Grain size fractionation in a Triassic dryland fluvial system: the Sherwood Sandstone Group, UK

Sediment routing systems record grain size changes from source to sink. Sediment grain size decreases downstream due to the selective deposition of sediment, but can also increase when material is added from tributaries. Importantly, quantifying downsystem changes in grain size helps elucidate the dynamics of sediment routing systems in the geological past, and understand the spatial heterogeneity of deposited strata.

The Sherwood Sandstone Group (SSG) and Mercia Mudstone Group (MMG) of the British Isles are regionally significant units deposited during the breakup of Pangea. The SSG is a key unit for groundwater resources, geothermal energy and carbon capture and storage. For our source-to-sink analysis, we use a chronostratigraphically defined interval of the SSG and MMG, which was deposited by a long-distance dryland river system active during the mid-Triassic (c.240 Ma).

We interpret 130 sections through the SSG and MMG using geophysical well logs, outcrops and cored boreholes. Using these data, existing palaeogeographies and isopach maps, we generate upsystem-to-downsystem volumetric grain size profiles (gravel, sand and mud) for this sediment routing system. We convert these profiles into a dimensionless mass balance framework. These results provide a detailed characterisation of this river system and its deposits, supported by existing studies on sediment routing, sandstone petrography and quantitative paleohydrology. Crucially, we are able to explore the spatial dynamics of this sediment routing system, including the locations of sediment inputs and downsystem sediment bypass, demonstrating the generic utility of our approach in reconstructing ancient source to sink systems.