Argyll Group, Grampian Caledonides - Revision history

Dbk at 15:35, 31 January 2018

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	'''From: Stephenson, D, and Gould, D. 1995. [[British regional geology: Grampian Highlands\|British regional geology: Grampian Highlands.]] Fourth edition. Keyworth, Nottingham: British Geological Survey.'''		{{GHRG}}

	== Argyll Group, introduction ==		== Argyll Group, introduction ==

Scotfot at 17:45, 27 July 2015

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	In the Loch Awe Syncline and the subsidiary Tayvallich Syncline, the ''Tayvallich Slate and Limestone Formation ''exhibits marked facies changes from'' ''north-west to south-east comparable to those in the underlying Crinan Subgroup. The overall thickness varies considerably, mainly due to variations in the amounts of slate and volcanic material. The formation reaches a maximum of 1200 m, but the total thickness of limestones is relatively constant, at about 100 m. On the north-western limb of the Loch Awe Syncline fine-grained, dark, partly oolitic limestones and coarse-grained, graded, gritty limestones are interbedded with dark blue-grey phyllites or slates. These lithologies probably represent shelf sedimentation. Coarse, slumped limestone breccias, conglomerates and gritty psammites, all suggestive of an unstable shelf margin, attain maximum development along the axial zone of the syncline. On the south-eastern limb thinner lenses of limestone-breccias and conglomerates are interbedded with turbiditic psammites (Knill, 1963). Around the north-eastern closure of the Loch Awe Syncline, the Tayvallich Limestones are succeeded by the ''Kilchrenan Grit'' and the ''Kilchrenan Conglomerate'' (Borradaile, 1973). The former is a poorly graded, feldspathic psammite with mudstone flakes, whereas the latter is a matrix-supported ‘boulder bed’, up to 30 m thick, consisting of well-rounded quartzite boulders in a gritty black slate matrix. It has been interpreted as a slump conglomerate (Kilburn et al., 1965).		In the Loch Awe Syncline and the subsidiary Tayvallich Syncline, the ''Tayvallich Slate and Limestone Formation ''exhibits marked facies changes from'' ''north-west to south-east comparable to those in the underlying Crinan Subgroup. The overall thickness varies considerably, mainly due to variations in the amounts of slate and volcanic material. The formation reaches a maximum of 1200 m, but the total thickness of limestones is relatively constant, at about 100 m. On the north-western limb of the Loch Awe Syncline fine-grained, dark, partly oolitic limestones and coarse-grained, graded, gritty limestones are interbedded with dark blue-grey phyllites or slates. These lithologies probably represent shelf sedimentation. Coarse, slumped limestone breccias, conglomerates and gritty psammites, all suggestive of an unstable shelf margin, attain maximum development along the axial zone of the syncline. On the south-eastern limb thinner lenses of limestone-breccias and conglomerates are interbedded with turbiditic psammites (Knill, 1963). Around the north-eastern closure of the Loch Awe Syncline, the Tayvallich Limestones are succeeded by the ''Kilchrenan Grit'' and the ''Kilchrenan Conglomerate'' (Borradaile, 1973). The former is a poorly graded, feldspathic psammite with mudstone flakes, whereas the latter is a matrix-supported ‘boulder bed’, up to 30 m thick, consisting of well-rounded quartzite boulders in a gritty black slate matrix. It has been interpreted as a slump conglomerate (Kilburn et al., 1965).
	[[File:P219459.jpg\|thumbnail\|P219459]]		[[File:P219459.jpg\|thumbnail\|Basaltic pillow lavas, Tayvallich Volcanic Formation, Argyll Group, coast south-west of Tayvallich, Argyll. P219459.]]
	Volcanic rocks occupy much of the core of the Loch Awe syncline between the Pass of Brander in the north-east and Loch Crinan in the south-west. Smaller outcrops occur farther to the south-west, notably in the core of the Tayvallich Syncline. The ''Tayvallich Volcanic Formation'' has a very sharp base in its more northern outcrops, just above the Kilchrenan Conglomerate, but in the south the lower parts consist of a complex interdigitation of limestones, clastic sediments and volcanic rocks. The volcanic formation consists of up to 2000 m of commonly pillowed basic lavas (P219459), hyaloclastites and a variety of epiclastic volcanic rocks (Borradaile, 1973; Graham, 1976). The epiclastic rocks include breccias and waterlain pebbly deposits such as the ''Loch na Cille'' ''‘Boulder Bed''’ of the Tayvallich Peninsula (Gower, 1977). Extrusion of the main'' ''volcanic pile was clearly submarine but away from the main centre of volcanicity, which corresponds to the axis of the Loch Awe Syncline, air-fall tuffs have been recognised. A suite of metabasic sills with a similar geographical distribution to the volcanic rocks intrudes the whole succession from the Craignish and Ardrishaig phyllites upwards; their total thickness attains 3000 m in places. These were originally thought to be contemporaneous with the lavas (Borradaile, 1973; Graham, 1976), but they may well be near-contemporaneous with their host sediments, building upwards with time as shallow intrusions into soft sediments (Wilson and Leake, 1972; Graham and Borradaile, 1984; Graham, 1986). A suite of NW-trending metabasalt dykes on Jura may represent feeders for the lavas and sills (Graham and Borradaile, 1984).		Volcanic rocks occupy much of the core of the Loch Awe syncline between the Pass of Brander in the north-east and Loch Crinan in the south-west. Smaller outcrops occur farther to the south-west, notably in the core of the Tayvallich Syncline. The ''Tayvallich Volcanic Formation'' has a very sharp base in its more northern outcrops, just above the Kilchrenan Conglomerate, but in the south the lower parts consist of a complex interdigitation of limestones, clastic sediments and volcanic rocks. The volcanic formation consists of up to 2000 m of commonly pillowed basic lavas (P219459), hyaloclastites and a variety of epiclastic volcanic rocks (Borradaile, 1973; Graham, 1976). The epiclastic rocks include breccias and waterlain pebbly deposits such as the ''Loch na Cille'' ''‘Boulder Bed''’ of the Tayvallich Peninsula (Gower, 1977). Extrusion of the main'' ''volcanic pile was clearly submarine but away from the main centre of volcanicity, which corresponds to the axis of the Loch Awe Syncline, air-fall tuffs have been recognised. A suite of metabasic sills with a similar geographical distribution to the volcanic rocks intrudes the whole succession from the Craignish and Ardrishaig phyllites upwards; their total thickness attains 3000 m in places. These were originally thought to be contemporaneous with the lavas (Borradaile, 1973; Graham, 1976), but they may well be near-contemporaneous with their host sediments, building upwards with time as shallow intrusions into soft sediments (Wilson and Leake, 1972; Graham and Borradaile, 1984; Graham, 1986). A suite of NW-trending metabasalt dykes on Jura may represent feeders for the lavas and sills (Graham and Borradaile, 1984).

