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

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