The Grampian Group: a Major Late Proterozoic Clastic Sequence in the Central Highlands of Scotland

Journal of the Geological Sociev, London, Vol. 146, 1989, pp. 85-96, 9 figs. Printed in Northern Ireland

The Grampian Group: a major Late Proterozoic clastic sequence in the Central Highlands of Scotland

B. W.GLOVER & J. A. WINCHESTER Department of Geology, University of Keele, Keele, Staffs ST5 SBG, UK

Abstract: A stratigraphic framework is proposed for the Late Proterozoic Grampian Group of the Scottish Highlands. The Grampian Group is divided into three subgroups, each defined by distinctive lithofacies associations reflecting different environments of deposition. The thin Ord Ban Subgroup, which is locally developed at the base of the Grampian Group on Speyside and Strath Dearn, consists of shallow marine shelf sediments in association with concordant amphibolite sheets. Elsewhere the CorrieyairackSubgroup, which is thickest around Loch Laggan,forms the lowerportion of the Grampian Group and comprises a thick turbiditic clastic sequence laid down in deeper water during rapid basin subsidence. The Glen Spean Subgroup comprises the upper part of the Grampian Group andis thickest in the Atholl District. It is composed of shallowwater tidal and deltaic deposits, marking a shallowing of the Grampian Group basin. Lateral facies changes and diachronous contacts are indicative of differential subsidence and the Grampian Group basin apparently developed and filled before the onset of regional subsidence which heralded deposition of the overlying Dalradian sediments. The possibility that the Grampian Group was unconformable on the structurally underlying Central Highland Division and Glenshirra Succession is currently unresolved.

TheGrampian Group crops outover approximately Slide and below the Appin Group in the Central Highlands. 4250 km2 within theGrampian Highlands of Scotland, Piasecki did not map west of the Markie Fault (Fig. l), extending from Glen Orchy in the south-west to near Elgin an extension of the Laggan Fault (Johnson & Frost 1977), in the north-east.Previous investigations correlated which is one of the persistent Caledonian fractures trending Grampian Group rocks with the ‘Moine Schists’ of the NW parallel to the Great Glen Fault. Between the Markie Fault Highlands onthe basis of their superficial lithological and the Great Glen Fault, Haselock (1982) mapped a zone resemblance (Barrow 1904). In subsequentmore detailed of high strain which she termed the Gairbeinn Slide. This work the rocks underlying theAppin Group of the forms the base of a succession assigned to the Grampian DalradianSupergroup were named as ‘Eilde Flags’ in the Group.Beneath the Gairbeinn Slide the Glenshirra Lochaber area (Bailey 1910; Bailey & Maufe 1960; Hickman Succession is dominated by arkosic, locally pebbly 1975) and as‘Struan Flags’ inPerthshire (Barrow et al. psammites (P. J. Haselock 1982 and pers. comm.). It crops 1905). Because clear marker horizons werelacking, the out in two main areas: in Upper Strathspeywhere it is Grampian Group was initially considered as a monotonous exposed in an antiformal structure (the Glenshirra Dome in flaggy psammitic sequence (Central HighlandGranulites) Fig. 2), andnorth-east of Fort Augustusadjacent tothe and little attempt was made to subdivide it stratigraphically. Foyers Granite (Fig. 1). At no point is a clear stratigraphic The nature of its base is still being debated. While the relationship between the Glenshirra Sucession andthe existence of anolder migmatitic basement (theCentral overlying GrampianGroup rocks seen, andhence the Highland Division) hasbeen advocated on petrological, affinities of the former remain unresolved. In addition there structuraland isotopic evidence (Piasecki & van Breemen are distinct geochemical and sedimentological differences 1979; Piasecki 1980; Piasecki et al. 1981), it hasnot been between the Glenshirra Succession andGrampian Group universally accepted.Because of the generally migmatitic rocks (Haselock 1984; Okonkwo 1985) and for these reasons natureand lack of clearstratigraphy and preserved discussion of the Glenshirra Succession is omitted here. sedimentarystructures in rocks assigned tothe Central Highland Division by Piasecki (1980), theserocks are not discussed here. Piasecki (1980) defined the base of the Previous work and the status of the Grampian Grampian Group (also referred to in the literature as the Group Grampian Division or YoungerMoines) as a zone of In recent years structural and stratigraphic studies have been enhanced strain or a slide, which he termed the Grampian undertaken in many parts of the Central Highlands. As a Slide. result of detailed mapping, dominantly migmatitic gneisses The Grampian Group is overlain by the Appin Group of in the area between Strath Nairn and Speyside (Fig. 1) were the Dalradian Supergroup (Harris et al. 1978) and, where considered by Piasecki (1980) to form an olderbasement the boundary is locally preserved, as in the River Leven, a (the Central Highland Division) to the remaining rocks in sedimentary transition can be seen. However, deformation the area (the Grampian Group). InAtholl a local succession during the Grampian and Caledonian orogenies has in most was established in a structurally complex area of Grampian places modified and obscured the nature of this boundary. Group rocks underlying the Appin Group (Thomas 1980), In this paper,therefore, we include within theGrampian while mapping in the MonadhliathMountains led tothe Group all the rocks lying structurally above the Grampian establishment of a local GrampianGroup succession 85

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Fig. 1. Map showing the outcrop of the Grampian Groupin the Central Highlands of Scotland. Additional maps show the Glen Spean Subgroup relationships in the Black Mount and the River Leven Inlier. Abbreviations [onall maps]: AE, Aonach Eagach; CF, Corran Ferry; CT,Coire Toaig; FA, Fort Augustus, FG, Foyers Granite; FWS, Fort William Slide; G., Grantown-on-Spey; GO, Glen Orchy; GS, Gairbeinn Slide; K, Killin; KH, Kyllachy House; L, Lochaber; LEM, Loch Eilde Mor; LL, Loch Laggan; MCD,Meall Cian Dearg; OB, (3rd Ban; RC, Rubha Cuil-cheanna; S, Glen Spean; SB, Spean Bridge; SE, Strath Errick; SEM, Sgurr EildeMor; SG, Stob Gabhar;T, Torness.

