Harrow and Hillingdon Geological Society

Previous Meetings

An introduction to the geology of Assynt

Bernard J. Skillerne de Bristowe

	View from Badcall towards Quinag, one of the major Torridonian sandstone mountains resting unconformably on Lewisian Gneiss. Courtesy Chris Fone of the RGS
	Two generations of pegmatite dykes cutting the Lewisian Gneiss. The relative ages of the rocks can bee seen from what cuts what. Courtesy Chris Fone of the RGS
	A recumbent, isoclinal, intrafolial, Scourian fold within the Lewisian Gneiss. Courtesy Chris Fone of the RGS

This presentation told the story of a field trip by the Reading Geological Society, which provided an introduction to field mapping in the ancient geological area of Assynt in the north-west of Scotland, about 15 miles from Cape Wrath. The geology comprises Moine Schists to the east with Cambrian sediments, Torridonian Sandstones, Lewisian gneisses and the Moine Thrust zone. Particular attention was paid to the Torridonian Sandstone of the Stour Peninsula, the Lewisian gneiss at Badcall Bay and the Moine Thrust at Knockan Crag.

Torridonian

The Torridonian sandstones give rise to spectacular mountain scenery above the cnoc and lochan scenery (low, rugged hills separated by lochs) of the Lewisian gneiss. They comprise the Stoer Group (which may have suffered burial metamorphism), and the Torridon Group, with an unconformity between, which represents a 200million year gap.

The importance of observation was stressed with the example of a coastal exposure where the only unconformity seen was actually with the beach sand at the base of the exposure. The first important stage is to make graphic logs by looking at each bed in turn and recording information about it, such as the vertical thickness, the form of the base of the bed (whether it is flat or cross-cutting, representing a channel fill), grain size, minerals, clasts etc. Sedimentary structures need to be recorded such as trough cross-bedding, ripple marks, desiccation cracks and algal mats. At 1200 million years, algal mats in the Stour Group were formed by cyanobacteria with eukaryotic cells and are the oldest fossils to be found in the UK.

Having recorded the beds, it is possible to divide the sequence into separate units, which are mappable and to work out the sedimentary environment. Some of the Stour Group are ephemeral lake deposits in what was a desert (in terms of absence of vegetation, which did not arise till millions of years later) but with high rainfall. Trough cross-bedded units were produced by braided streams similar to the lower courses of the Brahmaputra River in Bangladesh.

The Stac Fada Member is a very massive rock with no structures, which is full of large angular green clasts in a red matrix plus veins of feldspar, which looks almost as if it had been pushed between the other sandstones. The clasts are volcanic glasses and there are spherulites now completely replaced by chlorite. It was described for many years as a lahar deposit (volcanic mudflow), which are usually associated with calc-alkaline igneous rocks. However, it has about a 50km long outcrop, almost in a straight line with a constant thickness, and there is no evidence of a channel which could have enabled a lahar to flow that far. 4 or 5 years ago two groups from Oxford and Aberdeen Universities found that it contained shocked quartz, which had first been recognised from the Nevada Desert after nuclear tests. It was also anomalously high in Chromium, nickel and platinum group minerals, particularly iridium. The conclusion was that it was caused by meteorite impact and represents the blanket of shocked ejecta thrown out by the impact (suevite). It is the only known impact site in the UK, though there is a possible one in the North Sea at Silverpit and possibly the largest one in Europe. Although they have not yet been recorded, nano-scale diamonds caused by the high pressures involved in impact may possibly occur.

Preparation for mapping – Traditionally, geological mapping is carried out at 1:10,000 scale but this is very expensive and modern technology such as GPS (global positioning system) can be used independently of the base map. Google Earth is a fantastic resource since it enables a preliminary interpretation of the geology and the planning of a strategy for mapping. The importance of graded bedding as an indicator of the right way-up of beds was stressed since it shows that there is a sequence of graded beds and that this sequence is not due to folding. It was suggested that different colours should be used to distinguish structural sequence and geomorphological features, that overfolds should be differentiated by the younging direction. Mapping of the faults shows that the Stour Group sediments were deposited in a mini-graben formed by faulting, similar to the sedimentary basin formed by faulting in which the younger Cambrian sediments were deposited.

The Moine Thrust

This comprises one nappe fold thrust over another, with windows, where the upper nappe has been eroded to expose the lower nappe, being the equivalent of inliers and klippes, where remnants of the upper nappe remain isolated from its main body, being the equivalent of outliers.

At Skiag Bridge, Loch Assynt, there is a whole series of nappes, one on top of another. The Loch Ailsh pluton sits in one of the nappe sheets, having been detached from its roots by the thrusting. A new kind of rock has also developed, with a new fabric due to dynamic metamorphism, known as a mylonite. The Torridonian Sandstone, in contrast has suffered burial metamorphism of very low grade. Wavy lines of micas form the foliation, quartz has broken down to very fine-grained material but the feldspars have not recrystallised because the temperature was not high enough. If the rock had suffered very high temperature, it would have melted to form pseudo-tachylite. Detailed examination of thin sections to determine these features forms the new discipline of micro-tectonics, which has developed as a cross between structural geology and petrology.

Lewisian

The Lewisian gneiss has no bedding so it is not possible to work out a nice sequence as in the Torridonian. However, folds are clearly more recent than the rocks that are folded, as are dykes and faults that cut them and the metamorphism, so it is possible to work out the sequence of events.

At Badcall Bay, the Sourian was mapped by Professor Janet Watson for her PhD thesis as a sequence of early metamorphic rocks cut by the Scourie Dykes, which are in turn cut by pegmatites and all were metamorphosed in the later Laxfordian event. The Scourie Dykes are 40-50m across and there are two generations of pegmatites, the older biotite pegmatite is cut by the Scourie Dykes and the younger muscovite pegmatite is cut by the dykes. The pegmatites contain lots of quartz and feldspar (but no potassium feldspar).

The felsic or grey gneiss contains quartz with thin dark layers of feldspar and pyroxene and the feldspars have deformation twinning rather than crystallisation twinning. The rocks are folded into very tight isoclinal folds that are lying flat and are refolded folds. The mafic gneiss contains pyroxene and garnets with a corona structure of feldspar produce from reaction with pyroxene in granulite facies metamorphism. There are ultramafic pods containing olivine and pyroxene, which have a much higher melting point than quartz and feldspar so this rock stayed solid and fractured into boudinage when the felsic material reached a temperature high enough for them to flow.

The Scourie dyke has a sharp contact which is near vertical. It is a dolerite/gabbro with lineation of the plagioclase and mafic minerals, which at the bottom of the exposure is virtually a schist. Anhedral crystals of pyroxene have converted to hornblende. Folds have developed within the dyke which have formed an axial plane cleavage due to deformation by a Laxfordian shear zone. (Shear zones are faults formed at high temperature with plastic flow producing folds instead of brittle fractures of faults.)

The chronology of this sequence can thus be summarised as:

 

• Intrusion of tonalite magmas 3,000 million years
• Prograde metamorphism to form gneiss 2,700 million years
• Isoclinal folding
• Intrusion of biotite pegmatite
• Intrusion of Scourie dolerite dykes 2,000 million years
• Intrusion of muscovite pegmatite
• Laxfordian retrogressive metamorphism and shearing 1,750 million years

Brittle fracture and faulting

 

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