ARCHITECTURE OF THE LOWER OLIGOCENE SCENIC MEMBER OF THE BRULE FORMATION IN THE NORTH UNIT OF BADLANDS NATIONAL PARK, SOUTH DAKOTA

The 50 m thick Scenic Member of the Brule Formation, White River Group, is exposed for 60 km along the Badlands Wall in the North Unit of Badlands National Park. These outcrops provide an exceptional example of the regional architecture of a distal volcaniclastic, fluvial sequence. The Scenic is composed mainly of tuffaceous mudstones and fine-grained sandstones, with minor amounts of claystone, siltstone and limestone. The Scenic mudrocks were derived from the alteration of volcanic glass that fell in the Great Plains as dust. The base of the Scenic is a broadly undulatory contact with as much as 24.5 m of relief, reflecting an erosional topography cut into the underlying Chadron Formation. This topography was initially blanketed by mud derived from the clay-rich Chadron deposits and by volcanic dust weathered into clay. Deposition shifted into an active fluvial system represented by light gray sandstones deposited first in paleovalleys then coalescing into broad blankets. These blankets are capped by thin, very widespread mudstones that extend along the entire length of the Badlands Wall and are important marker beds. These mudstone markers have evidence of intense pedogenesis, including well defined Bt horizons and soil fabrics suggestive of modern Alfisols , and represent a temporary shift from active fluvial deposition to slow accumulation and weathering of volcanic dust. Paleosols that formed during active periods of aggradation between individual marker beds are thinner, less well developed, and similar to modern entisols and inceptisols. Three thick sandstone blankets are capped by these thin but widepread mudstone beds, and together they comprise three depositional packages. Finally, the top of the Scenic is marked by the shift from mudstone beds to thick siltstone beds of the overlying Poleslide Member. The change from mudstone to siltstone accumulation reflects a shift from wet, subhumid conditions where the volcaniclastic dust was weathered, to drier semiarid conditions where the dust accumulated as loess. This climatic shift is also evident within changing paleosol morphologies.