EOLIAN VS FLUVIAL ORIGINS OF THE LATE EOCENE, EARLY OLIGOCENE WHITE RIVER SEQUENCE OF THE GREAT PLAINS AND MIDDLE ROCKY MOUNTAINS

White River sequence covers an area of about 400,000 cubic kilometers from western Wyoming to eastern Nebraska, and east central Colorado to western North Dakota. The region was in a post-Laramide erosional episode that resulted in the late Eocene unconformity. The tremendous influx of fine-grained volcaniclastic sediment during the ignimbrite flare-up from volcanic areas farther to the west overwhelmed the fluvial systems and resulted in aggradation in the older, basal paleovalleys. These valleys were filled and thick sheets of this fine-grained material spread across the Great Plains and the basins in the Middle Rockies by the end of White River deposition, roughly 30 Ma. Some of this material was reworked by fluvial processes. Fluvial deposits include long ribbon sandstone beds filled with coarse crystalline debris derived from the Rocky Mountains, large widespread sandstone blankets in the White River Badlands of South Dakota, and paleovalleys that were cut into older White River deposits and filled in part by fluvial deposits. However all of these fluvial deposits grade laterally into fine-grained deposits, ranging from claystone beds at the base of the White River sequence, mudstone beds in the middle, and massive siltstone beds at the top. Coarse-grained fluvial deposits comprise less than 10% of the total White River deposits, and are typically concentrated in the lower third to half of the sequence. The upper beds of the White River are typically massive, structureless, very thick, and widespread siltstone beds that represent volcaniclastic loess deposits. Even in the lower White River, vast expanses of stacked claystone and mudstone beds that locally blanket paleotopography and contain few or no fluvial channel deposits represented volcaniclastic dust deposits that were weathered in subhumid to moist semiarid climates. Thus, eolian deposits are the dominant sediments of the White River sequence.