Fault activity and sedimentation in a marine rift basin (Upper Jurassic, Wessex Basin, UK)

Shallow-marine carbonates and siliciclastics of the Corallian Formation (Ox fordian-Early Kimmeridgian) accumulated on and around an intrabasinal high in the extensional Wessex Basin. Four sequences can be recognized. Sequence s 1-3 accumulated under conditions of thermal subsidence on a ramp-type mar gin. The initial sequence was siliciclastic. Highstand sedimentation in thi s sequence reflects the supply of sandy mud from a recently emergent intrab asinal high. During transgression and regression this muddy sediment was re worked into cleaner sandstone bodies by landward or basinward migrating zon es of shoreface erosion. Carbonates dominate the second and third sequences when rising sea level increased the area of carbonate production and reduc ed siliciclastic input. Oolite bodies developed as both transgressive barri er bars and highstand sheets. The forth sequence formed during the activati on of major normal faults. This caused the breakdown of the ramp system, an d patterns of sediment accumulation were strongly controlled by tectonic su bsidence patterns.