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

Abstract

ing planforms in modern alluvial rivers (Galeazzi et al., 2021). Rooted plants are hypothesized to have dramatically increased channel bank strength, decreased the ability of rivers to rework overbank deposits, and increased the production of mud through enhanced chemical weathering (Long, 1977;McMahon & Davies, 2018a)all factors that promote channel stabilization and consequently the global dominance of single-thread river Abstract 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 × 10 −4 to 4.5 × 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. Plain Language SummaryThe earliest appearance of rooted plant fossils coincides with widespread evidence of meandering rivers in the geologic record. This correlation has led researchers to suggest that meandering rivers only existed on our planet since terrestrial plants colonized the continents, and that pre-vegetation rivers were predominantly characterized by shallow and multi-thread channels. While there is growing evidence of deep, single-thread rivers predating the rise of land plants, it is currently unclear how these rivers maintained stable banks. Here, we combine observations of 1.2-billion-year-old river sediments in NW Scotland with mechanistic theories of river dune formation to constrain the geometry of pre-vegetation rivers. We show that our field observations are consistent with deposition by deep, low-sloping, and single-thread rivers, whose reconstructed geometry is similar to modern-day meandering, rather than braided, rivers. We also demonstrate that the mud fraction of the floodplain sediment could have provided sufficient cohesion to resist erosional forces in deep, low-sloping rivers. Together, our results indicate that single-thread rivers could have been prevalent before the rise of land plants, and that mud can provide sufficient bank strength for the development of deep rivers.

Vegetation enhances curvature-driven dynamics in meandering rivers

“…The vegetation-related changes in curvature-driven planform dynamics that we observe are likely to be preserved in the stratigraphic record, necessitating their inclusion in depositional models for unvegetated single-channel rivers 19 , 22 , 29 to enhance the identification of ancient unvegetated meandering rivers in the rock record and to improve paleohydrological reconstructions on early Earth and Mars. Furthermore, our results provide new insights into channel mobility and floodplain reworking in meandering rivers—a key aspect to understanding watershed-scale biogeochemistry, particularly in relation to weathering processes and floodplain carbon stocks and fluxes 5 – 8 .…”

“…However, recent empirical evidence 19 – 22 and physics-based modeling 23 indicate that meandering rivers can also develop in landscapes devoid of plant life—including on Mars, where records of ancient vegetation are missing 23 , 24 . Unvegetated meandering river systems have now been recognized on several continents, displaying a diverse set of geomorphic units akin to those observed in vegetated alluvial floodplains, including subtle channel levees, crevasse splays, and point bars 5 , 15 , 20 , 21 , 25 – 29 .…”