Pre-vegetation fluvial sheet sands explained: Bedform and bar architecture evidence for 1.2 Ga rapidly migrating, meandering, and high-sinuosity wandering rivers

The Silurian radiation of land plants fundamentally altered fluvial stratigraphy and is often associated with a shift in river planform. Recent work challenges the long-held view that pre-vegetation rivers were predominantly braided; however, geologic evidence reconciling the potential for pre-vegetation meandering rivers with the laterally amalgamated, sheet-like sandstones characteristic of pre-Silurian fluvial strata remains limited. Here, we test the hypothesis that pre-Silurian strata instead record evidence for mobile, meandering, and high-sinuosity wandering rivers characterized by rapid floodplain reworking through detailed outcrop scale analysis of the 1.2 Ga Clachtoll Formation (Stoer Group, NW Scotland), one of the best-preserved Mesoproterozoic fluvial systems. Our analysis reveals that bars predominantly accrete orthogonally to dune paleocurrent directions (i.e., lateral accretion), circular variance in dune paleocurrent data is consistent with modern meandering and high-sinuosity wandering river patterns, and channel bodies are isolated within, and in sharp contact with, muddy floodplains. Critically, we find that 87% of fluvial bars are poorly preserved, with no evidence for fully preserved bars, suggesting rapid river mobility. These findings support the interpretation that laterally extensive, poorly preserved sandstones in pre-vegetation strata may represent deposits of mobile meandering and high-sinuosity wandering rivers prone to rapid floodplain reworking.