Pre-Palaeogene rocks of the Isle of Skye - Revision history

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	[[File:GSG_SKYE_02.jpg\|thumbnail\|Figure 2 Geological sketch-map of the Tarskavaig and Moine Thrust Sheets of the Sleat Peninsula (modified from Cheeney and Matthews 1965)]]		[[File:GSG_SKYE_02.jpg\|thumbnail\|Figure 2 Geological sketch-map of the Tarskavaig and Moine Thrust Sheets of the Sleat Peninsula (modified from Cheeney and Matthews 1965)]]
	[[File:GSG_SKYE_48.jpg\|thumbnail\|Table 2 Stratigraphic column for Jurassic rocks on Skye.]]		[[File:GSG_SKYE_48.jpg\|thumbnail\|Table 2 Stratigraphic column for Jurassic rocks on Skye.]]
			== Chapter 2 Pre-Tertiary rocks ==
	== (A) Lewisian Gneiss ==		== (A) Lewisian Gneiss ==

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	The following groupings were recognised by Peach et al. (1910):		The following groupings were recognised by Peach et al. (1910):


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References

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	Several dykes of Pre-Tertiary age crop out within the Pre-Triassic rocks of Skye, especially along the east side of the Sleat Peninsula, and in the district of Strath. They typically trend E-W, and are cut by NW-SE trending dykes of the Lower Tertiary regional swarm (see Section [[The minor intrusions of the Skye Central Complex#(B) The regional dyke swarm and associated subswarms\|(9B)]] of Chapter 9). Intrusion of the Pre-Tertiary dykes post-dates all movements of the Moine and associated thrusts (see Section [[Tectonic and stratigraphic evolution of the Isle of Skye#(B) The Moine Thrust Zone\|(10B)]] of Chapter 10). They are presumed to be of Upper Palaeozoic (Devonian) age and related to one, or more, of the Caledonian intrusive complexes which crop out on the mainland (for example, Glenelg-Ratagan). As such, they are better developed on the mainland. The following rock-types are reported by Peach et al. (1910): felsite, porphyry, vogesite, minette, and other extremely altered lamprophyres (referred to as mica-traps). Several of these dykes crop out around the summit of Ben Aslak, at the northern end of the Sleat Peninsula.		Several dykes of Pre-Tertiary age crop out within the Pre-Triassic rocks of Skye, especially along the east side of the Sleat Peninsula, and in the district of Strath. They typically trend E-W, and are cut by NW-SE trending dykes of the Lower Tertiary regional swarm (see Section [[The minor intrusions of the Skye Central Complex#(B) The regional dyke swarm and associated subswarms\|(9B)]] of Chapter 9). Intrusion of the Pre-Tertiary dykes post-dates all movements of the Moine and associated thrusts (see Section [[Tectonic and stratigraphic evolution of the Isle of Skye#(B) The Moine Thrust Zone\|(10B)]] of Chapter 10). They are presumed to be of Upper Palaeozoic (Devonian) age and related to one, or more, of the Caledonian intrusive complexes which crop out on the mainland (for example, Glenelg-Ratagan). As such, they are better developed on the mainland. The following rock-types are reported by Peach et al. (1910): felsite, porphyry, vogesite, minette, and other extremely altered lamprophyres (referred to as mica-traps). Several of these dykes crop out around the summit of Ben Aslak, at the northern end of the Sleat Peninsula.
	==[[Excursion guide to the geology of the Isle of Skye#References\|References]] ==		==[[Excursion guide to the geology of the Isle of Skye#References\|References]] ==
			== [[Glossary of petrological names and terms found on the Isle of Skye\|Appendix 1: Glossary of petrological names and terms]] ==

			== [[Glossary of fossil names found on the Isle of Skye\|Appendix 2: Glossary of fossil names]] ==

			== [[Glossary of place names and grid references found on the Isle of Skye\|Appendix 3: Glossary of place names and grid references]] ==
	{{EGwalks}}		{{EGwalks}}

