Grampian Group, Grampian Caledonides - Revision history

Dbk at 15:40, 31 January 2018

2018-01-31T15:40:23Z

← Older revision		Revision as of 16:40, 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}}
	== Grampian Group, introduction ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.jpg\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.jpg\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.

Scotfot at 17:33, 27 July 2015

2015-07-27T17:33:46Z

← Older revision		Revision as of 18:33, 27 July 2015
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	== Grampian Group, introduction ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.jpg\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.jpg\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.
	[[File:P915416.png~~\|600px~~\|thumbnail\|P915416]]		[[File:P915416.png\|thumbnail\|Distribution of the Central Highland Migmatite Complex (modified after Piasecki and Temperley, 1988). P915416.]]
	The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).		The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).

	Piasecki (1980) recognised early structures only in the migmatitic rocks which led him to interpret the relationship with the Grampian Group as one of basement and cover. In the road cutting at Slochd Summit, the contact between migmatitic and non-migmatitic rocks is well exposed and has been interpreted as a locally preserved unconformable relationship (Piasecki and Temperley, 1988).		Piasecki (1980) recognised early structures only in the migmatitic rocks which led him to interpret the relationship with the Grampian Group as one of basement and cover. In the road cutting at Slochd Summit, the contact between migmatitic and non-migmatitic rocks is well exposed and has been interpreted as a locally preserved unconformable relationship (Piasecki and Temperley, 1988).
	[[File:P001195.jpg\|thumbnail\|P001195]]		[[File:P001195.jpg\|thumbnail\|Grampian Group country. In the distance are the rounded peat-covered Drumochter Hills, formed of Grampian Group psammites. P001195.]]
	An alternative interpretation of the contact has been made by Lindsay et al. (1989) who did not recognise the presence of early structures confined to the migmatites, instead describing an apparent continuity of stratigraphy and early tectonic structures from the Grampian Group of the Atholl Nappe through the Central Highlands area to the Great Glen. The contrasts between the Grampian Group and the migmatites was attributed to a gradualincrease in metamorphic grade from south-west to north-east, with the contact being interpreted in terms of a sedimentary passage locally modified by zones of higher tectonic strain and selective migmatisation, i.e. downgrading the role and significance of the Grampian Slide.		An alternative interpretation of the contact has been made by Lindsay et al. (1989) who did not recognise the presence of early structures confined to the migmatites, instead describing an apparent continuity of stratigraphy and early tectonic structures from the Grampian Group of the Atholl Nappe through the Central Highlands area to the Great Glen. The contrasts between the Grampian Group and the migmatites was attributed to a gradualincrease in metamorphic grade from south-west to north-east, with the contact being interpreted in terms of a sedimentary passage locally modified by zones of higher tectonic strain and selective migmatisation, i.e. downgrading the role and significance of the Grampian Slide.

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	== Ord Ban Subgroup ==		== Ord Ban Subgroup ==
	This subgroup consists of the distinctive assemblage of limestone, pelitic schist (partly kyanite-bearing) and amphibolite immediately overlying the Central Highland Migmatite Complex in Strathspey; it was originally called the Grantown Series by Hinxman and Anderson (1915) and, in recognition of this, it has been renamed the ''Grantown Formation''. It is overlain by rhythmites and thick psammites but the relationship of these units to the Corrieyairack Subgroup farther west is uncertain at present.		This subgroup consists of the distinctive assemblage of limestone, pelitic schist (partly kyanite-bearing) and amphibolite immediately overlying the Central Highland Migmatite Complex in Strathspey; it was originally called the Grantown Series by Hinxman and Anderson (1915) and, in recognition of this, it has been renamed the ''Grantown Formation''. It is overlain by rhythmites and thick psammites but the relationship of these units to the Corrieyairack Subgroup farther west is uncertain at present.
	[[File:P915417.png~~\|600px~~\|thumbnail\|~~left\|~~P915417]]		[[File:P915417.png\|thumbnail\|Lithostratigraphical units in the Grampian Group. P915417.]]
	== Corrieyairack Subgroup ==		== Corrieyairack Subgroup ==
	This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) [[Media:P915417.png\|(P915417]], column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.		This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) [[Media:P915417.png\|(P915417]], column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.

