A sequence stratigraphic model for an intensely bioturbated shallow‐marine sandstone: the Bridport Sand Formation, Wessex Basin, UK

Abstract

The Bridport Sand Formation is an intensely bioturbated sandstone that represents part of a mixed siliciclastic‐carbonate shallow‐marine depositional system. At outcrop and in subsurface cores, conventional facies analysis was combined with ichnofabric analysis to identify facies successions bounded by a hierarchy of key stratigraphic surfaces. The geometry of these surfaces and the lateral relationships between the facies successions that they bound have been constrained locally using 3D seismic data. Facies analysis suggests that the Bridport Sand Formation represents progradation of a low‐energy, siliciclastic shoreface dominated by storm‐event beds reworked by bioturbation. The shoreface sandstones form the upper part of a thick (up to 200 m), steep (2–3°), mud‐dominated slope that extends into the underlying Down Cliff Clay. Clinoform surfaces representing the shoreface‐slope system are grouped into progradational sets. Each set contains clinoform surfaces arranged in a downstepping, offlapping manner that indicates forced‐regressive progradation, which was punctuated by flooding surfaces that are expressed in core and well‐log data. In proximal locations, progradational shoreface sandstones (corresponding to a clinoform set) are truncated by conglomerate lags containing clasts of bored, reworked shoreface sandstones, which are interpreted as marking sequence boundaries. In medial locations, progradational clinoform sets are overlain across an erosion surface by thin (<5 m) bioclastic limestones that record siliciclastic‐sediment starvation during transgression. Near the basin margins, these limestones are locally thick (>10 m) and overlie conglomerate lags at sequence boundaries. Sequence boundaries are thus interpreted as being amalgamated with overlying transgressive surfaces, to form composite erosion surfaces. In distal locations, oolitic ironstones that formed under conditions of extended physical reworking overlie composite sequence boundaries and transgressive surfaces. Over most of the Wessex Basin, clinoform sets (corresponding to high‐frequency sequences) are laterally offset, thus defining a low‐frequency sequence architecture characterized by high net siliciclastic sediment input and low net accommodation. Aggradational stacking of high‐frequency sequences occurs in fault‐bounded depocentres which had higher rates of localized tectonic subsidence.

Quantitative characterisation of deltaic and subaqueous clinoforms

“…The Bridport Sand Formation is dominated by bioturbated storm-event beds with calcite-cemented bioclastic lag horizons, which were deposited in an offshore transition to lower shoreface environment. These strata form 10-40 m thick, upward-shallowing units that overlie laterallyextensive, mudstone-rich horizons (Morris et al, 2006). The Down Cliff Clay Member comprises calcareous bioturbated mudstones that record deposition in an offshore transition to offshore environment (Morris et al, 2006).…”

“…The Down Cliff Clay Member comprises calcareous bioturbated mudstones that record deposition in an offshore transition to offshore environment (Morris et al, 2006). 2D and 3D seismic data calibrated with wells from the onshore Wytch Farm Field indicate that each upwardshallowing, shoreface-to-shelf unit of the Bridport Sand Formation corresponds to a steeply-dipping (2-3°), delta-scale, progradational clinoform set that downlaps onto the topset of a similar progradational clinoform set in the Down Cliff Clay Member (Morris et al, 2006;Hampson et al, in press; Fig. 11BR).…”

Anatomy of a compound delta from the post-glacial transgressive record in the Adriatic Sea

“…; Ganges-Brahmaputra Rivers subaqueous delta (Kuehl et al, 1997;Palamenghi et al, 2011); Fly, Kikori and Purari Rivers (Walsh et al, 2004); Yellow River subaqueous delta (Yang and Liu, 2007); Mekong River subaqueous delta (Xue et al, 2010); Rhone River subaqueous delta (Fanget et al, 2014). While compound and subaqueous deltas are increasingly recognized on modern continental margins, their identification in ancient sedimentary records is more difficult, mainly for two reasons: the difficulty in pinpointing the position of the feeder system and the poor geometric resolution in depicting the low angle of muddy subaqueous progradations (Mellere et al, 2002;Morris et al, 2006;Hampson, 2010).…”

Three-dimensional modeling of clinoforms in shallow-marine reservoirs: Part 1. Concepts and application

“…The Jurassic Bridport Sand Formation, a close sedimentologic analog present onshore United Kingdom, contains similarly abundant calcite-cemented concretionary beds. These are observed at the outcrop to be laterally extensive (>80% areal coverage) along bedding planes and in a producing subsurface reservoir; their presence is marked by breaks in pressure and fluid saturation within seismically imaged clinoform sets (Morris et al, 2006;Hampson et al, 2014). Thus it appears probable that permeability barriers and baffles in the form of calcite-cemented concretionary layers occur along clinoforms in the Troll Field reservoir and could influence drainage patterns and recovery from the thin oil zone (Gibbons et al, 1993); this may have been recognized previously and shown to impact on well test interpretations (Lien et al, 1991;Haug, 1992).…”