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	To the north-east of Glen Isla the limestone can be traced as a 10 m-thick unit in Glen Doll, but it is absent east of the Glen Doll Fault. Here, calcareous semipelites around the head of Glen Mark pass NNE into ribbed calc-silicate rocks and limestones of the ''Water of Tanar Limestone''. In middle Deeside, this becomes the ''Deeside Limestone'' (Read 1927; 1928) which consists mainly of greenish calc-silicate rocks with calcareous psammite, hornblende-schist and thin layers of impure limestone. A discontinuous band of crystalline limestone, containing abundant pyrite and up to 15 m thick, has been quarried extensively. These calcareous rocks lie at the base of the Tarfside ‘group’ of Harte (1979), in which they are overlain by a diverse but dominantly psammitic unit, the ''Tarfside Psammite Formation'', consisting of quartzites, psammites, semipelites and pelites with locally abundant calc-silicate and amphibolite bands. Parts of the unit are gneissose and between Glen Clova and Glen Lee pelitic and semipelitic gneisses predominate.		To the north-east of Glen Isla the limestone can be traced as a 10 m-thick unit in Glen Doll, but it is absent east of the Glen Doll Fault. Here, calcareous semipelites around the head of Glen Mark pass NNE into ribbed calc-silicate rocks and limestones of the ''Water of Tanar Limestone''. In middle Deeside, this becomes the ''Deeside Limestone'' (Read 1927; 1928) which consists mainly of greenish calc-silicate rocks with calcareous psammite, hornblende-schist and thin layers of impure limestone. A discontinuous band of crystalline limestone, containing abundant pyrite and up to 15 m thick, has been quarried extensively. These calcareous rocks lie at the base of the Tarfside ‘group’ of Harte (1979), in which they are overlain by a diverse but dominantly psammitic unit, the ''Tarfside Psammite Formation'', consisting of quartzites, psammites, semipelites and pelites with locally abundant calc-silicate and amphibolite bands. Parts of the unit are gneissose and between Glen Clova and Glen Lee pelitic and semipelitic gneisses predominate.
	[[File:P219244.jpg\|thumbnail\|P219244]]		[[File:P219244.jpg\|thumbnail\|Minor folding in the metamorphosed Boyne Limestone Formation, Argyll Group, Boyne Bay, Banffshire. P219244.]]
	To the north of Deeside most of the gneissose units already described probably include Tayvallich Subgroup rocks, particularly in those areas where calc-silicate and limestone bands are common. Notable examples are the calcareous part of the ''Strichen Formation'' at the top of the Stuartfield division and its lateral equivalent, the ''Kinnairds Head ‘group’'' which is well exposed on the north coast at Fraserburgh (Read and Farquhar, 1956). In these units finely banded calc-silicate beds occur in a sequence of pelites, semipelites and psammites. The Strichen Formation also contains limestone bands up to 20 m thick. Grading in the coarser-grained lithologies, which include meta-greywacke, suggests a turbiditic origin and the units are transitional upwards into the more persistent turbidites of the Southern Highland Group.		To the north of Deeside most of the gneissose units already described probably include Tayvallich Subgroup rocks, particularly in those areas where calc-silicate and limestone bands are common. Notable examples are the calcareous part of the ''Strichen Formation'' at the top of the Stuartfield division and its lateral equivalent, the ''Kinnairds Head ‘group’'' which is well exposed on the north coast at Fraserburgh (Read and Farquhar, 1956). In these units finely banded calc-silicate beds occur in a sequence of pelites, semipelites and psammites. The Strichen Formation also contains limestone bands up to 20 m thick. Grading in the coarser-grained lithologies, which include meta-greywacke, suggests a turbiditic origin and the units are transitional upwards into the more persistent turbidites of the Southern Highland Group.

