Moine Succession, stratigraphy, Northern Highlands of Scotland
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Elucidation of the stratigraphy of the Moine over most of its outcrop is a matter of considerable difficulty, due to lack of good marker bands or distinctive sedimentary criteria, tectonic thinning, and change in aspect of the rock as the result of various degrees of metamorphism and migmatisation. Nevertheless a grouping of strata into the Morar Division, the Glenfinnan Division and Loch Eil Division (proposed by Johnstone and others, 1969) has proved useful (See the table and P915472). The divisions are based on regional associations of rocks of similar lithological type and structural aspect. The extent to which they represent a stratigraphical sequence is not proven and is the subject of current research. The tripartite grouping based on the West Highlands holds good over much of the Northern Highlands. North-west of a line joining Strath Carron and the Dornoch Firth, however, the incoming of numerous slices or infolds of Lewisian rocks, and the presence of migmatite and granitic complexes which obscure the original nature of the strata, renders the recognition of the Glenfinnan and Loch Eil divisions problematical.
A succession was worked out in the type area of Morar by Richey and Kennedy (1939) for the rocks which now form this division, and this forms the basis (with modifications) of the present stratigraphy of the division throughout the whole area. The three Formations are the Lower Morar Psammite, the Morar Schist (itself divided into three in the type area) and the Upper Morar Psammite. The lithological characteristics of these formations (and of those making up the other divisions in the south-west part of the Northern Highlands) are given in the table. Alternative names used by previous workers are given in P915472.
In addition to these three formations, a Basal Pelite is usually found below the Lower Morar Psammite. The true status of this ‘formation’ is not yet certain. It is probably a stratigraphical unit but its variable composition suggests that it may be in part a tectonic mélange of basal Moine strata and slices of underlying Lewisian basement. It is commonly characterised by the presence of numerous thin deformed quartz veins.
The succession can be followed on either side of the Morar Anticline (P915473). The centre of this structure is a tectonic window into a more complex structural level within which the Lower Morar Psammite can be seen to overlie infolded or thrust Lewisian strata (Kennedy, 1955; Poole, 1966). The Lower Morar Psammite between Attadale and the Coire nan Gall Lewisian inlier (p.28) comprises a pale (lower) and dark (upper) division made out with difficulty in a zone of multiple sliding. These divisions probably respectively represent the Arnisdale psammite and Rubha Ruadh semipelite of the Loch Hourn area (P915472), but north of Kintail the sequence of the Morar Division becomes less clear. This is in part because of a probable facies change of the upper part of the Lower Morar Psammite to a more mixed psammitic-semipelitic assemblage and, in part, due to the cut-out of the higher groups of the Division by a major slide or slide zone — the Sgurr Beag Slide — a feature which is taken as the boundary between the Glenfinnan and Morar divisions (See the table).
Between Kintail and Strath Oykell, however, the current state of knowledge suggests that the Moine rocks attributed to the Morar Division are essentially Lower Morar Psammite, with attenuated representatives of the higher groups around the north side of the Fannich inlier of Lewisian. It is unlikely, however, that the Morar strata in this area form a simple succession, because projecting into them from the north there is a complex series of thrust Lewisian slices traceable from the north coast down to, and past, the south end of Loch Shin. Langford (1980) considers that, east of Attadale, the apparently uniform sedimentary sequence is strongly folded, with laminar or platy zones representing what may be the continuation of the slides or sheared infolds, which are responsible for the interbanding of Lewisian rocks in the Kintail area. If this is so, then the ‘zone of Lewisian slices’ (see below, and P915471) of the north could be structurally equivalent to the tectonic interbanding in Kintail.
