COLD METHANE SEEPS AS POSSIBLE REFUGES FROM ASH FALLS: EVIDENCE FROM THE UPPER CRETACEOUS PIERRE SHALE, SOUTH DAKOTA

The Pierre Shale is well exposed in southwestern South Dakota, USA. A series of siltstones and mudstones represents offshore sedimentation in the Late Cretaceous Western Interior Seaway. Methane seep deposits, ranging from upper Campanian to lower Maastrichtian (~75-70 Ma), have been widely documented in the Pierre Shale. These seeps contain diverse faunal assemblages, representing highly productive, chemosynthetic ecosystems. Bentonite beds occur throughout the Pierre Shale, allowing for precise dating and correlation of strata. Bentonite layers are laterally continuous and range in thickness from 5-30 cm, indicating periodic yet intense volcanic events. Widespread ash deposition adversely affects marine ecosystems and can be catastrophic for benthic communities. Oxygen and nutrient availability becomes restricted on the seafloor and benthos are quickly buried in thick layers of ash. At several localities, bentonite layers cut across methane seep carbonates and become irregular. At the seeps, bentonites are discontinuous and brecciated; beds become mottled, containing clasts of shale and seep associated concretions. This may be the result of continuous methane seepage during ash deposition, thus providing an uninterrupted source of methane and sulfide, necessary compounds for chemosynthetic based communities. Therefore, cold seeps may have provided refuges during environmental perturbations, protecting communities from local extinction events. This study compares the biotic response of both a chemosynthetic (seep associated) and photosynthetic (non-seep associated) communities to a single volcanic event. By examining faunal assemblages below, within, and above the ash layer, this study elucidates pre- and post- event ecosystems, focusing on changes in macroinvertebrate diversity. Preliminary findings suggest seep associated deposits maintain similar faunal diversity and abundance following ash deposition as opposed to non-seep deposits.