Planview style and palaeodrainage of Torridonian channel belts: Applecross Formation, Stoer Peninsula, Scotland

Abstract

The sedimentary products of forty-three Neoproterozoic fluvial-channel belts are superbly exposed along the seaboard of Stoer Peninsula, Scotland, UK. The deposits belong to the Applecross Formation, part of the Torridonian succession, and their planview style and palaeodrainage are reconstructed at three sites. In-channel deposits record both active and abandonment stages, while soft-sediment deformation calls for local failure of cohesion-less deposits and/or effects of seismic events. Channel belts comprise compound-bar sheets and large foreset bars, as well as channel bodies with rarely preserved levees. In most cases bars show downstream accretion, though lateral and upstream growth is also evident along reactivated bar flanks and heads, respectively. While these features are distinctive of active braided-channel belts, scattered trunk-channel bodies with preserved levees point to local embankment of deep streams with somewhat limited lateral mobility.Profusion of thick cross-bedding calls for perennial discharge in channels with depth in the order of 13m, suggesting a mature, large-scale fluvial drainage capable of transporting sediment for hundreds to thousands of km. While entire channel belts yield width:thickness ratios exceeding 320, single channel bodies with ratios of 20 to 80 better reconcile with a channelled rather than sheet braided fluvial model, a feature in contrast with classic models for pre-vegetation rivers. Though some degree of uncertainty exists when applying Phanerozoic-derived models to Torridonian rivers, the latter could have been controlled by cycles of discharge to sediment supply that alternated aggradation and degradation, with patterns of bar buildup and reworking comparable to those observed in modern braided rivers.