North of Loch Shin no subdivision of the Moine rocks is possible with the present state of knowledge. West of a line from the Kyle of Tongue to Lairg, the rocks are mainly psammites, which in places are pebbly and feldspathic; they resemble much of the Lower Morar Psammite to the south, but Soper and Barber (1982) consider that some could be ‘young’ Moines, equivalent in age to the Grampian Group of the Dalradian. The conglomerate of Strathan has already been referred to. East of the Tongue- Lairg line, however, the country appears to be formed of interleaved slices of Moine and Lewisian basement (Mendum, 1979) and includes rock types unknown in the south. Of these, the gneisses of Strathy appear to be of higher metamorphic grade than any recorded elsewhere in the Moines (Harrison and Moorhouse, 1976) but are unlike Lewisian gneiss.
In contrast to the massive units which make up the Formations of the Morar Division, the Glenfinnan Division typically comprises an assemblage of pelitic schists and striped and banded rocks. Thick psammites are found from place to place but do not form identifiable ‘Formations’.
Indeed no clear stratigraphic units can be identified in the Glenfinnan Division, although in the Glenfinnan–Loch Ailort area itself two distinct rock formations are the Lochailort Pelite (Powell, 1964), composed dominantly of mica schist and the Glenfinnan Schist, the typical striped and banded rocks of the Division as a whole (for other names used in other areas see P915472). It is a matter of great difficulty to decide whether the broader pelite bands of the Division are facies variants of the Glenfinnan Schist, or whether they are infolds of, for example, the Lochailort Pelite.
Between Glen Moriston and Glen Affric the Division contains major psammitic units, while around the Glendessary Complex at the head of Loch Arkaig other thick siliceous psammites are present (and marbles are found within the complex itself). The stratigraphical status of these psammites depends on the structural interpretation of the area, as this is not clear everywhere. The allocation of the psammites is similarly uncertain; they could even be attributed to the Loch Eil Psammite of the adjacent division (see below). It is this kind of uncertainty which has given rise to the suspicion that the ‘Divisions’ of the Moines may be as much structural zones as a stratigraphical sequence (Johnstone and others, 1969; Roberts and Harris, 1983).
The Glenfinnan Division is bounded in the west by the Sgurr Beag Slide (P915471). South of Loch Hourn the slide separates pelitic or striped rocks typical of the Glenfinnan association from Morar Division rocks (Tanner and others, 1970) but, in central Ross-shire, broad outcrops of Lewisian strata are interfolded or interleaved with psammitic Moine rocks, and lie between the Divisions in places. These Lewisian–Moine complexes are taken to lie either above the Sgurr Beag Slide or within a zone of slides which separates the Divisions. Whatever the explanation, it seems that the pelites and striped rocks of the Glenfinnan association lie either in direct stratigraphic contact with Lewisian basement or are only separated from it by a relatively minor basal psammitic group. Where there are no large outcrops of Lewisian along the Sgurr Beag Slide zone this psammite persists as a typically gneissic rock whose gneissosity may be the result of the metamorphic effects associated with sliding. It is locally referred to as the Reidh Psammite, although its continuity along the slide as a single formation is questionable. (Similar gneissic psammites accompany the thrust slices of Lewisian rock within the Morar Division of Kintail.)
The rocks of the Glenfinnan Division are commonly highly deformed (P219846). In the striped lithology this deformation has, in places, resulted in the breakup of the psammitic bands in a mobile pelitic matrix. Curvilinear fold axes (which show no evidence of having been formed by repeated folding) are common, and highly elongate basin-and-dome structures are characteristic (p.78). Much of the Division lies within the area of regional migmatite complexes in which lit-par-lit banded gneisses are developed (p.88). Calc-silicate lenses are common and the rocks of the Division locally have numerous pods of coarse garnetiferous hornblende schist. Their occurrence helps to distinguish the Glenfinnan pelites from those of the Morar Division where, in the type area, such features are not seen. Winchester (1976) however, reports that hornblende schists do occur with increasing frequency in rocks of the Morar Division northwards towards Sutherland; he notes that these are geochemically different from the ones found in the Glenfinnan Division of the area around the Fannich Forest.