Scotfot: /* Metasedimentary rocks of uncertain stratigraphical affinity */

2015-07-11T14:22:19Z

Metasedimentary rocks of uncertain stratigraphical affinity

← Older revision		Revision as of 15:22, 11 July 2015
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	Structurally underlying the Corrieyairack Subgroup in the area south-east of Loch Lochy and Loch Ness is a succession of arkosic psammites with meta-conglomerates and pebbly psammites that is more than 2000 m thick north-east of Fort Augustus (Parson, 1982). Near Loch Lochy thin lenses of dolomitic marble, black schist, quartzite and rare metabasite also occur. These lithologies have been grouped together with the psammites as the ''Glen'' ''Buck Pebbly Psammite Formation, ''which is separated from the overlying Corrieyairack Subgroup by the Eilrig Shear Zone, a zone of mylonites up to 1 km thick locally (Phillips et al., 1993). The stratigraphical affinity of a fault-bounded outcrop of gneissose micaceous psammites with minor siliceous marble at Gleann Liath, near Foyers (Mould, 1946) remains problematical.		Structurally underlying the Corrieyairack Subgroup in the area south-east of Loch Lochy and Loch Ness is a succession of arkosic psammites with meta-conglomerates and pebbly psammites that is more than 2000 m thick north-east of Fort Augustus (Parson, 1982). Near Loch Lochy thin lenses of dolomitic marble, black schist, quartzite and rare metabasite also occur. These lithologies have been grouped together with the psammites as the ''Glen'' ''Buck Pebbly Psammite Formation, ''which is separated from the overlying Corrieyairack Subgroup by the Eilrig Shear Zone, a zone of mylonites up to 1 km thick locally (Phillips et al., 1993). The stratigraphical affinity of a fault-bounded outcrop of gneissose micaceous psammites with minor siliceous marble at Gleann Liath, near Foyers (Mould, 1946) remains problematical.

	Farther to the south-east the ''Glenshirra succession'' is separated from the overlying Corrieyairack Subgroup by a slide, or high-strain zone, the Gairbeinn Slide (Haselock et al., 1982) [[Media:P915417.png\|(P915417)]], column 3). The Glenshirra succession is subdivided into four formations in its type area between the Allt Crom Granite and the Corrieyairack Igneous Complex and is at least 2500 m thick. Psammites with interbanded semipelites are the main rock types in the succession which includes an upper unit of pebbly psammites. Rapid lateral facies changes occur and Haselock (1984) described geochemical and sedimentological differences between the Glenshirra succession and the overlying Corrieyairack Subgroup. Both the Glenshirra succession and the Glen Buck Pebbly Psammite Formation are separated from the Corrieyairack Subgroup by tectonic discontinuities and their stratigraphical relationships with the overlying Grampian Group are consequently uncertain.		Farther to the south-east the ''Glenshirra succession'' is separated from the overlying Corrieyairack Subgroup by a slide, or high-strain zone, the Gairbeinn Slide (Haselock et al., 1982) [[Media:P915417.png\|(P915417]], column 3). The Glenshirra succession is subdivided into four formations in its type area between the Allt Crom Granite and the Corrieyairack Igneous Complex and is at least 2500 m thick. Psammites with interbanded semipelites are the main rock types in the succession which includes an upper unit of pebbly psammites. Rapid lateral facies changes occur and Haselock (1984) described geochemical and sedimentological differences between the Glenshirra succession and the overlying Corrieyairack Subgroup. Both the Glenshirra succession and the Glen Buck Pebbly Psammite Formation are separated from the Corrieyairack Subgroup by tectonic discontinuities and their stratigraphical relationships with the overlying Grampian Group are consequently uncertain.

	On Islay, it is possible that the Bowmore Sandstone may be assigned to the Grampian Group. This problematical unit is discussed in Chapter 3.		On Islay, it is possible that the Bowmore Sandstone may be assigned to the Grampian Group. This problematical unit is discussed in Chapter 3.

