Date of this Version
Wooden, S. R., Jr., 2013. Fluvial and Stratigraphic Architecture of the Ash Hollow Formation, Ogallala Group, Lake C. W. McConaughy Nebraska, U.S.A. MS Thesis, University of Nebraska-Lincoln
The fluvial strata of the Ash Hollow Formation (AHF) constitute a major part 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 ca. 10 Ma, during an interval of major climatic, biotic, and physiographic change that shaped the ~1.3 million km2 Great Plains. The AHF consists of mostly massive sandstones and siltstones which represent stacked sheets of laterally accreted channel belt deposits and overbank massive sandstones and siltstones in the study area. Mudstones are essentially absent. Lateral-accretion surfaces can be identified through careful, serial observation in many otherwise nearly structureless, rhizolith-laden sandstones. Common and locally laterally-extensive, but thin and volumetrically minor, 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 is interpreted as the in-channel deposits of meso- and macroforms, with locally large (> 30 cm) intraformational clasts. Many large soft-sediment deformation structures (large dish-shaped structures, folds, contorted beds, etc.) appear in a single stratigraphic interval approximately 10 m in thickness. These structures appear to have been produced by synsedimentary slumping, sedimentary loading, and groundwater movement. Storey thicknesses range from 2.5 to 5.5 m. The average thickness of a sandbody in a fluvial storey is approximately 3 m, which suggests an average channel depth of approximately 1.95 m. Deposition in the study area represents a wide range of fluvial depositional environments, and the poorly studied sediments of the AHF provide great insight into the depositional systems that shaped the Great Plains.
Advisor: R. M. Joeckel