Scotfot at 17:42, 27 July 2015

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Scotfot at 13:37, 13 July 2015

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	[[File:P915411.png\|thumbnail\|P915411]]		[[File:P915411.png\|thumbnail\|P915411]]

	Rocks of the Argyll Group crop out over an area of some 5700 km<sup>2</sup> (P915411). Extensive outcrops occur on Islay and Jura, and Argyll Group rocks constitute almost the whole of the South-west Highlands to the north-west of Kintyre and Loch Fyne. The type successions for all four subgroups are found in this latter area where outcrops can be described in relation to three major structures, the Islay Anticline, the Loch Awe Syncline and the Ardrishaig Anticline [[Media:P915427.png\|(P915427)]]. To the north-east of these structures, between the Etive Granite Complex and the Bridge of Balgie Fault, the Islay Subgroup, like much of the preceding Appin Group, is absent, although younger units are continuous. Farther to the north-east a full succession is present through the Tummel Steep Belt to the Glen Shee area, with the higher units continuing to middle Deeside. In the North-east Highlands, west of the Portsoy Lineament, the lowest units have been traced intermittently to the north coast. East of the Portsoy Lineament, the higher parts of the group are believed to be present in an undivided gneissose sequence forming a horseshoe outcrop pattern in the Turriff Syncline.		Rocks of the Argyll Group crop out over an area of some 5700 km<sup>2</sup> [[Media:P915411.png\|(P915411)]]. Extensive outcrops occur on Islay and Jura, and Argyll Group rocks constitute almost the whole of the South-west Highlands to the north-west of Kintyre and Loch Fyne. The type successions for all four subgroups are found in this latter area where outcrops can be described in relation to three major structures, the Islay Anticline, the Loch Awe Syncline and the Ardrishaig Anticline [[Media:P915427.png\|(P915427)]]. To the north-east of these structures, between the Etive Granite Complex and the Bridge of Balgie Fault, the Islay Subgroup, like much of the preceding Appin Group, is absent, although younger units are continuous. Farther to the north-east a full succession is present through the Tummel Steep Belt to the Glen Shee area, with the higher units continuing to middle Deeside. In the North-east Highlands, west of the Portsoy Lineament, the lowest units have been traced intermittently to the north coast. East of the Portsoy Lineament, the higher parts of the group are believed to be present in an undivided gneissose sequence forming a horseshoe outcrop pattern in the Turriff Syncline.
	[[File:P915427.png\|thumbnail\|P915427]]		[[File:P915427.png\|thumbnail\|P915427]]
	== Islay Subgroup ==		== Islay Subgroup ==

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Tayvallich Subgroup

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	To the north of Deeside most of the gneissose units already described probably include Tayvallich Subgroup rocks, particularly in those areas where calc-silicate and limestone bands are common. Notable examples are the calcareous part of the ''Strichen Formation'' at the top of the Stuartfield division and its lateral equivalent, the ''Kinnairds Head ‘group’'' which is well exposed on the north coast at Fraserburgh (Read and Farquhar, 1956). In these units finely banded calc-silicate beds occur in a sequence of pelites, semipelites and psammites. The Strichen Formation also contains limestone bands up to 20 m thick. Grading in the coarser-grained lithologies, which include meta-greywacke, suggests a turbiditic origin and the units are transitional upwards into the more persistent turbidites of the Southern Highland Group.		To the north of Deeside most of the gneissose units already described probably include Tayvallich Subgroup rocks, particularly in those areas where calc-silicate and limestone bands are common. Notable examples are the calcareous part of the ''Strichen Formation'' at the top of the Stuartfield division and its lateral equivalent, the ''Kinnairds Head ‘group’'' which is well exposed on the north coast at Fraserburgh (Read and Farquhar, 1956). In these units finely banded calc-silicate beds occur in a sequence of pelites, semipelites and psammites. The Strichen Formation also contains limestone bands up to 20 m thick. Grading in the coarser-grained lithologies, which include meta-greywacke, suggests a turbiditic origin and the units are transitional upwards into the more persistent turbidites of the Southern Highland Group.

