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

Sequence 1

Sequence 1 is dominantly siliciclastic, around 30 m thick at Weymouth Bay, and can be divided into four systems tracts:

Lowstand systems tract (LST). At Weymouth Bay, 9 m of pervasively bioturbated, fine-grained clayey sandstone (Nothe Grit) sharply overlies marine mudstones of Oxford Clay Formation (Sun 1989; Coe 1995). On a larger scale, the sandstone body appears to be wedge shaped, probably pinching out in an updip position just east of the Southard Quarry borehole (Fig. 4). Basinwards, it grades into mudstone toward Hewish 1 (Fig. 4). Sun (1989) has interpreted the Nothe Grit as a lower-shoreface deposit. Sellwood et al. (1990) suggested that it formed at a sea-level lowstand. This interpretation is supported by the sharp lower boundary and abrupt basinward shift of fades. Coe (1995) has correlated this major siliciclastic influx across much of the Wessex Basin.

Transgressive systems tract (TST). At Weymouth Bay, the TST consists of two erosively-based beds, a lower argillaceous sandstone overlain by a highly-bioclastic, calcareous sandstone with many oolitic limestone intraclasts (Sun 1989). Both beds are included within the Preston Grit Member, which grades upwards into limestones and mudstones of the Nothe Clay. Updip from Weymouth Bay, the TST thickens, and in the Southard Quarry borehole, it contains thin oolitic limestones. Basinwards it appears to thin toward Hewish 1, and only mudstone is present at Abbotsbury (Fig. 4). Extending the conclusions of Sun (1989), the intraclastic-bioclastic sandstones of the Preston Grit are interpreted as part of a transgressive sheet that formed in a mid-ramp setting. This sheet was coeval with the development of thin oolitic bars in a high-energy, inner-ramp position. This system was drowned around maximum flooding and covered by mudstone which contains bored micrites, bioclastic limestones, sideritic nodules, encrusted pebbles and chamosite ooids (Wright 1986). These facies of the basal Nothe Clay are typical condensed-- zone features formed under very low rates of sedimentation around peak transgression (Louth et al. 1988).

Highstand systems tract (HST). Highstand sedimentation is probably represented by sandy mudstones in the upper part of the Nothe Clay. This is supported by their position in the sequence, overlying the condensed section, and their low diversity fauna of euryhaline bivalves (Hallam 1978). They may represent a prograding mud-dominated shoreline,

Falling stage systems tract (FSST). At Weymouth Bay, sandy mudstones of the Nothe Clay are sharply overlain by 6 m of oil-impregnated, well-sorted, fine-grained sandstone. This unit, the Bencliff Grit Member, can be broadly divided into four erosionally based sandbodies separated by thin discontinuous beds of bioturbated mudstone and cross-laminated sandstone (Goldring et al. 1998). The structure of each sandbody changes upwards from amalgamated hummocky (HCS), or swaley cross-stratification (SCS), into parallel lamination or climbing ripple lamination (see Allen & Underhill 1989; Sun 1989). Traced basinwards, the Bencliff Grit thins and pinches out around Hewish 1. At Kimmeridge 3, the sandstone body maintains its thickness before thinning rapidly toward Southard Quarry (Fig. 4).