MCMILLAN, MARGARET E., University of Wyoming, Dept. of Geology and Geophysics, Laramie, WY

ABSTRACT: Late Cenozoic Exhumation of the Central Rocky Mountains

Basins in the greater Wyoming region, including parts of Nebraska, Colorado, Utah, Montana, and South Dakota, contain remnants of sedimentary rocks of latest Eocene to Miocene, and possibly Pliocene age. These fluvial and eolian units likely filled structural topography created by the preceding Laramide orogeny. Remnants today are scattered along basin flanks and more rarely as isolated buttes out in basin centers. Deposits stand as much as 1500 meters above present day basin floors in north central Wyoming and to the east, merge with the modern flood plain in central Nebraska. The continuity of the basin fill, if demonstrated to be true, has major implications for the post-Laramide evolution of the Rocky Mountain region. First, the fill serves as a paleotopographic marker from which subsequent large-scale tectonic deflections can be determined. It also serves as a datum from which subsequent incision can be measured thereby providing a basis for comparison with hypothesized elevation vs. rock uplift changes in the Rockies. Additionally, incision into the fill provides potential evidence that canyon cutting observed in the Colorado Plateau is part of a far larger story of erosion and evacuation of the Rocky Mountain region in post-Laramide time. Finally, the large volume of the inferred Wyoming basin erosion produced a tremendous influx of terrigenous sediment throughout the Mississippi Valley and the Gulf Coast. This wholesale redistribution of sediment, if verified, radically changed the topography of the west as well as the geometry of the sedimentary prism along the Gulf of Mexico.

This abstract is on page 1868.

AAPG Search and Discovery Article #90909©2000 AAPG Foundation Grants-in-Aid