PRELIMINARY DESCRIPTION OF NEWLY RECOGNIZED OLIGOCENE AND EARLY MIOCENE STRATA NEAR WANBLEE, SOUTH DAKOTA

Based on fieldwork conducted during the spring and summer of 2009 by field parties from Oglala Lakota College and the University of Illinois, we report the newly recognized middle Oligocene Wanblee beds of the White River Group and the early Miocene Ghost Canyon beds of the Arikaree Group in Jackson County, South Dakota. These strata are widely exposed along the Pine Ridge Escarpment at Ghost Canyon, 16 km northeast of Wanblee, South Dakota. Ghost Canyon is a sacred site where Lakota people hid from smallpox and attacks from the US Calvary. It is a historical site on the Pine Ridge Reservation that holds both cultural and geological significance and is in need of preservation. Based on detailed lithologic descriptions and lithostratigraphic correlations, the stratigraphic sequence studied at Ghost Canyon includes (from the base) the Wanblee beds of the White River Group (Oligocene) and the Sharps Formation (Oligocene), Harrison Formation (Oligo-Miocene), and Ghost Canyon beds (Miocene) of the Arikaree Group. The Wanblee beds consist of 10 m massive to thickly bedded, pedogenically modified, brown silty volcaniclastic claystone. The Ghost Canyon beds consist of 25 m of massive to thinly bedded, pedogenically modified, pale brown volcaniclastic colluvial siltsone and interbeds of volcaniclastic quartz-biotite sandstone. Based on our lithostratigraphic correlations, the Wanblee beds are likely equivalent to the middle Oligocene Brule Formation (White River Group) of Nebraska, Wyoming, and South Dakota, and the Ghost Canyon beds are likely equivalent to the Anderson Ranch Formation (Arikaree Group) of northwestern Nebraska and Harksen & Macdonald’s (1969) “Rosebud Formation” (Arikaree Group) of south-central South Dakota. Additional research is planned to more completely describe the Cenozoic strata exposed in and around Ghost Canyon. 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.