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(Whittles 1981; Haselock 1982; Haselock et al. 1982). Within it may be included both theGlen Doe and Extension of mapping into the ground north of Loch Laggan Knockchoilum ‘semipsammites’ described by Haselock et al. by Okonkwo (1985) and Glover (this paper) has shown that (1982), which apart from the different types of calc-silicate the most completeand accessible stratigraphic succession pods and lenses present, are lithologically identical and are within the Grampian Group occurs between Glen Spean and thus assigned to the same formation. At the base of the Upper Strathspey. Its lithological formations may be formation the Fechlin Member, previously termedthe compared with stratigraphic successions elsewhere in the Fechlin Psammite (Haselock et al. 1982; Whittles 1981), Grampian Group, particularly the areas mapped in Atholl forms a laterally impersistent unit.At the top of the (Thomas 1980) andon Speyside (Piasecki 1980). Broad formation a distinctively striped member, here termed the similarities between the successions in these areasare Moy Member, is more extensively developed. Near Loch recognizable, but important lateralvariations also occur. Killin (Fig. 2) it is not observed: instead a thick quartzite Preserved sedimentary structuresare locally abundant, (Killin Quartzite, Figs, 2, 3) is presentat a similar level despite widespread deformation andupper greenschist to (Whittles 1981). The remainder of the Glen DoeFormation, amphibolite facies metamorphism. Their occurrencein which consists of monotonous turbiditic psammites, is here distinctive lithological associations has greatly facilitated described as the Laggan Member. lithostratigraphiccorrelation. The results of this work are The Fechlin Member (0-750 +m) crops out N of described below. Gairbeinn (NH 468030), but is best displayed below the dam on the River Fechlin (NH 497143). Locally exceeding 750 m Stratigraphy of the Monadhliath-Glen Spean area in thickness, it is characterized by a thick-bedded sequence of turbidites which exhibit Bouma A, B, and C divisions, A stratigraphic succession within the recently-mapped but is almost devoid of semipelites (Bouma D, E). groundbetween Loch Killin, Loch Laggan andthe River Convolute bedding and loading are also preserved, as noted Spean is shown in Fig. 2. Many lateral facies changes exist, previously (Haselock et al. 1982). Its impersistence (Fig. 2) but two subgroups may be distinguished by their may reflect deposition within submarinefan systems of characteristic lithofacies associations. The lower subgroup, limited lateral extent. Poor exposure across the Glen Doe heretermed the CorrieyairackSubgroup, comprises mot Formation outcrop may inhibit recognition of Fechlin-type rocks formerly termedthe Corrieyairack Succession lithologies at higher stratigraphic levels. (Haselock 1982; Haselock et al. 1982; Okonkwo 1985). The Laggan Member (1250-3000 + m) is well-exposed in Rock types range from dark micaceous psammites to road cuttings along the north shore of Loch Laggan and in semipelites. Incontrast, the upper subgroup, here named shoreexposures atRubha na Magach (NN 460849), the Glen Spean Subgroup, consists of discrete psammites, particularly when the level of Loch Laggan is low. It semipelites and quartzites. These rocks have not previously comprises turbiditic dark micaceous psammites,often been described in detail. grading into semipelitic tops. Beds range from 5 cm up to 1 m in thickness, but commonly tend to be between 20 to Corrieyairack Subgroup 40 cm thick. Calc-silicate pods occur throughout: the rather scarce green actinolitic calc-silicates appear tobe confined to The CorrieyairackSubgroup hasbeen divided into distinctive formations (Fig. 3). Contactsare gradational, the north and west of the area, while whitish clinozoisite- with the local development of subordinate members. bearing calc-silicates occur moreabundantly in the south and east. At Rubha na Magach sedimentary structures are The Coire nan Laogh Formation. (250111) overlies the clearly visible. They include convolutebedding, ripple Gairbeinn Slide and consists ofa lower semipelitic member, cross-lamination, rip-up clasts and scouring. However, and an upper ‘rhythmic’ member (Haselock 1982; Haselock elsewhere sedimentarystructures are rarely visible in the et al. 1982). field, but X-ray radiography of thin slabs has revealed that The Semipelitic Member is well-exposed on Stac Buidhe, they are frequently preserved (Fig. 4). south of Garva Bridge (Fig. 2) (NN 522940), and on The Moy Member (100-300 m), referred to previously as Gairbeinn (NN 460985) and consists of a variably migmatitic the Striped Transition(Haselock et al. 1982) reachesa semipelite in which no sedimentary structures are preserved. maximum thickness of 300m in Glen Tarff (NN 391021 in Haselock et al. (1982) give a more detailedpetrographic Fig. 2), but is usually much thinner. It is also well displayed in description. a quarry above Loch Spean W of Moy Lodge (NN 415825), The ‘rhythmic’ member overlies the semipelitic member where it consists of striped, white and grey psammites, around the Glenshirra Dome (Fig. 2). It also occurs in which are sometimescalcareous, and laterally persistent various localities south-east of Strath Errick (NH 540178), semipelitic units. Beds commonly range from 4 to 10 cm in where the lower semipelitic member is missing (P. J. thickness, butthicker units arepresent. Trough cross- Haselock pers. comm.). It is characterized by the presence bedding, ripple lamination and gradedbedding are of numerousgraded units of thin dark micaceous displayed in the exposures by Loch Spean. psammite-semipelite couplets up to lOcm thick, giving the memberrhythmica appearance.Rare quartzites and The Ardair Formation. (350 m), formerly described as the psammites occur as thin (less than 5 cm thick)lenticular ‘Monadhliath Semipelite’ (Okonkwo 1985), exhibits marked units up to 5 m long. These become scarcer towards the top lateralvariation. It was correlated initially with the of the member. Whitish clinozoisite-bearing calc-silicates are MonadhliathSemipelite north-west of the Corrieyairack also present. Granodiorite (Fig. 3) because it possesses a higher proportion of semipelite thanthe Glen Doe Formation, The Glen Doe Formation. (1350-3100111) comprisesmore which underlies both theArdair Formationand the than half the thickness of the CorrieyairackSubgroup. Monadhliath Semipelite.

