banked around local irregularities in the underlying syndepositional topography. Much thicker deposits, up to 500 m, occur in proximity to known, or suspected, early Carboniferous growth faults such as the North Solway Fault, the Kirkby/Foxfield Fault system north of the Duddon Estuary in Furness, the Butterknowle Fault on the West Pennine Escarpment and the Shag Rock Fault on the Isle of Man. Extensional faulting may also have been a controlling influence on eruption of the basaltic lavas that form the Cockermouth Volcanic Formation and lie near the base of the Marsett Formation on the south side of the Solway Basin. These lavas have been described earlier in association with the volcanic rocks that occupy a similar position in the Inverclyde Group.

The Marsett and Langness Conglomerate formations are imprecisely dated, but their palaeofloras indicates a Courceyan age in many areas. However, the sequences in west Cumbria and the Isle of Man contain internal disconformities and in their upper parts are interbedded with limestones that range in age from Chadian to Arundian, depending on location. Hence the sequences may represent accumulation, or cycles of erosion and re-accumulation, over a long period of time.

In Ravenstonedale, the Vale of Eden and south Cumbria, deposition of the Marsett Formation was ended by a late Chadian marine transgression, extending out from the Craven and Stainmore basins onto the southern margin of the Alston and Lake District blocks. Initially, conditions were supra-to peritidal and the lower parts of the Martin Limestone Formation in south and west Cumbria and the Stone Gill Limestone Formation in Ravenstonedale are largely composed of dolostone, algal limestone (both oncolites and stromatolites are present) and calcareous mudstone with bedding surfaces marked by desiccation features. In the Shap and south Cumbria areas, the marginal, shallow-water marine environment was maintained through to the end of the Chadian and the Shap Village Limestone Formation comprises dolostone, shelly ooidal limestone, cross-bedded calcareous sandstone and pebbly calcarenite. Mudstone seatearths occur in the lower part of the formation, whilst the top is defined by palaeosol developed from a breccia deposit.

The upper part of the Martin Limestone Formation was deposited in a broadly shallow-water marine environment and comprises partially dolomitised limestone, some of which is ooidal. Mudstone, sandstone and calcarenite are not present, and the lithological range is similar to that of the open marine, platform and ramp carbonates that typify the overlying Great Scar Limestone Group. Accordingly, in some accounts, the Martin Limestone Formation is included within the Great Scar Limestone Group. The top of the Martin Limestone Formation is prominently marked by a reddened palaeokarst zone.

== Great Scar Limestone Group ==
The type area of the Great Scar Limestone Group [[Media:P689698.jpg|(P689698)]] is the Askrigg Block, where it accumulated as a platform sequence about 400 m thick and Arundian to Asbian in age. Regionally, the group name is used to unify all equivalent, locally named, thick carbonate platform successions of Dinantian and earliest Namurian age in the north of England [[Media:P916068.jpg|(P916068)]]. The greatest exposed thickness is seen in Ravenstonedale where about 800 m of strata range from Chadian up to Asbian in age. The sequence in west Cumbria is a little thinner, at about 740 m, but ranges a little higher stratigraphically, from Chadian up to Pendleian (but with no evidence for Arundian rocks). In contrast, on the Alston Block only about 107 m of strata are present and are largely Asbian in age. In the south of the Isle of Man, the group is up to about 157 m thick and Arundian to Asbian in age. In the north of the island, approximately 7.3 m of strata at the bottom of the Ballavaarkish (Shellag North) Borehole (NX 460 007) are assigned to the Great Scar Limestone Group but may possibly be more closely related to the Yoredale Group; a broadly Dinantian or early Namurian age is the best biostratigraphical estimate that can be made from the poor evidence available.

=== Chadian to Arundian ===
The lowest formation in the Great Scar Limestone Group, forming the base of the sequence in the Stainmore Trough and extending north-west to just south of Shap, is the Coldbeck Limestone Formation. A dolostone with a characteristic abundance of algal structures (oncoliths) interbedded with mudstone, it was deposited on a west- to east-sloping marine shelf on which the characteristic Chadian coral ''Dorlodotia'' flourished. The conformably overlying Scandal Beck Formation comprises a succession of cyclically bedded bituminous limestone with thin siltstone interbeds.

At the end of the Chadian there was a general reversion to shallow water conditions and block areas suffered erosion. In the Stainmore Trough, where marine environments persisted until late in the Chadian, the Brownber Formation was deposited as a sequence dominantly made up of ooidal limestone, but interspersed with beds of calcareous sandstone and cross-bedded calcarenite. The sandstone beds are laterally discontinuous, due to channelised deposition, and contain distinctive layers of well-rounded pebbles of white quartz and calcite.

In the early Arundian, a fault-controlled acceleration in basin subsidence rates resulted in deeper marine conditions becoming re-established in the Stainmore Trough and across south Cumbria. At first, high-energy, shallow-marine conditions resulted in deposition of the cross-bedded ooidal limestone typical of the Red Hill Formation. Subsequently, water depths increased and the Dalton Formation of south Cumbria, and time equivalent Breakyneck Scar Limestone Formation of Ravenstonedale and the Stainmore Trough, are thick (200–300 m), marine shelf successions of dark grey, well-bedded crinoidal grainstone with siltstone interbeds.

The early Arundian deposition of peripheral shelf limestone was interrupted where rivers flowing from the north-east introduced terrigenous sediment, now represented around Edenside and Ravenstonedale by the Ashfell Sandstone Formation. It comprises the fluviodeltaic deposits of a river that flowed around and across the Alston Block, feeding a delta system within the Stainmore Trough. Within the upper part of the Dalton Formation in south Cumbria are sandstone beds that may have been introduced by the same fluvial system that deposited the Ashfell Sandstone farther to the east. A broad, regional link also seems likely with the approximately coeval fluvial system responsible for deposition of the Fell Sandstone Formation in the Northumberland–Solway Basin.

The early Arundian deposition of peripheral shelf limestones extended to the Castletown area in the southern part of the Isle of Man where a sequence about 90 m thick forms the Derbyhaven Formation. Three members are recognised. The lowest of these, the Turkeyland Member, consists of ooidal and bioclastic grainstone and has a sharply defined basal contact with the Langness Conglomerate. The middle, Sandwick Member, comprises dark, bioclastic packstone with an abundance of mudstone interbeds. At the top of the formation is the dark, pyritic packstone of the Skillicore Member, with an increasing proportion of interbedded fine-grained sandstone indicating a progressive shallowing of the depositional environment. The Turkeyland and Sandwick members correlate, respectively, with the Red Hill and Dalton formations of the south Cumbria succession. The Skillicore Member records a minor input of sandy sediment, though compositional differences make it unlikely to have been derived from the same source as the sands that formed similar interbeds in the Dalton Formation.

=== Holkerian to Pendleian ===
The Holkerian saw a significant extension of the shallow marine depositional environment onto the flanks of the Lake District and Alston blocks. Across north-west England the Ashfell Limestone and Frizington Limestone formations (and the equivalent Knockrushen Formation of the Isle of Man) are distinctive, dark grey, argillaceous limestone successions, cross- or tabular-bedded, with sandstone interbeds, especially towards the base. South of Penrith and into Ravenstonedale, the topmost beds record a marine regression and comprise a shallow marine to estuarine facies of cherty, porcellanous limestone, carbonaceous mudstone and ferruginous shell beds. On the Askrigg Block and in the Cockermouth area respectively, the Ashfell Limestone and Frizington Limestone formations are topped by thin fluviodeltaic intervals with a coal developed locally. In south Cumbria, the Holkerian Park Limestone Formation was deposited on a carbonate ramp with little terrestrial input, giving a lithologically uniform succession of pale grey, well-jointed, crinoidal grainstone, commonly crowded with robust gastropod and brachiopod shells.

The late Holkerian marine regression led to early Asbian emergence and erosion of the block and platform areas. However, later in the Asbian, the most extensive of the Dinantian marine transgressions re-established basinal or marine platform conditions across the entire region; even the Lake District Block was inundated at this time.

The character of the Asbian platform carbonate deposits is remarkably uniform across northern England and beyond. The limestones were formed on a sediment-starved marine carbonate platform, subject to a fluctuating relative sea level. This oscillation in water depth is expressed in a cycle of sediment type, with low-energy wackestone grading upwards into high-energy grainstone. Corals, mainly lithostrotionoids, occur commonly both fragmentally and as in situ colonies. During sea-level lowstands the exposed limestone surfaces were subject to karst development and a patchy form of diagenetic calcretisation, traditionally termed ‘pseudobreccia’. During more prolonged emergent intervals, the karst topography was wholly or partly infilled with claystone, formed from the accumulation of residual soils, decayed vegetation and subaerial deposits of volcanic ash. The Asbian limestones today form the most prominent of the scarps and upland plateaux of the north Pennines, and the spectacular glaciated limestone pavements of the Edenside and Kendal areas.

