ABSTRACT: Sedimentation and Geodynamics of the Gres d'Annot Foreland Basin (Southern Part of the Western Alps, France)

R. Vially, C. Ravenne

This report is based on sedimentological field surveys performed recently by Institut Francais du Petrole (IFP) and Ecole Nationale Superieur du Petrole et des Moteurs (ENSPM).

On a Mesozoic basement folded by the Pyreneo-Provencal orogeny (Late Cretaceous-Eocene) in the western part of the southern Alps nummulitic basin, the nummulitic transgression flooded the eastern area after the Lutetian (middle Eocene) and spread westward during the late Eocene. On a basin scale, flexural subsidence induced a rapid increase and made the basal carbonates pass from platform type to slope type. The overlying marls corresponded to slope facies wrapping around the inherited topography. Upward and laterally, the Gres d'Annot sensu lato is a turbiditic sandstone formation with a thickness of 1200 m, which fills up and onlaps the paleoslope of the basin. At the top of this sequence an erosional marine surface is marked by northeast-southwest submarine canyons related to olist strome sliding (nappe de l'Autapie).

We interpret this basin as a flexural one during the late Eocene for several reasons: (1) the migration and diachronism of sedimentation and deformation from the internal to the external part of the orogen; (2) the strong flexural subsidence responsible for its elongated shape parallel to the chain; and (3) its final filling by gravitational nappes.

Reconstructions at different stages of its length, depth, and detrital influxes are based on accurate and numerous field observations. All these new data hardly constrain our subsidence modeling, assuming a lithospheric flexuring in response to tectonic overloading in the inner part of the orogen.

This interpretation is backed up by comparison with flume-tank sedimentation experiments and seismic data on current offshore basins. Such a multidisciplinary study emphasizes the primordial importance of sedimentological studies for understanding the evolution of such basins.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990