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EI!I!ll Lochaber Subgroup El Caledonianlntrusives (Leven and Binnein Schists) S em ipe lite DevonianUilleim Semipelite Glenshirra Succession ’F2 FOLDS /

Fig. 2. Geological map of the Monadhliath-Glen Spean area. The map incorporates previous work by Haselock et al. (1982), Hickman (1975, 1978) and Okonkwo (1985) in addition to mapping undertaken by Glover (this paper). Abbreviations: ACG, Allt Crom Granite; BC, Beinna’ Chaoruinn; BT, Beinn Teallach; CG, Corrieyairack Granodiorite; CM, Creag Meagaidh; CU, Creag Uilleim; GB, Glen Brein; GD, Glenshirra Dome; GR, Glen Roy; GS, Gairbeinn Slide;IF Inverlair Falls; LL, Loch Laggan; LK, Loch Killin; LS, Loch Spean; OG, Ossian Granodiorite; RM, Rubha na Magach; UGT, Upper Glen Tarff.

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Loch Klllin GlenBreinUpper Glen Tarff Corrleyalrack Creag Meagaldh Loch Laggan 5000

4000

3000

Glen Doe Frnn

2000

1000

Fig. 3. Stratigraphic correlations in the Monadhliath-Glen Spean area. See Fig. 2 for column locations.

However, because the semipelitic rocks overlying the 5 cm in thickness. Thin white psammitelaminae occur Glen Doe Formationin the Laggan-Spean area differ in sporadically throughout. lithofacies association from the Monadhliath Semipelite to The BhaideanachMember (0-370 m) is restricted to the north-west, we suggest that they be termed the Ardair Meall Bhaideanach andthe southern slope of Beinn a’ Formation, eventhough its deposition may havebeen in Chaoruinn (Fig. 2). It consists exclusively of thick-bedded part contemporaneous with that of the Monadhliath dark micaceous psammite sequences which thin and fine Semipelite. Within the Ardair Formation the characteristic upwards, On account of their cyclic natureand lateral lithofacies comprisethick-bedded turbidites which are impersistence, we interpret thesesequences as aseries of frequentlyamalgamated. They consist dominantly of small-scale stacked submarine channel fills. Calc-silicates are micaceous psammitesinterspersed with semipelitic units rare. measuring up to 2 m in thickness and graded darkmicaceous The Uamha Member (0-480 m), which is restricted in psammites with semipelitic tops 2-1Ocm thick. Micaceous occurrence tothe large east-facing corries on Beinn psammites commonly containpersistent white clinozosite- a’chaoruinn(NN 385855), locally comprises theentire bearing calc-silicate bands, which are well-exposed by the thickness of the Creag Meagaidh Formation. It displays a road above Loch Spean-(NN 413822). Weinterpret this rapid lateral transition into adjacent members both to the formationas having beendeposited within the submarine northand south.It consists dominantly of stacked, fan environment. parallel-sided gritty turbiditesequences up to 15 m thick. Individual turbidites exhibit coarsetail grading and faint The Creag Meagaidh Formation. (400-400 m) crops out in planar lamination towards thetop of each bed.Bed an area extending from Loch Roy (NN 417893) across Creag thicknesses range from 10-50 cm.seriesA of thin Meagaidh and Loch Spean as far as the Ossian-Geal Charn semipelites and dark micaceous psammites similar to those Steep Belt in the south-east (Fig. 2). It is well-exposed in a seen in theQuarry Member separate eachstacked unit. small roadside quarry (NN 399819) and in the nearby Allt These may represent more ‘distal’ turbidite sedimentation. na Uamha (NN 400820). Facies associations are variable and three members have been recognized on Beinn a’ Chaoruinn (Fig. 2). Glen Spean Subgroup The Quarry Member (0-600 m) forms most of the Creag Towards thetop of theCreag MeagaidhFormation Meagaidh Formation. North of Loch Spean it forms the semipelite becomes more common, forming a passage up lower part of the Formation and is overlain by the into the overlying formation. Onthe top of Beinn a’ Bhaideanach member. It is traceable south of Loch Spean Chaoruinn (NN 386850) a thick, often pink-stained quartzite and to the north it crops out on Creag Meagaidh (Fig. 2). In is taken to mark the base of the Clachaig Formation and the both of these areas it forms the entirethickness of the Creag GlenSpean Subgroup. Inthe type areathe Glen Spean Meagaidh Formation. It consists of thin, flaggy, graded Subgrouphas been divided into two lithologically distinct units, usually between 2-5 cm thick, and planar-laminated formations (Fig. 3). The lower one,here named the dark micaceous psammites. Whitish calc-silicates are less Clachaig Formation, whichis dominantly semipelitic, is abundant than in the Ardair Formation and rarely exceed overlain by thick psammites and quartzites with subordinate

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calc-silicates in the Killin area.We consider thatthese associations suggest deposition within a shallow marine A environment, with slow sediment accumulation.