Platform carbonate rocks of early Asbian age comprise the Potts Beck Limestone Formation in Ravenstonedale and the lower part of the Urswick Limestone Formation in south Cumbria. The successions have a relatively shallow-water depositional aspect and siltstone–mudstone interbeds are common. Prominent amongst the latter is the ‘Woodbine Shale’, a 4–5 m thick bed of black pyritous mudstone located about 30 m above the base of the Urswick Limestone Formation; it forms a marked topographical break in many parts of south Cumbria and north Lancashire. A sandstone bed marks the top of the Potts Beck Limestone Formation above Orton Scar (NY 634 097), south of Appleby.

There was no early Asbian sedimentation on the Alston Block nor in west Cumbria west of the Bothel Fault, and in these areas, respectively, the Melmerby Scar Limestone Formation and the lower part of the Eskett Limestone Formation are condensed late Asbian successions. Thicker developments of lithologically identical rocks comprise the upper part of the Urswick Limestone Formation in south Cumbria and the Knipe Scar Limestone Formation of Ravenstonedale and Edenside. The Robinson Limestone Member, the highest component of the Melmerby Scar Formation (and also represented at the top of the Knipe Scar Formation) is separated from the limestones below by a prominent sandstone–mudstone interval at its base. In the Isle of Man, the upper Asbian Balladoole Formation comprises a platform carbonate succession equivalent to the Urswick Limestone Formation of south Cumbria. Locally however, the Balladoole Formation includes bioherms, indicating that the southern margin of the carbonate platform lay nearby.

In general, the youngest beds of the Great Scar Limestone Group are of Asbian age, and are succeeded by the uppermost Asbian to Brigantian strata of the Yoredale Group. An exception to this pattern is seen in west Cumbria where the Eskett Limestone Formation (Great Scar Limestone Group) continues up through the Brigantian and into the Pendleian as a dominantly limestone succession but with abundant thin interbeds of mudstone and sandstone. This upper part of the Eskett Limestone Formation is a time-equivalent deposit to much of the Alston Formation and is conformably overlain by sandstone — the ‘Hensingham Grit’ — at the base of the Stainmore Formation [[Media:P916067.jpg|(P916067)]].

== Craven Group ==
The Craven Group is most fully developed in the Craven Basin area of Lancashire, to the south-west of the Craven Fault system at the margin of the Askrigg Block [[Media:P916068.jpg|(P916068)]], but its strata extend northwards and have limited outcrop on the Isle of Man. There, in the south, reactivation of the basin margin fault system caused the Knockrushen Formation to be succeeded by a 14 m-thick band of hemipelagic, cherty wackestone and mudstone. This band has a distinctive late Holkerian fauna and is the equivalent of the Hodderense Limestone Formation seen farther south in the main part of the Craven Basin. The succeeding Asbian to Brigantian succession in the Isle of Man and north Lancashire is largely of prodelta or basinal facies, with interbedding of black, hemipelagic claystone and limestone turbidites; it is accommodated in the Bowland Shale Formation. The Asbian, Balladoole Limestone Formation (Great Scar Limestone Group) was partly coeval with the Bowland Shale Formation and was subsequently overstepped by it. In the south of the Isle of Man, the lowermost Asbian part of the Bowland Shale Formation includes the Scarlett Point Member [[Media:P703191.jpg|(P703191)]], wherein hemipelagic claystone is interbedded with coarser-grained sediment, including large blocks (olistoliths) of reef carbonate, introduced by turbidity currents and submarine slides from the platform margin to the north. Higher in the Isle of Man succession, in the Brigantian part of the Bowland Shale Formation, submarine debris flows and slides of volcaniclastic rocks and basaltic pillow lavas form the Scarlett Volcanic Member [[Media:P703196.jpg|(P703196)]], and indicate that the basin margin was volcanically active at this time.
== Bibliography ==
Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). 1989. The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. ''Occasional Publication of the Yorkshire Geological Society'', No. 6.

Barclay, W J, Riley, N J, and Strong, G E. 1994. The Dinantian rocks of the Sellafield area, West Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 37–49.

Bott, M H P, Swinburne, P M, and Long, R E. 1984. Deep structure and origin of the Northumberland and Stainmore troughs. ''Proceedings of the Yorkshire Geological Society'', Vol. 44, 479–495.

Burgess, I C. 1986. Lower Carboniferous sections in the Sedbergh district, Cumbria. ''Transactions of the Leeds Geological Association'', Vol. 11, 1–23.

Calver, M A. 1968. Distribution of Westphalian marine faunas in Northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society'', Vol. 37, 1–72.

Cleal, C J, and Thomas, B A. 1996. British Upper Carboniferous Stratigraphy. ''Geological Conservation Review Series'', No. 11. (Peterborough: Joint Nature Conservation Committee.)

Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A and Wright, V P. 2004. British Lower Carboniferous Stratigraphy. ''Geological Conservation Review Series'', No. 29. (Peterborough: Joint Nature Conservation Committee.)

Dickson, J A D, Ford, T D, and Swift, A. 1987. The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man. ''Proceedings of the Yorkshire Geological Society'', Vol. 46, 203–229. 268

Fairbairn, R A. 2001. The stratigraphy of the Namurian Great/Main Limestone on the Alston Block, Stainmore Trough and Askrigg Block of northern England. ''Proceedings of the Yorkshire Geological Society'', Vol. 53, 265–274.

Fielding, C R. 1984. A coal depositional model for the Durham Coal Measures of North East England. ''Journal of the Geological Society of London'', Vol. 141, 917–931.

Fraser, A J, and Gawthorpe, R L. 2003. An Atlas of Carboniferous Basin Evolution in Northern England. ''Geological Society of London, Memoir'', No. 28.

Garwood, E J. 1913. The Lower Carboniferous succession in the north-west of England. ''Journal of the Geological Society of London'', Vol. 68 (for 1912), 449–586.

Guion, P D, Fulton, I M, and Jones, N S. 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant High. 45–78 ''in ''European Coal Geology. Whateley, M K G, and Spears, D A (editors). ''Geological Society of London Special Publication'', No. 82.

Heckel P H, and Clayton, G. 2006. Use of the new official names for the Subsystems, Series and Stages of the Carboniferous System in international Journals. Correspondence. ''Proceedings of the Geologists’ Association'', Vol. 117, 393–396.

Johnson, G A L. 1984. Subsidence and sedimentation in the Northumberland Trough. ''Proceedings of the Yorkshire Geological Society'', Vol. 45, 71–83.

Johnson, G A L, and Dunham, K C. 1963. The geology of Moor House. ''Nature Conservancy Monograph'', No. 2. (London: HMSO.)

Johnson, G A L, and Nudds, J R. 1996. Carboniferous biostratigraphy of the Rookhope Borehole, Co. Durham. ''Transactions of the Royal Society of Edinburgh: Earth Sciences'', Vol. 86, 181–226.

Jones, N S, and Holliday, D W. 2006. The stratigraphy and sedimentology of Upper Carboniferous Warwickshire Group red-bed facies in the Canonbie area of SW Scotland. ''British Geological Survey Internal Report'', IR/06/043.

O’Mara, P T, and Turner, B R. 1999. Sequence stratigraphy of coastal alluvial plain Westphalian B Coal Measures in Northumberland and the southern North Sea. ''International Journal of Coal Geology'', Vol. 42, 33–62.

Owens, B, and Burgess, I C. 1965. The stratigraphy and palynology of the Upper Carboniferous outlier of Stainmore, Westmorland. ''Bulletin of the Geological Survey of Great Britain'', Vol. 23, 17–44.

Reynolds, A D. 1992. Storm, wave and tide-dominated sedimentation in the Dinantian Middle Limestone Group, Northumbrian Basin. ''Proceedings of the Yorkshire Geological Society'', Vol. 49, 135–148.

Rippon, J H. 1996. Sand body orientation, palaeoslope analysis, and basin fill implications in the Westphalian A–C of Great Britain. ''Journal of the Geological Society of London'', Vol. 153, 881–900.

Rippon, J H. 1998. The identification of syndepositionally active structures in the coalbearing Upper Carboniferous of Great Britain. ''Proceedings of the Yorkshire Geological Society'', Vol. 52, 73–93.

