TEN YEARS AFTER: REVISED LITHOSTRATIGRAPHY OF THE EOCENE-OLIGOCENE WHITE RIVER GROUP, NEBRASKA AND SOUTH DAKOTA

Based on over 10 years of 1:24,000 geologic mapping, lithologic descriptions, and lithostratigraphic correlations we provisionally revise and redescribe the lithostratigraphy of the White River Group in Nebraska and adjacent South Dakota. Overall, the volcaniclastic claystones and siltstones of the White River Group consist of 38-90% volcanic ash and intermixed continental clastic debris. The White River Group overlies various Cretaceous strata but principally the Pierre Shale. Based on criteria established in the North American Stratigraphic Code (NACSN, 1983), the revised White River Group includes the Chamberlain Pass, Chadron, Brule, and Rosebud fms. The Chamberlain Pass Fm. consists of gray silty claystone (pedogenically modified into the Weta and Interior paleosols) and thin ribbons of diagenically altered sandstone. The Chadron Fm. includes the Ahearn (fluvial sandstones), Crazy Johnson (fluvial siltstones and sandstones), Peanut Peak (eolian silty claystone), and Big Cottonwood Creek (eolian silty claystones and lacustrine interbeds) mbrs., the Trunk Butte beds (colluvial siltstones), and the Buffalo Gap beds (sandstone and cobble conglomerates). The Brule Fm. includes the Orella (fluvial siltstones and sandstones), Scenic (nodular fluvial siltstones and sandstones), Whitney (eolian siltstone), and Poleslide (eolian and fluvial siltstone and sandstones)mbrs. and the Sharps beds (eolian sandy siltstone, formerly Arikaree Group). The Rosebud Fm. (also formerly Arikaree Group) consists of the St. Francis (eolian siltstone), Parmalee (fluvial claystones and lenticular sandstones), Smith Falls (colluvial siltstone), and Meadeville beds (fluvial siltstone and claystone). The White River Group is overlain by volcaniclastic (18-35%) siltstones and sandstones of the Arikaree Group and epiclastic claystones and sandstones of the Ogallala Group. Following this revision, White River Group lithostratigraphy no longer corresponds to traditional divisions based in part on North American land mammal ages. Based on this improved lithostratigraphic framework, more detailed correlations of rock strata to climate events and mammalian faunal evolution are possible. This research was funded from 1991-1995 by the Nebraska National Forest and from 1996-2006 by the USGS STATEMAP program and the University of Nebrska-Lincoln.