A new perspective of the Sherwood Sandstone: spatio-temporal dynamics of a fluvial system revealed by quantitative paleohydrology

The regionally significant lower-middle Triassic Sherwood Sandstone Group (SSG) of the British Isles records the early stages of the breakup of Pangea and is an important aquifer and subsurface reservoir. It was deposited by a major north-flowing river system, which was sourced in northern France and flowed towards the East Irish Sea Basin, and reaches a maximum thickness of 1.6 km. However, the vertical and lateral amalgamation of fluvial sandstones and conglomerates of the SSG, coupled with its interpreted arid continental environment of deposition, have traditionally rendered the impacts of climate and tectonics on sedimentation uncertain. Developing a better understanding of these interactions for this palaeodrainage system is key to appraising source-to-sink sediment routing trends. This would allow improved predictions to be made of the volumetrics and heterogeneity of sandstone reservoir fairways for carbon capture and storage (CCS).

To this end, we apply quantitative paleohydrological methods to reconstruct key characteristics of the fluvial system throughout its depositional fairway using both architectural and bedform-based analyses. We collected outcrop measurements from 37 key field sites across England from south to north, of dune-scale bedforms (n=1278), architectural elements (n=270), palaeocurrent (n=820), and grain size (n=157). From these data, bankfull flow depths, palaeoslopes, unit discharge and river planform are quantified from empirical-numerical approaches.

We firstly illustrate the temporal evolution of the fluvial system through the chronostratigraphically dated Devon coast section in the south-west of England. Our results quantitatively develop on previously inferred climatic trends in the British Isles. We recover median bankfull flow depths of 1.5 to 2m, and palaeoslopes of 0.0006 to 0.001. The lowest SSG unit illustrates the presence of large, pebble-grade rivers with high bankfull discharge: a consequence of the fluvial system’s response to the Permo-Triassic Extinction at 252 Ma. The upper SSG reflects a return to more uniform hydrological and sedimentological conditions and a decrease in palaeoslope, due to progressive topographic decay and a climatic recovery by the mid-Triassic.

To the north, the spatial variation of sedimentological and palaeohydraulic character indicates that the fluvial system that deposited the SSG was more complex than previously interpreted. Results indicate the trunk river was fed by multiple tributaries that drained local sediment sources in addition to the typically identified source from northern France. These findings may have substantial implications for palaeoclimate, regional drainage patterns and CCS, with reservoir properties in the SSG likely variable as a result.