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

Shallow-marine carbonates and siliciclastics of the Corallian Formation (Oxfordian–Early Kimmeridgian) accumulated on and around an intrabasinal high in the extensional Wessex Basin. Four sequences can be recognized. Sequences 1–3 accumulated under conditions of thermal subsidence on a ramp-type margin. The initial sequence was siliciclastic. Highstand sedimentation in this sequence reflects the supply of sandy mud from a recently emergent intrabasinal high. During transgression and regression this muddy sediment was reworked into cleaner sandstone bodies by landward or basinward migrating zones of shoreface erosion. Carbonates dominate the second and third sequences when rising sea level increased the area of carbonate production and reduced siliciclastic input. Oolite bodies developed as both transgressive barrier bars and highstand sheets. The forth sequence formed during the activation of major normal faults. This caused the breakdown of the ramp system, and patterns of sediment accumulation were strongly controlled by tectonic subsidence patterns.