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

Highstand systems tract. The HST is dominated by oolitic limestone. At Abbotsbury, it is around 35 m thick and consists of four parasequences (Fig. 4). Each parasequence records progressive shoaling from an environment initially below mean storm wave base (mudstone), through sub-fairweather wave base (mud-sand couplets with HCS), into shallow-water oolitic limestone or quartz sandstone shoals. At Weymouth Bay the HST comprises a number of cross-bedded oolitic grainstone sheets separated and overlain by mudstones and nodular wackestones (Shortlake and Nodular Rubble members) (Sun 1989). In updip positions (e.g. Kimmeridge 3) the oolites amalgamate, with little intervening mudstone. This stacking pattern is continued eastwards over the rest of the section, although there is substantial thinning of the oolite bodies over the topography created by the TST (Fig. 4). A short core in this unit at Marchwood 1, shows that here the oolitic limestone is brecciated and pisolitic. These features could result from subaerial weathering (Handford & Loucks 1993).

Sun (1989) has discussed the regressive character of the oolitic grainstones and bioturbated wackestones of the Shortlake Member and Nodular Rubble in terms of their meteoric-influenced diagenesis. The regional fades pattern is also typical of many oolite bodies formed under highstand conditions (Aigner 1984; Burchette et al. 1990; Tucker et al. 1993). Highstands are generally the optimum time for carbonate production, because the area of shallow-marine carbonate production has reached its maximum extent (Hanford & Loucks 1993). The amalgamated carbonate body seen in Kimmeridge 3 is typical of high-energy, inner ramp zones which are the main site of ooid generation. The occurrence of bipolar trough and planar cross-bedding, mud drapes and channel scours at Weymouth Bay indicates the importance of tidal processes in ooid formation (Sun 1989). In outer ramp zones, such as at Abbotsbury, ooid bodies interfinger with offshore mudstones, in a series of upward-shoaling cycles formed during phases of abrupt marine flooding and shoreline progradation.

Sequence 3

Sequence 3 is a mixed siliciclastic/carbonate unit, around 15 m thick at Weymouth Day, and can be divided into two systems tracts:

Transgressive systems tract. The development of the TST at both Abbotsbury and Weymouth Bay follows the same pattern of underlying sequences, with a sandy bioclastic wackestone at the base (Sandy Block), overlain by an erosionally based skeletal-oolitic-intraclastic grainstone (Chief Shell Beds). This transgressive sheet is overlain by mudstone, bioclastic wackestone and sideritic micrite (Red Beds), interpreted as a condensed interval. In updip locations (Southard Quarry) the TST is a more uniform stack of bioclastic and oolitic limestones, possibly representing an inner-ramp barrier bar.

Highstand systems tract. At Weymouth Bay, the condensed zone is overlain by 5-15 m of calcareous, intensively bioturbated, sandy mudstone (Sandsfoot Clay Member). Sun (1989) and Wright (1998) have suggested an offshore environment for this mudstone. However, Talbot (1973) and Brookfield (1978) have favoured low-salinity, nearshore conditions based on the extremely low-diversity bivalve fauna.