Rowley, C R. 1969. The stratigraphy of the Carboniferous Middle Limestone Group of West Edenside, Westmorland. ''Proceedings of the Yorkshire Geological Society'', Vol. 37, 329–350.

Smith, T E. 1968. The Upper Old Red Sandstone–Carboniferous junction at Burnmouth, Berwickshire. ''Scottish Journal of Geology'', Vol. 4, 349–354.

Tucker, M E, Gallagher, J, Lemon, K, and Leng, M. 2003. The Yoredale Cycles of Northumbria: High-Frequency Clastic-Carbonate Sequences of the Mid-Carboniferous Icehouse World. ''Open University Geological Society Journal'', Vol. 24, 5–10.

Turner, B R, Younger, P L, and Fordham, C E. 1993. Fell Sandstone Group lithostratigraphy south-west of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone. ''Proceedings of the Yorkshire Geological Society'', Vol. 49, 269–281.

Ward, J. 1997. Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore, Cumbria, England. 277–296 ''in ''Petroleum Geology of the Irish Sea and Adjacent Areas. Meadows, N S, and others (editors). ''Geological Society of London Special Publication'', No. 124.

Waters, C N, Browne, M A E, Dean, M T, and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). ''British Geological Survey Research Report'', RR/07/01.

[[Category: Northern England]]

Northern ‘Dinantian’ successions, Carboniferous, Northern England

2016-03-23T19:27:20Z

BobMcIntosh:

Depositional controls, Carboniferous, Northern England

2016-03-23T19:23:02Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__

== The structural framework ==
[[File:P916069.jpg|thumbnail|Sketch cross-section from the Cheviot Block to the Stainmore Trough, showing the half-graben structure of the Northumberland Trough (after Chadwick et al., 1995. The Northumberland–Solway Basin and adjacent areas. BGS Subsurface Memoir). P916069.]]
[[File:P916037.jpg|thumbnail|3D model showing depth to the Lower Palaeozoic ‘basement’ across northern England, viewed from the west. The principal structural features that influence Upper Palaeozoic and later geology are identified and related to a ‘blocks and basins’ sketch map rotated into a top-to-north orientation. P916037.]]
[[File:P693033.jpg|thumbnail|Detail of the Concretionary Limestone Member of the Roker Formation from a coastal exposure near Sunderland [NZ 412 555]. (P693033).]]
[[File:P916071.jpg|thumbnail|Illustrative logs and interpretations for some types of high-frequency clastic sequences within the Yoredale and Pennine Coal Measures groups of northern England (after Tucker et al., 2003). P916071.]]

Over a hundred years ago, it was appreciated that the Carboniferous of northern England was deposited in a series of troughs or ‘geosynclines’, separated by higher areas. Towards the end of the 20th century, the recognition that these basins and blocks were formed as extensional or transtensional features during a period of lithospheric stretching was a major development in our understanding of the region. This period of crustal extension, in a stress-field with a dominantly horizontal component of north–south tension, began in the Late Devonian and continued until late Visean times; it gradually produced a rifted topography of fault-bounded blocks with intervening graben and half-graben basins. Rifts opened along preexisting lines of structural weakness embedded in the underlying crust, with the Iapetus Suture Zone providing a primary control. For example, the fault system forming the southern margin of the Northumberland–Solway trough is rooted into the Iapetus Suture so that the trough effectively formed by extension in the suture’s hanging wall. The extensional regime allowed the local occurrence of basaltic magmatism.

Rifting was pulsed, with particularly active episodes in Courceyan, Chadian to early Arundian and in mid to late Asbian times, although the magnitude and perhaps the timing of each pulse, appears to have varied significantly from basin to basin. The buoyant and rigid behaviour of the blocks is usually ascribed to the presence beneath them of low-density granitoid plutons, emplaced during Late Ordovician or Early Devonian magmatic events. Within this framework, the Carboniferous record of uplift, tilting and submergence of individual blocks points to a complex history of fault-block rotation and lateral as well as vertical movements along the boundary faults. Further, block boundaries are not always fault-controlled, but may be transitional across the hinge zone into a half-graben. In these cases, large areas between the main blocks and flanking the troughs are now covered by a shelf carbonate succession deposited in a regime that was neither wholly ‘block’ nor wholly ‘basin’. The principal structural units [[Media:P916037.jpg|(P916037)]] are described below.

The Southern Uplands Block and its offshore continuation into the Mid North Sea High, broadly separated the Peel–Solway–Northumberland Basin from the Midland Valley of Scotland. Evidence suggests that, at times, the barrier was breached by a series of narrow north-north-west-trending basins.

Traversing the north of England is the composite Peel–Solway–Northumberland Basin. Interpretation of geophysical profiles across the basin indicates that it developed over the inferred line of the Iapetus Suture, with extension and growth faulting further facilitated by pre-existing intracrustal detachment surfaces such as the Causey Pike Fault. The southern margin of the basin is defined by the Maryport–Stublick–Ninety Fathom fault system. Throughout much of its length the northern margin is also formed by a system of en échelon synsedimentary dislocations including the North Solway, Gilnockie and Alwinton faults. In the early Carboniferous, the Cheviot Block separated the Tweed Basin from the main Northumberland Trough, although the extent of its continuation offshore is still uncertain. The eastern side of the Cheviot Block, onshore, was submerged in the Asbian and its boundary with the Northumberland Trough is poorly defined. The latter basin is a half-graben, with the rock succession thickest close to the major bounding fault in the south [[Media:P916069.jpg|(P916069)]]. Seismic data reveal a number of fault-controlled, linear intrabasinal highs that may have been exposed and subjected to contemporary erosion during the early, pre-Chadian period of basin evolution. To the west, the Peel and Solway basins formed as complex and roughly symmetrical grabens.

On its south side, the Northumberland Trough is bordered by the Alston and Lake District blocks. Underlain by the North Pennine Batholith and bounded by faults on three sides, the Alston Block formed a prominent high until the Asbian. The Lake District Block is underpinned by the Lake District granitic batholith and probably remained emergent during early Carboniferous times. Thereafter, southward tilting of its upper surface gradually allowed northward onlap of Arundian and younger strata and it is likely that latest Visean, Namurian and Westphalian strata were deposited over most if not all of the block. The Southern Lake District High is essentially the south-dipping flank of the Lake District Block, which from seismic evidence was dissected during Visean times by a series of small north-trending half-grabens. The Lake District block extends westwards as the Ramsey–Whitehaven Ridge, an elevated tilt-block, bounded to the north-west by the Maryport Fault and to the south-east by the Lagman and Eubonia faults. At its western end, the ridge merges with the Manx Block, a structural high underpinned by the granitic Manx Pluton.

To the west of the Alston Block, the Vale of Eden Basin developed from the Visean onwards as a half-graben structure adjacent to the Pennine Fault system. To the south of the block, the Stainmore Trough is an embayment open to the east. The northern margin of the trough is bounded by the Closehouse–Lunedale–Butterknowle fault system, which links northwards via the Pennine Fault to the Stublick Fault at the northern margin of the Alston Block. In contrast to the floor of the Northumberland Trough, seismic evidence indicates that the floor of the Stainmore Trough is relatively flat, with little evidence of significant intrabasinal faulting. The southern margin of the Stainmore Trough is formed by the Askrigg Block, a northward dipping massif underpinned by the Wensleydale Granite. Thence the basin network extends through the Craven and Lancaster Fells basins and continues westward into the East Irish Sea Basin. The northern margins of the Askrigg, Craven and Lancaster Fells structural units define the southern limit of the Northern England region described here; these units are described in the companion volume for the Pennines and adjacent areas.

The timing of early Carboniferous extension remains a matter for debate. Geophysical interpretation indicates that the earliest, synrift Carboniferous strata in northern England were deposited in the axes of the main troughs. Very thick, early Carboniferous deposits appear to form the lower part of the synrift succession in the Northumberland–Solway Basin and the Stainmore Trough, but their character is unknown since they are located at depths beyond the reach of existing exploratory boreholes. From likely correlations with surface outcrops in northern Cumbria, these early deposits (at least in the Northumberland Trough) may well include fault-related breccio-conglomerate and basaltic lavas erupted at the onset of rifting. Siliciclastic rocks interbedded with limestones of Arundian and Holkerian age were proved in the Seal Sands Borehole (NZ 5379 2380) and may be typical of much of the basin fill in the Stainmore Trough. The synextensional rocks of the region are largely confined to the basinal areas, and are thickest close to the major bounding faults. Up to around 5000 m of synrift strata lie adjacent to both the Maryport–Stublick–Ninety Fathom fault system along the southern margin of the Solway–Northumberland Trough, and to the Closehouse–Lunedale– Butterknowle fault system at the northern margin of the Stainmore Trough. Equivalent strata are largely absent from the structural highs though relatively complete but thin, synextensional sequences do occur on the eastern part of the Cheviot Block and on some of the main intrabasin highs. Only relatively small thicknesses, up to 200 m of beds from the later part of the extensional phase, occur around the margins of the Alston and Lake District blocks.

