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Stratigraphic Architecture of the Ash Hollow Formation, Ogallala Group, Western Interior, USA

Steven R. Wooden
University of Nebraska

The Ash Hollow Formation is a major component of the Ogallala Group, a critical aquifer in the western interior, United States. The strata hold an important, but poorly studied sedimentary record of Late Miocene fluvial systems and terrestrial paleoenvironments during an interval of major climatic, biotic, and physiographic change that shaped the ~1.3 million km2 Great Plains. The Ash Hollow Formation consists of mostly massive sandstones and siltstones, which represent stacked sheets of laterally accreted channel belt and overbank deposits in the study area. Lateral-accretion surfaces can be identified through careful, serial observation in many otherwise nearly structureless, rhizolith-laden sandstones. Common and laterally-extensive thin ponded-water deposits dominated by laminated siltstone and massive, impure carbonates, record the existence of recurrent floodbasins and shallow abandoned channels distal to trunk paleostreams. A rare, gravelly lithofacies record in-channel deposits, with locally large (> 30 cm) intraformational clasts. Many large soft-sediment deformation structures (dish-shaped structures, folds, contorted beds, etc.) appear to have been produced by synsedimentary slumping, sedimentary loading, and groundwater movement. Storey thicknesses range from 2.5 to 5.5 meters with average thicknesses of sand bodies of approximately 3 meters, suggesting an average channel depth of approximately 1.95 meters.

Deposition in the study area represents a wide range of fluvial depositional environments, and the poorly studied sediments of the Ash Hollow Formation provide great insight into the depositional systems that shaped the Great Plains, allowing a much improved characterization of fluvial deposition during the Late Miocene, when the tectonic, physiographic, meteorological, and biological setting of the Great Plains was established. Results suggest that deposits of modestly sized rivers make up far greater sediment volumes of the Ogallala Group than do deposits of very large ancient rivers in the Late Miocene.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013