FAUNAL ASSOCIATIONS WITH FLUID FLOW RATES IN LATE CRETACEOUS METHANE COLD-SEEPS FROM THE PIERRE SHALE, SOUTH DAKOTA

Faunal diversity increases with time and duration of fluid flow in modern methane cold-seeps as substrate shifts from a soft to a hard ground environment. However, faunal successional stages have yet to be determined in ancient seep assemblages. We propose faunal stages for 25 Late Cretaceous methane cold-seeps in the Pierre Shale, South Dakota. Over 8000 organisms were identified to the genus or species level where possible, and analyzed using multiple diversity indices (Shannon-Weiner (H) Diversity, Margalef’s Richness (S), Pielou’s Evenness (J) Index, and percentage of organism type in the preserved population found at a seep and then compared to the basic substrate of the seep. Preserved population density (D) ranges from 4 – 21, H ranges from 0.376 – 2.237, S ranges from 0.163 – 0.452, and J ranges from 0.609 – 3.174. Baculitids, lucinids, and inoceramids are common (10 - 20% of all specimens each) and represent foundation organisms based upon their abundance in soft-substrate seep assemblages. Secondary organisms (number of specimens ≥ 4%) include oysters, carnivorous gastropods, and scaphitids. Shifts in the kind of secondary organisms correlate with changes in substrate and mineralogy, which correlate, in turn, with relative fluid flow. Faunal successional stages include: 1) a foundation stage where the dominant organisms are baculitids, lucinids, and inoceramids with a clay-based substrate; 2) a seral stage where the dominant secondary organisms are oysters and the substrate is changing from clay-dominated to carbonate; 3) a climax stage where gastropods are the dominant secondary organisms and the substrate is a calcite hard ground, 4) a poly-climax stage where scaphitids are the dominant secondary organisms and the substrate is a Mg-calcite hard ground; and 5) a senescent stage where scaphitids are the sole secondary organisms among foundation species and the substrate is a calcite hard ground. Unlike modern faunal assemblages, secondary organisms at these ancient seeps never outnumber foundation organisms despite apparent changes in fluid flow or duration of methane-rich fluids. The increase in scaphitids accompanying a decrease in methane fluid flow suggests a shift towards a more hospitable environment with a readily available food source.