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

Journal of the Geological Society, Jan 2000 by Newell, Andrew J

Fault activity and marine sedimentation

Sequences 1, 2 and 3 can be correlated across major faults without significant changes in thickness. Each forms a gently offshore thickening wedge. This supports the conclusion of Chadwick (1986) that in the Oxfordian there was little or no syndepositional faulting and the Wessex Basin was undergoing regional flexural subsidence. Sequence 4 has a much more variable thickness distribution pattern that can be related to the position of major faults (Fig. 6). Maximum accumulation in the tilt-block basin of the Abbotsbury-Ridgeway Fault may be related to the fact that this is a listric structure that soles on Triassic salt. The Purbeck Fault is planar and linked to its counterpart basement structure (Stewart et aL 1996). Marine sedimentation under the contrasting regimes of thermal and fault-driven subsidence is discussed below.

Thermal subsidence (Sequences 1-3)

Under conditions of thermal subsidence, gradients may be slight over large areas and ramp profiles can extend from highs into basinal areas (Burchette & Wright 1992). Regional changes in sea level (e.g. eustatic) may be the dominant control on stratigraphy.

Thermal subsidence may have favoured carbonate sedimentation in the Wessex Basin. However, this also required suitable climatic conditions and a sea level that maximized the area and water circulation of the `carbonate factory' (Handford & Loucks 1993). The change from the siliciclastic-- dominated sequence 1 to the carbonate-dominated sequence 2 may represent a long-term rise in relative sea level that promoted widespread shallow flooding of the ramp (Fig. 7). Two distinct types of oolite body can be recognized in the Corallian Formation. A shoestring barrier body (merging offshore into a transgressive sheet) formed under conditions of rising relative sea level (Swift et al. 1991). A sheet-like body formed under highstand conditions when ooid shoals expanded laterally to fill available accommodation space. These morphologies compare closely with those described from other ramp margins (Burchette et al. 1990; Tucker et al. 1993). The depositional topography created by the TST had an important control on the geometry of the HST. In inner ramp zones, the amalgamated HST oolite body thins significantly over the high created by the TST barrier bar. A reverse situation can occur in mid-outer ramp zones, where a topographic low can be generated by sediment starvation during transgression. In this case, the oolite in the lowermost parasequence of the highstand may be substantially thicker than those above as this excess accommodation is filled.

Siliciclastic facies deposited during this ramp phase were strongly controlled by source area characteristics. The intrabasinal high described here had previously been a site for muddy marine sedimentation (Oxford Clay). These poorly consolidated sediments were cannibalized to form mud-prone highstand and lowstand deposits. From a commercial perspective, these deposits would have limited potential as hydrocarbon reservoirs. However, their sorting (and permeability) characteristics may be improved by shoreface reworking during transgression and forced regression.