ISOTOPE AND FAUNAL COMPOSITION OF COLD METHANE SEEP DEPOSITS IN THE WESTERN INTERIOR SEAWAY, SOUTH DAKOTA

Methane seep deposits are common in the Upper Cretaceous (Baculites scotti to Baculites compressus ammonite zones) of the Pierre Shale of South Dakota and support a diverse community of mollusks, echinoderms, serpulids, and other invertebrates. The development of the seeps and the dynamics of the communities are still poorly understood. Several morphotypes of seep associated concretions (SACs) occur as a result of methane-enriched flow. These include platy/tabular fossiliferous pavements, pipes, tubes, irregular masses, balls, and globular masses. Study of concretion shapes, isotope signatures, and faunal composition can provide important clues into the development of the seeps and their plumbing systems. Carbonate concretions were collected from five localities in South Dakota to identify the fauna and determine the oxygen and carbon isotope composition. Selected samples were also analyzed for the strontium isotope composition. A single concretion from an age-equivalent non-seep deposit was examined for comparison. Tubular and sub-spherical seep concretions are composed of calcitic micrite with light d¹³C (to -48‰), typical of methane-influenced carbonate precipitation. The concretions also contain sparry calcite, with slightly heavier values of d¹³C(-10‰), suggesting that they formed later with less influence of methane-derived C. Comparison of ⁸⁷Sr/⁸⁶Sr in seep carbonates with coeval seawater values provides information on fluid flow in the seep systems. In at least one seep concretion, values are elevated above the seawater value, suggesting equilibration of the fluids with a radiogenic source of Sr at depth in the deposit and advective fluid transfer toward the sediment-water interface. In general, pipes, tubes, and balls are relatively unfossiliferous whereas the irregular carbonate masses contain up to 15 species of molluscs. These differences may reflect where the concretions formed (at or below the sediment-water interface) and by what processes (diffusive or advective transport of methane).