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	'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''		'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''

	[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]		[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]
	[[File:P916227.jpg\|thumbnail\|Ordovician series. P916227.]]		[[File:P916227.jpg\|thumbnail\|Ordovician series. P916227.]]

Ajhil at 08:53, 14 April 2016

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			__notoc__
	'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''		'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''
	[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]		[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]

Ajhil at 07:38, 14 April 2016

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	YOUNG, T P, GIBBONS, W, and MCCARROLL, D. 2002. Geology of the country around Pwllheli. ''Memoir of the British Geological Survey'', Sheet 134 (England and Wales).		YOUNG, T P, GIBBONS, W, and MCCARROLL, D. 2002. Geology of the country around Pwllheli. ''Memoir of the British Geological Survey'', Sheet 134 (England and Wales).

	[[Category:Wales]]		{{British regional geology of Wales - contents}}
			[[Category:Regional Geology of Wales]]

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BobMcIntosh at 19:59, 6 March 2016

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← Older revision		Revision as of 09:53, 14 April 2016
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	'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''		'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''

	[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]		[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]
	[[File:P916227.jpg\|thumbnail\|Ordovician series. P916227.]]		[[File:P916227.jpg\|thumbnail\|Ordovician series. P916227.]]

← Older revision		Revision as of 09:53, 14 April 2016
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	~~__notoc__~~
	'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''		'''From: Howells, M F. 2007. [[British regional geology: Wales\|British regional geology: Wales]]. Keyworth, Nottingham: British Geological Survey.'''

	[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]		[[File:P916159.jpg\|thumbnail\|Palaeogeography of the Welsh Basin in Longvillian (mid-Caradoc) (adapted from Cope et al., 1992). P916159.]]
	[[File:P916227.jpg\|thumbnail\|Ordovician series. P916227.]]		[[File:P916227.jpg\|thumbnail\|Ordovician series. P916227.]]
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	Following this early eruption, the north-east-directed palaeoslope was maintained, and the strata deposited in the interval before the next significant eruption thicken in that direction. North of the Pass of Llanberis, these strata comprise a thick succession of coarse- to medium- grained, cross-bedded sandstones with intercalations of siltstone and fine-grained siliceous tuffs that probably represent distal air-fall and water-settled vitric dust. In Cwm Idwal, the narrow quarry excavation south of Ogwen Cottage indicates their past importance as ‘honestones’. Late in the interval, basaltic volcanism was extensive, and is indicated by basalt lavas (and intrusions), hyaloclastites and tuffs, which are well developed from Cwm Cneifion to the Pass of Llanberis and farther into Cwm Llan, on the south side of Snowdon, where they are the sole element. The basalts form a coherent geochemical group, distinct from those erupted later in the cycle.		Following this early eruption, the north-east-directed palaeoslope was maintained, and the strata deposited in the interval before the next significant eruption thicken in that direction. North of the Pass of Llanberis, these strata comprise a thick succession of coarse- to medium- grained, cross-bedded sandstones with intercalations of siltstone and fine-grained siliceous tuffs that probably represent distal air-fall and water-settled vitric dust. In Cwm Idwal, the narrow quarry excavation south of Ogwen Cottage indicates their past importance as ‘honestones’. Late in the interval, basaltic volcanism was extensive, and is indicated by basalt lavas (and intrusions), hyaloclastites and tuffs, which are well developed from Cwm Cneifion to the Pass of Llanberis and farther into Cwm Llan, on the south side of Snowdon, where they are the sole element. The basalts form a coherent geochemical group, distinct from those erupted later in the cycle.