Scotfot at 14:21, 11 July 2015

2015-07-11T14:21:29Z

← Older revision		Revision as of 15:21, 11 July 2015
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	The local stratigraphies generally have been divided into formations which may be amalgamated into informal successions [[Media:P915417.png\|(P915417)]] such as the Ben Alder, Drumochter and Strathtummel successions for the area north of Schiehallion (Thomas, 1980). Further work is needed before a more formal stratigraphical framework for the whole of the Grampian Group is possible, although Winchester and Glover (1988) have suggested a regional tripartite subdivision, into the Ord Ban, Corrieyairack and Glen Spean subgroups.		The local stratigraphies generally have been divided into formations which may be amalgamated into informal successions [[Media:P915417.png\|(P915417)]] such as the Ben Alder, Drumochter and Strathtummel successions for the area north of Schiehallion (Thomas, 1980). Further work is needed before a more formal stratigraphical framework for the whole of the Grampian Group is possible, although Winchester and Glover (1988) have suggested a regional tripartite subdivision, into the Ord Ban, Corrieyairack and Glen Spean subgroups.
	~~==[[References, Grampian Highlands\|Full list of references]]==~~
	~~[[Category:Grampian Highlands]]~~

	== Ord Ban Subgroup ==		== Ord Ban Subgroup ==
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	[[File:P915417.png\|600px\|thumbnail\|left\|P915417]]		[[File:P915417.png\|600px\|thumbnail\|left\|P915417]]
	== Corrieyairack Subgroup ==		== Corrieyairack Subgroup ==
	This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) (P915417, column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.		This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) [[Media:P915417.png\|(P915417]], column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.

	== Glen Spean Subgroup ==		== Glen Spean Subgroup ==
	This subgroup consists of a mixed sequence of semipelite and psammite with quartzite. At Spean Bridge the subgroup is approximately 4000 m thick (Glover and Winchester, 1991) but farther south, in the River Leven, and on the Black Mount, the exposed psammites and semipelites of the subgroup are only about 100 m in thickness. The rocks of this subgroup are thought to reflect a change of depositional environment from the deep water turbidites of the Corrieyairack Subgroup to shallow marine shelf sedimentation (Glover, 1993). Similar lithologies farther north-east, exposed in the mountainous tract bisected by the A9 road (which provides excellent exposures in cuttings), have been described by Thomas (1980; 1988). Here a ''Drumochter'' ''succession ''of monotonously flaggy psammites and semipelites is overlain by'' ''the predominantly psammitic ''Strathtummel succession'' (P915417, column 5); they may be lateral equivalents of the Glen Spean Subgroup (Glover and Winchester, 1989). Shallow water sedimentary structures are well preserved in the upper psammites which are up to 3000 m thick.		This subgroup consists of a mixed sequence of semipelite and psammite with quartzite. At Spean Bridge the subgroup is approximately 4000 m thick (Glover and Winchester, 1991) but farther south, in the River Leven, and on the Black Mount, the exposed psammites and semipelites of the subgroup are only about 100 m in thickness. The rocks of this subgroup are thought to reflect a change of depositional environment from the deep water turbidites of the Corrieyairack Subgroup to shallow marine shelf sedimentation (Glover, 1993). Similar lithologies farther north-east, exposed in the mountainous tract bisected by the A9 road (which provides excellent exposures in cuttings), have been described by Thomas (1980; 1988). Here a ''Drumochter'' ''succession ''of monotonously flaggy psammites and semipelites is overlain by'' ''the predominantly psammitic ''Strathtummel succession'' [[Media:P915417.png\|(P915417]], column 5); they may be lateral equivalents of the Glen Spean Subgroup (Glover and Winchester, 1989). Shallow water sedimentary structures are well preserved in the upper psammites which are up to 3000 m thick.

	In Strathspey the relations between the Grantown Formation of the Ord Ban Subgroup and the overlying succession are uncertain and the contact is at least in part tectonic. The overlying succession consists for the most part of micaceous psammites with some white quartzites but there is one distinctive unit consisting of an alternation of quartzites and semipelites. Exposure in the area is generally poor and the structure is consequently not fully understood but the succession appears to be several kilometres in thickness (P915417, column 6).		In Strathspey the relations between the Grantown Formation of the Ord Ban Subgroup and the overlying succession are uncertain and the contact is at least in part tectonic. The overlying succession consists for the most part of micaceous psammites with some white quartzites but there is one distinctive unit consisting of an alternation of quartzites and semipelites. Exposure in the area is generally poor and the structure is consequently not fully understood but the succession appears to be several kilometres in thickness [[Media:P915417.png\|(P915417]], column 6).

