The Stratigraphy and Structure of the Moine Rocks N of the Schiehallion Complex, Scotland
Total Page：16
File Type：pdf, Size：1020Kb
J. geol. SOC. London, Vol. 137, 1980, pp. 469-482, 8 figs., 1 table. Printed in Northern Ireland. The stratigraphy and structure of the Moine rocks N of the Schiehallion Complex, Scotland P. R. Thomas SUMMARY: Sedimentary structures are used to establish a ‘Younger’ Moine stratigraphical succession in Strathtummel. Polyphase re-folding of the Moine sequence between Schiehallion and Drumochter can be clearly subdivided into primary and secondary deformations. Primary NE-SW trending deformational events D, and D, include the formation of the south-eastward facing D, Atholl nappe re-folded by strongly overturned D, folds which become upright on the crest of theDrumochter dome. Secondary deformation resulted in D, (oblique)and D, (transverse) major folds, both of which plunge into the SE quadrant. Since much of the published work on the tectonics of ferred to as the Struan or EildeFlags by most authors. theCentral Highlands has involved Dalradian sequ- It is composed largely of siliceous schists whose gen- ences, very little is known about the underlying Moine eral flaggy character varies with the proportion of the succession and structure. Considering the monotonyof constituentminerals and their grain size. The main the psammitic lithologies, the paucity of clean expos- minerals are quartz, plagioclase feldspars, biotite and ureon moorlands and the remoteness of manykey some muscovite, with very few porphyroblasts, other localities, this is not surprising. Except for the lengthy than microcline. paper by Barrow (1904), much of the published work Granularpsammitic schist is themost common on the main tract of Moine in the W Central High- lithologyin theGlen Garry-Strathtummel area, landsconsists of shortnotes on tunnel sections thoughwhite quartzites, pelitic and semi-pelitic (Robertson1939; Anderson 1951; Johnstone & schists, heavy mineral and calc-silicate bands are also Wright 1955) and isolated quarry exposures (McIntyre present. The term ‘schist’ has been adopted here since 1950). there are few of the discrete stromatic segregations of N of the Drumochter Pass the Moine and Lower quartzo-feldspathic material that may be found in the Dalradian rocks of the Monadhliath have been studied ‘gneisses’ of the ‘Central Highland Division’ (Piasecki byAnderson (1947, 1956), Whitten (1959), Smith & van Breemen 1976b). Unfortunately, most of the (1968) and Piasecki (1975), while to the S, the Dalra- useful non-psammitic bands are too thin to be traced dian of the Schiehallion Complex has been mapped in betweenriver sections. However, the obvious litho- detail by E. M. Anderson (1923), Bailey & McCallien stratigraphicaldisadvantages are partly alleviated by (1937) and Rast (1958). Treagus & King (1978) re- the abundance of sedimentarystructures and by the vised some of the stratigraphical detail in the Dalra- fact that these monotonous Moine lithologies react to dian around Strathfionan, while the continuing work stress more uniformly than the Dalradian succession, of Piasecki and van Breemen (1979 a, b) in the NE in which mineralogy and competence vary markedly. CentralHighlands is bringing to lightevidence of a Althoughmetamorphic recrystallization up to lower mucholder Moine stratigraphy at lower structural amphibolite facies has obliterated most of the original levels. Other publications relevant to the present studysedimentary grain boundaries, it is consideredthat are shown on Fig. 1. much of the lithological variation represents true bed- Thispaper is a summary of thework carried out ding. during 3 years of mappingand research from the Distinct ‘zones’ of banded schist, up to 4 km wide, Dalradian boundary NW to Loch Garry, an area of occur mainly in the proximity of the Boundary Slide over350 km2, together with observations in the (Fig.4) andDalnacardoch (NN 722 703) where the ground between Loch Garry and Loch Ericht. Subse- Moine contains thin regular platy units of psammitic quentwork by the author covering the important andpelitic material (each 20-300 mm inthickness) structures found more recently between Loch Ericht which may be equivalent to rocksof the ‘Moine Phase’ andLoch Treig is thesubject of a separatepaper referred to by King & Rast (1955). Within the ‘banded (Thomas 1979), which attempts to constructa continu- zones’ lithological changes lie sub-parallel to the schis- ous section across the W Central Highlands without tosity, sedimentary structures are usually absent, and obliteration by plutonic masses. minor isoclinalfolds of highamplitude and low wavelength are a commonfeature. Away from the Moine lithologies ‘bandedzones’, semi-pelitic and psammitic Moine is often laminated with fine ribs of biotite. The Central Highland