Negligible microbial matground influence on pre-vegetation river functioning: Evidence from the Ediacaran-Lower Cambrian Series Rouge, France

The pre-Silurian alluvial rock record is dominated by accumulations of laterally-extensive, sheet-like sandstone strata with minimal mudrock; a depositional style frequently explained as representing fluvial processes particular to “pre-vegetation” Earth. While the sedimentological and geomorphological influence of Palaeozoic embryophytes and other higher vegetation has been commonly inferred, the influence of the non-marine microbial matgrounds that preceded them has been less well studied. The ?Ediacaran-Cambrian Series Rouge of northern France and the Channel Islands is a rare example of a predominantly alluvial succession which exhibits both pre-vegetation sedimentary motifs and evidence for the existence of terrestrial microbial mats. The latter include likely microbial sedimentary surface textures, the enigmatic matground “pseudofossils” Aristophycus and Arumberia, and probable mat fragments and mat-related microtextures preserved in argillaceous sediment. The sedimentological characteristics of the Series Rouge are described and analysed in order to assess the role of microbial influences on pre-vegetation alluvial systems. Near ubiquitously trough-cross bedded sheet-braided facies, with rarely preserved channel-forms, indicate that alluvial sedimentation was dominated by in-channel dune migration, and depositional-dip exposures reveal the periodic downstream migration of complex bar-forms. Lateral accretion elements and minor discontinuous lenses of more argillaceous material are locally present. Thus, despite the evidence for matgrounds, sedimentary architecture was essentially 'abiotic'. Using this evidence from the Series Rouge, we argue that the surficial cohesion provided by matgrounds did not exceed thresholds for reworking by hydrodynamic processes thus having little or no effect on their preserved sedimentary architecture.