PRELIMINARY LITHOLOGIC DESCRIPTION OF THE CHADRON FORMATION NEAR RED SHIRT, SOUTH DAKOTA, WITH NOTES ON THE INVERTEBRATE AND VERTEBRATE TRACE FOSSILS

Based on preliminary observations of lithology and sedimentary structures during the summer of 2009, we describe the lithostratigraphy and trace fossils of the Chadron Formation on the Pine Ridge Reservation and adjacent Buffalo Gap National Grassland along Battle Creek near Red Shirt, South Dakota. In this area the Chadron Formation (Eocene) unconformably overlies the Yellow Mounds Paleosol equivalent developed in the Pierre Shale (Cretaceous). We recognize (from bottom) the Ahearn, Crazy Johnson, Peanut Peak, and Big Cottonwood Creek members of the Chadron Formation exposed in the canyons and small hills on the north side of the Battle Creek Valley. The Chadron Formation is the youngest stratum exposed in this area. The Ahearn Member consists of coarse-grained white fluvial sandstone with stringers and lags of rounded pebble to cobble-sized clasts of various Black Hills igneous and metamorphic lithologies. The Crazy Johnson Member consists of interbedded red claystones and red, brown, green, and white fluvial sandstones with locally abundant coarse clasts. The Peanut Peak Member consists of olive green volcaniclastic claystone and interbedded brown fluvial sandstones. The Big Cottonwood Creek Member consists of grayish green volcaniclastic silty claystones and interbedded brown fluvial sandstones. The fluvial sandstone interbeds within the Big Cottonwood Creek Member contain locally abundant tubular and meniscate invertebrate burrows, and multiple sets of poorly defined footprints of mammalian quadrupeds of varying sizes. Additional work is being planned to inventory and describe invertebrate traces and vertebrate trackways within Paleogene strata exposed on the Pine Ridge Reservation. This research was supported by the NSF Model Institutes for Excellence Phase III and NSF Tribal College and Universities Program Phases II & III at Oglala Lakota College.