Reconstructing Proterozoic Planforms and Channel Dynamics: New insights from the 1.2 Ga Stoer Group, NW Scotland

A fundamental shift in fluvial architecture is often argued to coincide with the evolution of terrestrial plants in the Silurian period. This shift away from “sheet-braided” conditions has traditionally been attributed to the influence of vegetation on river geometry and planform, resulting in Earth’s first single-threaded rivers. However, recent paleohydraulic reconstructions of Proterozoic rivers suggest that rivers of this time commonly attained aspect ratios and slopes similar to modern meandering and anastomosing rivers. At the same time, a wider range of channel planforms is increasingly recognised for pre-Silurian strata, although these interpretations have resulted from varied methodologies. Thus, a consistent and multi-faceted approach, applied to a series of fluvial successions, is needed to develop a unified model of pre-vegetation fluvial morphodynamics. Here, we present field observations and paleohydraulic reconstructions of fluvial strata from the exceptionally well-preserved Mesoproterozoic (1.2-1 Ga) Stoer Group located in NW Scotland. These strata host a range of fluvial architectures, from low aspect-ratio channel bodies isolated within muddy floodplain sediments, to amalgamated channel facies forming apparent “sheet-braided” successions with high sand/mud ratios. Reconstructions and bar-dune orientations from the Clachtoll, Bay of Stoer, and Meall Dearg Formations of the Stoer Group reveal a range of channel morphologies, including meandering, wandering, and braided planforms. Furthermore, we show that mud, even in relatively low volumes, was capable of providing sufficient cohesion to foster single-threaded planforms. We propose that evolving channel kinematics post the greening of the continents was as important as changing planform for determining the fluvial architectures we see preserved in the rock record.