ORIGIN OF SOME OFFSHORE SAND BODIES AROUND FRANCE

Serge Brrn€1, Tania Mnnssetl, Gilles Lerlcolarsl, Jean Frangois, Bo Pnulert, Marc De Bnrtsr2, Jean Yves, Reynaud, Bernadette Tessrer
unpublished
Offshore sanci bodies are described in many continental shelves in the world, as well as in the stratigraphic record where they represent interesting reservoirs. Many of the latest, initially interpreted as sand ridges built by physical processes, are now re-interpreted as lowstand shorefacies. In contrast, relatively few attention has been paid to the origin of recenf offshore sand ridges, mainly because of the lack of data about their internal structure. lmproved techniques in acquisition and
more » ... processing of very high resolution seismic and some shallow cores allow to reconstruct the architecture of "offshore sand bodies" from the Celtic Sea (tide dominated) and the Gulf of Lions iwave dominated) shelves around France. In both cases, our investigation demonstrates that the sand bodies mainly consist of lowstand deposits (estuarine/deitaic systems, sharp-based shorefaces), instead of transgressive deposits as proposed by several workers. However , the shape and position of the "ridges" mainly results from control by physical processes. In the Celtic Sea, intense erosion resulted in the shaping of shore-normal ridges "cannibalizing" lowstand deposits, while the shore-parallel orientation of the lowstand shorefaces has been preserved in the Gulf of Lions. The understanding of the architecture and distribution of offshore sand bodies requires to take into account not only the classical sequence stratigraphic concepts (relative sea-level changes, sediment supply...) but also hydrodynamic processes. The erosional sand bodies we describe represent a new category of outer shelf sand bodies, in between the purely hydrody-namic examples described by Houbolt (1968) in the Southern North Sea and the purely "allocyclic" lowstand shorefaces mainly described in the stratigraphic record of the Western Interior Seaway. This finding may have interesting applications for predicting the geometry, orientation and position of fossil sand bodies with respect to paleo-shorelines. The magnitude of erosional processea evidenced by our investigations also implies that a large amount of shelf sediments (mainly sand) has been transferred to the adjacent.deep sea fans. R6sum6. Des corps sableux de plate-forme sont d6crits en diverses r6gions du monde, ainsi que dans de nombreuses roches s6di-mentaires oi ils constituent des rdservoirs int6ressants. En Mer Celtique et dans le Golfe du lion, on observe ainsi, i des profondeurs comprises entre l00 et 160 m mdtres, des accumulations sableuses d'Epaisseur impodante (>30m). L'analyse sismique d haute r6solu-tion et quelques carottages suggdrent que ces accumulations correspondent d des d6p6ts r6gressifs ou de bas niveau, remani6s et tor-tement 6rod6s par la transgression post-glaciaire. Une nouvelle cat6gorie de corps sableux, r6sultant i la fois de processus autocycliques (remaniement par des courants) et allocycliques (glacio-eustatisme) est ainsi mise en 6vidence. Cette d6couverte d des implications quant d la g6om6trie et ir I'orientation de ces corps s6dimentaires ; elle implique aussi qu'un volume s6dimentaire important a 6t6 6rod6 et transferr6 au domaine profond.
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