A new source-to-sink synthesis of the Middle Triassic Helsby Sandstone Formation (Sherwood Sandstone Group) river systems in the British Isles

Xiang Yan, Gary J Hampson, Alexander C Whittaker

Sediment grain size and mineralogy change in sediment routing systems from source to sink. A better understanding of sediment routing allows improved predictions to be made of the bulk grain-size and mineralogy of sandstone fairways. We present a new appraisal of sediment routing in the Triassic Helsby Sandstone Formation (Sherwood Sandstone Group) and lowermost Mercia Mudstone Group of the British Isles, which constitute a key play for geological sequestration of CO2. These strata were deposited and supplied by a major, north-flowing river system, which is traced from its source region in north France to beyond the Irish Sea. We construct a new, integrated litho- and chronostratigraphic model to correlate key units across the British Isles. We then present sediment isopachs and volumes for this chronostratigraphic interval, and resolve paleogeographic discrepancies using published sedimentological datasets, supplemented by a new synthesis of bulk sandstone mineralogy. Finally, we present a unified, updated sediment routing map for the Helsby Sandstone Formation. Importantly, differences in bulk sandstone mineralogy between the south and north of the studied fairway signifies that aside from its primary source area in France, substantial tributaries modified the composition of the Helsby Sandstone Formation along the course of the sediment routing system.