Probably proximal pebbles? An outcrop-constrained quantitative analysis of clast transport distances in the lower Triassic Sherwood Sandstone Group, UK

The lower Triassic Sherwood Sandstone Group fluvial strata in England record the northward flow of the Budleighensis river system through fault-bounded synrift graben from the Wessex Basin to the Irish Sea Basin. The pebbles in the Sherwood Sandstone Group are typically argued to originate mostly from the Armorican Massif, northern France. Northwards transport should mean that the pebbles decrease in concentration northwards. However, it can also be argued that the pebble-rich strata found in several basins across central and northern England suggest that a number of different sediment source areas contributed pebbles along the river system. To further test and constrain this multiple source hypothesis, pebble concentration data were used to constrain a simple source-to-sink exponential decay model predicting pebble concentration v. distance from source. The model was inverted on the number and location of clast input sources to calculate the best-fit matches. The results indicate that three distinct pebble sources best-fit these data, with major tributaries originating in the Welsh Massif and the London–Brabant Massif, supporting a multiple sediment source interpretation for the early Triassic river system. Multiple sediment input points increase the prediction uncertainty for Triassic strata. Unanticipated heterogeneity has implications for practical subsurface prediction related, for example, to proposed carbon and nuclear waste storage sites.