ELLIOTT, T., Department of Earth Sciences, University of Liverpool, England; * G. M. APPS, BP Exploration, Houston, Texas, USA; H. DAVIES, Shell Research, Rijswijk, Netherlands; M. J. EVANS, Tonbridge, England; N. JONES, Department of Earth Sciences, University of Liverpool, England; and R. JONES, BP Exploration, Glasgow, Scotland

ABSTRACT: Sequence Stratigraphy of the Tertiary Foreland Basins in the External Alps, S.E. France

In the S.W. Alpine foreland, Eocene compressional basins developed behind the thrust front as dominantly extensional basins formed ahead of it. The focus of Eocene to Quaternary deposition migrated with the westward propagation of Alpine thrusts. Alpine structures controlled sediment supply and entry points into the basins.

Sequence boundaries, traced through all the basins, define seven sequences. The stratigraphy of each basin can now be understood in the context of regional evolution. The earlier sequences are primarily driven by tectonics; later sequences record a stronger eustatic signature.

The onset of Alpine flexural subsidence in the Eocene was recorded as the basal transgressive surface of the Gres d'Annot basin to the east; in the foreland, it caused the expansion of lakes. A major tectonic reorganization in the Early Oligocene emplaced thrust sheets across the Western Alps, and large rift systems propagated through the foreland. Deep water deposition ceased abruptly, as most of the accommodation space was filled with thrust sheets. A rapid basinward shift in deposition was recorded in basins on the leading edge of the thrust system and in the foreland. The focus of deposition shifted to the rapidly subsiding, extensional basins beyond the thrust front, in which thick alluvial sections accumulated.

The foreland was a region of bypass through much of the Aquitanian, and deposition was re-established above a diachronous transgressive surface during the Burdigalian, with a dominantly carbonate succession in Provence. Subsequent regional surfaces have been interpreted as unconformities driven by eustatic sea level falls. One such surface, preserved within a thick conglomeratic section, records the Messinian dessication of the Mediterranean.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.