Both the Peel–Solway–Northumberland Trough and the Craven Basin were inundated from a seaway to the west, while the Stainmore Trough was probably flooded from the east. Borehole evidence shows that deep marine conditions were maintained throughout Dinantian times in the Craven Basin and concealed eastern part of the Stainmore Trough. However, the exposed Dinantian successions of both the Solway–Northumberland and west Stainmore troughs were deposited in relatively shallow marine conditions and there were frequent fluviodeltaic incursions. During later phases of extension, deposition gradually spread more widely until, Asbian to Brigantian times, rapid subsidence gave way to slow downwarping of the major troughs and their adjacent bounding block areas. Minor extensional faulting continued into the ‘postextensional’ phase.

== Tectonic influences on local sea level ==
The position of relative sea level was a primary control on the nature of the Carboniferous sedimentary successions. Active growth of the major structures beneath the region caused the sea to be persistently deeper in some areas, thus influencing both the type and thickness of sediment deposited. In general, the block areas subsided more slowly than the intervening basins, resulting in the accumulation of thinner sequences of Carboniferous rocks over the blocks than in the basins. Despite these marked variations in subsidence rates, it seems that during much of the Carboniferous, sedimentation across northern England everywhere kept pace with subsidence so that the depositional surface across both blocks and basins was at any time almost horizontal.

During the Dinantian and most of the Namurian, limestone and marine mudstone were deposited in maximum water depths of a few tens of metres; coals, seatearths and many of the sandstones represent emergence of a few metres. Lateral changes in lithofacies and stratal thickness point to recurrent syndepositional activity along basin margins and inherited fault lines such as the Closehouse–Lunedale fault system. From the late Namurian, significant marine influence was progressively lost over the entire region and subsidence and river-borne sedimentation were balanced, maintaining a stable delta-top environment through to the late Westphalian. Deposition was increasingly dominated by sand, silt and mud, carried into the region by large prograding river deltas draining a land area far to the north. With time, marine intervals became less frequent and of shorter duration. The likely changes in palaeogeography inherent in this situation are summarised in [[Media:P916070.jpg|(P916070)]].

The initial, rapid fault-controlled subsidence along the early Carboniferous rift axes, was locally accompanied by the eruption of basaltic lavas, now preserved in northern Cumbria and Northumberland, and along the southern margin of the Southern Uplands. Earliest Carboniferous (Tournaisian) sedimentary successions include variable thicknesses of unfossiliferous conglomerate and sandstone derived from erosion of the Late Devonian landscape. The lithofacies closely resemble that of the upper Old Red Sandstone in southern Scotland (Stratheden Group) where, in the Tweed Basin, there is locally a conformable passage from uppermost Devonian into lowermost Carboniferous strata.

Limestone deposition predominated during Visean times in much of south and west Cumbria, in Ravenstonedale and on the Alston and Askrigg blocks. Sedimentary environments ranged from shallow shelf seas to ramp and slope areas, though the central parts of the Alston and Lake District blocks were only partially and briefly submerged. Differential subsidence between the Alston, Cheviot and Southern Uplands blocks and the Tweed and Northumberland basins had been most active during the early part of the Tournaisian and reduced progressively from the Visean onwards.

In late Visean times, uplift of source areas to the north led to southwards progradation of a giant clastic delta complex that rapidly filled the basinal areas of northern England. The Cheviot Block was the first to lose structural independence in the Asbian, with deposition between the Northumberland and Tweed basins becoming uniform and continuous. At the same time, the Alston Block became more closely linked with the Northumberland Trough as the degree of differential subsidence across the Stublick–Ninety Fathom line gradually decreased. Thereafter, from the Brigantian onwards, a similar pattern of cyclic sedimentation developed throughout the region, although relatively thin Brigantian and Namurian successions over the Alston Block indicate that it was still subsiding more slowly than the adjacent Northumberland Trough. This situation persisted until the beginning of the Westphalian, but then uniform subsidence affected both block and basin until at least the Bolsovian (Westphalian C). The Westphalian D red beds of the Canonbie area are the youngest Carboniferous strata now preserved in northern England and their lithofacies shows establishment of a fluvial–terrestrial environment.

== Climate ==
Palaeomagnetic and lithofacies evidence (the latter including worldwide facies distributions) independently suggest that Britain was situated in near-equatorial latitudes for much of the Carboniferous Period [[Media:P916033.jpg|(P916033)]] c–d). In northern England, Tournaisian terrestrial strata laid down in small isolated basins include pedogenic horizons (cornstones) indicative of a semi-arid climate; the coeval offshore deposits preserved in the Northumberland Trough are assemblages of interbedded mudstone (some with halite and gypsum pseudomorphs), sandstone and argillaceous dolostone (‘cementstone’) deposited in a marginal marine environment subject to periodic desiccation. The discovery of thick, early Visean anhydrite beds in the Easton Borehole (NY 4412 7170) confirmed largely arid climatic conditions and shallow marine deposition. By the late Visean, Britain lay at the southern margin of the equatorial belt, but experienced fluctuations of climate with the possibility of monsoonal-type rains. Thereafter, during the later part of the Carboniferous, Britain moved into humid, equatorial latitudes, as confirmed by the extent of coal within the sedimentary sequence that accumulated. The end of the Westphalian saw a return to more arid conditions.

From late Visean times onward, the southern hemisphere experienced repeated phases of glaciation as its continental mass (Gondwana, by then the southern part of Pangaea — [[Media:P916033.jpg|(P916033)]] drifted across the South Pole. The coldest intervals may have brought about short-term, seasonally drier climate farther north, whereas melting ice may have resulted in a wetter equatorial climate, and would certainly have caused a eustatic rise in sea level. It has been proposed, but not established, that glacial fluctuations in southern Gondwana, controlled sedimentary cyclicity elsewhere. Across northern Britain, at least, this glacial/eustatic influence would have interacted with other, more local tectonic effects. A factor in the southern Gondwana glaciation may have been the rapid Carboniferous rise in the atmospheric O2/CO2 ratio that was coincident with the proliferation of land plants. At around this time there was a marked evolutionary expansion of several groups of spore-bearing plants such as sphenopsids (horsetails), lycopsids (clubmosses) and filicopsids (ferns). Two groups of seed producing plant — cordaites and pteridosperms — also expanded, whilst the first conifers, cycads and bennettitales appeared in late Carboniferous times. Other side-effects of the increase in atmospheric oxygen were seen in the coal swamps: the evolutionary rise of large insects, and the high frequency of wild fires.

== Sedimentary cyclicity ==
Sedimentary cyclicity is a feature of the Carboniferous successions. The phenomenon is particularly developed in parts of the upper Visean and lower Namurian successions of the Northumberland and Stainmore troughs and the Alston and Askrigg blocks, which are characterised by the ‘Yoredale facies’; an assemblage wherein each cyclothem is thicker, more extensive, and can be more widely correlated, than is the case for cyclothems elsewhere in the Carboniferous succession. Each Yoredale cyclothem has a limestone at the base, which is overlain sequentially by mudstone, sandstone, seatearth and coal [[Media:P916071.jpg|(P916071)]]. The cycles have an average thickness of around 20 m, range up to a maximum of several hundred metres, and are generally thicker in the basins and thinner on the blocks. Their immediate cause was a marine transgression followed by a progressive shallowing and change to fluvial conditions with subsequent emergence and the growth of delta-top swamp vegetation; a series of events repeated many times. In terms of sequence stratigraphy, the limestone was deposited during the high-stand phase with the base representing the sequence boundary, coincident with the transgressive surface. Lowstand facies are represented by palaeosols and coal at the top of some of the cycles.

The origin of typical Yoredale cycles has been much discussed, with tectonic, eustatic and sedimentary mechanisms all proposed. Since broadly similar patterns of relative cycle thickness are found for the Yoredale successions in both block and basin localities, a control on deposition that affected the whole region seems likely and from this perspective glacioeustatic sea-level oscillations are attractive. However, the advance and migration of delta lobes, or local tectonism such as that associated with syndepositional fault movement, both of which mechanisms are independent of sea-level change, can also result in cyclical deposits of mudstone and sandstone. It is most likely that a complex interaction of all of these mechanisms resulted in the deposition of the distinctive Yoredale lithofacies and the many other examples of Carboniferous cyclothems.

