Benefits of New Offshore Seismics on the Understanding of a Western Mediterranean Continental Shelf Geodynamics and Eustatic Controls During the Neogene (Provence Margin, SE France)

Outcrops in Provence have been exhaustively studied, however few studies focused on the marine geology and the possible offshore continuity of onshore structures. It is a complex geological domain situated between the Alpine arc and the continental margin of the Liguro-Provençal back-arc basin which was also influenced by the high amplitude Neogene eustatic changes, especially during the Messinian Salinity Crisis (MSC). To improve our knowledge on this key domain within the context of the characterization of coastal karst system, it is critical to integrate data from onshore geology and offshore seismics. A major asset of this study has been the acquisition since 2007 of high resolution offshore seismic lines and shallow coring. Given the lack of offshore wells in this area, the seismic lines have been mainly interpreted on the basis of the stratigraphical and structural comparisons to the near-by outcrops (qualitative seismic inversion) and locally with the support of shallow coring. Onshore-offshore geological cross-sections help to validate the seismic interpretations. A variety of seismo-facies have been interpreted in terms of stratigraphic and structural characteristics, sedimentary and diagenetic heterogeneities of the formations and rock properties measured on outcrop samples. We propose a new geological map of the offshore Provence Continental margin, a completed depth map of the Messinian Erosionnal Surface, and a new land-sea depth map of the top Mesozoic carbonate Formation that is the main karstic aquifer in the area. Such results imply new considerations for the tectonic, stratigraphic and hydrographic framework of the Provence area. We integrate the present day bathymetry and altimetry, the surfaces of the Messinian erosional unconformity and the top Mesozoic carbonate in a 3D onshore-offshore surfacic model. Successive sea-level changes of the Mediteranean since the Burdigalian period are modeled and compared to the altitudes of sedimentary deposits and marine erosional surfaces onland. This method evidences anomalies between altitude of Tertiary deposits and corresponding sea-level, relative to the actual topography and structures. This implies significant differential vertical movements of the Provence margin during the Plio-Quaternary. The causes of these tectonic movements are debated, probably gravity alpine tectonics, isostatic rebound after the MSC, or subduction and slab detachment under the Corsica and Sardinia.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California