	Quartzite becomes the dominant component of the upper Grampian Group succession at the north-east limit of its outcrop, around Rothes and on the Moray Firth coast at Cullen. Here, quartzites up to 2500 m thick, directly underly Appin Group lithologies (P915417, columns 8 and 9). The ''Cullen'' ''Quartzite Formation ''crops out along 12 km of the Moray Firth coast, between'' ''Buckpool and Logie Head. It is divided into the ''Findochty Quartzite Member'', consisting of hummocky-bedded and thickly bedded quartzites with garnet-mica-schist interbeds, and an overlying ''Logie Head Quartzite Member'', comprising planar-bedded, flaggy quartzites, also with finely interbanded garnet-mica-schists. Shallow water sedimentary structures are preserved in the various quartzites. Inland, the ''Ben Aigan Quartzite Member'' is equivalent to the Logie Head Member.		Quartzite becomes the dominant component of the upper Grampian Group succession at the north-east limit of its outcrop, around Rothes and on the Moray Firth coast at Cullen. Here, quartzites up to 2500 m thick, directly underly Appin Group lithologies (P915417, columns 8 and 9). The ''Cullen'' ''Quartzite Formation ''crops out along 12 km of the Moray Firth coast, between'' ''Buckpool and Logie Head. It is divided into the ''Findochty Quartzite Member'', consisting of hummocky-bedded and thickly bedded quartzites with garnet-mica-schist interbeds, and an overlying ''Logie Head Quartzite Member'', comprising planar-bedded, flaggy quartzites, also with finely interbanded garnet-mica-schists. Shallow water sedimentary structures are preserved in the various quartzites. Inland, the ''Ben Aigan Quartzite Member'' is equivalent to the Logie Head Member.
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	Structurally underlying the Corrieyairack Subgroup in the area south-east of Loch Lochy and Loch Ness is a succession of arkosic psammites with meta-conglomerates and pebbly psammites that is more than 2000 m thick north-east of Fort Augustus (Parson, 1982). Near Loch Lochy thin lenses of dolomitic marble, black schist, quartzite and rare metabasite also occur. These lithologies have been grouped together with the psammites as the ''Glen'' ''Buck Pebbly Psammite Formation, ''which is separated from the overlying Corrieyairack Subgroup by the Eilrig Shear Zone, a zone of mylonites up to 1 km thick locally (Phillips et al., 1993). The stratigraphical affinity of a fault-bounded outcrop of gneissose micaceous psammites with minor siliceous marble at Gleann Liath, near Foyers (Mould, 1946) remains problematical.		Structurally underlying the Corrieyairack Subgroup in the area south-east of Loch Lochy and Loch Ness is a succession of arkosic psammites with meta-conglomerates and pebbly psammites that is more than 2000 m thick north-east of Fort Augustus (Parson, 1982). Near Loch Lochy thin lenses of dolomitic marble, black schist, quartzite and rare metabasite also occur. These lithologies have been grouped together with the psammites as the ''Glen'' ''Buck Pebbly Psammite Formation, ''which is separated from the overlying Corrieyairack Subgroup by the Eilrig Shear Zone, a zone of mylonites up to 1 km thick locally (Phillips et al., 1993). The stratigraphical affinity of a fault-bounded outcrop of gneissose micaceous psammites with minor siliceous marble at Gleann Liath, near Foyers (Mould, 1946) remains problematical.

	Farther to the south-east the ''Glenshirra succession'' is separated from the overlying Corrieyairack Subgroup by a slide, or high-strain zone, the Gairbeinn Slide (Haselock et al., 1982) (P915417, column 3). The Glenshirra succession is subdivided into four formations in its type area between the Allt Crom Granite and the Corrieyairack Igneous Complex and is at least 2500 m thick. Psammites with interbanded semipelites are the main rock types in the succession which includes an upper unit of pebbly psammites. Rapid lateral facies changes occur and Haselock (1984) described geochemical and sedimentological differences between the Glenshirra succession and the overlying Corrieyairack Subgroup. Both the Glenshirra succession and the Glen Buck Pebbly Psammite Formation are separated from the Corrieyairack Subgroup by tectonic discontinuities and their stratigraphical relationships with the overlying Grampian Group are consequently uncertain.		Farther to the south-east the ''Glenshirra succession'' is separated from the overlying Corrieyairack Subgroup by a slide, or high-strain zone, the Gairbeinn Slide (Haselock et al., 1982) [[Media:P915417.png\|(P915417)]], column 3). The Glenshirra succession is subdivided into four formations in its type area between the Allt Crom Granite and the Corrieyairack Igneous Complex and is at least 2500 m thick. Psammites with interbanded semipelites are the main rock types in the succession which includes an upper unit of pebbly psammites. Rapid lateral facies changes occur and Haselock (1984) described geochemical and sedimentological differences between the Glenshirra succession and the overlying Corrieyairack Subgroup. Both the Glenshirra succession and the Glen Buck Pebbly Psammite Formation are separated from the Corrieyairack Subgroup by tectonic discontinuities and their stratigraphical relationships with the overlying Grampian Group are consequently uncertain.