Arthurton, R S, Gutteridge, P, and Nolan, S C (editors). 1989. The Role of Tectonics in Devonian and Carboniferous Sedimentation in the British Isles. ''Occasional Publication of the Yorkshire Geological Society'', No. 6.

Barclay, W J, Riley, N J, and Strong, G E. 1994. The Dinantian rocks of the Sellafield area, West Cumbria. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 37–49.

Bott, M H P, Swinburne, P M, and Long, R E. 1984. Deep structure and origin of the Northumberland and Stainmore troughs. ''Proceedings of the Yorkshire Geological Society'', Vol. 44, 479–495.

Burgess, I C. 1986. Lower Carboniferous sections in the Sedbergh district, Cumbria. ''Transactions of the Leeds Geological Association'', Vol. 11, 1–23.

Calver, M A. 1968. Distribution of Westphalian marine faunas in Northern England and adjoining areas. ''Proceedings of the Yorkshire Geological Society'', Vol. 37, 1–72.

Cleal, C J, and Thomas, B A. 1996. British Upper Carboniferous Stratigraphy. ''Geological Conservation Review Series'', No. 11. (Peterborough: Joint Nature Conservation Committee.)

Cossey, P J, Adams, A E, Purnell, M A, Whiteley, M J, Whyte, M A and Wright, V P. 2004. British Lower Carboniferous Stratigraphy. ''Geological Conservation Review Series'', No. 29. (Peterborough: Joint Nature Conservation Committee.)

Dickson, J A D, Ford, T D, and Swift, A. 1987. The stratigraphy of the Carboniferous rocks around Castletown, Isle of Man. ''Proceedings of the Yorkshire Geological Society'', Vol. 46, 203–229. 268

Fairbairn, R A. 2001. The stratigraphy of the Namurian Great/Main Limestone on the Alston Block, Stainmore Trough and Askrigg Block of northern England. ''Proceedings of the Yorkshire Geological Society'', Vol. 53, 265–274.

Fielding, C R. 1984. A coal depositional model for the Durham Coal Measures of North East England. ''Journal of the Geological Society of London'', Vol. 141, 917–931.

Fraser, A J, and Gawthorpe, R L. 2003. An Atlas of Carboniferous Basin Evolution in Northern England. ''Geological Society of London, Memoir'', No. 28.

Garwood, E J. 1913. The Lower Carboniferous succession in the north-west of England. ''Journal of the Geological Society of London'', Vol. 68 (for 1912), 449–586.

Guion, P D, Fulton, I M, and Jones, N S. 1995. Sedimentary facies of the coal-bearing Westphalian A and B north of the Wales–Brabant High. 45–78 ''in ''European Coal Geology. Whateley, M K G, and Spears, D A (editors). ''Geological Society of London Special Publication'', No. 82.

Heckel P H, and Clayton, G. 2006. Use of the new official names for the Subsystems, Series and Stages of the Carboniferous System in international Journals. Correspondence. ''Proceedings of the Geologists’ Association'', Vol. 117, 393–396.

Johnson, G A L. 1984. Subsidence and sedimentation in the Northumberland Trough. ''Proceedings of the Yorkshire Geological Society'', Vol. 45, 71–83.

Johnson, G A L, and Dunham, K C. 1963. The geology of Moor House. ''Nature Conservancy Monograph'', No. 2. (London: HMSO.)

Johnson, G A L, and Nudds, J R. 1996. Carboniferous biostratigraphy of the Rookhope Borehole, Co. Durham. ''Transactions of the Royal Society of Edinburgh: Earth Sciences'', Vol. 86, 181–226.

Jones, N S, and Holliday, D W. 2006. The stratigraphy and sedimentology of Upper Carboniferous Warwickshire Group red-bed facies in the Canonbie area of SW Scotland. ''British Geological Survey Internal Report'', IR/06/043.

O’Mara, P T, and Turner, B R. 1999. Sequence stratigraphy of coastal alluvial plain Westphalian B Coal Measures in Northumberland and the southern North Sea. ''International Journal of Coal Geology'', Vol. 42, 33–62.

Owens, B, and Burgess, I C. 1965. The stratigraphy and palynology of the Upper Carboniferous outlier of Stainmore, Westmorland. ''Bulletin of the Geological Survey of Great Britain'', Vol. 23, 17–44.

Reynolds, A D. 1992. Storm, wave and tide-dominated sedimentation in the Dinantian Middle Limestone Group, Northumbrian Basin. ''Proceedings of the Yorkshire Geological Society'', Vol. 49, 135–148.

Rippon, J H. 1996. Sand body orientation, palaeoslope analysis, and basin fill implications in the Westphalian A–C of Great Britain. ''Journal of the Geological Society of London'', Vol. 153, 881–900.

Rippon, J H. 1998. The identification of syndepositionally active structures in the coalbearing Upper Carboniferous of Great Britain. ''Proceedings of the Yorkshire Geological Society'', Vol. 52, 73–93.

Rowley, C R. 1969. The stratigraphy of the Carboniferous Middle Limestone Group of West Edenside, Westmorland. ''Proceedings of the Yorkshire Geological Society'', Vol. 37, 329–350.

Smith, T E. 1968. The Upper Old Red Sandstone–Carboniferous junction at Burnmouth, Berwickshire. ''Scottish Journal of Geology'', Vol. 4, 349–354.

Tucker, M E, Gallagher, J, Lemon, K, and Leng, M. 2003. The Yoredale Cycles of Northumbria: High-Frequency Clastic-Carbonate Sequences of the Mid-Carboniferous Icehouse World. ''Open University Geological Society Journal'', Vol. 24, 5–10.

Turner, B R, Younger, P L, and Fordham, C E. 1993. Fell Sandstone Group lithostratigraphy south-west of Berwick-upon-Tweed: implications for the regional development of the Fell Sandstone. ''Proceedings of the Yorkshire Geological Society'', Vol. 49, 269–281.

Ward, J. 1997. Early Dinantian evaporites of the Easton-1 well, Solway Basin, onshore, Cumbria, England. 277–296 ''in ''Petroleum Geology of the Irish Sea and Adjacent Areas. Meadows, N S, and others (editors). ''Geological Society of London Special Publication'', No. 124.

Waters, C N, Browne, M A E, Dean, M T, and Powell, J H. 2007. Lithostratigraphical framework for Carboniferous successions of Great Britain (Onshore). ''British Geological Survey Research Report'', RR/07/01.

[[Category: Northern England]]

Stratigraphical principles, Carboniferous, Northern England

2016-03-23T19:19:09Z

BobMcIntosh:

Old Red Sandstone, Devonian, Northern England

2016-03-23T19:06:10Z

BobMcIntosh:

Early Devonian magmatism, Acadian Orogeny, Devonian, Northern England

2016-03-23T19:04:58Z

BobMcIntosh:

Acadian Orogeny, Devonian, Northern England

2016-03-23T19:02:13Z

BobMcIntosh:

'''From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. [[British regional geology: Northern England|British regional geology: Northern England]]. Fifth edition. Keyworth, Nottingham: British Geological Survey.'''