	[[Category:3. Palaeogene volcanic districts of Scotland]]		[[Category:3. Palaeogene volcanic districts of Scotland]]

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	The Capistal Psammite Group overlies deformed Lewisian Gneiss and contains rare pelitic beds near its base, which increase in number near the transitional contact with the overlying Laidhe na Greine Group. The middle unit of the Tarskavaig Moine sequence consists of psammites, semi-pelites and pelites, some of which show well-developed graded-bedding. A return to dominant psammites is recorded in the Aruisg Psammite Group, with pelites only locally developed. Cross-bedding is present, although tectonic deformation has frequently lead to the destruction of these features.		The Capistal Psammite Group overlies deformed Lewisian Gneiss and contains rare pelitic beds near its base, which increase in number near the transitional contact with the overlying Laidhe na Greine Group. The middle unit of the Tarskavaig Moine sequence consists of psammites, semi-pelites and pelites, some of which show well-developed graded-bedding. A return to dominant psammites is recorded in the Aruisg Psammite Group, with pelites only locally developed. Cross-bedding is present, although tectonic deformation has frequently lead to the destruction of these features.

	Various generations of folds are present within these rocks, including a set of early, large, isoclinal folds, the Capistal Antiform and Synform. The Capistal folds and the subsequent smaller Doire na h-Achlais folds developed before the main movements of the Tarskavaig Thrust Plane, as some of the brecciated rocks within the thrust plane contain Doire na h-Achlais folds. The Tarskavaig Thrust Plane was then folded by the later Caradal folds, giving rise to the Tarskavaig Synform and the Caradal Synform and Antiform. Finally, generations of small folds, named the Port a' Chuil and Conjugate folds by Cheeney and Matthews (1965), have been recognised to post-date the Caradal folds. Following this, movement on the Moine Thrust Plane took place, involving the tectonic transport of Lewisian Gneiss westwards over Torridonian sedimentary rocks (see Section (10B) of Chapter 10).		Various generations of folds are present within these rocks, including a set of early, large, isoclinal folds, the Capistal Antiform and Synform. The Capistal folds and the subsequent smaller Doire na h-Achlais folds developed before the main movements of the Tarskavaig Thrust Plane, as some of the brecciated rocks within the thrust plane contain Doire na h-Achlais folds. The Tarskavaig Thrust Plane was then folded by the later Caradal folds, giving rise to the Tarskavaig Synform and the Caradal Synform and Antiform. Finally, generations of small folds, named the Port a' Chuil and Conjugate folds by Cheeney and Matthews (1965), have been recognised to post-date the Caradal folds. Following this, movement on the Moine Thrust Plane took place, involving the tectonic transport of Lewisian Gneiss westwards over Torridonian sedimentary rocks (see Section [[Tectonic and stratigraphic evolution of the Isle of Skye#(B) The Moine Thrust Zone\|(10B]] of Chapter 10).

	Possible correlations between the Moine Schists and the Torridonian sedimentary rocks (see Section [[Pre-Palaeogene rocks of the Isle of Skye#(B) Torridonian sedimentary rocks\|(2B)]], above) are as follows:		Possible correlations between the Moine Schists and the Torridonian sedimentary rocks (see Section [[Pre-Palaeogene rocks of the Isle of Skye#(B) Torridonian sedimentary rocks\|(2B)]], above) are as follows:
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	# On the Strathaird Peninsula and in Camasunary Bay		# On the Strathaird Peninsula and in Camasunary Bay
	# North Skye, along the east and west coasts, including the ground south of Loch Sligachan		# North Skye, along the east and west coasts, including the ground south of Loch Sligachan



	An excellent summary of the geology of Jurassic sedimentary rocks throughout the Inner Hebrides is provided by Hudson (1983). Details of the succession are presented in [[Media:GSG_SKYE_48.jpg\|(Table 2)]].		An excellent summary of the geology of Jurassic sedimentary rocks throughout the Inner Hebrides is provided by Hudson (1983). Details of the succession are presented in [[Media:GSG_SKYE_48.jpg\|(Table 2)]].

