EROSIONAL OFFSHORE SAND RIDGES AND LOWSTAND SHOREFACES - EXAMPLES FROM TIDE-DOMINATED AND WAVE-DOMINATED ENVIRONMENTS OF FRANCE

Offshore sand bodies are described from many continental shelves in th e world, as well as in the stratigraphic record, where they shelves in the world, as well as in the stratigraphic record, where they commonl y are productive reservoirs. Same of these ancient sand bodies, initia lly interpreted as sand ridges, are now reinterpreted as lowstand shor eface deposits. Modern sand bodies, in contrast, have received relativ ely little attention with regard to reinterpretation of their origin, largely because of the lack of information about their internal struct ure. Improved techniques in acquisition and processing of very high-re solution seismic profiles, along with some shallow cores, allow us to reconstruct the architecture of ''offshore sand bodies'' from the Celt ic Sea (tide dominated) and the Gulf of Lions (wave-dominated) shelves of France, in water depths of 100-170 m. In both areas, our investiga tions 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-ob lique ridges by cannibalization of older lowstand deposits. In the Gul f of Lions, the shore-parallel orientation of the lowstand shorefaces has been preserved, leaving an ancestral sand body with reworked (tran sgressive) surface deposits.Understanding the architecture and distrib ution of offshore sand bodies requires taking into consideration not o nly 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 com bination of the ''autocyclic'' examples described by Houbolt (1968) in the southern North Sea and the ''allocyclic'' lowstand shorefaces mai nly described in the stratigraphic record of the Western Interior Seaw ay 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 boor during the early transgressi on.