	On the Banffshire coast the Tayvallich Subgroup comprises a 1200 m-thick sequence, termed the ''Boyne Limestone Formation'', which includes the ''Boyne Castle'' ''Limestone'', a thickly bedded but finely banded limestone, some 200 m thick'' ''[[Media:P219244.jpg\|P219244)]]. Beneath the main limestone are subordinate purple phyllites, mica-schists and several thin bands of white limestone (Read, 1923; Sutton and Watson, 1955). Inland the limestone can be traced for only 2.5 km. The sequence above the Boyne Castle Limestone consists of semipelites, laminated and striped calc-silicate rocks, minor thin limestones and lenses of reworked carbonate material. These rocks were formerly described as the lower part of the Whitehills ‘group’ (Read, 1923; Harris and Pitcher, 1975). Concordant sheets and lenses of amphibolite probably represent metabasaltic sills.		On the Banffshire coast the Tayvallich Subgroup comprises a 1200 m-thick sequence, termed the ''Boyne Limestone Formation'', which includes the ''Boyne Castle'' ''Limestone'', a thickly bedded but finely banded limestone, some 200 m thick'' ''[[Media:P219244.jpg\|(P219244)]]. Beneath the main limestone are subordinate purple phyllites, mica-schists and several thin bands of white limestone (Read, 1923; Sutton and Watson, 1955). Inland the limestone can be traced for only 2.5 km. The sequence above the Boyne Castle Limestone consists of semipelites, laminated and striped calc-silicate rocks, minor thin limestones and lenses of reworked carbonate material. These rocks were formerly described as the lower part of the Whitehills ‘group’ (Read, 1923; Harris and Pitcher, 1975). Concordant sheets and lenses of amphibolite probably represent metabasaltic sills.

	==[[References, Grampian Highlands\|Full list of references]]==		==[[References, Grampian Highlands\|Full list of references]]==
	[[Category:Grampian Highlands]]		[[Category:Grampian Highlands]]