The CZuchaig Formation (200-350m), crops out west of theCreag Meagaidh Formation andis well exposed on Meall Clachaig (NN 375830) (Fig. 2). It consists of semipelitic units upto 2 m thick. Microscopic studyhas shown that some semipelitescontain apatite-rich laminae, indicating that the original argillaceous sediment contained phosphatic bands. Interbedded with the semipelite are scarce psammites and quartzites, ranging up to 5 m thick and locally exhibiting tabular cross-bedding. These tend to become thicker and more abundant towards the top of the formation. Thin-bedded ripple and planar laminated white psammites less than 10 cm thick occur throughout the formation.

The Inverlair Formution (400-800 m), overlies the Clachaig Formation. The contactbetween them is transitional, the base of the InverlairFormation being marked by the development of thicker, laterally more extensive psammites and the increasing scarity of semipelitic units. North of Glen Spean the base of the Inverlair Formation is identified by a B reddish quartzite marker at least 10 m thick. This rock is well exposed by a cairn north of Beinn a’ Chaoruinn (NN 381874) and is traceableacross the westernslopes of the mountain. The type lithofacies associations are well displayed at Inverlair Falls (NN 341806). Sedimentological variations are illustrated by a representative log (Fig. 5). The base of thick psammites is commonly marked by an erosive surface. Such psammites locally fine and thin upwards, perhaps indicating channel abandonment and infilling. Slumping at the base of channel fills has also been observed. Thicknesses vary from 3 to 15 m, while widths may exceed 100 m. Trough cross and Fig. 4. Planar and cross-lamination present within a 3 mm slab. (A) planarbedding with setsup to 20cm thick are also shows the seemingly homogeneous natureof the dark micaceous sometimespreserved within thesepsammites (Fig. 6). psammite. (B)is a positive print of an X-ray radiographof the same Parallel-sided psammites and semipelites are also present, slab, showing sedimentary structures. Specimen from a thin-bedded consisting of sharp-topped planar bedded psammites up to sequence within the Laggan Member. 40cm thick separated by thinner beds of semipelite, and are interpreted as overbank deposits.

Other rocks Inthe RiverSpean (NN 315809), on Creag Uilleim (NN semipelite, here termed the Inverlair Formation. However, 344836) andeast of the summit of Beinn Teallach(NN in the north-west of the area thelower semipelitic formation 362859) a series of variable grey speckled semipelitic rocks (MonadhliathSemipelite) rests conformably onthe Glen occurs between the InverlairFormation andthe Leven Doe Formation. Therefore the uppermostformations of the Schist to the west (Fig. 2). These rocks, here termed the Comeyairack Subgroup in GlenSpean are absent in the Uilleim Semipelite (0-460 m), havenot been observed north-west (Figs 2, 3). elsewhere, andbear scant lithological resemblance to semipelitic formationsin eitherthe Appin or Grampian The Monadhliath Semipelite (380-650 m), together with Groups. Their stratigraphic position is unclear.because they numerous other semipelitic rocks, was originally referred to appear to have sharp contacts with boththe Inverlair as the ‘Monadhliath Schist’ andequated with theAppin Formation and the Leven Schist, and because they occur in Group Leven Schist in GlenRoy (Anderson 1956). This association with a zone of enhanced strain, which may be correlation was discredited by both geochemistry (Lambert traced continuously around the Corrieyairack Synform into et al. 1982) and field mapping which showed the the Fort William Slide. Monadhliath Semipelite to lie at a lower stratigraphic level than the Leven Schist at the head of Glen Roy (Haselock & Winchester 1982). North-west of the Corrieyairack Stratigraphic correlation within the Grampian Group Granodiorite the MonadhliathSemipelite is dominated by The stratigraphic succession established in the Loch semipeliteassociated with quartzite in the Corrieyairack Laggan-Glen Spean area is recognizable in a modified form area,and by semipelitecontaining abundant white elsewhere within the Grampian Group outcrop.

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sand/mud 0.- 0.-

channel

sheet sands

abandonment

channel

overbanks 5m ?crevasse splays

Fig. 6. Tabular and trough cross bedding in Inverlair 1 Formation psammite from Meallhire (NN 334791). Thin, dark pelitic laminae mark each foreset.