	The emplacement of the Lower Rhyolitic Tuff Formation was concentrated about the Snowdon Centre. Magma movement prior to the first eruption domed and disrupted the partly unlithified sediments in an area about the Beddgelert Fracture Zone, and the earliest, acidic ash-flow tuff activity developed at a centre close to Yr Arddu (Yr Arddu Tuffs) [[Media:P662409.jpg\|(P662409)]] and the Nantmor Fracture. The ash-flow tuffs, with a patchy concentration of blocks, accumulated close to their eruptive centre, which was later intruded by a rhyolite. Subsequently, the main caldera-forming activity within the Lower Rhyolitic Tuff Formation [[Media:P916162.jpg\|(P916162)]] developed progressively along the Beddgelert Fracture Zone. Early fissure eruptions, probably along the line of the Gwynant valley, produced welded ash-flow tuffs with large rafts of the the earlier Pitts Head Tuff, south-west of Cwm Tregalan and between Beddgelert and Moel Ddu. To the east of the fracture zone, thick wedges of sedimentary and volcanic breccias, such as those on the south-east side of Llyn Dinas, developed as a result of instability caused by enhanced tectonism in the vicinity of the fracture zone. The activity progressed into the main eruptive phase and the emplacement of more than 500 m of homogeneous, nonwelded acidic ash-flow tuff about the Snowdon massif. The distribution of these tuffs, the associated facies and thickness variations, allowed the caldera structure to be clearly defined. The thickest intracaldera sequence, at Lliwedd, lay close to the eruptive centre and to the north margin of the caldera, along the line of the Llanberis Pass. There the base of the tuffs, marked by a coarse breccia, is exposed in Dinas Mot and Dinas Gromlech. The primary intracaldera tuffs thin south-westwards to Moel Hebog, where they include large rafts of the Pitts Head Tuff, close to the south-western margin of the caldera.		The emplacement of the Lower Rhyolitic Tuff Formation was concentrated about the Snowdon Centre. Magma movement prior to the first eruption domed and disrupted the partly unlithified sediments in an area about the Beddgelert Fracture Zone, and the earliest, acidic ash-flow tuff activity developed at a centre close to Yr Arddu (Yr Arddu Tuffs) [[Media:P662409.jpg\|(P662409)]] and the Nantmor Fracture. The ash-flow tuffs, with a patchy concentration of blocks, accumulated close to their eruptive centre, which was later intruded by a rhyolite. Subsequently, the main caldera-forming activity within the Lower Rhyolitic Tuff Formation [[Media:P916162.jpg\|(P916162)]] developed progressively along the Beddgelert Fracture Zone. Early fissure eruptions, probably along the line of the Gwynant valley, produced welded ash-flow tuffs with large rafts of the the earlier Pitts Head Tuff, south-west of Cwm Tregalan and between Beddgelert and Moel Ddu. To the east of the fracture zone, thick wedges of sedimentary and volcanic breccias, such as those on the south-east side of Llyn Dinas, developed as a result of instability caused by enhanced tectonism in the vicinity of the fracture zone. The activity progressed into the main eruptive phase and the emplacement of more than 500 m of homogeneous, nonwelded acidic ash-flow tuff about the Snowdon massif. The distribution of these tuffs, the associated facies and thickness variations, allowed the caldera structure to be clearly defined. The thickest intracaldera sequence, at Lliwedd, lay close to the eruptive centre and to the north margin of the caldera, along the line of the Llanberis Pass. There the base of the tuffs, marked by a coarse breccia, is exposed in Dinas Mot and Dinas Gromlech. The primary intracaldera tuffs thin south-westwards to Moel Hebog, where they include large rafts of the Pitts Head Tuff, close to the south-western margin of the caldera.

	From the north of Llanberis Pass to Cwm Idwal, a nonwelded ash flow tuff outside the caldera margin, the sole primary representative of the caldera-forming activity, is overlain by tuffs and volcaniclastic sediments reworked from the caldera margin. The base of the outflow tuff includes numerous blocks in irregular patches and entrained in layers, and the overlying reworked tuffs display a progressive increase in the sedimentary component. A thick rhyolite near the top of the sequence in the core of the syncline at Cwm Idwal forms a prominent feature along the east limb to Esgair Felen, high above Llanberis Pass; the fine-grained, columnar jointed, devitrified rhyolite is autobrecciated at its top contact, and is overlain by cross-bedded coarse-grained sandstones crowded with rhyolitic debris.		From the north of Llanberis Pass to Cwm Idwal, a nonwelded ash flow tuff outside the caldera margin, the sole primary representative of the caldera-forming activity, is overlain by tuffs and volcaniclastic sediments reworked from the caldera margin. The base of the outflow tuff includes numerous blocks in irregular patches and entrained in layers, and the overlying reworked tuffs display a progressive increase in the sedimentary component. A thick rhyolite near the top of the sequence in the core of the syncline at Cwm Idwal forms a prominent feature along the east limb to Esgair Felen, high above Llanberis Pass; the fine-grained, columnar jointed, devitrified rhyolite is autobrecciated at its top contact, and is overlain by cross-bedded coarse-grained sandstones crowded with rhyolitic debris.