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Easdale Subgroup

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	In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].		In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].
	[[File:P219043.jpg\|thumbnail\|P219043]]		[[File:P219043.jpg\|thumbnail\|P219043]]
	On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.		On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|(P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.
	[[File:P915423.png\|thumbnail\|P915423]]		[[File:P915423.png\|thumbnail\|P915423]]
	In the Benderloch area [[Media:P915422.png\|(P915422]]; [[Media:P915423.png\|P915423)]], the top part of the Ardmucknish succession (600 to 1200 m thick) consists predominantly of black slates with a few thin black limestones and pebbly mudstones, the ''Selma Slate Formation'' (Litherland, 1980). The slates contain several beds of calcareous grit and a sedimentary breccia with a variety of exotic clasts, the ''Selma Breccia'' (up to 100 m thick). The whole sequence is interpreted as equivalent to the Jura Slate and Scarba Conglomerate to the south-west. At the top of the Ardmucknish succession is a 200 m-thick, graded, pebbly quartzite, the ''Culcharan'' ''Quartzite''. Correlation with the top part of the Creran succession to the east is'' ''difficult. Here black slates and striped siltstones of the ''Salachan Formation'' (1000 m thick) are overlain by the ''Beinn Donn Quartzite Formation'' (1500 m thick) (Litherland, 1980). South of Loch Creran the quartzites are pebbly and are intercalated with pelites, pebbly mudstones, calcareous grits and thin limestone breccias; Anderton (1985) regards the whole succession as typical of the Easdale Subgroup. A massive, pebbly quartzite with graded bedding and intraformational breccias, within the Beinn Donn Quartzite Formation, is correlated by Litherland (1980) with the Cairn Mairg Quartzite of the Southern Highlands However, the same quartzite can be traced around the Beinn Donn Syncline [[Media:P915422.png\|P915422)]] where it changes in facies to cross-bedded quartzites and flags, regarded by Litherland (1980; 1987) as equivalent to the Islay and Jura quartzites. Regardless of whether this part of the succession is assigned to the Islay or the Easdale subgroup, it seems likely that the incoming of Easdale-type facies, marked by slide deposits, pebbly quartzites, pebbly limestones and pebbly mudstones, is diachronous, occurring earlier in the east than the west.		In the Benderloch area [[Media:P915422.png\|(P915422]]; [[Media:P915423.png\|P915423)]], the top part of the Ardmucknish succession (600 to 1200 m thick) consists predominantly of black slates with a few thin black limestones and pebbly mudstones, the ''Selma Slate Formation'' (Litherland, 1980). The slates contain several beds of calcareous grit and a sedimentary breccia with a variety of exotic clasts, the ''Selma Breccia'' (up to 100 m thick). The whole sequence is interpreted as equivalent to the Jura Slate and Scarba Conglomerate to the south-west. At the top of the Ardmucknish succession is a 200 m-thick, graded, pebbly quartzite, the ''Culcharan'' ''Quartzite''. Correlation with the top part of the Creran succession to the east is'' ''difficult. Here black slates and striped siltstones of the ''Salachan Formation'' (1000 m thick) are overlain by the ''Beinn Donn Quartzite Formation'' (1500 m thick) (Litherland, 1980). South of Loch Creran the quartzites are pebbly and are intercalated with pelites, pebbly mudstones, calcareous grits and thin limestone breccias; Anderton (1985) regards the whole succession as typical of the Easdale Subgroup. A massive, pebbly quartzite with graded bedding and intraformational breccias, within the Beinn Donn Quartzite Formation, is correlated by Litherland (1980) with the Cairn Mairg Quartzite of the Southern Highlands However, the same quartzite can be traced around the Beinn Donn Syncline [[Media:P915422.png\|(P915422)]] where it changes in facies to cross-bedded quartzites and flags, regarded by Litherland (1980; 1987) as equivalent to the Islay and Jura quartzites. Regardless of whether this part of the succession is assigned to the Islay or the Easdale subgroup, it seems likely that the incoming of Easdale-type facies, marked by slide deposits, pebbly quartzites, pebbly limestones and pebbly mudstones, is diachronous, occurring earlier in the east than the west.

	On the south-eastern side of the Loch Awe Syncline, only the top part of the South-west Highlands Easdale Subgroup succession is exposed in the core of the Ardrishaig Anticline. Here, the ''Ardrishaig Phyllite'' is lithologically similar to the Craignish Phyllite and is regarded as stratigraphically equivalent (Roberts, 1966; Borradaile, 1973). Sedimentary structures are not so well preserved as in the Craignish Phyllite owing to a higher metamorphic grade but pseudomorphs after gypsum have been recorded and sulphur isotopes are consistent with a shallow-water, mildly reducing marine environment (Hall et al., 1994). The amount of deformation makes estimates of thickness difficult but over 4000 m of succession are recorded in places. In Knapdale a diachronous, predominantly quartzitic unit, the ''Erins Quartzite'', has an apparent thickness of up to 5000 m. The lower part is included in the Easdale		On the south-eastern side of the Loch Awe Syncline, only the top part of the South-west Highlands Easdale Subgroup succession is exposed in the core of the Ardrishaig Anticline. Here, the ''Ardrishaig Phyllite'' is lithologically similar to the Craignish Phyllite and is regarded as stratigraphically equivalent (Roberts, 1966; Borradaile, 1973). Sedimentary structures are not so well preserved as in the Craignish Phyllite owing to a higher metamorphic grade but pseudomorphs after gypsum have been recorded and sulphur isotopes are consistent with a shallow-water, mildly reducing marine environment (Hall et al., 1994). The amount of deformation makes estimates of thickness difficult but over 4000 m of succession are recorded in places. In Knapdale a diachronous, predominantly quartzitic unit, the ''Erins Quartzite'', has an apparent thickness of up to 5000 m. The lower part is included in the Easdale

Scotfot: /* Easdale Subgroup */

2015-07-11T14:50:05Z

Easdale Subgroup

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	The base of the subgroup is marked in most places by rapid change to finer-grained rocks showing features typical of deep-water sedimentation and turbidity currents'','' with local incursions of coarse-grained mass-flow deposits. Higher parts of the subgroup are dominated by calcareous pelites and semipelites with local limestones, representing shallower-water sedimentation. These general characteristics are preserved throughout the outcrop from Islay to the north coast and individual units can be traced for up to100 km, but the detailed stratigraphy is less continuous than in preceding subgroups.		The base of the subgroup is marked in most places by rapid change to finer-grained rocks showing features typical of deep-water sedimentation and turbidity currents'','' with local incursions of coarse-grained mass-flow deposits. Higher parts of the subgroup are dominated by calcareous pelites and semipelites with local limestones, representing shallower-water sedimentation. These general characteristics are preserved throughout the outcrop from Islay to the north coast and individual units can be traced for up to100 km, but the detailed stratigraphy is less continuous than in preceding subgroups.