Northwards from Glen Affric the Glenfinnan Division is, as already described, separated from Morar-type rocks by large and small inliers of Lewisian gneiss, including the major inliers of Monar, Scardroy and Fannich (see p.28). It contains fairly thick psammite units, in places strongly cross-bedded which, for want of contrary evidence, are taken to be variants of the Glenfinnan stratigraphical succession. Glenfinnan Division rocks with marginal Lewisian slivers along a possible extension of the Sgurr Beag Slide have been mapped just north of the Dornoch Firth. Further north their identity is lost in the migmatite complexes of Strath Halladale and Loch Coire (p.90).
Loch Eil Division
Although local subdivision is possible (e.g. Strachan, 1982), the rocks of this division are usually referred to a single formation, the Loch Eil Psammite. This is a quartzofeldspathic granulite which can vary with increased biotite content to a ‘pepper-and-salt’ type and also, by diminished feldspar, to a quartzite. It is notably quartzitic along its junction with the Glenfinnan rocks south of Loch Eil. Some zones are characterised by thin pelitic bands.
The Loch Eil Psammite strongly resembles parts of the Upper Morar Psammite, both in its general aspect and in its content of calc-silicate lenses. Cross- bedding is present from place to place, but is much less common than in the Morar rocks; its relative scarcity could be attributed to obliteration due to the higher degree of deformation suffered by the Loch Eil rocks, which are commonly flaggy.
It has been pointed out that the grouping of the Moines into divisions is not entirely on a stratigraphical basis; however the Loch Eil Psammite, on the evidence of well preserved cross-bedding, certainly appears to be younger than the adjacent Glenfinnan Division pelites. Roberts and Harris (1983) state that there is a stratigraphical passage between the two at Loch Quoich and, in the Glen Cannich area, BGS surveyors hold that there is no sharp separation of the rocks of the two divisions. In Ardgour, Stoker (1983) also suggests a stratigraphical passage between rocks of Glenfinnan and ‘Loch Eil’ types through an intermediate passage zone with mappable lithological subdivisions. Near Achnacarry, at the east end of Loch Arkaig, an area of striped schists has been tentatively assigned to an upfold of Glenfinnan rocks.
Rocks of uncertain affinity
The marbles of Blairnahenachrie, Rebeg, Glendessary and Ardgour have already been referred to (p.66); in the Ardgour area, they are associated with black schists and other pelites of Dalradian aspect. A similar assemblage uncharacteristic of the Moine rocks is found in the Glen Urquhart area, near Drumnadrochit (Rock, 1985). Other limestones are found at Kirkton (5 km W of Inverness) and South Clunes (12 km WSW of Inverness). The possibility that these rocks (with the quartzite of Scaraben) are of Dalradian or other non- Moine affinity is under current examination (Rock, in press).
The Tarskavaig Moines
Although by definition the Moine rocks are found ‘east or south-east of the Caledonian Front Thrust belt’ (Johnstone, 1975), within the thrust belt itself, between the Kishorn Thrust and the Moine Thrust in the Sleat peninsula of Skye, a minor thrust-bounded nappe — the Tarskavaig Nappe — contains strata intermediate in character between the little-altered Torridonian rocks of the Kishorn Nappe and the overlying Moines above the Moine Thrust (Clough, 1910; Bailey, 1955; Cheeney and Matthews, 1965). Various correlations have been made between these Tarskavaig Moines and the Torridonian and Moine rocks, but bed-for-bed matching in nappes of uncertain displacement must always be speculative.
These rocks reach the garnet grade of metamorphism and comprise three ‘groups’ (Cheeney and Matthews, 1965), of which the upper and lower are mainly psammites and the middle group mixed psammitic, semipelitic and pelitic rocks. Pebbly beds are common and the rocks show varying degrees of granulation and deformation. Although less deformed that the immediately adjacent Moines above the Moine Thrust, they seem to have much in common with the pebbly psammites of Mull, west Ardnamurchan and west Morar. Cheeney and Matthews note, however, that they may never have been affected by the ‘very early’ folds of the Mainland. The possibility that they represent ‘Young Moines’ equivalent to the Grampian Group (Dalradian) of Harris and others (1978) must thus be considered.