	On Islay, it is possible that the Bowmore Sandstone may be assigned to the Grampian Group. This problematical unit is discussed in Chapter 3.		On Islay, it is possible that the Bowmore Sandstone may be assigned to the Grampian Group. This problematical unit is discussed in Chapter 3.

Scotfot: /* Grampian Group, introduction */

2015-07-11T14:18:20Z

Grampian Group, introduction

← Older revision		Revision as of 15:18, 11 July 2015
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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.'''		'''From: Stephenson, D, and Gould, D. 1995. [[British regional geology: Grampian Highlands\|British regional geology: Grampian Highlands.]] Fourth edition. Keyworth, Nottingham: British Geological Survey.'''
	== Grampian Group, introduction ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.~~png~~\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.jpg\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.
	[[File:P915416.png\|600px\|thumbnail\|P915416]]		[[File:P915416.png\|600px\|thumbnail\|P915416]]
	The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).		The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).
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	Stratigraphical successions have been established in a number of separate parts of the Grampian Group outcrop but at present no complete picture of the sedimentation and stratigraphy has emerged. Correlation between the established local successions is incomplete owing to the possible effects of lateral changes of lithofacies, of the high-strain zones and thrusts, and of penecontemporaneous faults which exerted an influence on sedimentation.		Stratigraphical successions have been established in a number of separate parts of the Grampian Group outcrop but at present no complete picture of the sedimentation and stratigraphy has emerged. Correlation between the established local successions is incomplete owing to the possible effects of lateral changes of lithofacies, of the high-strain zones and thrusts, and of penecontemporaneous faults which exerted an influence on sedimentation.

	The local stratigraphies generally have been divided into formations which may be amalgamated into informal successions (P915417) such as the Ben Alder, Drumochter and Strathtummel successions for the area north of Schiehallion (Thomas, 1980). Further work is needed before a more formal stratigraphical framework for the whole of the Grampian Group is possible, although Winchester and Glover (1988) have suggested a regional tripartite subdivision, into the Ord Ban, Corrieyairack and Glen Spean subgroups.		The local stratigraphies generally have been divided into formations which may be amalgamated into informal successions [[Media:P915417.png\|(P915417)]] such as the Ben Alder, Drumochter and Strathtummel successions for the area north of Schiehallion (Thomas, 1980). Further work is needed before a more formal stratigraphical framework for the whole of the Grampian Group is possible, although Winchester and Glover (1988) have suggested a regional tripartite subdivision, into the Ord Ban, Corrieyairack and Glen Spean subgroups.
			==[[References, Grampian Highlands\|Full list of references]]==
			[[Category:Grampian Highlands]]

	== Ord Ban Subgroup ==		== Ord Ban Subgroup ==

Scotfot at 14:16, 11 July 2015

2015-07-11T14:16:39Z

← Older revision		Revision as of 15:16, 11 July 2015
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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.'''		'''From: Stephenson, D, and Gould, D. 1995. [[British regional geology: Grampian Highlands\|British regional geology: Grampian Highlands.]] Fourth edition. Keyworth, Nottingham: British Geological Survey.'''
	== Grampian Group, introduction ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen [[Media:P915416.png\|(P915416]], [[Media:P001195.png\|P001195)]]. The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.
	[[File:P915416.png\|600px\|thumbnail\|P915416]]		[[File:P915416.png\|600px\|thumbnail\|P915416]]
	The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).		The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).