channel except for a gap east of Spean Bridge, a near-continuous exposure of Grampian Group semipelites, with quartzites up to 10 m thick, occurs downstream as far as Torness (Fig. 1, NN 187837). The abundance of semipelite and the existence of subordinate quartzites suggest a lithological resemblance to the Clachaig Formation and the Monadhliath Semipelite north-west of the Comeyairack Pass. However, whereas the Facies associations: Inverlair Formation Clachaig Formation is only 200 m thick in the type area, in the River Spean gorgesection, where the metamorphic Fig. 5. Representative sedimentary log from the Inverlair grade is relatively low (mid-upper greenschist), numerous Formation, based on data recorded at Inverlair Falls (NN preserved younging indicators suggest an apparent thickness 341806). of about4000m, inspite of the effects of major deformation. NearTorness a series of quartzites and psammites Spean Bridge-Rubha Cuil-cheanna separated by subordinate semipelites are exposed in the Grampian Group rocks crop out along the shore of Loch gorge. They are in faultedcontact with the mainly Linnhe and by the main road south-west of Fort William, semipelitic rocks seen to the east. This sequence, at least although they are usually affected by late deformation and 150m thick, closely resembles the InverlairFormation. veining associated with movement on the Great GlenFault. These rocks pass upinto a pelite-semipelite succession Deformation associated with the Fort William Slide (Fig. 1) lithologically similar to the Leven Schist 400 m from the provides further complication at Rubha Cuil-cheanna. Most west end of the section (NN 185837). of these rocks, which are pale and extensively chloritized, contain a high proportion of micas (Bailey & Maufe 1960). Psammites andquartzites were observed throughoutthe River Leven Inlier sequence by Bailey & Maufe (1960), who referred to the The Grampian Group exposedin the River Leven (NN LinnheQuartzite andDark Schist exposed nearCorran 203610) occurs within an inlier separatedfrom the main Ferry (Fig. 1). Grampian Group outcrop (inset in Fig. 1, & Fig. 7). This The most complete section is seen in the gorge of the inlier occurs in the overturned limb of the F1 Kinlochleven RiverSpean west of Spean Bridge wherethe exposed Anticline and is preserved in the core of the F2 Blackwater Grampian Group rockscan best be equated lithologically Synform (Hickman 1978). Extending north-east for 20 km to with theGlen Spean Subgroup (Fig. 7).In the east the Meall Cian Dearg (NN 331759), west of Loch Treig, the contact with theAppin Group is exposed beneaththe River Leven Inlier is crucially important in revealing the railway bridge overthe River Spean (NN 238817) and, original relationshipbetween theGrampian and Appin

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lntefpretaltonStratlgraphyType Black Mount RlverLagganSpeanLochSpeyslde GlenLowerLeven Atholl

5200- Shallow rnarme- deltalc Kynachan Psarnrnlte 5000- and lntertldal Kynachan Quartzlte ?outer shelf

Deep water- submarlne fans etc 4000- Turnrnel Psamrnlte

Deep water- submarme fans etc 3000- ~

m Turnrnel I Quartzites Shallow marme wlth larnlnaled psarnrnttes

2000-

ICOO-

0- Shallow marme

Fig. 7. Regional stratigraphic correlations proposed for the Grampian Group. See Figs 1 and 2 for column locations.

Groups, because of the absence of a high strain zone at the 236662 on Fig. l),but is absent from the north-east end of contact. the inlier. In the RiverLeven almost continuous exposure of The remaining rocks of the inlier consist dominantly of GrampianGroup rocks occursbetween the junction with the flaggy semipelitic association, which locally exhibits the EildeQuartzite (NN 203610) for360m upstream. marked lithological variations. Southeast of Loch Eilde Mor Grampian Grouprocks, referred to previously as Eilde (NN240640), inverted flaggy gradedunits consisting of Flags (Bailey 1910; Treagus 1974; Roberts 1976; Roberts & white psammite grading into pelite are exposed, while at the Treagus 1977), consist of acoarse-grained, occasionally north-east end of the inlier on Meal1 Cian Dearg laminated pebbly psammitic association,exhibiting varietya of dark micaceous psammites and semipelites are associated sedimentary structures. These include dune-bedding, ripple with thin pinkish quartzites.Despite this variability, we cross-lamination, slumping and dewateringstructures. consider that these rocks in the River Leven Inlier closely Throughout the section pelite and semipelite occurs as thin resemble assemblages seen in the Clachaig Formation of the partingsand drapes on foresets and dune tops. However, Glen Spean Subgroup. It is clear from the evidence in the towards thetop of theGrampian Group two thicker River Leven Inlier that these two associations are in part semipelitic beds are present. These occur within the lateralequivalents (Fig. 7). The pebbly psammite transition between theGrampian Group and the Eilde association seen in the RiverLeven forms atransition Quartzite. between theGrampian Group and the Eilde Quartzite. A series of flaggy semipelites and thin white psammites is However, on the east limb and in the core of the Blackwater exposed in the River Leven between NN226604 and NN Synform in the NE the pelitic association forms a transition 206606 (Inset in Fig. 1). These rocksstratigraphically between the Grampian Groupand LochaberSubgroup underlie the pebbly psammitic association exposed below pelites with subordinate quartzites. theEilde Quartziteon the west limb of the Blackwater Synform. However, on the east limb this association extends to the eastern contact with the Lochaber Subgroup of the Black Mount Appin Group (NN 236608). Within the River Leven Inlier Grampian Group rocks are best seen north-west of Loch there are thus two distinctive facies associations. While the Tulla on Stob Gabhar (NN 230455) and Beinn Toaig (Inset pebbly psammite association resembles the Inverlair on Fig. 1 at NN 262455). The contact with the Appin Group Formation in Glen Spean, the flaggy semipelitic association is marked by a series of banded psammitic rocks closely is more akin to the Clachaig Formation (Fig. 7). resembling the ‘Banded Zone’ seen below the Iltay The relationshipbetween the two associations in the Boundary Slide north of Schiehallion (Thomas 1980). The inlier is significant. The pebbly psammiteassociation is succession is repeated by upright F2 folding so that confined to the uppermost 200m of the Grampian Group Grampian-Appincontacts are exposed on both Stob succession in the west only. It is traceable north-east as far Gabhar and north-west of Beinn Toaig. as lochans onthe eastern flank of SgurrEilde Mor (NN On Stob Gabhar 1OOOm of theGrampian Group is

