Field and seismic images of sharp-based shoreface deposits: Implications for sequence stratigraphic analysis

Sharp-based shoreface sandstones are of considerable interest because of th eir potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills. The sharp-based shorefac e sandstones studied herein are Upper Jurassic (Kimmeridgian-Tithonian) and exposed along the coastal cliffs of the Dover Strait in northwestern Franc e. These series consist of tens of meter-thick alternations of sandstones b odies and organic-rich shales that can be correlated for over 30 km along c oastal cliff exposures and tied to high-resolution (ca. 1 in of sediment) m arine-seismic profiles obtained several hundred meters offshore. The units described here comprise two sharp-based sandstone bodies. Each is composed of a basal progradational set of shoreface parasequences overlain by a progradational-aggradational shoreface succession. Each sharp-based s and body lies on a marine regressive surface of erosion and is truncated by a marine transgressive surface of erosion, which in turn is overlain by a thin retrogradational ravinement lag or coarse-grained, planar-laminated be dset. The two progradational packages are separated by a third surface, a s ubaerial exposure surface that is interpreted as a sequence boundary. Two d istinct types of seismic units, referred to as type A and type B, have been identified in the study area. Seismic unit A has conformable upper and low er boundaries and parallel (aggradational) configurations; seismic unit B i s characterized by downlap and toplap boundaries and simple or compound, si gmoid and oblique-tangential (progradational) configurations. A single shar p-based shoreface sandstone body makes up the type B unit and typically con sists of two compound superimposed progradational sets (B1, B2). The lowerm ost set, B1, corresponds in the field to the progradational set sensu stric to, whereas B2 corresponds to a progradational-aggradational set. B1 and B2 are separated in outcrop by a sequence boundary. These observations led us to reevaluate the sequence stratigraphic interpre tation of sharp-based shoreface sandstones. It is proposed here that comple te, single, sharp-based shoreface sandstones bodies can be separated into t wo different systems tracts: (1) a progradational set (131 seismic body) at the base, which corresponds to the forced regressive wedge systems tracts (FRWST) of Hunt and Tucker (1992), and (2) a progradational-aggradational s et (B2) at the top, above the sequence boundary, which corresponds to the l owstand systems tract (LST) of Posamentier et al. (1992). A complete sharp- based shoreface sandstone body is bounded below the FRWST by a regressive s urface of marine erosion caused by the downward shift of wave base, and by a transgressive surface of marine erosion, or ravinement surface, at the to p of the LST.