__FORCETOC__
== Introduction ==
[[File:P916060.jpg|thumbnail|Simplified structural cross-section of the Lake District inlier, showing the likely overburden at the time of Acadian deformation. This was mostly eroded away prior to the deposition of the Carboniferous strata (after Woodcock and Soper. Figure 6.5 in Brenchley and Rawson (editors). The Geology of England and Wales, 2006). P916060.]]
[[File:P916119.jpg|thumbnail|Cleavage development in Skiddaw Group (Buttermere Formation) strata near Hollows Farm, Borrowdale [NY 249 174]. The regional Acadian cleavage (S1) is inclined from top left to bottom right and is deformed by minor open folds with an axial planar crenulation cleavage inclined from top right to bottom left (A H Cooper, MNS8736).]]
[[File:P916062.jpg|thumbnail|Principal structures affecting the Skiddaw Group modelled in terms of a deep-seated flower structure rooted along the northern margin of the Lake District Batholith. To clarify the major structures, the Lower Palaeozoic strata are projected above the present surface level (cf. Figure 30). The unconformity at the base of the Carboniferous sequence is shown as a reference level (after Woodcock and Soper. Figure 6.5 in Brenchley and Rawson (editors). The Geology of England and Wales, 2006). P916062.]]
[[File:P104236.jpg|thumbnail|Regional Acadian cleavage forming a convergent fan in a syncline of sandstone beds from the Niarbyl Formation (Dalby Group) near Glen Maye, Isle of Man [SC 2237 7987]. (P104236)]]
[[File:P104237.jpg|thumbnail|Tight to isoclinal folds refolded by recumbent open folds in laminated to thinly bedded mudstone and siltstone of the Lonan Formation, Manx Group, at Port Erin, Isle of Man [SC 1959 6929]. (P104237).]]
[[File:P916063.jpg|thumbnail|Sketch cross-section illustrating the main structural elements of the Manx Group. P916063.]]
[[File:P916054.jpg|thumbnail|Generalised distribution of the component groups of the Windermere Supergroup in its southern Lake District outcrop.]. P916054.]]
[[File:P916064.jpg|thumbnail|Variation in the relationship between Acadian cleavage and fold orientation across the outcrop of the Windermere Supergroup in the southern Lake District and Craven inliers (after Soper, Webb and Woodcock, 1987). P916064.]]
[[File:P916065.jpg|thumbnail|Pattern of faults cutting the Windermere Supergroup in the southern Lake District (after Soper and Woodcock, 2003). P916065.]]

Stratigraphically, the Acadian Orogeny is constrained as post-Pridoli, pre-Late Devonian. Cleavage development overlapped the generation of metamorphic aureoles around the Devonian granites at Skiddaw and Shap, and these have been dated radiometrically at about 399 and 404 Ma respectively. Further south, in the Ribblesdale (Craven) inlier, metamorphic white mica developed along the cleavage in a Ludlow bentonite has given radiometric ages in the 397–418 Ma range. All the evidence points to the Acadian Orogeny being an Early to Mid Devonian event, most probably of Emsian to Eifelian age.

The Acadian tectonic deformation was superimposed on pre-existing, Late Ordovician and Silurian disruption of the strata in both the Skiddaw and Manx groups. Not surprisingly, the structures of both groups are highly complex, and their apparently polyphase character led initially to proposals for a protracted, polyphase deformation history. The recognition of the earliest deformations as soft-sediment, slump-related phenomena, and definitive confirmation that the first, regional tectonic cleavage was indeed Acadian, have largely resolved the regional interpretational controversies. The possibility of a diachronous cleavage being developed in a southward prograding thrust belt, in continuity with the Southern Uplands accretionary complex and linked to the Windermere Supergroup foreland basin, cannot be sustained. However, at a more local scale, the detailed structures of the Manx and Skiddaw groups still remain open to different interpretations.

The Borrowdale and Eycott Volcanic groups were affected by synvolcanic faulting associated with piecemeal caldera collapse. The most important Acadian structures superimposed on the previously block-faulted volcanic tracts are regional monoclines facing north in the Eycott Volcanic Group and south in the Borrowdale Volcanic Group. These structures define the regional Lake District ‘anticline’ of which the Skiddaw Group forms the core, and are asymmetrically disposed above the margins of the subvolcanic batholith [[Media:P916060.jpg|(P916060)]]. The northern ‘monocline’, affecting the Eycott Volcanic Group may well have been initiated much earlier, with geophysical evidence and age relationships suggesting that at least some of the volcanic rocks were rotated to a near-vertical attitude soon after their eruption. In this case, Acadian effects may simply have been to modify the existing structure. A better-defined and more extensive Acadian structure is the southern, Westmorland Monocline; it affects the southern margin of the Borrowdale Volcanic Group outcrop and the overlying Windermere Supergroup strata, and lies above the southern margin of the batholith. Cleavage is strongly developed in the volcanic rocks close to the monocline hinge zone, forming the Tilberthwaite slate belt in which the ‘green slate’ industry is concentrated (see Chapter 12). Another zone of enhanced cleavage, the Honister slate belt, lies further north and may coincide with a step, downwards to the north, in the top surface of the batholith. Overall, the style of deformation is strongly influenced by the disposition of the more rigid rocks in the volcanic succession and by the protective effects of the underpinning batholith.

Across the southern Lake District the steep limb of the Westmorland Monocline forms the northern limb of the Bannisdale Syncline, the large-scale, regional structure that is the primary influence on disposition of Windermere Supergroup strata [[Media:P916060.jpg|(P916060)]]. A single slaty cleavage is ubiquitous and congruous to the folding, though the relationship is commonly transecting (albeit by only a small amount) rather than axial planar.

== Skiddaw Group structure ==
The dominant structural feature is a series of polyphase fault-and-thrust zones that trend approximately east-north-east across the main Skiddaw Group outcrop, roughly parallel to the regional strike of bedding and cleavage. Of these, the Causey Pike Fault has the longest demonstrable history and a geographical extent that spans the main Lake District inlier and extends as far as the Cross Fell inlier 30 km to the east. It has a profound stratigraphical effect (equivalent to at least 2 km vertical downthrow to the south), separating discrete parts of the Skiddaw Group succession with marked differences in sedimentary provenance and slump-fold orientation; hence it may well have had a synsedimentary role, partitioning the original depositional basin. Some sinistral strike-slip movement seems likely during the Early Devonian transpressive phase, whilst south-directed thrust movement cutting across the approximately 400 Ma Crummock Water thermal aureole is likely to be an Acadian (sensu stricto) effect.

Farther north, the Gasgale, Loweswater and Watch Hill thrust faults were all most probably initiated as the slide planes of large-scale, synsedimentary slumps. Thereafter, subsequent reactivation during Acadian deformation has created a situation such that, in the hanging walls, minor, upright folds have an axial-plane cleavage that progressively swings into an alignment parallel with the thrust plane, but is then crenulated by a uniformly shallow-dipping cleavage. The Watch Hill thrust is a particularly complex structure and is most probably a compound plexus of faults having different attitudes, movement senses, and ages.

Acadian deformation of the Skiddaw Group occurred against this regional background of pre-existing stratigraphical disruption. The main Acadian cleavage forms a regional arc in the Skiddaw Group, with a west to east variation in trend between north-east–south-west and east–west. It is a variably developed, pressure-solution fabric that is almost slaty in some places but is absent in others. Where a bedding-parallel, sedimentary compaction fabric is well developed, this regional Acadian cleavage may have the appearance of a crenulation fabric. The main cleavage is axial planar to gently plunging, steeply inclined, open to isoclinal folds with amplitudes of hundreds of metres. To the north of the Causey Pike Fault the cleavage mostly dips to the north with associated folds overturned towards the south; the folds are commonly hanging-wall anticlines to south-directed reverse faults. To the south of the Causey Pike Fault (though as far north as Carrock Fell in the east of the outcrop and Mellbreak (NY 144 195) in the west) folds associated with the main cleavage are less common and, where present, are upright or overturned towards the north. In this southern zone, the dip of the main cleavage is more variable, to both north and south. Further south, for example in the Bampton inlier, the Acadian cleavage is only weakly developed and mostly dips steeply to the north; in this inlier the dominant ‘cleavage’ is a strong bedding-parallel fissility.

In the Skiddaw Group of the northern Lake District, the regional Acadian cleavage is commonly crenulated by fabrics that are axial planar to open, gently plunging minor folds with gently inclined axial planes. These may have been generated by vertical shortening in the tectonically thickened sedimentary pile. However, in some cases the attitude of these later, minor folds is variable [[Media:P916119.jpg|(P916119)]]; more than one generation may be present or perhaps conjugate sets of crenulations have developed rather than a single cleavage. Nonetheless, the widespread, gently dipping crenulation plane is itself crenulated by later, more variable fabrics, some of which are associated with minor folds linked to a set of south-directed thrusts. These may possibly be related to the late thrust movement established in the Causey Pike fault zone. A range of minor, small-scale structures such as kink bands is also widespread; it postdates all of the cleavage development, and may have been generated late in the Acadian or even long thereafter.

In the south of the Lake District, in the Black Combe inlier, the Skiddaw Group has been severely deformed within a major, south-directed thrust zone. On the north side of this zone the rocks are intensely cleaved, sheared and metasomatised with much quartz-tourmaline veining; south of the thrust zone the combination of slaty and crenulation cleavages is very similar to that seen in the main, northern inlier. However, only a short distance south from Black Combe, in the Furness inlier, only the regional slaty cleavage is present. The Black Combe inlier lies within the hinge zone of the Westmorland Monocline, in a position to experience the maximum focus of Acadian strain above the margin of the subvolcanic batholith. The Furness area, lying south of the monocline, escaped much of the late Acadian deformation.

