Field and Seismic Images of Sharp-Based Shoreface Deposits: Implications for Sequence Stratigraphic Analysis

Abstract

ABSTRACT Sharp-based shoreface sandstones are of considerable interest because of their potential as hydrocarbon reservoirs and because they play an important role in the stratigraphic analysis of basin fills. The sharp-based shoreface sandstones studied herein are Upper Jurassic (Kimmeridgian-Tithonian) and exposed along the coastal cliffs of the Dover Strait in northwestern France. These series consist of tens of meter-thick alternations of sandstones bodies and organic-rich shales that can be correlated for over 30 km along coastal cliff exposures and tied to high-resolution (ca. 1 m of sediment) marine-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 sand 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 bedset. The two progradational packages are separated by a third surface, a subaerial exposure surface that is interpreted as a sequence boundary. Two distinct 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 lower boundaries and parallel (aggradational) configurations; seismic unit B is characterized by downlap and toplap boundaries and simple or compound, sigmoid and oblique-tangential (progradational) configurations. A single sharp-based shoreface sandstone body makes up the type B unit and typically consists of two compound superimposed progradational sets (B1, B2). The lowermost set, B1, corresponds in the field to the progradational set sensu stricto, 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 interpretation of sharp-based shoreface sandstones. It is proposed here that complete, single, sharp-based shoreface sandstones bodies can be separated into two different systems tracts: (1) a progradational set (B1 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 set (B2) at the top, above the sequence boundary, which corresponds to the lowstand systems tract (LST) of Posamentier et al. (1992). A complete sharp-based shoreface sandstone body is bounded below the FRWST by a regressive surface of marine erosion caused by the downward shift of wave base, and by a transgressive surface of marine erosion, or ravinement surface, at the top of the LST.