	To the east, the outflow tuff, a single primary, nonwelded, ash-flow tuff, about 40 m thick, was transported into deeper water, and forms a persistent outcrop from Betws y Coed and Capel Curig in the south to Conwy in the north. Subsequently the Crafnant Centre (Crafnant Volcanic Group) developed in north-east Snowdonia in a basin dominated by mud deposition. The influence of the Snowdon Centre was minimal. The main activity was of acidic ashflow tuff volcanism, with much slumping and soft-sediment disruption, but to the north, at Tal y Fan, a contemporaneous basic centre is marked by a restricted sequence of basaltic lavas and tuffs with a coeval dolerite sill (Tal y Fan Volcanic Formation). At the Snowdon Centre, the form of the caldera established from the distribution of the tuff facies is corroborated by the distribution of five geochemically distinct groups of rhyolites, which were successively emplaced along specific annular and linear patterns within the structure. The earliest rhyolites were related to a resurgent phase within the caldera, which initiated a period of shallow marine reworking of the intracaldera tuffs, prior to an episode of basaltic volcanic activity (Bedded Pyroclastic Formation) [[Media:P916163.jpg\|(P916163)]]. In Cwm Glas, rhyolitic pebble and cobble conglomerates that lie on the reworked top of the intracaldera tuffs are overlain by a rhyolite lava, which was itself eroded into small stacks. The contemporaneous emplacement of a large rhyolite intrusion, exposed in the face of Cyrn Las, deformed the intracaldera tuffs into a well-defined syncline.		To the east, the outflow tuff, a single primary, nonwelded, ash-flow tuff, about 40 m thick, was transported into deeper water, and forms a persistent outcrop from Betws y Coed and Capel Curig in the south to Conwy in the north. Subsequently the Crafnant Centre (Crafnant Volcanic Group) developed in north-east Snowdonia in a basin dominated by mud deposition. The influence of the Snowdon Centre was minimal. The main activity was of acidic ashflow tuff volcanism, with much slumping and soft-sediment disruption, but to the north, at Tal y Fan, a contemporaneous basic centre is marked by a restricted sequence of basaltic lavas and tuffs with a coeval dolerite sill (Tal y Fan Volcanic Formation). At the Snowdon Centre, the form of the caldera established from the distribution of the tuff facies is corroborated by the distribution of five geochemically distinct groups of rhyolites, which were successively emplaced along specific annular and linear patterns within the structure. The earliest rhyolites were related to a resurgent phase within the caldera, which initiated a period of shallow marine reworking of the intracaldera tuffs, prior to an episode of basaltic volcanic activity (Bedded Pyroclastic Formation) [[Media:P916163.jpg\|(P916163)]]. In Cwm Glas, rhyolitic pebble and cobble conglomerates that lie on the reworked top of the intracaldera tuffs are overlain by a rhyolite lava, which was itself eroded into small stacks. The contemporaneous emplacement of a large rhyolite intrusion, exposed in the face of Cyrn Las, deformed the intracaldera tuffs into a well-defined syncline.

	The Bedded Pyroclastic Formation forms extensive outcrops about the Snowdon massif and in the cores of the Cwm Idwal, Moel Hebog and Dolwyddelan synclines. Its base is variably conformable to unconformable on the Lower Rhyolitic Tuff Formation. It is dominated by basaltic volcaniclastic sedimentary rocks, with high level intrusive and extrusive basalts, hyaloclastites and basic tuffites. The formation is well exposed in the outcrops between Cwm Glas and the north-east face of Snowdon [[Media:P213255.jpg\|(P213255)]]; [[Media:P662410.jpg\|(P662410)]]. The succession records the complex interplay between uplift, subsidence, volcanism and sedimentation in a shallow- marine environment, and developed from the shoreline reworking of small Strombolian-type island volcanoes. Shoreline cliffs formed and debris was reworked into offshore fan deltas and distal turbidite aprons.		The Bedded Pyroclastic Formation forms extensive outcrops about the Snowdon massif and in the cores of the Cwm Idwal, Moel Hebog and Dolwyddelan synclines. Its base is variably conformable to unconformable on the Lower Rhyolitic Tuff Formation. It is dominated by basaltic volcaniclastic sedimentary rocks, with high level intrusive and extrusive basalts, hyaloclastites and basic tuffites. The formation is well exposed in the outcrops between Cwm Glas and the north-east face of Snowdon [[Media:P213255.jpg\|(P213255)]]; [[Media:P662410.jpg\|(P662410)]]. The succession records the complex interplay between uplift, subsidence, volcanism and sedimentation in a shallow- marine environment, and developed from the shoreline reworking of small Strombolian-type island volcanoes. Shoreline cliffs formed and debris was reworked into offshore fan deltas and distal turbidite aprons.
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	YOUNG, T P, GIBBONS, W, and MCCARROLL, D. 2002. Geology of the country around Pwllheli. ''Memoir of the British Geological Survey'', Sheet 134 (England and Wales).		YOUNG, T P, GIBBONS, W, and MCCARROLL, D. 2002. Geology of the country around Pwllheli. ''Memoir of the British Geological Survey'', Sheet 134 (England and Wales).

	{{British regional geology of Wales - contents}}		{{British regional geology of Wales - contents}}
	[[Category:Regional Geology of Wales]]		[[Category:Regional Geology of Wales]]

Caradoc Series, Ordovician, Wales - Revision history

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BobMcIntosh at 19:59, 6 March 2016