SEISMIC MODELS OF A PROGRADING CARBONATE PLATFORM - VERCORS, SOUTH-EAST FRANCE

The southern Vercors area (south-east France) displays superb exposure s of a Lower Cretaceous prograding carbonate platform. These exposures allow a geometrical sequence analysis to be combined with detailed ou tcrop observations. Two-dimensional seismic modelling techniques were used to determine the seismic response of three of these exposures: th e Montagnette, the Archiane Valley and the Rocher de Combau. The outcr ops display prograding and retreating series of clinoforms interfinger ing with fine-grained basinal sediments. Photographs from different an gles of view, combined with detailed field observations, were used to construct lithological profiles with a resolution of a few metres. P-w ave velocities and bulk densities were measured from cored hand specim ens and selected values were assigned to lithostratigraphic units in e ach profile. The vertical incidence modelling technique was then used to compute migrated time sections of reflectivity. These were convolve d with source wavelets of different frequencies to produce the final s ynthetic seismic profiles. None of the modelled outcrop geometries is correctly portrayed seismically at 25-50 Hz frequencies. Not only do t he synthetic seismic profiles show less detail, they also show mislead ing geometries. In most seismic models, for instance, pseudo-unconform ities a re present that correspond to rapid changes in dip and in faci es in outcrop. Furthermore, the seismic tool is not able to resolve be tween stratal patterns punctuated by unconformities and those with gra dual shifts in facies belts accompanied by complex interfingering rela tionships. This implies that an unconformity recognized on a real seis mic line should not automatically be assumed to represent a true strat al unconformity. Large outcrops are often used to show the validity of sequence stratigraphy, a concept mainly based on the analysis of seis mic lines. This study, however, shows that there is no one to one rela tionship between geometries observed in outcrop and in seismic lines. Seismic modelling of outcrops therefore provides a means for evaluatin g outcrop geometries in terms of seismic stratigraphy.