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exposed over a distance of 1.5 km E of the summit, along other rocks in the Grampian Group. Its impersistent nature Aonach Eagach and into Coire Toaig (Figs 1, 7). In Coire may partly resultfrom deformation, but may also reflect Toaig an upright F2 antiform folds the uninverted Grampian sedimentation within relatively small basins. Widespread Group succession (P. R. Thomas pers. comm.). The lowest sedimentationsubsequently produced the rhythmic as- rocks exposed consist dominantly of semipelite and schistose semblage forming the base of the Corrieyairack Subgroup. micaceous psammite, with rare psammitesand pinkish Where the Ord Ban Subgroup is present, it forms the lowest quartzites less than 1 mthick. Within the schistose recognizable subgroup of the Grampian Group on Speyside. micaceous psammiteripple cross lamination, trough cross On Speyside, the Comeyairack Subgroup is represented bedding, planar bedding and dewatering structures occur. by the basal rhythmic assemblage overlain conformably by a Towards the top of the succession on Aonach Eagach thick sequence of dark micaceous psammites (Markie (NN 240453), thick, fine-grained massive quartzites become Micaceous Psammite) lithologically identical to the Laggan an important part of the succession. They occur interbedded Member of theGlen Doe Formation, with which it is with schistose micaceous psammite. juxtaposed along the MarkieFault (Fig. 2).We therefore On Beinn Toaig theGrampian Group succession is consider that the Markie Micaceous Psammite, which is the exposed onthe crest of the F2 ToaigAntiform (P.R. highest Grampian Group formation exposed on Speyside, is Thomaspers. comm). It consists almost exclusively of an eastward continuation of the Glen Doe Formation. semipelite and schistose micaceous psammite associated with rare pinkish quartzites. At the top of the succession a banded psammite is succeeded by athick, persistent pink Atholl massive fine-grained quartzite. Most of this area has been mapped by Thomas (1980) who In bothareas the semipelite and micaceous psammite established a local stratigraphy comprising a lower association is overlain by a quartzite-bearing succession, but (Drumochter) and upper (Strathtummel) succession (Fig. west of BeinnToaig thelatter is much thinnerand 7). Recent A9(T) road cuttings have since produced some of comprises a single quartzite. A shallow marine environment the best evidence for sedimentological interpretation within is postulated for this sequence, which is similar to that seen theGrampian Group.The Drumochter Succession differs in Glen Spean andthe RiverLeven Inlier. We therefore from the overlying Strathtummel Succession in having a consider theserocks to form part of the Glen Spean generally higher mica content, suggesting thatit was Subgroup, although they may in part be equivalents to the formerly a muddier succession. Sedimentary structures are base of the Lochaber Subgroup. rarely seen in this sequence as bedding is attenuated at the most accessible outcrops. The base of the Strathtummel Succession is marked by a Speyside quartzitesequence (Tummel Quartzite andLaminated Grampian Group rocks havebeen mapped north-west of Psammite, Thomas 1980) (Fig. 7). Within parts of this Kincraig (NH 826064) intectonic contact with gneissose sequenceand in the overlying succession a variety of rocks assigned to the Central Highland Division (Piasecki sedimentary structures (Figs 8, 9) indicate tidal-dominated 1980). In this areathe rocksimmediately overlying the deltaic and interdeltaic sedimentation. Towards the top of Grampian Slide (Piasecki 1980) are in places unlike any the Strathtummel Succession attenuation increases and a observed in the Glen Spean-Monadhliath area. They consist banded zone up to 1km thick is developed in association of a thin association of impersistent quartzite, marble, pelite with the Iltay Boundary Slide (Thomas 1980). The increased andconcordant amphibolite. Similar rockshave been strain and loss of sedimentary structures renders assessment recorded at Ord Ban (NH 892086), north of Grantown on of depositionalenvironment difficult. The Atholl area Spey and near Kyllachy House in Strath Dearn (NH 783262 exposes the uppermost part of the Grampian Group, with a in Figs 1, 7) (seealso Piasecki 1975). Althoughthey are combined thickness of up to 5 km. In many respects the often found in an area of enhanced strain associated with Grampian Group inAtholl resembles theGlen Spean theGrampian Slide, the metasedimentary rocks are Subgroup of the type area (Fig. 7). Correlationmust, nowhereseen to exceed 50 m in thickness, while the however, be only tentativein view of the structural associated amphibolite, normally 2-30mthick, may reach complexity of the interveningBen Alderarea (Thomas 100m in thickness (Piasecki 1980). Abovethe Grampian 1979). Slide near Kincraig, an impersistent assemblage of marble, pelite and calc-silicate rocks is overlain by a coarse pelitic schist which is rapidly succeeded by thinly bedded, rhythmic Discussion semipeliteand dark micaceous psammite commonly The major stratigraphic divisions established are based upon exhibiting gradedbedding. The rhythmic assemblage is environment of deposition inferred from a combination of widespread on Speyside and commonly occurs adjacent to lithological association andother available evidence. the Grampian Slide. It bears astrong resemblance to the Correlationsalready suggested are lithostratigraphic. No ‘rhythmic’ semipelite anddark micaceous psammite precise contemporaneity is inferred. assemblage seen in the Coire nan Laogh Formation to the TheOrd Ban Subgroup comprises quartzites and west. limestones. This association suggests deposition within a The impersistentunderlying marble-quartzite as- shallow waterenvironment. The overlying rocks of the semblage is thought to be sufficiently distinctive for it to be Corrieyairack Subgroup were deposited in deeper water and recognized as a separate subgroup of the Grampian Group. are characterised by a thick sequence of turbidites.A On account of its thicker development and accessibility at change of environment to shallow marine shelf sedimenta- Ord Ban, we here term it the Ord Ban Subgroup (Fig. 7). tion marks the base of theGlen Spean Subgroup, which Rocks within thissubgroup differ lithologically from all recordsa progressive shallowing upwardssequence,

