ASHFALL TEPHRA IN THE OGALLALA GROUP OF THE GREAT PLAINS: CHARACTERISTICS AND SIGNIFICANCE

The Miocene Ogallala Group blankets the Great Plains west of the Rocky Mountains. This sheet of largely fluvial deposits, lying downwind of major silicic volcanic fields to the west, was ideally located to receive and preserve tephra from these volcanic fields. This investigation brings modern methods of tephrochronology to bear on the age and identity of Ogallala tephra. Results indicate ~40 separate tephra layers, ranging in age from ~16.5–5 Ma, in the Ogallala. Most tephra came from Yellowstone hotspot (YHS) sources. The relative frequency of YHS tephra in the Ogallala matches that in more proximal regions with peak intensities in the intervals ~16.5–15 Ma and ~13–8.5 Ma. About 30 of the Ogallala tephra are correlated with tephra of known age. These tephra show that sediment accumulation rates were low to moderate — in the range of 3–25 m/Ma. Rates are lowest (3–9 m/Ma) are in the Cap Rock Mbr. of the Ash Hollow Fm. in north-central Nebraska and in the undifferentiated Ash Hollow/Ogallala along the North Platte River and undifferentiated Ogallala Gp. in NW Kansas. Rates of 40–80 m/Ma characterize the Valentine Fm. beneath the Cap Rock Mbr. Finally, one tephra, the 11.37 Ma Cougar Point Tuff XI, is recognized at 6 localities. This key horizon provides the first detailed structure contours within the Ogallala. These contours show a sharply increasing slope of the Ogallala west of 101ºW longitude that reflects post–6 Ma tilt of the western Great Plains.