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	[[File:GSG_SKYE_02.jpg\|thumbnail\|Figure 2 Geological sketch-map of the Tarskavaig and Moine Thrust Sheets of the Sleat Peninsula (modified from Cheeney and Matthews 1965)]]		[[File:GSG_SKYE_02.jpg\|thumbnail\|Figure 2 Geological sketch-map of the Tarskavaig and Moine Thrust Sheets of the Sleat Peninsula (modified from Cheeney and Matthews 1965)]]
			[[File:GSG_SKYE_48.jpg\|thumbnail\|Table 2 Stratigraphic column for Jurassic rocks on Skye.]]

	== (A) Lewisian Gneiss ==		== (A) Lewisian Gneiss ==

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(F) Jurassic sedimentary rocks

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	An excellent summary of the geology of Jurassic sedimentary rocks throughout the Inner Hebrides is provided by Hudson (1983). Details of the succession are presented in (Table 2).		An excellent summary of the geology of Jurassic sedimentary rocks throughout the Inner Hebrides is provided by Hudson (1983). Details of the succession are presented in [[Media:GSG_SKYE_48.jpg\|(Table 2)]].

	In the Strath district, Jurassic strata disconformably overlie Triassic sedimentary rocks, although locally they rest upon parts of the Torridonian and Cambro-Ordovician successions. In Camasunary Bay, at the southern end of the Blaven ridge, Jurassic strata unconformably overlie Torridonian sedimentary rocks. No bases to the successions on the Strathaird Peninsula and in north Skye are seen.		In the Strath district, Jurassic strata disconformably overlie Triassic sedimentary rocks, although locally they rest upon parts of the Torridonian and Cambro-Ordovician successions. In Camasunary Bay, at the southern end of the Blaven ridge, Jurassic strata unconformably overlie Torridonian sedimentary rocks. No bases to the successions on the Strathaird Peninsula and in north Skye are seen.

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	In the Strath district, 6km west of Broadford, a small inlier of gneiss is preserved between the Inner and Outer Granites of the Eastern Red Hills Centre. It has a surface area of approximately 0.5km<sup>2</sup> and has a faulted contact to the NW against Jurassic strata. Lying unconformably above the gneiss are subaerially-erupted basic lavas of Lower Tertiary age. The unconformity is somewhat irregular, but can be traced from the base of Creagan Dubh, NE to where it is exposed in the Allt na Teangaidh. This gneiss shows the development of a strong mineral banding, with alternating salic and mafic portions. It is agmatitic in places and, when examined in thin-section, there is evidence that a melt phase has developed. Abundant micrographic intergrowths of quartz and alkali feldspar are readily noted, suggesting that sufficient heat was present during Lower Tertiary times to enable partial fusion to take place (see Section (12D) of Chapter 12). Furthermore, breakdown of primary mafic minerals such as pyroxene and amphibole has occurred, resulting in the extensive development of secondary chlorite and epidote.		In the Strath district, 6km west of Broadford, a small inlier of gneiss is preserved between the Inner and Outer Granites of the Eastern Red Hills Centre. It has a surface area of approximately 0.5km<sup>2</sup> and has a faulted contact to the NW against Jurassic strata. Lying unconformably above the gneiss are subaerially-erupted basic lavas of Lower Tertiary age. The unconformity is somewhat irregular, but can be traced from the base of Creagan Dubh, NE to where it is exposed in the Allt na Teangaidh. This gneiss shows the development of a strong mineral banding, with alternating salic and mafic portions. It is agmatitic in places and, when examined in thin-section, there is evidence that a melt phase has developed. Abundant micrographic intergrowths of quartz and alkali feldspar are readily noted, suggesting that sufficient heat was present during Lower Tertiary times to enable partial fusion to take place (see Section (12D) of Chapter 12). Furthermore, breakdown of primary mafic minerals such as pyroxene and amphibole has occurred, resulting in the extensive development of secondary chlorite and epidote.