The overall impression is that a regional cleavage and subsequent, highly domainal crenulation fabrics were all developed during the Acadian Orogeny. It is likely that the sequence of fabrics records a continuum of deformation rather than separate tectonic pulses, with deformation being effected within a regime controlled by south-directed reverse faults.

The interpretation of thrust geometry in the Skiddaw Group is important in establishing the overall convergence regime. The sequence of thrusts seen at outcrop was previously modelled as an imbricate network extending southwards from the leading edge of the Southern Uplands thrust belt [[Media:P916061.jpg|(P916061)]] but, in order to accommodate the apparently steep dips that some of these structures show at outcrop, subsequent reorientation was required. It could only be achieved by compression buttressed against the subvolcanic Lake District Batholith during the propagation of a deep thrust dislocation at the base of the batholith. This in turn was thought to have driven the Westmorland Monocline into mountain front proportions, with commensurate acceleration of subsidence in the Windermere Supergroup foreland basin phase. Such reorientation would have been essentially pre-Acadian and is effectively ruled out by the relationship of the major fault structures to the Acadian cleavage.

A more likely, alternative view of the Skiddaw Group thrusts sees them as early, synsedimentary slump planes caught up in wholly Acadian reactivation entirely unrelated to the Southern Uplands thrust belt. In this interpretation the dominant Acadian components are part of a compressional flower structure generated along the northern margin of the batholith [[Media:P916062.jpg|(P916062)]]. If this view is accepted, it follows that a considerable thickness of Skiddaw Group strata probably exists below the oldest exposed level; the tentative recognition of a single Cambrian-aged locality (see discussion in Chapter 2) may be supportive. It also follows that the southward propagation of thrusts at the leading edge of the Southern Uplands thrust belt must have largely taken place at a structural level higher than that currently exposed.

== Manx Group structure ==
The Manx Group succession on the Isle of Man has a relatively simple outcrop pattern due to predominantly steep dip and consistent north-east strike, although the latter swings to the east in the north-east of the outcrop. However, at the exposure scale, dip may vary widely due to two main phases of deformation: the first is a pervasive slaty cleavage associated with gently to moderately plunging folds on a range of scales; the second is a widely developed, gently dipping crenulation cleavage associated with small folds that verge in the direction of dip of bedding. These affect both the Ordovician Manx Group [[Media:P104237.jpg|(P104237)]] and the upper Silurian Dalby Group. The earlier deformation also affects many of the minor intrusions, the Dhoon Grandiorite Pluton, and possibly the metamorphic phases in the aureole of the Early Devonian Foxdale Granite, although not the granite itself. Overall, the broad style and Acadian age of the deformation seen in the Manx Group are both very similar to the equivalent features of the Skiddaw Group, including the cleavage–aureole relationships around the Shap and Skiddaw granite plutons. A late phase of localised deformation affecting the Manx Group as a broadly north-trending, steeply dipping crenulation cleavage associated with upright folds may be Carboniferous in age, although somewhat similar fabrics and structures in the Skiddaw Group are regarded as having a late Acadian origin.

A number of kilometre-scale folds have been identified in the outcrop of the Lonan Formation, but determination of the overall structure of the Isle of Man has, until recently, been limited by the absence of a well-established stratigraphical framework. Early interpretations suggested fold models based on lithological correlations that have since been eliminated on biostratigraphical grounds. More recently, the possibility that major strike-parallel faults may separate tracts of reasonably coherent stratigraphy has been developed into an interpretation wherein a broad zone of south-east-directed thrusting has imbricated the upper part of the sequence and carried it over a folded ‘foreland’ succession [[Media:P916063.jpg|(P916063)]].

Several ductile shear zones run subparallel to the north-east-orientated tract boundary faults within the Manx Group and are generally marked by narrow zones of intense fabric development with indications of predominantly sininstral shear. They would seem to indicate a transition from orthogonal compression to transpression during the later stages of Acadian deformation. In this respect the Isle of Man has more in common with the Scottish Southern Uplands terrane than with the Lake District inlier.

Geophysical data suggest a zone of complex structure in the mid crust, underlying and trending parallel to the zone of faulting in the north-western part of the Isle of Man (see Chapter 2). This, together with the apparent north-westward increase in the structural complexity of the Manx Group, may be related to the proximity of the Isle of Man to the Iapetus Suture. Deep seismic profiles across the suture show a number of north-dipping reflectors in its footwall and these might link with the tract boundary faults in the Isle of Man — and also, by inference, with the major fault structures in the Skiddaw Group.

== Borrowdale Volcanic Group structure ==
Acadian tectonic structures were superimposed on the pre-existing volcanotectonic framework of the Borrowdale Volcanic Group; the main basins were tightened, their contained strata were folded, and a cleavage was locally superimposed. Fault reactivation seems likely, with clear examples in the Coniston and Troutbeck zones, and in general the geometry of the Acadian structures is superficially congruous with that of their volcanotectonic precursors. Three large-scale features of regional importance are the Haweswater, Scafell and Ulpha synclines. The first of these synclines, at Haweswater, is much broken-up by faulting, but the other two are more clearly defined. The Scafell Syncline is several kilometres across and dominates the structure of the Lake District’s Central Fells. Though broken-up by many volcanotectonic faults, it has an overall north-east, Caledonoid trend and axial plunge that decreases north-eastwards from about 30° to subhorizontal. The Ulpha Syncline, further to the south-west, formed as Acadian sinistral transpression was superimposed on the pre-existing, volcanotectonic and extensional Duddon Basin. It is a large but poorly defined, eastward-plunging fold within a high-strain zone on the south side of the Lake District Batholith. It is truncated by the unconformity at the base of the Windermere Supergroup, but the uniform cleavage spanning the unconformity attests to the Acadian influence. A further complication is the position of the syncline close to the hinge zone of the Westmorland Monocline, the major Acadian structure of the region. The attempts to fold the relatively rigid volcanic strata resulted in sheared-out anticline–syncline pairs, for example in association with the Greenburn Thrust and the east-north-east faults to the north of the Shap Granite; these are demonstrably Acadian structures.

It is significant that the large-scale structure of the Borrowdale Volcanic Group is dominated by synclines, and worth noting that previously described structures such as the Nan Bield and Wrynose anticlines are now thought to arise from the cumulative effects of faulting. This general absence of anticlines reinforces the regional interpretation of the major synclines as reactivated and tightened, volcanotectonic basins. It further emphasises the generally rigid response to deformation of the volcanic succession overlying the Caradoc batholith.

Cleavage fabric varies from spaced to slaty and penetrative, with the intensity varying between different lithologies; it is best developed in volcaniclastic sandstones and phyllosilicate-rich rocks, and may show strong refraction across lithologically contrasting strata. Since most of the central Lake District is underlain by the granitoid batholith, the Acadian strain there was relatively low, and so cleavage is only weakly developed. In contrast, zones of relatively high strain occur in places, particularly above the margins of the batholith and in the belt of steeply dipping strata in the Coniston Fells. There, all lithologies possess a steeply inclined cleavage with a uniform strike trend of about 065°, though there is a regionally arcuate trend from about 030° in the south-west of the outcrop (e.g. Millom Park) to approaching 080° in the east (e.g. Haweswater). The cleavage dip shows systematic variations in zones influenced by the distribution of the more significant faults.

== Windermere Supergroup structure ==
The disposition of Windermere Supergroup strata in the southern Lake District is largely influenced by a single large-scale, regional structure — the Bannisdale Syncline. This forms the lower hinge line of the Westmorland Monocline [[Media:P916054.jpg|(P916054)]] and [[Media:P916060.jpg|(P916060)]], for which the overall maximum amplitude is about 10 km. Farther east, in the Howgill Fells, similar-scale structures occur, but their continuity with those seen in the southern Lake District cannot be confirmed. The axis of the Bannisdale Syncline changes in trend and plunge along its length; eastwards, it swings from 055° with a plunge of 30° to the east-north-east (about half of which can be attributed to post-Carboniferous fault rotation), to become subhorizontal with an axial trend of about 065°. The regional structural arc is even more pronounced when minor fold hinges and cleavage strikes are considered. Then, the structural trend varies from about 060° in the south-west, east–west in the Howgill Fells, and about 110° in the Craven inliers [[Media:P916064.jpg|(P916064)]].