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I I A 0.5m

B 1 0.6m

C 1 0.5m

l I E 1.Om

t- l l Fig. 8. Small-scale trough cross bedding, with localized scouringin 0.3m the Tummel Psammitic Schistat Stalcair Cut on theA9 [NN Fig. 9. Types of cross bedding and lamination observedin the 6817171. Note that the variabilityin shade isa function of biotite Strathtummel Successionin Atholl, Perthshire. (A)Scour and fill content which partly reflectsthe originalmud content of the associated with ripple lamination. (B)Small-scale dune bedding sediment. with regular foreset drapes. (C)Ripple drift cross stratification with smaller starved ripples. (D) Trough cross bedding. (E)Cross bedding with reactivation surface. (F) Wave formed bedding. culminating in deltaicand interdeltaicdeposition. These broad characteristics seem to have been present throughout time-planes: on account of marked thickness variations they the basin andform the basis for correlation.Preliminary are more likely to be diachronous. work suggests thatthe thickness of subgroups is highly TheComeyairack Subgroup is thickest around Loch variable, even with allowance made for subsequent tectonic Laggan where a maximum thickness of 4500mhas been thickening orattenuation. Furthermore, there nois estimated. To the north in the Glen Doe-Loch Killin area evidence thatthe contactsbetween subgroups represent its thickness is apparently reduced to approximately 3000 m.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/146/1/85/4897596/gsjgs.146.1.0085.pdf by guest on 28 September 2021 T H E GRAMPIAN GROUP, SCOTLANDGROUP, GRAMPIAN THE 95

