3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of France

Abstract

In the SE Basin of France (Drome), fossil cold seep structures made of fossil-rich carbonate lenses were identified in the Oxfordian (Late Jurassic) Terres Noires Formation about 30 years ago. To date, all seep sites have generally been studied individually without consideration of any link to neighboring or more distant sites. Based on a detailed fieldwork in the Beauvoisin area and comparison with the modern, active giant Regab pockmark and sandbox experiments, we suggest that this site can now be considered as a 800 m wide fossil analogue of a giant pockmark. It comprises several 4-6 m deep, 80-120 m wide coalesced sub-seep sites. A spatio-temporal 3D reconstruction of the position of these sub-sites shows that the carbonate lenses are organized in clusters with only one cluster active at a given period. Two periods of seep activity are separated by a period of quiescence due to the lateral shift of the feeder conduit beneath the structure. This suggests that even if a seafloor pockmark appears to be inactive, this could be due to the lateral shift of the feeder conduit meaning that the sub-seafloor is still charged in gas.