	The only other direct evidence of a Lewisian Gneiss basement below Skye is the presence of gneiss xenoliths within the ferrodiorite intrusion of the Western Red Hills Centre (see Section [[The Western Red Hills Centre, Skye Central Complex#(H) The Marscoite Suite\|(6H)]] of Chapter 6). Most of these xenoliths are 5m in largest dimension and are leucocratic to mesocratic. They contain quartz, sodic plagioclase and alkali feldspar in the light-coloured bands, whilst pyroxene is present within the darker bands. Most of these xenoliths have been found in Harker's Gully, on Marsco. Also, Thompson (1981) recognises a two-pyroxene granulite xenolith within the ferrodiorite intrusion at the head of the Allt Coire nam Bruadaran.		The only other direct evidence of a Lewisian Gneiss basement below Skye is the presence of gneiss xenoliths within the ferrodiorite intrusion of the Western Red Hills Centre (see Section [[The Western Red Hills Centre, Skye Central Complex#(H)The Marscoite Suite\|(6H)]] of Chapter 6). Most of these xenoliths are 5m in largest dimension and are leucocratic to mesocratic. They contain quartz, sodic plagioclase and alkali feldspar in the light-coloured bands, whilst pyroxene is present within the darker bands. Most of these xenoliths have been found in Harker's Gully, on Marsco. Also, Thompson (1981) recognises a two-pyroxene granulite xenolith within the ferrodiorite intrusion at the head of the Allt Coire nam Bruadaran.

	== (B) Torridonian sedimentary rocks ==		== (B) Torridonian sedimentary rocks ==

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			[[File:GSG_SKYE_02.jpg\|thumbnail\|Figure 2 Geological sketch-map of the Tarskavaig and Moine Thrust Sheets of the Sleat Peninsula (modified from Cheeney and Matthews 1965)]]

	== (A) Lewisian Gneiss ==		== (A) Lewisian Gneiss ==

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	== (H) Pre-Tertiary minor intrusions ==		== (H) Pre-Tertiary minor intrusions ==

	Several dykes of Pre-Tertiary age crop out within the Pre-Triassic rocks of Skye, especially along the east side of the Sleat Peninsula, and in the district of Strath. They typically trend E-W, and are cut by NW-SE trending dykes of the Lower Tertiary regional swarm (see Section [[The minor intrusions of the Skye Central Complex#(B) The regional dyke swarm and associated subswarms\|(9B)]] of Chapter 9). Intrusion of the Pre-Tertiary dykes post-dates all movements of the Moine and associated thrusts (see Section (10B) of Chapter 10). They are presumed to be of Upper Palaeozoic (Devonian) age and related to one, or more, of the Caledonian intrusive complexes which crop out on the mainland (for example, Glenelg-Ratagan). As such, they are better developed on the mainland. The following rock-types are reported by Peach et al. (1910): felsite, porphyry, vogesite, minette, and other extremely altered lamprophyres (referred to as mica-traps). Several of these dykes crop out around the summit of Ben Aslak, at the northern end of the Sleat Peninsula.		Several dykes of Pre-Tertiary age crop out within the Pre-Triassic rocks of Skye, especially along the east side of the Sleat Peninsula, and in the district of Strath. They typically trend E-W, and are cut by NW-SE trending dykes of the Lower Tertiary regional swarm (see Section [[The minor intrusions of the Skye Central Complex#(B) The regional dyke swarm and associated subswarms\|(9B)]] of Chapter 9). Intrusion of the Pre-Tertiary dykes post-dates all movements of the Moine and associated thrusts (see Section [[Tectonic and stratigraphic evolution of the Isle of Skye#(B) The Moine Thrust Zone\|(10B)]] of Chapter 10). They are presumed to be of Upper Palaeozoic (Devonian) age and related to one, or more, of the Caledonian intrusive complexes which crop out on the mainland (for example, Glenelg-Ratagan). As such, they are better developed on the mainland. The following rock-types are reported by Peach et al. (1910): felsite, porphyry, vogesite, minette, and other extremely altered lamprophyres (referred to as mica-traps). Several of these dykes crop out around the summit of Ben Aslak, at the northern end of the Sleat Peninsula.
			==[[Excursion guide to the geology of the Isle of Skye#References\|References]] ==
			{{EGwalks}}

