Abstract
Synopsis Two cycles of sedimentation are present in these Precambrian strata. First cycle sediments were lithified and tilted 25–30 degrees to the west before those of the second cycle were deposited. The first cycle begins with a thin mantle of massive, unsorted gneiss breccia (I) deposited on a hilly, unweathered surface of Lewisian gneiss. Then follow tabular-bedded breccias and red sandstones (IIa) grading upwards and laterally into red mudstone (IIc). A brown mudflow about 32 m thick, containing accretionary lapilli, is intercalated (IIb). These sediments probably margined a sea or lake. Fades IIc is erosively overlain by large-scale planar cross-bedded, medium to coarse-grained red sandstones (III), perhaps of fluvial origin. Sediments of the second cycle rest on an irregular unconformity cutting both gneiss and first cycle sediments. Tabular-bedded, (locally massive) breccias, containing gneiss, sandstone and breccia clasts (IVa), pass upwards and laterally into red and grey flaggy sandstones and siltstones (IVb). Facies IV originated like facies II. It is erosively overlain by the fluvial trough cross-bedded pebbly red arkoses (V) of the Applecross Formation. Facies I–IV probably belong to the Geological Survey’s Diabaig Formation. It follows that the ‘Diabaig’ (and the Torridonian) have not the stratigraphic unity implied by the existing nomenclature.
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