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	== Grampian Group, introduction ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.
	[[File:P915416.png\|~~400px~~\|thumbnail\|P915416]]		[[File:P915416.png\|600px\|thumbnail\|P915416]]
	The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).		The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).

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	== Grampian Group, introduction ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.
			[[File:P915416.png\|400px\|thumbnail\|P915416]]
	The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).		The nature of the contact between the migmatitic, gneissose lithologies of the Central Highland Migmatite Complex and the essentially non-gneissose lithologies of the Grampian Group has become the subject of differing interpretations. The two groups of rocks are separated by a complex zone of high strain and ductile shears of regional extent, the Grampian Slide Zone (Piasecki and Temperley, 1988). The Slide is well exposed at Lochindorb and at Kincraig House there are thin zones of mylonite and ultramylonite. The zones of high strain are characterised by the presence of concordant, highly deformed, apparently syntectonic pegmatite and quartz-muscovite veins which have yielded Rb/Sr mineral ages of 750 Ma (Piasecki and van Breemen, 1983).

	Piasecki (1980) recognised early structures only in the migmatitic rocks which led him to interpret the relationship with the Grampian Group as one of basement and cover. In the road cutting at Slochd Summit, the contact between migmatitic and non-migmatitic rocks is well exposed and has been interpreted as a locally preserved unconformable relationship (Piasecki and Temperley, 1988).		Piasecki (1980) recognised early structures only in the migmatitic rocks which led him to interpret the relationship with the Grampian Group as one of basement and cover. In the road cutting at Slochd Summit, the contact between migmatitic and non-migmatitic rocks is well exposed and has been interpreted as a locally preserved unconformable relationship (Piasecki and Temperley, 1988).
			[[File:P001195.jpg\|thumbnail\|P001195]]
	An alternative interpretation of the contact has been made by Lindsay et al. (1989) who did not recognise the presence of early structures confined to the migmatites, instead describing an apparent continuity of stratigraphy and early tectonic structures from the Grampian Group of the Atholl Nappe through the Central Highlands area to the Great Glen. The contrasts between the Grampian Group and the migmatites was attributed to a gradualincrease in metamorphic grade from south-west to north-east, with the contact being interpreted in terms of a sedimentary passage locally modified by zones of higher tectonic strain and selective migmatisation, i.e. downgrading the role and significance of the Grampian Slide.		An alternative interpretation of the contact has been made by Lindsay et al. (1989) who did not recognise the presence of early structures confined to the migmatites, instead describing an apparent continuity of stratigraphy and early tectonic structures from the Grampian Group of the Atholl Nappe through the Central Highlands area to the Great Glen. The contrasts between the Grampian Group and the migmatites was attributed to a gradualincrease in metamorphic grade from south-west to north-east, with the contact being interpreted in terms of a sedimentary passage locally modified by zones of higher tectonic strain and selective migmatisation, i.e. downgrading the role and significance of the Grampian Slide.

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	== Ord Ban Subgroup ==		== Ord Ban Subgroup ==
	This subgroup consists of the distinctive assemblage of limestone, pelitic schist (partly kyanite-bearing) and amphibolite immediately overlying the Central Highland Migmatite Complex in Strathspey; it was originally called the Grantown Series by Hinxman and Anderson (1915) and, in recognition of this, it has been renamed the ''Grantown Formation''. It is overlain by rhythmites and thick psammites but the relationship of these units to the Corrieyairack Subgroup farther west is uncertain at present.		This subgroup consists of the distinctive assemblage of limestone, pelitic schist (partly kyanite-bearing) and amphibolite immediately overlying the Central Highland Migmatite Complex in Strathspey; it was originally called the Grantown Series by Hinxman and Anderson (1915) and, in recognition of this, it has been renamed the ''Grantown Formation''. It is overlain by rhythmites and thick psammites but the relationship of these units to the Corrieyairack Subgroup farther west is uncertain at present.
			[[File:P915417.png\|600px\|thumbnail\|left\|P915417]]
	== Corrieyairack Subgroup ==		== Corrieyairack Subgroup ==
	This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) (P915417, column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.		This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) (P915417, column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.