			[[Category:3. Palaeogene volcanic districts of Scotland]]

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	The Torridonian sedimentary rocks of the Sleat Peninsula are preserved within the Kishorn Thrust Sheet and are exposed in the tract of ground between Kyleakin, on Loch Alsh, in the NE, and Loch a' Ghlinne (4km NW of Ardvasar), in the SW. These rocks form an important part of the Lochalsh Syncline which is overturned towards the west. The axial plane of this structure can be traced from Loch na Beiste, south of Kyleakin, to the Sound of Sleat (at Meall Port Mealary). West of this line the strata are the correct way up and show few signs of deformation, whilst to the east they are inverted, and in places are heavily fractured and contain a cleavage.		The Torridonian sedimentary rocks of the Sleat Peninsula are preserved within the Kishorn Thrust Sheet and are exposed in the tract of ground between Kyleakin, on Loch Alsh, in the NE, and Loch a' Ghlinne (4km NW of Ardvasar), in the SW. These rocks form an important part of the Lochalsh Syncline which is overturned towards the west. The axial plane of this structure can be traced from Loch na Beiste, south of Kyleakin, to the Sound of Sleat (at Meall Port Mealary). West of this line the strata are the correct way up and show few signs of deformation, whilst to the east they are inverted, and in places are heavily fractured and contain a cleavage.

	The lowest exposed part of the sequence is the Epidote Grit Formation, which crops out on the Sound of Sleat, at Meall Port Mealary, where approximately 100m of strata are preserved. The name of this formation is derived from the presence of epidote and epidotised feldspar, often in the form of pebbles, which represent eroded debris from the Lewisian basement gneisses (see Section [[Pre-Palaeogene rocks of the Isle of Skye#(A) Lewisian Gneiss\|(2A)]], above). Grits and sandstones, as well as mudstones and shales, are present within the sequence and it has been concluded that they were deposited in fast-flowing, braided channels and shallow seas, respectively.		The lowest exposed part of the sequence is the Epidote Grit Formation, which crops out on the Sound of Sleat, at Meall Port Mealary, where approximately 100m of strata are preserved. The name of this formation is derived from the presence of epidote and epidotised feldspar, often in the form of pebbles, which represent eroded debris from the Lewisian basement gneisses (see Section [[Pre-Palaeogene rocks of the Isle of Skye#(A)Lewisian Gneiss\|(2A)]], above). Grits and sandstones, as well as mudstones and shales, are present within the sequence and it has been concluded that they were deposited in fast-flowing, braided channels and shallow seas, respectively.