	In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418)]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].		In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].
	[[File:P219043.jpg\|thumbnail\|P219043]]		[[File:P219043.jpg\|thumbnail\|P219043]]
	On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.		On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.

Scotfot: /* Easdale Subgroup */

2015-07-11T14:49:44Z

Easdale Subgroup

← Older revision		Revision as of 15:49, 11 July 2015
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	The base of the subgroup is marked in most places by rapid change to finer-grained rocks showing features typical of deep-water sedimentation and turbidity currents'','' with local incursions of coarse-grained mass-flow deposits. Higher parts of the subgroup are dominated by calcareous pelites and semipelites with local limestones, representing shallower-water sedimentation. These general characteristics are preserved throughout the outcrop from Islay to the north coast and individual units can be traced for up to100 km, but the detailed stratigraphy is less continuous than in preceding subgroups.		The base of the subgroup is marked in most places by rapid change to finer-grained rocks showing features typical of deep-water sedimentation and turbidity currents'','' with local incursions of coarse-grained mass-flow deposits. Higher parts of the subgroup are dominated by calcareous pelites and semipelites with local limestones, representing shallower-water sedimentation. These general characteristics are preserved throughout the outcrop from Islay to the north coast and individual units can be traced for up to100 km, but the detailed stratigraphy is less continuous than in preceding subgroups.

	In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''(P915418 and P915421; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; P915421). The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].		In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418)]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].
	[[File:P219043.jpg\|thumbnail\|P219043]]		[[File:P219043.jpg\|thumbnail\|P219043]]
	On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.		On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.

Scotfot: /* Islay Subgroup */

2015-07-11T14:47:17Z

Islay Subgroup

← Older revision		Revision as of 15:47, 11 July 2015
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	This largely psammitic subgroup is dominated in most areas by a thick quartzite formation. The quartzite and the underlying basal tillite-bearing formation are persistent and distinctive, enabling a good correlation of beds from Connemara to the Moray Firth.		This largely psammitic subgroup is dominated in most areas by a thick quartzite formation. The quartzite and the underlying basal tillite-bearing formation are persistent and distinctive, enabling a good correlation of beds from Connemara to the Moray Firth.
	[[File:P215702.jpg\|thumbnail\|P215702]]		[[File:P215702.jpg\|thumbnail\|P215702]]
	In the type area for the subgroup on Islay the basal ''Port Askaig Tillite'' ''Formation ''consists of a sequence of sandstones, siltstones, conglomerates and'' ''dolostones in which boulder beds have been recognised (Spencer, 1971; 1981; [[Media:P215702.~~png~~\|P215702)]]. These beds range from 0.5 m to 65 m in thickness and contain boulders up to 2 m in diameter. One particular bed, the ‘Great Breccia’ contains large rafts of dolostone up to 320 m long. The lower beds contain clasts of dolostone, probably derived locally from within the formation or from the underlying Blair Atholl Subgroup. However, the higher boulder beds contain clasts of granite and granite-gneiss of extrabasinal origin. This division on the basis of clast content is also recognisable in boulder bed sequences outwith the type area. The boulder beds are generally regarded as tillites but isolated clasts in associated varved siltstones have been interpreted as dropstones from floating ice. Polygonal sandstone wedges have been interpreted as ice wedges formed during periods of emergence (Eyles and Clark, 1985).		In the type area for the subgroup on Islay the basal ''Port Askaig Tillite'' ''Formation ''consists of a sequence of sandstones, siltstones, conglomerates and'' ''dolostones in which boulder beds have been recognised (Spencer, 1971; 1981; [[Media:P215702.jpg\|P215702)]]. These beds range from 0.5 m to 65 m in thickness and contain boulders up to 2 m in diameter. One particular bed, the ‘Great Breccia’ contains large rafts of dolostone up to 320 m long. The lower beds contain clasts of dolostone, probably derived locally from within the formation or from the underlying Blair Atholl Subgroup. However, the higher boulder beds contain clasts of granite and granite-gneiss of extrabasinal origin. This division on the basis of clast content is also recognisable in boulder bed sequences outwith the type area. The boulder beds are generally regarded as tillites but isolated clasts in associated varved siltstones have been interpreted as dropstones from floating ice. Polygonal sandstone wedges have been interpreted as ice wedges formed during periods of emergence (Eyles and Clark, 1985).