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Grampian Group

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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.'''		'''From: Stephenson, D, and Gould, D. 1995. [[British regional geology: Grampian Highlands\|British regional geology: Grampian Highlands.]] Fourth edition. Keyworth, Nottingham: British Geological Survey.'''
	== Grampian Group ==		== Grampian Group, introduction ==
	The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.		The Grampian Group crops out over an area of approximately 4250 km<sup>2</sup> in a broad NE-trending zone extending from Glen Orchy to near Elgin, with an isolated outcrop on the Moray coast around Cullen (P915416, P001195). The group consists mainly of psammites and semipelites at amphibolite-facies metamorphic grade.

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	The local stratigraphies generally have been divided into formations which may be amalgamated into informal successions (P915417) such as the Ben Alder, Drumochter and Strathtummel successions for the area north of Schiehallion (Thomas, 1980). Further work is needed before a more formal stratigraphical framework for the whole of the Grampian Group is possible, although Winchester and Glover (1988) have suggested a regional tripartite subdivision, into the Ord Ban, Corrieyairack and Glen Spean subgroups.		The local stratigraphies generally have been divided into formations which may be amalgamated into informal successions (P915417) such as the Ben Alder, Drumochter and Strathtummel successions for the area north of Schiehallion (Thomas, 1980). Further work is needed before a more formal stratigraphical framework for the whole of the Grampian Group is possible, although Winchester and Glover (1988) have suggested a regional tripartite subdivision, into the Ord Ban, Corrieyairack and Glen Spean subgroups.

	=== Ord Ban Subgroup ===		== Ord Ban Subgroup ==
	This subgroup consists of the distinctive assemblage of limestone, pelitic schist (partly kyanite-bearing) and amphibolite immediately overlying the Central Highland Migmatite Complex in Strathspey; it was originally called the Grantown Series by Hinxman and Anderson (1915) and, in recognition of this, it has been renamed the ''Grantown Formation''. It is overlain by rhythmites and thick psammites but the relationship of these units to the Corrieyairack Subgroup farther west is uncertain at present.		This subgroup consists of the distinctive assemblage of limestone, pelitic schist (partly kyanite-bearing) and amphibolite immediately overlying the Central Highland Migmatite Complex in Strathspey; it was originally called the Grantown Series by Hinxman and Anderson (1915) and, in recognition of this, it has been renamed the ''Grantown Formation''. It is overlain by rhythmites and thick psammites but the relationship of these units to the Corrieyairack Subgroup farther west is uncertain at present.

	=== Corrieyairack Subgroup ===		== Corrieyairack Subgroup ==
	This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) (P915417, column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.		This subgroup is the major component of the Grampian Group of the southern Monadhliath Mountains where it is about 4500 m thick and consists of a thick succession of psammitic rocks overlying a basal semipelite (Haselock et al., 1982; Okonkwo, 1988) (P915417, column 3). Here it is separated from the underlying Glenshirra succession, which has uncertain stratigraphical status (see next section). Southwards, towards Glen Spean, quartzite forms several distinctive units of varying thickness.

	=== Glen Spean Subgroup ===		== Glen Spean Subgroup ==
	This subgroup consists of a mixed sequence of semipelite and psammite with quartzite. At Spean Bridge the subgroup is approximately 4000 m thick (Glover and Winchester, 1991) but farther south, in the River Leven, and on the Black Mount, the exposed psammites and semipelites of the subgroup are only about 100 m in thickness. The rocks of this subgroup are thought to reflect a change of depositional environment from the deep water turbidites of the Corrieyairack Subgroup to shallow marine shelf sedimentation (Glover, 1993). Similar lithologies farther north-east, exposed in the mountainous tract bisected by the A9 road (which provides excellent exposures in cuttings), have been described by Thomas (1980; 1988). Here a ''Drumochter'' ''succession ''of monotonously flaggy psammites and semipelites is overlain by'' ''the predominantly psammitic ''Strathtummel succession'' (P915417, column 5); they may be lateral equivalents of the Glen Spean Subgroup (Glover and Winchester, 1989). Shallow water sedimentary structures are well preserved in the upper psammites which are up to 3000 m thick.		This subgroup consists of a mixed sequence of semipelite and psammite with quartzite. At Spean Bridge the subgroup is approximately 4000 m thick (Glover and Winchester, 1991) but farther south, in the River Leven, and on the Black Mount, the exposed psammites and semipelites of the subgroup are only about 100 m in thickness. The rocks of this subgroup are thought to reflect a change of depositional environment from the deep water turbidites of the Corrieyairack Subgroup to shallow marine shelf sedimentation (Glover, 1993). Similar lithologies farther north-east, exposed in the mountainous tract bisected by the A9 road (which provides excellent exposures in cuttings), have been described by Thomas (1980; 1988). Here a ''Drumochter'' ''succession ''of monotonously flaggy psammites and semipelites is overlain by'' ''the predominantly psammitic ''Strathtummel succession'' (P915417, column 5); they may be lateral equivalents of the Glen Spean Subgroup (Glover and Winchester, 1989). Shallow water sedimentary structures are well preserved in the upper psammites which are up to 3000 m thick.

