Quantitative sedimentological analysis of the aeolian–fluvial Triassic Sherwood Sandstone Group, Cheshire Basin, UK

Sedimentological analysis of the outcropping Sherwood Sandstone Group, northern Cheshire Basin, UK, documents the preserved stratigraphic expression of water table-controlled aeolian-dominated erg-margin successions (Wilmslow Sandstone Formation and lower Helsby Sandstone Formation), overlain by dryland fluvial deposits (middle Helsby Sandstone Formation). A quantitative assessment of sedimentary architectures and a comparison with analogous depositional systems reveals that strata of the Sherwood Sandstone Group are similar to other aeolian-dominated successions that also accumulated in rift basins, in erg-margin settings and where the water table interacted with the sediment surface. In stratal successions of erg-margin origin in the Wilmslow Sandstone Formation and Helsby Sandstone Formation, the dominant facies associations comprise: (i) thin cross-bedded dune sets characterized by foreset–toeset grainflow strata; and (ii) lenticular bodies of dry and damp-to-wet interdune strata. Together, these associations record accumulation of small, rapidly migrating transverse or oblique dunes, and intervening isolated interdune depressions. Two types of interdune deposits are observed: (i) near-horizontal wind-ripple laminated sandstone; and (ii) irregular to wavy-laminated sandstone with adhesion strata. These deposits indicate dry and damp–wet surface conditions, respectively. Dune climbing is demonstrated by the interfingering between dune toesets and interdune strata; this was enabled by accommodation generation driven by fault-related subsidence and associated relative water-table rise. Accumulation via climbing at very low angle but net positive, else by non-climbing mechanisms, is demonstrated by erosional, sharp dune–interdune bounding surfaces, which likely developed during slowdowns in subsidence rate. Episodic absolute water-table rise may have occurred during wet seasons or due to intense precipitation events, potentially in a monsoon climate. Water-table fluctuations may have also occurred in response to longer-term climatic perturbations following the Permian–Triassic mass extinction event. This study helps to constrain the factors controlling the accumulation of Triassic aeolian and mixed aeolian–fluvial successions, and their long-term preservation. Quantitative models are proposed to explain aeolian dune–interdune facies architectures. These can be used to guide predictions of three-dimensional sedimentary heterogeneity in the subsurface.