Rocks of the Late Triassic and Early Jurassic age in the Carlisle Basin, Cumbria (north-west England)

Abstract

SUMMARY The thickness and nature of the rocks of Late Triassic (Penarth and Lias groups) and Early Jurassic (Lias Group) age in a poorly exposed outlier west of Carlisle, Cumbria, have been proved in two recent boreholes. A third borehole proved that the Mercia Mudstone Group (Triassic) crops out, beneath superficial deposits, within an area previously mapped as Lias. Cores from the lower part of the Lias Group have yielded biostratigraphical evidence for the position of the base of the Psiloceras planorbis Zone and, consequently, of the Hettangian Stage and the Jurassic System. Almost 70 m of Lias Group rocks were penetrated above 13–14 m of rocks of the Penarth Group which is proved for the first time in this area. The borehole data, combined with seismic reflection data, suggest that the outlier is both more extensive and is located further west than was previously envisaged, and that it is fault bounded to the west, to the south, and to the north and north-east. However, to the east, the Penarth Group-Lias Group succession may crop out beneath superficial deposits.