	Within the type area the formation varies considerably in thickness and in the number of tillites recognised. Around Port Askaig on Islay and on the Garvellach Islands, the formation is about 750 m thick and the combined sequence from the two areas may be up to 870 m, with up to 47 separate tillites recognised. However, to the south-west, on the Mull of Oa, the sequence is incomplete and less than 40 m thick. The presence of glacial deposits and their relationship to the adjacent dolomitic beds has considerable implications for the palaeogeography of Argyll Group time and for the age of the group, both of which are discussed in later sections.		Within the type area the formation varies considerably in thickness and in the number of tillites recognised. Around Port Askaig on Islay and on the Garvellach Islands, the formation is about 750 m thick and the combined sequence from the two areas may be up to 870 m, with up to 47 separate tillites recognised. However, to the south-west, on the Mull of Oa, the sequence is incomplete and less than 40 m thick. The presence of glacial deposits and their relationship to the adjacent dolomitic beds has considerable implications for the palaeogeography of Argyll Group time and for the age of the group, both of which are discussed in later sections.

Scotfot at 14:46, 11 July 2015

2015-07-11T14:46:41Z

Show changes

← Older revision		Revision as of 15:51, 11 July 2015
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	In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].		In south-east Islay and south Jura the Islay and Jura quartzites are overlain by the ''Jura Slate'' which constitutes the basal part of the ''Scarba Conglomerate'' ''Formation ''[[Media:P915418.png\|(P915418]] and [[Media:P915421.png\|P915421]]; Anderton, 1979). Here this formation is about'' ''450 m thick consisting of grey and black slates with thick lenses of quartzite and conglomerate which pass upwards into persistent quartzite and conglomerate units. Sedimentary structures such as graded beds and erosional bases suggest deposition from turbidity currents on a subsiding offshore platform shelf. Farther north, on Scarba and adjoining islands, there is a pronounced increase in thickness and a change of facies. Proximal turbidites similar to those of the southern outcrops are interbedded with thick, coarse debris flows which are considered to have slumped downslope north-wards into a fault-bounded basin. The debris flows consist of locally derived blocks, up to 6 m in diameter, consisting of quartzite, limestone and phyllite, all in a gritty matrix. To the north of Scarba the debris flows and associated quartzites become finer grained and thinner as they become interbedded with and pass upwards into the deeper-water ''Easdale Slate'' (Anderton, 1979; [[Media:P915421.png\|P915421)]]. The Easdale Slate consists predominantly of black, carbonaceous, pyritic slates, best developed on Seil and Luing where they have been quarried extensively (Baldwin and Johnson, 1977). Incursions of distal turbidites are seen as thin, poorly graded sandstones and occasional dolomitic carbonate beds [[Media:P219043.jpg\|(P219043)]].
	[[File:P219043.jpg\|thumbnail\|P219043]]		[[File:P219043.jpg\|thumbnail\|P219043]]
	On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.		On the island of Shuna and on Degnish Point, the Easdale Slate passes upwards into the 20 m-thick ''Degnish Limestone'' [[Media:P915418.png\|(P915418)]], a complex unit composed of interbedded limestones, mudstones and calcareous sandstones which marks a passage into the shallow marine facies of the ''Craignish Phyllite ''(Knill, 1959; Roberts, 1966; Borradaile, 1973; Anderton,'' ''1975). This phyllite unit, which is correlated with the ''Port Ellen Phyllite'' of Islay and Jura, consists of abundant alternations of laminated phyllites, quartzites, limestones and pebbly sandstones, with a total thickness of up to 4500 m. Many of the sediments were deposited on tidal flats and in subtidal environments with pseudomorphs after gypsum that indicate periodical emergence. Thick cross-bedded, dark grey limestones, intercalated with dark phyllites at the top of the formation comprise the ''Shira Limestone'' ''and Slate'', which is a persistent marker unit, up to 300 m thick, in parts of'' ''the South-west Highlands.
	[[File:P915423.png\|thumbnail\|P915423]]		[[File:P915423.png\|thumbnail\|P915423]]
	In the Benderloch area [[Media:P915422.png\|(P915422]]; [[Media:P915423.png\|P915423)]], the top part of the Ardmucknish succession (600 to 1200 m thick) consists predominantly of black slates with a few thin black limestones and pebbly mudstones, the ''Selma Slate Formation'' (Litherland, 1980). The slates contain several beds of calcareous grit and a sedimentary breccia with a variety of exotic clasts, the ''Selma Breccia'' (up to 100 m thick). The whole sequence is interpreted as equivalent to the Jura Slate and Scarba Conglomerate to the south-west. At the top of the Ardmucknish succession is a 200 m-thick, graded, pebbly quartzite, the ''Culcharan'' ''Quartzite''. Correlation with the top part of the Creran succession to the east is'' ''difficult. Here black slates and striped siltstones of the ''Salachan Formation'' (1000 m thick) are overlain by the ''Beinn Donn Quartzite Formation'' (1500 m thick) (Litherland, 1980). South of Loch Creran the quartzites are pebbly and are intercalated with pelites, pebbly mudstones, calcareous grits and thin limestone breccias; Anderton (1985) regards the whole succession as typical of the Easdale Subgroup. A massive, pebbly quartzite with graded bedding and intraformational breccias, within the Beinn Donn Quartzite Formation, is correlated by Litherland (1980) with the Cairn Mairg Quartzite of the Southern Highlands However, the same quartzite can be traced around the Beinn Donn Syncline [[Media:P915422.png\|P915422)]] where it changes in facies to cross-bedded quartzites and flags, regarded by Litherland (1980; 1987) as equivalent to the Islay and Jura quartzites. Regardless of whether this part of the succession is assigned to the Islay or the Easdale subgroup, it seems likely that the incoming of Easdale-type facies, marked by slide deposits, pebbly quartzites, pebbly limestones and pebbly mudstones, is diachronous, occurring earlier in the east than the west.		In the Benderloch area [[Media:P915422.png\|(P915422]]; [[Media:P915423.png\|P915423)]], the top part of the Ardmucknish succession (600 to 1200 m thick) consists predominantly of black slates with a few thin black limestones and pebbly mudstones, the ''Selma Slate Formation'' (Litherland, 1980). The slates contain several beds of calcareous grit and a sedimentary breccia with a variety of exotic clasts, the ''Selma Breccia'' (up to 100 m thick). The whole sequence is interpreted as equivalent to the Jura Slate and Scarba Conglomerate to the south-west. At the top of the Ardmucknish succession is a 200 m-thick, graded, pebbly quartzite, the ''Culcharan'' ''Quartzite''. Correlation with the top part of the Creran succession to the east is'' ''difficult. Here black slates and striped siltstones of the ''Salachan Formation'' (1000 m thick) are overlain by the ''Beinn Donn Quartzite Formation'' (1500 m thick) (Litherland, 1980). South of Loch Creran the quartzites are pebbly and are intercalated with pelites, pebbly mudstones, calcareous grits and thin limestone breccias; Anderton (1985) regards the whole succession as typical of the Easdale Subgroup. A massive, pebbly quartzite with graded bedding and intraformational breccias, within the Beinn Donn Quartzite Formation, is correlated by Litherland (1980) with the Cairn Mairg Quartzite of the Southern Highlands However, the same quartzite can be traced around the Beinn Donn Syncline [[Media:P915422.png\|(P915422)]] where it changes in facies to cross-bedded quartzites and flags, regarded by Litherland (1980; 1987) as equivalent to the Islay and Jura quartzites. Regardless of whether this part of the succession is assigned to the Islay or the Easdale subgroup, it seems likely that the incoming of Easdale-type facies, marked by slide deposits, pebbly quartzites, pebbly limestones and pebbly mudstones, is diachronous, occurring earlier in the east than the west.

