PROPOSED LITHOSTRATIGRAPHIC REVISION, REDESCRIPTION, AND REDEFINITION OF THE WHITE RIVER GROUP (EOCENE-OLIGOCENE), SOUTH DAKOTA

Based on 1996-2009 lithologic descriptions and lithostratigraphic correlations in Nebraska and South Dakota, we revise, redescribe, and redefine the White River Group (Eocene-Oligocene) in South Dakota. The White River Group consists of the Chamberlain Pass, Chadron, Brule, and Rosebud formations. The Chamberlain Pass Formation is redescribed in part as pedogenically modified volcanic ash. The Chadron Fm. consists of the Ahearn, Crazy Johnson, Peanut Peak, and Big Cottonwood Creek members and the Buffalo Gap beds. The Buffalo Gap beds are fluvial sheet gravels that blanket the eastern flank of the Back Hills uplift and laterally intertongue with the Peanut Peak and Big Cottonwood Creek members. The Brule Fm. consists of the Scenic, Whitney, Poleslide, and Rockyford Ash members, the Sharps beds, and the Wanblee beds. The Rockyford Ash Member was considered part of the Sharps Formation. The Sharps beds are the remaining Sharps Fm. which we redefine as a member of the Brule Formation based on volcaniclastic content and correlations to Nebraska. The Wanblee beds are pedogenically modified, brown silty volcaniclastic claystones. The Rosebud Fm. consists of early Miocene volcaniclastic silty sandstones we consider to be the Anderson Ranch Formation of the Arikaree Group, and this use of the term "Rosebud" is rejected. Rosebud was also used for Oligocene volcaniclastic siltstones at Rosebud, South Dakota, that were correlated along the Niobrara River in Nebraska. This use is retained and the strata redefined as part of the White River Group based on volcaniclastic content and intercalation with the underlying Sharps beds. The Rosebud Formation includes the St. Francis beds, which are volcaniclastic eolian siltstones, and the Parmalee beds, which are volcaniclastic fluvial overbank siltstones and claystones, large interbedded fluvial sandstones, and pedisediment. Additional work is planned to map and describe the remainder of the Pine Ridge Reservation and Pennington, Jackson, Shannon, Bennett, and Todd counties in southern South Dakota. 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.