Pre‐Vegetation, Single‐Thread Rivers Sustained by Cohesive, Fine‐Grained Bank Sediments: Mesoproterozoic Stoer Group, NW Scotland

A Silurian shift in fluvial stratigraphic architecture, coincident with the appearance of terrestrial vegetation in the fossil record, is traditionally cited as evidence for exclusively shallow, braided planforms in pre-vegetation rivers. While recent recognition of deep, single-thread channels in pre-Silurian strata challenge this paradigm, it is unclear how these rivers maintained stable banks. Here, we reconstruct paleohydraulics and channel planform from fluvial cross-strata of the 1.2 Ga Stoer Group. These deposits are consistent with deep (4-7 m), low-sloping rivers (2.7 x 10(-4) to 4.5 x 10(-5)), similar in morphometry to modern single-thread rivers. We show that reconstructed bank shear stresses approximate the cohesion provided by sand-mud mixtures with 30%-45% mud-consistent with Stoer floodplain facies composition. These results indicate that sediment cohesion from mud alone could have fostered deep, single-thread, pre-vegetation rivers. We suggest that the Silurian stratigraphic shift could mark a kinematic change in channel migration rate rather than a diversification of planform.