	Above the Epidote Grit Formation, Peach et al. (1910) recognised an alternating coarse- and fine-grained sedimentary sequence, with rapid variations in grain-size. These rocks, referred to as the Loch na Dal Formation, are best exposed on the NE side of the loch of that name, on the east coast of the Sleat Peninsula. Thicknesses of individual units are very variable, ranging from a few centimetres, up to 2 or 3m. The presence of desiccation-cracks, ripple-structures and fine laminations lead Sutton and Watson (1964) to conclude that these sediments were deposited in some form of intertidal environment.		Above the Epidote Grit Formation, Peach et al. (1910) recognised an alternating coarse- and fine-grained sedimentary sequence, with rapid variations in grain-size. These rocks, referred to as the Loch na Dal Formation, are best exposed on the NE side of the loch of that name, on the east coast of the Sleat Peninsula. Thicknesses of individual units are very variable, ranging from a few centimetres, up to 2 or 3m. The presence of desiccation-cracks, ripple-structures and fine laminations lead Sutton and Watson (1964) to conclude that these sediments were deposited in some form of intertidal environment.
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	Sandstones and arkoses of Torridonian age are also preserved on the islands of Scalpay and Longay, NW of Broadford Bay, and between Sconser and Maol na Gainmhich, in the vicinity of Loch Sligachan, on Skye. In both these areas Mesozoic strata unconformably cover the Torridonian rocks and it would appear reasonable from field relationships on the island of Raasay, to the north, that they rest directly on foreland Lewisian Gneiss.		Sandstones and arkoses of Torridonian age are also preserved on the islands of Scalpay and Longay, NW of Broadford Bay, and between Sconser and Maol na Gainmhich, in the vicinity of Loch Sligachan, on Skye. In both these areas Mesozoic strata unconformably cover the Torridonian rocks and it would appear reasonable from field relationships on the island of Raasay, to the north, that they rest directly on foreland Lewisian Gneiss.

	The only other outcrops of Torridonian strata are found south of the Cuillin Hills, in Camasunary Bay, and between Eilean Reamhar and An Leac, on the Sound of Soay, and on the island of Soay itself. The Camasunary Bay outcrops dip at shallow angles to the NW and are composed of massive sandstones and arkoses, together with occasional gritty horizons in the east side of the bay, and more shaly material on the west side, especially on the west bank of the Abhainn Camas Fhionnairigh. Clough and Harker (1904) suggest that these strata belong to the Diabaig Group, although they have, in places, been subjected to intense thermal effects by the nearby Cuillin Complex, resulting in rheomorphism and partial melting of material up to a distance of 100m from the margin of the intrusion. (see Section [[The Cuillin Complex, Skye Central Complex#(M) Thermal effects of the Cuillin Complex\|(4M)]] of Chapter 4). The strata exposed on both sides of the Sound of Soay are coarse, pebbly sandstones and arkoses, generally dipping to the west at angles of 15°, or less. From their lack of tectonic features, Clough and Harker (1904) suggest that these rocks lie to the west of the Moine Thrust Zone and have not suffered tectonic transport. Cross-bedding, ripples and water-escape structures are all present within these rocks. From their general appearance it is possible to correlate them with the Applecross Group.		The only other outcrops of Torridonian strata are found south of the Cuillin Hills, in Camasunary Bay, and between Eilean Reamhar and An Leac, on the Sound of Soay, and on the island of Soay itself. The Camasunary Bay outcrops dip at shallow angles to the NW and are composed of massive sandstones and arkoses, together with occasional gritty horizons in the east side of the bay, and more shaly material on the west side, especially on the west bank of the Abhainn Camas Fhionnairigh. Clough and Harker (1904) suggest that these strata belong to the Diabaig Group, although they have, in places, been subjected to intense thermal effects by the nearby Cuillin Complex, resulting in rheomorphism and partial melting of material up to a distance of 100m from the margin of the intrusion. (see Section [[The Cuillin Complex, Skye Central Complex#(M)Thermal effects of the Cuillin Complex\|(4M)]] of Chapter 4). The strata exposed on both sides of the Sound of Soay are coarse, pebbly sandstones and arkoses, generally dipping to the west at angles of 15°, or less. From their lack of tectonic features, Clough and Harker (1904) suggest that these rocks lie to the west of the Moine Thrust Zone and have not suffered tectonic transport. Cross-bedding, ripples and water-escape structures are all present within these rocks. From their general appearance it is possible to correlate them with the Applecross Group.

	== (C) Moine schists ==		== (C) Moine schists ==

Scotfot: Created page with "== (A) Lewisian Gneiss == The oldest rocks preserved on Skye are gneisses of Lewisian age (Archaean–Middle Proterozoic), the bulk of which are exposed on the east side of t..."

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