A single Acadian cleavage is developed throughout the Windermere Supergroup, as a penetrative, mica-defined fabric in the mudstones, but as a more widely spaced, pressure-solution fabric in the sandstones [[Media:P223323.jpg|(P223323)]]. The cleavage is generally inclined steeply (though strongly refracted through different lithologies), but has a less arcuate strike trend than the axial planes to the associated minor folds. Hence, the cleavage shows a small clockwise transection of the fold hinges in the west of the outcrop, passing eastwards into a small anticlockwise transection [[Media:P916064.jpg|(P916064)]]. This is part of a wider pattern of transecting cleavage seen across the Avalonian Lower Palaeozoic strata of Britain and Ireland. It has been interpreted in terms of transpressive strains created by the indenting of a rigid basement block that was driven northward during Acadian continental collision at the southern margin of Avalonia.

In the Isle of Man, the Dalby Group strata have a deformation history similar to that seen in the Manx Group, emphasising the Acadian age of the folding and cleavage affecting both units. A strong axial planar cleavage is ubiquitous across the first generation of folds in the Dalby Group, and in the hinge zones is commonly developed as a convergent cleavage fan [[Media:P104236.jpg|(P104236)]]. Later folds are more variably associated with crenulation cleavages that are usually gently inclined.

A network of broadly north-trending faults cuts across the main outcrop of the Windermere Supergroup, and many of the individual faults separate tracts with different fold and cleavage patterns. They have accommodated variable flexure and so have changes in displacement along their length. These faults were probably pre-existing, basement fractures that propagated upwards during Acadian deformation, partitioning strain in the Windermere Supergroup strata. The larger examples (e.g. Coniston, Brathay and Troutbeck faults; [[Media:P916065.jpg|(P916065)]] give rise to significant offsets of the supergroup’s basal unconformity, but their displacement decreases upwards into its younger stratigraphical components. Overall, the faults appear to have had early, subvolcanic and/or Early Devonian histories, and then to show Acadian reactivation, before many also fulfilled a post-Acadian role, particularly during Permo-Carboniferous extension.

== Regional metamorphism ==
Only low grades of regional metamorphism pertain in the Lower Palaeozoic strata. They are best defined by illite crystallinity in terms of the anchizone and epizone and were achieved by a combination of sedimentary burial and Acadian tectonism. An important variable during the former was the thermal regime affecting the basins in which sedimentary burial occurred. So, for example, likely extensional basins such as those in which the Skiddaw and Manx groups were deposited, would have had a higher heat flow (up to 50°C/km) than the Windermere Supergroup’s foreland basin that formed in response to convergence and loading. One effect of this contrast might be the widespread presence of a bedding-parallel, burial-induced compaction fabric in the (warm) Skiddaw and Manx groups, but its virtual absence in the (cool) Windermere Supergroup.

In the Skiddaw Group, there is a complicated distribution of grade, some of which can be attributed to post-metamorphic faulting. However, the underlying broad pattern of diagenetic to low epizonal variation is hard to reconcile with a single metamorphic episode. Instead, it would seem likely that the effects of burial have been overprinted by regional metamorphism, developed contemporaneously with folding and cleavage formation and locally influenced by the composition of the affected rock. Similarly, throughout the Manx Group, low-grade regional metamorphism can be linked with deformation, although again the pattern seems strongly influenced by the original composition of the various units.

The western part of the Borrowdale Volcanic Group outcrop lies within a thermal aureole surrounding the Caradoc, Eskdale and Ennerdale intrusions; an inner zone is characterised by secondary hornblende with biotite, and an outer zone by hornblende with actinolite, epidote and chlorite. Beyond these obvious contact metamorphic effects, the volcanic rocks in the eastern part of the outcrop carry a metamorphic mineral assemblage that includes epidote, chlorite and white mica, locally and rarely with prehnite and/or pumpellyite. It is not easy to disentangle the overlapping effects of thermal, contact metamorphism as opposed to those induced by high heat flow during synvolcanic burial, entirely postvolcanic burial, or Acadian metamorphism during deformation but still beneath a thick cover of now-eroded sedimentary rocks. Overall, post-Caradoc, sub-greenschist facies metamorphism of the Borrowdale Volcanic Group seems likely, broadly comparable in grade to the anchizonal metamorphism described below from the Windermere Supergroup. An alternative interpretation of the 420–430 Ma, reset Rb-Sr ages from the volcanic and intrusive rocks relates the resetting to isotopic re-equilibration during burial diagenesis, rather than the hydration during structural flexure discussed previously.

The Windermere Supergroup shows a more uniform pattern of low-grade, burial metamorphism; low anchizone grade in the east, rising westwards to upper anchizone or low epizone. The change can be at least partially explained by the overall eastwards plunge of the Bannisdale Syncline, the regionally dominant structure, supplemented by post-metamorphic faulting. The nature of the sedimentary basin in which burial occurred would suggest a low geothermal gradient of about 25°C/km and, if this is adopted, a burial depth of several kilometres (at least 4 km, maybe more depending on other assumptions) is required for the top of the Bannisdale Formation in order to achieve the metamorphic conditions seen therein. This cannot be achieved from the known upper parts of the Windermere Supergroup stratigraphy, and so requires a considerable thickness of Old Red Sandstone strata to have been originally present across the Lake District, but which has since been lost through erosion.
== Bibliography ==
Cox, R A, Dempster, T J, Bell, B R, and Rodgers, G. 1996. Crystallisation of the Shap Granite: evidence from zoned K-feldspar megacrysts. ''Journal of the Geological Society of London'', Vol. 153, 625–635.

Dewey, J F, and Strachan, R A. 2003. Changing Silurian–Devonian relative plate motion in the Caledonides: sinistral transpression to sinistral transtension. ''Journal of the Geological Society of London'', Vol. 160, 219–229.

Fortey, N J, Roberts, B, and Hirons, S R. 1993. Relationship between metamorphism and structure in the Skiddaw Group, English Lake District. ''Geological Magazine'', Vol. 130, 631–638.

Hughes, R A, Cooper, A H, and Stone, P. 1993. Structural evolution of the Skiddaw Group (English Lake District) on the northern margin of eastern Avalonia. ''Geological Magazine'', Vol. 130, 621–629.

Kneller, B C, King, L M, and Bell, A M. 1993. Foreland basin development and tectonics on the northwest margin of eastern Avalonia. ''Geological Magazine'', Vol. 130, 691–697.

Merriman, R J, Rex, D C, Soper, N J, and Peacor, D R. 1995. The age of Acadian cleavage in northern England, UK: K-Ar and TEM analysis of a Silurian metabentonite. ''Proceedings of the Yorkshire Geological Society'', Vol. 50, 255–265.

Soper, N J, and Woodcock, N H. 2003. The lost Lower Old Red Sandstone of England and Wales: a record of post-Iapetan flexure or Early Devonian transtension? ''Geological Magazine'', Vol. 140, 627–647.

Soper, N J, Webb, B C, and Woodcock, N H. 1987. Late Caledonian (Acadian) transpression in North West England: timing, geometry and geotectonic significance. ''Proceedings of the Yorkshire Geological Society'', Vol. 46, 175–192.

Soper, N J, Strachan, R A, Holdsworth, R E, Gayer, R A, and Greiling, R O. 1992. Sinistral transpression and the Silurian closure of Iapetus. ''Journal of the Geological Society of London'', Vol. 149, 871–880.

Vaughan, A P M. 1996. A tectonomagmatic model for the genesis and emplacement of Caledonian calc-alkaline lamprophyres. ''Journal of the Geological Society of London'', Vol. 153, 613–623.

Woodcock, N H, Soper, N J, and Strachan, R A. 2007. A Rheic cause for Acadian deformation in Europe. ''Journal of the Geological Society of London'', Vol. 164, 1023–1036.

[[Category: Northern England]]

Convergence of Avalonia and Laurentia, Devonian, Northern England

2016-03-23T18:57:13Z

BobMcIntosh:

Windemere Supergroup, Southern Uplands and Isle of Man

2016-03-23T18:55:59Z

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Depositional environments, Windemere Supergroup, late Ordovician to Silurian, Northern England

2016-03-23T18:55:09Z

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Kendal Group succession, Windemere Supergroup, late Ordovician to Silurian, Northern England

2016-03-23T18:53:48Z

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Coniston Group succession, Windemere Supergroup, late Ordovician to Silurian, Northern England

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Tranearth Group succession, Windemere Supergroup, late Ordovician to Silurian, Northern England

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Stockdale Group succession, Windemere Supergroup, late Ordovician to Silurian, Northern England

2016-03-23T18:32:47Z

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Dent Group succession, Windemere Supergroup, late Ordovician to Silurian, Northern England

2016-03-23T18:31:19Z

BobMcIntosh: