Architecture, heterogeneity, and origin of late Miocene fluvial deposits hosting the most important aquifer in the Great Plains, USA

The Ash Hollow Formation (AHF) of the Ogallala Group is an important sedimentary archive of the emergence of the Great Plains and it contains major groundwater resources. Stratal patterns of constituent alluvial lithofacies demonstrate that the AHF is much more heterogeneous than is commonly assumed. Very fine- to fine-grained sandstone dominate overall, chiefly lithofacies Sm (massive to locally stratified sandstone). Stacked, thin sheets of Sm with accretionary macroform surfaces are common, indicating that many sandstone architectural elements originated as compound-bar deposits in dominantly sand-bed streams. Channel forms are difficult to identify and steep cutbanks are absent. Multiple units of lithofacies Sm show dense, and sometimes deep, burrowing by insects well above water tables under ancient floodplains. Massive, pedogenically modified siltstones (Fm), which compose floodplain fine architectural elements, are subsidiary in volumetric abundance to sandstones. Paleosols in these siltstones lack evidence for well-developed B horizons and advanced stages of maturity. Thin lenses of impure carbonate and laminated mud (lithofacies association Fl + C), which appear in most exposures, are deposits of ponded water in abandoned channels. Paleosols, ponded-water elements, and large vertebrate burrows in both Sm and Fm indicate that episodes of floodplain deposition, bar accretion, and channel filling were regularly followed by intervals of nondeposition on floodplains and by channel migration and abandonment. This study documents a major downdip change in the Ogallala Group overall, from source-proximal gravelly successions in the Wyoming Gangplank and deep, narrow paleovalley fills extending eastward into the Nebraska Panhandle. The lithofacies composition, stratigraphic architecture, and stratal dimensions of the AHF in the present study area are compatible with the planform geometries and floodplain soils of modestly-sized, sandy, low-sinuosity braided streams in Nebraska today, namely the modern North, South, and Middle Loup Rivers, rather than being the signatures of “big rivers.”