STAGE OF MATURITY AND FAUNAL COMPOSITION OF COLD METHANE SEEPS IN THE UPPER CRETACEOUS OF THE U.S. WESTERN INTERIOR BASIN

The stage of maturity (SOM) of modern methane seeps can be determined by evaluating the species diversity and hardness of the substrate. Our investigation compares ancient methane seeps to modern equivalents in the Pierre Shale of the upper Cretaceous sequence of the U.S. Western Interior Basin. We investigate five localities in South Dakota, with respect to the nature of the substrate, number of organisms, species richness, and how evenly the abundance is distributed among species,(using standard diversity indices (Shannon-Weiner, Margalef’s, Pielou’s). Two localities (American Museum of Natural History (AMNH) localities AMNH 3418 and 3405) are comprised of softer substrate material and contain 548 and 207 organisms respectively, with a low species richness and diversity. Three other localities (AMNH 3419, 3420, 3440) are comprised of a hard, calcareous substrate and contain 894, 464, and 1010 organisms, respectively, with a medium to high species richness and diversity. However, species are not evenly distributed at any of the five localities. The differences in the substrate, diversity, and richness of the fauna at these localities reflect aspects of modern methane seeps and their relative SOM and duration. Baculites and several species of bivalves (e.g. Ostrea, Inoceramus, and Nymphalucina) are pioneering species and appear at newly established methane seeps. As the seep matures, species richness and diversity increase. Furthermore, AMNH loc. 3440, which we interpret as a very mature seep based upon lithology, displays few pioneering species but the number of ammonite species is much higher, suggesting a more normalized marine environment. Therefore, the SOM of ancient methane seeps can be determined by both the analysis of substrate material and faunal content and involves a succession from pioneering species to late arriving species; leading to a multistage classification of these ancient seeps- incipient, immature, mature, and senescent.