Abstract

The Mesoproterozoic–Neoproterozoic Torridonian Sandstone of NW Scotland has renewed sedimentological significance following recent advances in the understanding of pre-vegetation alluvium, as it is one of the most extensive and easily accessible successions of such strata worldwide. This paper presents the first modern sedimentological analysis of the unit's constituent Meall Dearg Formation (late Mesoproterozoic), recognizing a dominance of alluvial facies, with subordinate aeolian facies. Alluvial strata within the Meall Dearg Formation contain direct evidence for event deposition by high-energy floods, including the following: (1) widespread upper and transitional upper flow regime elements; (2) frequent stacking of successively lower flow regime elements; (3) common subcritical subaqueous dune fields with superimposed ripple marks; (4) occasional thin, desiccated mudstones; (5) evidence that microbial mats colonized substrates during intervals of sedimentary stasis. Together these strands of primary sedimentary geological evidence indicate that the alluvial deposition of the Meall Dearg Formation was typified by supercritical flows during high-energy floods, punctuated by prolonged intervals of sedimentary stasis. The preservation potential of all of the features was boosted by highly aggradational sedimentary conditions. These primary observations have implications for the ‘norm’ in pre-vegetation alluvium and confirm that, despite increasing recognition of diversity within Precambrian fluvial systems, classical pre-vegetation motifs of high-energy alluvial flood deposits preserved as ‘sheet-braided’ alluvium are still an archetypal sedimentary signature in some instances. Using supportive evidence from the Meall Dearg Formation, we recommend that the term ‘sheet-braided’ be used in inverted commas in future studies; this emphasizes the polygenetic depositional nature of the 20:1 width:thickness fluvial style, while maintaining the value of the term in the isolation of a key characteristic of pre-vegetation sedimentary architecture.

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