AMMONITES AS AN INTEGRAL PART OF COLD METHANE SEEP FAUNAS: COMPARISON OF SITES FROM THE UPPER JURASSIC OF FRANCE AND THE UPPER CRETACEOUS OF NORTH AMERICA

Cold methane seeps have been a persistent feature in marine environments over the last 500 MY. One of the perennial questions is the faunal composition of the seep-associated community, and how this may have changed over time and in different settings. In an effort to answer this question, we compared the methane seep deposits from the Upper Jurassic Terres noires (Oxfordian) of southeastern France (Drome) and those from the Upper Cretaceous Pierre Shale (Campanian) of North America (South Dakota, Wyoming, Colorado, Montana). Both sites are characterized by large micritic masses and small, seep associated concretions (SAC’s). Isotopic analyses of the SAC’s reveal the characteristically low values of δ¹³C typical of cold methane seeps. The fauna is dominated by lucinid bivalves, ammonites, and gastropods at both sites. In North America, the ammonites mostly consist of baculitids, scaphitids, didymoceratids, and placenticeratids. In France, the ammonites mostly consist of perisphinctids and phylloceratids. However, the preponderance of ammonites including both adults and juveniles as well as the presence of ammonite jaws at both sites suggest that ammonites were an integral part of the seep ecosystem in both time periods. Based on the species composition of the ammonites, the sites in North America formed at water depths of ≤100 m whereas those in southwestern France formed at water depths between 150-200 m.