ABSTRACT

Offshore sand bodies are described from many continental shelves in the world, as well as in the stratigraphic record, where they commonly are productive reservoirs. Some of these ancient sand bodies, initially interpreted as sand ridges, are now reinterpreted as lowstand shoreface deposits. Modern sand bodies, in contrast, have received relatively little attention with regard to reinterpretation of their origin, largely because of the lack of information about their internal structure. Improved techniques in acquisition and processing of very high-resolution seismic profiles, along with some shallow cores, allow us to reconstruct the architecture of "offshore sand bodies" from the Celtic Sea (tide-dominated) and the Gulf of Lions (wave-dominated) shelves of France, in water depths of 100-170 m.

In both areas, our investigations demonstrate that these particular sand bodies consist mainly of lowstand deposits (estuarine/deltaic systems, sharp-based shorefaces), reworked during transgressions. In the Celtic Sea, intense erosion by combined waves and tidal currents resulted in the shaping of shore-oblique ridges by cannibalization of older lowstand deposits. In the Gulf of Lions, the shore-parallel orientation of the lowstand shorefaces has been preserved, leaving an ancestral sand body with reworked (transgressive) surface deposits.

Understanding the architecture and distribution of offshore sand bodies requires taking into consideration not only the effects of relative sea-level changes and sediment supply, but also the role of hydrodynamical processes. The erosional sand bodies we describe represent a new category of outer-shelf sand bodies, a combination of the "autocyclic" examples described by Houbolt (1968) in the southern North Sea and the "allocyclic" lowstand shorefaces mainly described in the stratigraphic record of the Western Interior Seaway of North America. Our findings have applications for predicting the geometry of ancestral sand bodies and their orientation and position with respect to paleo-shorelines. The magnitude of erosional processes also implies that a large amount of shelf sediment (mainly sand) was transferred to the adjacent deep-sea floor during the early transgression.