Natural Resources, School of


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Bulletin of the Geological Society of America Abstracts with Programs (2014) 46(4): 51


Copyright 2014, Michael E. Perkins, Robert F. Diffendal, Jr. Michael R. Voorhies, Barbara P. Nash, and Bruce E. Bailey


The Miocene Ogallala Group blankets the Great Plains east 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 fields. This investigation brings modern methods of tephrochronlogy to bear on the age and identity of Ogallala tephra. Results indicate that ~40 separate tephra layers, ranging in age from ~16.5–5.0 Ma, in the Ogallala. Most tephra came from Yellowstone hotspot sources. The relative frequency of hotspot tephra in the Ogallala matches that in more proximal regions to the west with peak intensities in the intervals ~16.5−15 Ma nd ~13.0−8.5 Ma. About 30 of the Ogallala tephra are correlated with tephra of known age the the Basin and Range to the west. Using the ages of correlative tephra to the west insight into the age of the Ogallala, the correlation of Ogallala tephra from region to region in the Great Plains, and sedimentation rates within the Ogallala. In the Ogallala sedimentation rates vary. The rates are lowest (3–9 m/Ma) in the Cap Rock Mbr. of the Ash Hollow Fm. along the Niobrara River and in undifferentiated Ogallala strata and in the undifferentiated Ogalala 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 localies. This key horizon provides the first detailed structure contours within the Ogallala. These contours show a sharply increasing slope of the Ogallala west of 101° W that reflects the post–6 Ma tilt along the western edge of the Ogallala. East of 101º W the gradients mirror the gradients of the major rivers (1.3 to 1.6 m/km.). West of 101º W gradients increase and reach a maximun of 4.6 m/km at the crest of the Gangplank.