Sequence stratigraphy, seismic profiles, and cores of Pleistocene deposits on the Rhône continental shelf

Abstract

High resolution seismic profiles on the Rhône deltaic shelf has shown the existence of a complex of Pleistocene superimposed prograding wedges interrupted by a major incised valley system cutting across the shelf. The individual wedges are interpreted, from internal geometry and coring data, as prograding shoreface deposits accumulated during periods of relative sea level lowering punctuated by small-scale relative sea level falls or forced regressions. They are related to 5th-order glacio-eustatic cycles and are stacked to form a composite 4th-order sequence. Radiocarbon dating (Tandetron AMS) of the upper wedge (to 40 ka BP) indicates a stratigraphic hiatus across the shelf corresponding to the glacial maximum lowstand at the end of the Würmian cycle (isotopic stage 2) during which sea level had dropped to 120 m below present sea level. Seaward tilting of the wedge boundaries indicate that the shelf has been subjected to differential subsidence that provides the necessary accommodation space for deposits to accumulate on the mid-outer shelf. The seaward tilting results from a cumulative effect of sediment and hydrostatic loading. Tectonism is also expressed by a westward shore-parallel tilting in the area east of the incised valleys. The amount of tilting, the stacking pattern of the wedges, and the location of incised valleys are structurally controlled, by the alpine orogen which dips westward under the Rhône margin. The tectonic component interacts with glacio-eustatic sea level fluctuations and accounts for the partition of the shelf into two areas with specific features separated by the Rhône incised valleys.