In other areas, however, only an incomplete stratigraphy of the British Geological Survey, R. Anderton, P. J. Haselock and P. the Corrieyairack Subgroup has been recorded and hence no R. Thomascontributed significantly towards thesuccessful estimates of its total thickness are available. completion of this work. Technical assistance at the Department of TheGlen SpeanSubgroup in theGlen Spean- Geology, Keele, has been greatly appreciated. Monadhliath area is relatively thin, but it is apparently much thicker both in the east (Atholl area) and west (the Lower River Spean section, Fig. 7). In areas where the Glen Spean References Subgroup is especially thick the base of theGrampian ANDERSON,J. G. C. 1956. The Moinian and Dalradian rocks between Glen Group is never seen,and hence it is not possible to Roy andthe Monadhliath Mountains, Inverness-shire. Transactions of determinewhether a thickening of theGlen Spean the Royal Society of Edinburgh, 63, 15-36. BAILEY,E. B. 1910. Recumbent folds in the schists of the Scottish Highlands. Subgroup is accompanied by a thinning of the Corrieyairack Quarterly Journal of the Geological Society of London, 66, 586-620. Subgroup.However, the available evidence suggests that -& MAUFE,H. B. 1960. The Geology of Ben Nevis and Glencoe and the the Monadhliath Semipelite north-west of the Corrieyairack surrounding country.Memoir of the Geological Survey of Scotland, Sheet Granodiorite is equivalent toboth the uppermosttwo 53. formations of the Corrieyairack Subgroup and the Clachaig BARROW,G. 1904. Moine gneisses of the East-Central Highlands and their position in the Highland Sequence. Quarterly Journal of the Geological Formationsouth-east of the Corrieyairack Granodiorite, Sociev of London, 60,400-44. and hence the boundarybetween the Corrieyairack and -, GRANT-WILSON,J. S. & CUNNINGHAM-CRAIG,E. H. 1905. The Geology Glen Spean Subgroups is diachronous (Fig. 7). of the country around Blair Atholl, Pitlochry and Abe$eldy. Memoir of The following conclusions may thus be drawn.The the Geological Survey of Scotland, Sheet 55. CABY,R. & BERTRAND-SARFATI,J. 1987. The Eleonore Bay Group (central thickness variations suggest differential subsidenceduring EastGreenland). In: WINCHESTER,J. A.(ed.) Later Proterozoic deposition as well as diachronouschanges of sedimentary stratigraphy of the Northern Atlantic Regions, Blackie, 212-36. environment. The boundaries of the Grampian Group basin GRAHAM,C. M. 1986. The role of the Cruachan Lineament during Dalradian are not clearly indicated:its extent was considerable. evolution. Scottish Journal of Geology, 22, 257-70. HARRIS,A. L., BALDWIN,C. T., BRADBURY, J., H. JOHNSON, H.D. & SMITH, Sedimentary facies indicate thatthe Grampian Group R. A. 1978. Ensialic basin sedimentation: the Dalradian Supergroup. In: representsnear-complete a basin fill sequence. The BOWES,D. R. & LEAKE,B. E. (eds). Crustal evolution in northwestern Corrieyairack Subgroup represents the phase of deepening, Britain and adjacent regions, GeologicalJournal, Special Issue 10, while theGlen SpeanSubgroup records basin shallowing 115-38. HASELOCK,P. J. 1982. The Geology of the Corrieyairack Pass Area, and ultimate infilling by deltaic and interdeltaic sediments. Inverness-shire. PhD Thesis, University of Keele. It therefore provides evidence of a separate pulse of - 1984. Thesystematic geochemical variation between two tectonically extension and basin fill which preceded the main Dalradian separatesuccessions in thesouthern Monadhliaths, Inverness-shire. Sconish Journal of Geology, 191-205. sedimentation in the Scottish Highlands. - U), Studies in other parts of the Caledonides suggest that & WINCHESTER,J. A. 1982. A note on the stratigraphic relationship of the Leven Schist and the Monadhliath Schist in the Central Highlands of clastic sequences similar to and broadly contemporaneous Scotland. Geological Journal, 16, 237-41. with theGrampian Group occur elsewhere in the -& WH~LES,K. H. 1982. The stratigraphy and structureof the southern Caledonian orogen. A thick clastic sequence (theErris MonadhliathMountains between Loch Killin and Glen Roy. Scottish Group) underlies theAppin Group in NW Ireland Journal of Geology, 18,275-90. HICKMAN,A. H. 1975. The stratigraphy of late Precambrian metasediments (Winchester et al. 1987). It resembles the Grampian Group betweenGlen Roy andLismore. Scottish Journal of Geology, 11, in both lithology and geochemistry. However, in the Ox 117-42. Mountains the Dalradian is in direct contact with gneissose -1978. Recumbent folds between Glen Roy and Lismore. Scottish Journal basement: theErris Group is absent. Geophysical data of Geology, 14, 191-212. JOHNSON, M. R.W. & FROST,R. T. C. 1977. Fault and lineament patterns in suggest thattheGrampian Group may notextend the southern Highlands of Scotland. Geologie en Mijnbouw, 56, 287-94. south-west of the Cruachan Line (Graham 1986). Thus links KUMPULAINEN,& R. NYSTUEN, J. P.1985. Late Proterozoic basin evolution between the Grampian Group and Erris Group basins are and sedimentation in the westernmost part of Baltoscandia. In: GEE, D. notestablished. In southern Scandinavia theHedmark G. & STURT,B. A. (eds) The Caledonide Orogen-Scandinavia and related areas, 213-32. Group comprises a lower turbiditic sequence succeeded by LAMBERT,R. STJ., WINCHESTER, J. A. & HOLLAND,J. G. 1982. A shallow marine and fluvial deposits. These deposits are geochemicalcomparison of theDalradian Leven Schistsand the considered to betime equivalents of theGrampian and Grampian DivisionMonadhliath Schists of Scotland. Journal of the Appin Groups in Scotland (Kumpulainen & Nystuen 1985) Geological Society, London, l39, 71-84. and it is possible that asneither the eastern limits of the NYSTUEN,J. P. & SIEDLECKA,A. 1987. The‘sparagmites’ of Norway. In: WINCHESTER,J. A. (ed.). Later Proterozoic stratigraphy of the Northern Grampian Group basin northe western margin of the Atlantic Lands. Blackie, 237-52. HedmarkGroup basin are visible both clastic sequences OKONKWO,C. T. 1985. The geology and geochernbtry of the metasedimentary could have beendeposited in the same basin (Winchester rocks of the Loch Laggan-Upper Strathspey area, Inverness-shire. PhD 1987). Similar intracratonic basins developedpenecontem- Thesis, University of Keele. PIASECKI,M. A. J. 1975. Tectonicand metamorphic history of theupper poraneously in both northern Scandinavia (Nystuen & Findhorn,Inverness-shire, Scotland. Scottish Journal of Geology, 11, Siedlecka 1987) and in East Greenland where the Eleonore 87-115. Bay Group was laid down (Caby & Bertrand-Sarfati 1987). - 1980.New lighton the Moine rocks of theCentral Highlands of We therefore conclude thatthe Grampian Group was Scotland. Journal of the Geological Society, London, l37, 41-59. -& VANBREEMEN, 0. 1979. A Morarian age for the “Younger Moines” deposited in one of severalintracratonic basins which of Central and Western Scotland. Nature, 278, 734-6. developed across a wide area of the Proterozoic _-, & ~RIGHT, A. E. 1981. Late Precambrian Geology of Scotland, Supercontinentbetween the main Laurentianand Baltic England and Wales. In: KERR,J. W. & FERGUSSON,A. J. (eds) Geology cratonic blocks during the late Riphaean. of the North Atlantic Borderlands, Memoir of the Canadian Society of Petroleum Geologists, 7, 59-94. ROBERTS,J. L. 1976. Thestructure of theDalradian rocks in the N B. W. Gloverwas financed by a NERC Studentship,which is Ballachulishdistrict of Scotland. Journal of the Geological Society, gratefullyacknowledged. Many useful discussions with officers of London, l32, 139-54.

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-& TREAGUS,J. E. 1977. Polyphase generation of nappe structures in the rocks of theKinlochleven area, Scotland. Journal of the Geological Dalradian rocks of the southwest Highlands of Scotland. Scortish Journal Society, London, WO, 525-44. of Geology, 13, 237-254. WHIITLES,K. H. 1981. The geology and geochemishy of the area west of Loch THOMAS,P. R. 1979.New evidence for a CentralHighland root zone. In: Killin, Inverness-shire. PhD Thesis, University of Keele. HARRIS,A. L., HOLLAND,C. H. & LEAKE,B. E. (eds.) The Caledonides WINCHESTER,J. A. 1987. LaterProterozoic environments and tectonic of the British Isles-reviewed, GeologicalSociety, London, Special evolutionin the northern Atlantic lands. In: WINCHESTER,J. A. (ed.) Publication, 8, 205-12. Later Proterozoic stratigraphy of the Northern Atlantic regions. Blackie, -, 1980. Thestratigraphy and structure of theMoine rocks N of the 253-70. SchiehallionComplex, Scotland. Journal of the GeologicalSociety, -, MAX, M. D. & LONG,C. B. 1987. The Erns Group,Ireland. In: London, 137,469-82. WINCHESTER,J. A. (ed.). Later Proterozoic stratigraphy of the Northern TREAGUS,J. E. 1974. A structural cross-section of the Moine and Dalradian Atlantic regions, Blackie, 162-76.

Received 16 February 1988; revised typescript accepted 18 July 1988.

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