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	Quartzite becomes the dominant component of the upper Grampian Group succession at the north-east limit of its outcrop, around Rothes and on the Moray Firth coast at Cullen. Here, quartzites up to 2500 m thick, directly underly Appin Group lithologies (P915417, columns 8 and 9). The ''Cullen'' ''Quartzite Formation ''crops out along 12 km of the Moray Firth coast, between'' ''Buckpool and Logie Head. It is divided into the ''Findochty Quartzite Member'', consisting of hummocky-bedded and thickly bedded quartzites with garnet-mica-schist interbeds, and an overlying ''Logie Head Quartzite Member'', comprising planar-bedded, flaggy quartzites, also with finely interbanded garnet-mica-schists. Shallow water sedimentary structures are preserved in the various quartzites. Inland, the ''Ben Aigan Quartzite Member'' is equivalent to the Logie Head Member.		Quartzite becomes the dominant component of the upper Grampian Group succession at the north-east limit of its outcrop, around Rothes and on the Moray Firth coast at Cullen. Here, quartzites up to 2500 m thick, directly underly Appin Group lithologies (P915417, columns 8 and 9). The ''Cullen'' ''Quartzite Formation ''crops out along 12 km of the Moray Firth coast, between'' ''Buckpool and Logie Head. It is divided into the ''Findochty Quartzite Member'', consisting of hummocky-bedded and thickly bedded quartzites with garnet-mica-schist interbeds, and an overlying ''Logie Head Quartzite Member'', comprising planar-bedded, flaggy quartzites, also with finely interbanded garnet-mica-schists. Shallow water sedimentary structures are preserved in the various quartzites. Inland, the ''Ben Aigan Quartzite Member'' is equivalent to the Logie Head Member.

	=== Metasedimentary rocks of uncertain stratigraphical affinity ===		== Metasedimentary rocks of uncertain stratigraphical affinity ==
	Structurally underlying the Corrieyairack Subgroup in the area south-east of Loch Lochy and Loch Ness is a succession of arkosic psammites with meta-conglomerates and pebbly psammites that is more than 2000 m thick north-east of Fort Augustus (Parson, 1982). Near Loch Lochy thin lenses of dolomitic marble, black schist, quartzite and rare metabasite also occur. These lithologies have been grouped together with the psammites as the ''Glen'' ''Buck Pebbly Psammite Formation, ''which is separated from the overlying Corrieyairack Subgroup by the Eilrig Shear Zone, a zone of mylonites up to 1 km thick locally (Phillips et al., 1993). The stratigraphical affinity of a fault-bounded outcrop of gneissose micaceous psammites with minor siliceous marble at Gleann Liath, near Foyers (Mould, 1946) remains problematical.		Structurally underlying the Corrieyairack Subgroup in the area south-east of Loch Lochy and Loch Ness is a succession of arkosic psammites with meta-conglomerates and pebbly psammites that is more than 2000 m thick north-east of Fort Augustus (Parson, 1982). Near Loch Lochy thin lenses of dolomitic marble, black schist, quartzite and rare metabasite also occur. These lithologies have been grouped together with the psammites as the ''Glen'' ''Buck Pebbly Psammite Formation, ''which is separated from the overlying Corrieyairack Subgroup by the Eilrig Shear Zone, a zone of mylonites up to 1 km thick locally (Phillips et al., 1993). The stratigraphical affinity of a fault-bounded outcrop of gneissose micaceous psammites with minor siliceous marble at Gleann Liath, near Foyers (Mould, 1946) remains problematical.