3D ARCHITECTURE OF LOWSTAND AND TRANSGRESSIVE QUATERNARY SAND BODIES ON THE OUTER SHELF OF THE GULF OF LION, FRANCE

A restricted area (700 km(2)) of the outer continental shelf in the Gu lf of Lions (Western Mediterranean Sea) has been surveyed in detail, i n order to reconstruct the 3D architecture of large Quaternary sand bo dies that are exceptionally well preserved in the western part of the Rhone delta. Data sources include digital 'very high resolution' seism ic, swath bathymetry and some shallow cores. In two dimensions, the se ismic data display a complex superposition of alternating seismic unit s, consisting of high and low angle prograding clinoforms, correspondi ng to 'high' and 'low' energy environments. The combination of faunal and grain size analysis from shallow cores, geometry of sedimentary bo dies, showed that these two types of seismic facies can be attributed, respectively, to upper shoreface and prodeltaic settings. Despite a s imilar geometry in 2D, it appears that the prograding sand units have very different 3D geometries (lateral extent, direction of progradatio n with respect to the paleoshorelines...), implying different sediment ary processes. At least two major sand units have shore parallel orien tations and can be mapped along the entire study area. The mechanism r esponsible for the formation of these sand bodies seems similar to tha t described for many 'sharp-based' sand bodies from the stratigraphic record, especially in the Western Interior Seaway; their isolated posi tion on the shelf being related to (forced) regressions. In contrast, some other sand units have lobate shapes, with divergent directions of progradation and limited lateral extent. They are often overlying (fl uvial) incisions and they pinch out landward; it is still not clear wh ether they are related to some allocyclic or autocyclic processes, lik e shifting of deltaic lobes as observed on the Texas coast. The very h igh resolution, and high quality of the seismic data have enabled us t o characterise the three-dimensional architecture of small-scale sedim entary bodies. The study has pointed out the variability of deposition al processes and has given an insight on reservoir characteristics. (C ) 1998 Elsevier Science Ltd. All rights reserved.