Offshore Cover Collapse Fields Related to Deep Messinian Karsts
Johanna Lofi, Michel Tesson, Michel Séranne, and Bernard Gensous
Sinkholes are frequently observed where soluble rocks are present, in meteoritic recharge areas or when subsurface dissolution causes ground collapse. Seismic profiles on the inner shelf of the Gulf of Lions (W. Mediterranean) display a giant atypical sub-circular structure, up to 800-m thick and 2-km wide. It is characterized by down-warped internal seismic reflections forming a concave V-shaped depression rooted in the substratum, most likely carbonated. This structure is interpreted as a poly-phased solution-subsidence-collapse structure of Plio- Quaternary age. The last phase of deformation probably occurred during the last sea-level low-stand phase. The mechanism proposed for the creation of this structure is collapse-dissolution of the soluble substratum, as a result of a combination of several factors including pre-existing deep Messinian palaeo-karst with active groundwater flows during Plio-Quaternary lowstands, and deep-rooted acidic fluids ascending through the substratum.
At least 14 similar collapse structures are present elsewhere on the inner shelf, as identified from high-resolution seismic profiles. Additional work is now required on these structures, to test the proposed mechanisms and refine our understanding of fluid migration and pathways at the scales of both the margin and the collapse structures. Such structures could provide an indirect marker of Messinian karst conduit location. At the present time, around the Mediterranean basin, several conduits or outlets are located too deep to have ever been exposed above sea-level during the PQ lowstands and are classically related to the dramatic drop in sea-level of the MSC. These deep karst conduits may constitute a new target as they have a high storage capacity of economic interest. This contribution has been funded by the Actions Marges French research program.
AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013