STRATIGRAPHY AND CLIMATIC SIGNIFICANCE OF THE WHITE RIVER GROUP, BADLANDS NATIONAL PARK, SOUTH DAKOTA

The type section of the latest Eocene and early Oligocene White River Group is in Badlands National Park, South Dakota. The White River Group is a series of mudrocks and sandstones that were derived from the Black Hills and volcanic sources much farther to the west. The latest Eocene rocks were deposited on a surface of low relief locally cut by deep paleovalleys. Most of the uppermost Eocene rocks are represented by the claystone beds of the Chadron Formation. The claystones of the Chadron Formation indicate a time of high weathering associated with humid conditions. A surface of low to moderate relief was cut prior to deposition of the lower Oligocene rocks of the Brule Formation. The paleomagnetic stratigraphy of the upper Chadron and lower Brule indicates an unconformity representing at least 1.2 million years. The Brule Formation includes two members, the lower Scenic Member and the upper Poleslide Member. The Scenic Member is composed of thick sequences of mudstone separated by a thick sandstone sequences in the middle. The individual sandstone units are separated by widespread mudstone marker beds that extend along the entire badland wall. The mudstone beds of the Scenic Member indicate subhumid conditions. The Poleslide Member is composed primarily by massive siltstone beds separated by two thick, laterally persistent sandstone sequences. The siltstone units of the Poleslide Member are very persistent laterally and represent dryland loess deposits that accumulated in semiarid conditions. The lateral relations between the Scenic and Poleslide rocks indicate that the onset of loess accumulation was earlier to the west and younger to the east. The Brule Formation is capped by sandy siltstone beds of the Sharps Formation that also accumulated in relatively dry conditions. A series of paleovalley cuts and fills ended deposition of the Sharps Formation in the middle Oligocene.