	On the south-eastern side of the Loch Awe Syncline, only the top part of the South-west Highlands Easdale Subgroup succession is exposed in the core of the Ardrishaig Anticline. Here, the ''Ardrishaig Phyllite'' is lithologically similar to the Craignish Phyllite and is regarded as stratigraphically equivalent (Roberts, 1966; Borradaile, 1973). Sedimentary structures are not so well preserved as in the Craignish Phyllite owing to a higher metamorphic grade but pseudomorphs after gypsum have been recorded and sulphur isotopes are consistent with a shallow-water, mildly reducing marine environment (Hall et al., 1994). The amount of deformation makes estimates of thickness difficult but over 4000 m of succession are recorded in places. In Knapdale a diachronous, predominantly quartzitic unit, the ''Erins Quartzite'', has an apparent thickness of up to 5000 m. The lower part is included in the Easdale		On the south-eastern side of the Loch Awe Syncline, only the top part of the South-west Highlands Easdale Subgroup succession is exposed in the core of the Ardrishaig Anticline. Here, the ''Ardrishaig Phyllite'' is lithologically similar to the Craignish Phyllite and is regarded as stratigraphically equivalent (Roberts, 1966; Borradaile, 1973). Sedimentary structures are not so well preserved as in the Craignish Phyllite owing to a higher metamorphic grade but pseudomorphs after gypsum have been recorded and sulphur isotopes are consistent with a shallow-water, mildly reducing marine environment (Hall et al., 1994). The amount of deformation makes estimates of thickness difficult but over 4000 m of succession are recorded in places. In Knapdale a diachronous, predominantly quartzitic unit, the ''Erins Quartzite'', has an apparent thickness of up to 5000 m. The lower part is included in the Easdale