THE UPPER EOCENE TO LOWER OLIGOCENE WHITE RIVER SEQUENCE OF THE NORTHERN GREAT PLAINS AND MIDDLE ROCKY MOUNTAINS

The White River is a fine-grained volcaniclastic sequence that originally covered an area of 400,000 km², extending from North Dakota to central Colorado, and western Wyoming to eastern Nebraska. The sequence is composed primarily of tuffaceous claystone, mudstone, and siltstone beds that are interbedded with scattered lenticular to sheet sandstone bodies, very localized thin limestone sheets, and widespread, typically thin, tuff beds. The White River was the first widespread unit to cover the post-Laramide, late Eocene unconformity as a huge influx of volcaniclastic dust derived from volcanoes in Nevada and Utah covered the region starting about 36 Ma. The White River contains an estimated 25,000 km³ of volcanic ash that covered the basins and filled the valleys in the Rocky Mountains, and blanketed the Great Plains. Local sources of epiclastic materials became so covered by volcanic dust that volcanic ash amounts changed from near 45% at the base of the sequence to over 80% at the top. Wind carried the volcanic ash into the region where initially it was weathered and reworked by streams, but later accumulated as massive silt beds representing volcaniclastic loess. This transition was caused by a shift from early humid, middle subhumid to semiarid conditions near the top. This transition from mudstones to siltstones is time-transgressive, with the fluvial mudstone and loessic siltstone transition occurring earlier in White River deposits to the west and south, and later in deposits to the east and north. Tectonically the area was relatively quiet, with the main depositional basin centered in western Nebraska and southern South Dakota where the most complete White River sequence (ending about 30 Ma) is preserved. Deposition was not continuous in the White River, but was punctuated by recognizable and widespread erosional gaps. Much of the Great Plains sequence has such a gap at the Eocene-Oligocene boundary.