Moine succession has been re- There are few pelitic schist units greater than 30 m 0016-7649/80/0700-0469$02.00 @ 1980 The Geological Society Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/4/469/4886679/gsjgs.137.4.0469.pdf by guest on 23 September 2021 470 P. R. Thomas FIG. 1. Location map indicating post-war research in the W Central Highlands. in thickness,but several thin heavy mineral bands, Salmon Leap at Struan Gorge (NN804 656) and on similar to those in the Western Highlands (Ramsay & Craig nan Caisean (NN 776 607) (Fig. 2). Spring 1962; Powell 1964) have been found at a few These sedimentary structures are valuable in Moine localities in the Rivers Garry and Tummel. Magnetite, geology since they provide evidenceof younging direc- sphene and biotite are contained within the bands but tionsand the facing of folds. The preservation of no true hornblende schists have been mapped. Calc- cross-cuttingsedimentary dykes at highangles to silicate bands and lenses are recognized most readily slumped semi-pelitic beds indicates that at least some in the clean water-worn surfaces of the River Garry of the bedding in the Moine does not appear to have and in thenew cuttings for the A9 road. They are suffered from major tectonic dislocation parallel to the fairly widespread, but are less commonly encountered original sedimentary layering. Since many sedimentary in the upper parts of the succession. structuresare modified by internaldeformation and recrystallization, detailed sedimentological work is li- mited and interpretations of sedimentary environmen- Sedimentary shctores tal conditions necessarily vague. However, the general Sedimentarystructures are mainly preserved in absence of coarse-grained clasts, the uniform nature of laminated psammitic and semi-pelitic lithologies where the mineralogy and the abundance of dunes and rip- relativelyundeformed units occur outside banded ples are all worthy of note. Together with the presence zones. Trough cross-bedding forms the most reliable of calc-silicates, current scours and the soft sediment way-up indicator, as tabular truncations can be con- deformationstructures, they may indicatea shallow fused with sheared-outminor isoclinal folds. water marine or deltaic environment with a constant Moreover,what appear to be non-curved foreset source of fine- to medium-grained material which has laminaemay, in fact,be the result of metamorphic been influenced by variable conditions of current flow. differentiation parallel to cross-cutting schistosities (cf. In contrast,graded bedding has been reported by Bowes & Jones 1958). Dune bedding up to 0.4m in Piasecki (1980) from the lower part of the Grampian height is commonly observed in association with smal- Division in Strathspey. Although graded beds have not ler ripples, some of which display ‘flaser-like’ trough been observed in the uppermost 4 km of the Moine, laminations.More rarely the cross-bedded units are thinly-beddedunits withpossible traces of grading accompanied by scour and fill structures, convolutions, havebeen recognized in theunderlying rocks of slumpsand even sedimentary dykes (e.g. nearthe Drumochter and Ben Alder. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/137/4/469/4886679/gsjgs.137.4.0469.pdf by guest on 23 September 2021 Moine rocks N of Schiehallion Complex, Scotland 47 1 0.1 m l 1 FIG. 2. Sedimentary structures in the Strathtummel Succession: a, Trough cross-bedding; Craig nan Caisean. b, Dune bedding truncating trough cross laminations; Ches. c, Convolute bedding in laminated psammite; Craig nan Caisean. d, Sedimentary dykes cutting psammite between two slumped semi-pelitic beds; cross bedding and scour and fill structures below; River Garry, Struan. Stratigraphy Successions within the Grampian Division Therehave been few attempts to subdivide the Theupper part of theGrampian Division, im- Struan or Eilde Flags stratigraphically. However, the mediatelyunderlying theDalradian of Strathfionan rocks in Strathtummelhave been found to young (Treagus & King 1978) can be subdivided on the basis consistently through nearly 4000 m into the Lochaber of quartzitic units of sufficiently consistent thickness to Subgroup of the Dalradian in Strathfionan (Treagus & be mapped over many km of strike. It is proposed to King 1978), and may be subdivided on a lithostratig- call this the ‘Strathtummel Succession’ as it has been raphical basis into a number of formations (Table 1). established on the uninverted SE limb of the Meal1 It has been suggested that these ‘younger’ Moines Reamhar foldin the hinge of thelater Bohespic shouldbe included ina ‘GrampianGroup’ of the Antiform. Here, the succession reaches its maximum Dalradian(Harris et al. 1978)since they appear to thickness of over 3000 m and contains cross-bedding havebeen affected by asimilar polyphase