The evolution of the Carlisle Basin, NW England and SW Scotland

Seismic reflection profiles show that the Permo-Triassic rocks of the ENE-trending Carlisle Basin occur in several en echelon synclines and intervening anticlines, rather than the relatively simple syncline generally shown in the literature. At least two of the synclines correspond to Permo-Triassic depocentres, which probably continued into the Early Jurassic. The western depocentre is the more pronounced, interpreted as locally containing >500 m of Appleby Group strata (Brockram and Penrith Sandstone) that are completely concealed beneath younger strata. Rocks of this age were not deposited in the other synclines and there is no subsurface continuity with Appleby Group strata in the nearby Vale of Eden Basin. The western depocentre may have been formerly contiguous with the NNW-trending Dumfries Basin prior to post-Triassic footwall uplift and erosion on the crosscutting North-East Axial-Waterbeck Fault. Both the Sherwood Sandstone and Mercia Mudstone groups locally exceed 500 m in thickness in the western syncline and 400 m in the central syncline, reducing to c. 250 m in the intervening saddle area. The depocentres were also the principal loci of Cainozoic inversion during which c. 2500 m of mainly Jurassic and Cretaceous strata were removed locally. Unlike other nearby Permo-Triassic basins, there is no evidence of synsedimentary displacements on basin margin faults. The NNW-trending Crummock Fault, which cuts across the basin, provides the best evidence for synsedimentary movement, but only during deposition of the Appleby Group. This fault, and a suite of adjacent parallel faults, may be a southerly continuation of the Dumfries Fault, which forms the western boundary of the Dumfries Basin. The outlier of the youngest preserved rocks, the Penarth and Lias groups, is preserved in the saddle area between the main depocentres where less inversion is inferred to have occurred.