FLUVIAL SEDIMENTS OF THE UPPER MIOCENE ASH HOLLOW FORMATION (OGALLALA GROUP) AT LAKE C.W. MCCONAUGHY, NEBRASKA, USA

The lower contact of the Ogallala Group (OG) in the vicinity of Lake C. W. McConaughy exhibits at least 70 m of relief along strike in the form of broad subsurface paleovalleys and paleohighs eroded into Oligocene strata. Borehole data suggest that sandbodies 15-30 m in thickness lie deeper in the succession, and that fining-upward packages 3-9 m in thickness dominate the remainder of the OG in the subsurface. A ~50 m composite section of the Ash Hollow Formation (AHF) of the OG is exposed on the lake’s south shore atop one of the paleohighs. These exposures trend chiefly along dip (regional paleoflow) and they exhibit multiple lithofacies and upward-fining trends, even though they are dominated by thin sheets of sandstone. Several of the thin sandstone sheets exhibit large, inclined bedding surfaces produced by the migration of fluvial macroforms; a few channel elements (≤ 3 m thick) and macroform accretion elements can also be identified. Sheets of cross-stratified gravels and conglomerates (Gt) are very rare. Channel, macroform, and gravel-bar elements in the lake’s emergency spillway suggests a bankfull paleochannel depth of 3.8 m; this stratal measurement and others are comparable with the bankfull depths of the modern North Platte, South Platte, and Republican Rivers in Nebraska today. Many massive sandstones (Sm) and siltstones (Fm), the latter constituting a floodplain fines architectural element, exhibit pervasive pedogenic features (fine rhizoliths, cracks, soil structure, and horizonation) as well as vertebrate and invertebrate burrows, the latter including Naktodemasis. Thin ponded-water architectural elements, which are common in the lower half of the composite section, consist of minor, laminated claystones to siltstones (Fl) and impure to pure, micritic to microsparitic carbonates (C).