LITHOFACIES AND STRATIGRAPHIC ARCHITECTURE OF THE ASH HOLLOW FORMATION, OGALLALA GROUP IN THE GENERAL TYPE AREA

Fluvial sediments of the Ash Hollow Formation at Lake McConaughy, Nebraska, mapped under USGS STATEMAP, are mostly massive fine to very fine sandstone (Sm) and massive coarse siltstone to sandy siltstone (Fm). These lithofacies exist in sheets generally less that 3 m thick and in some channel fills. Both Sm and Fm contain fine siliceous rhizoliths (pervasive in some Sm units), insect burrows, and long, low-angle vertebrate burrows ≤ 40 cm in diameter. Vertebrate burrows sometimes contain laminated “megageopetal” sediment fills. Sm exhibits filled vertical and horizontal desiccation cracks and columnar jointing. Fm frequently exhibits strong platy to moderate subangular blocky soil structure and reddening. Low-angle lateral accretion surfaces extending several meters are common in Sm and Fm units, and bar forms exist in Sm units.

Much rarer are trough cross-stratified, fine to very coarse pebbly sandstone (St), and trough cross-stratified (sets as thick as 1.5+ m), granule to boulder conglomerate (Gt). These lithofacies appear in sheets and in at least one lateral-accretion unit. Intraformational clasts in Gt reach ~90 cm in length and can be imbricated. Gt appears with St, but St appears in some cases without Gt. Very fine to fine sandstone with low-angle to horizontal lamination (Sh) and fine to coarse sandstone with ripple cross lamination in 1-3 cm sets (Sr) also appear in sheets generally < 0.5 m in thickness. Lithofacies Fl (laminated claystone to medium siltstone) and C (calcareous siltstone to silty carbonate) occupy the least volume overall, but they appear frequently.

St and Gt record deposition of coarse bedload on channel beds and point bars. Lateral accretion surfaces (LAS) appear within an association of St+Sm±Sh (overlying St±Gt) and indicate the predominance of deposition on point bars in laterally migrating stream channels. Active channels probably were ≤ 4.5 m deep. Association Fl+C (±Sr) appears in widespread thin sheets or very shallow (~30-60 cm) lenses that are the terminal fills of abandoned channels. Lacustrine diatomites also appear in the regional succession. Fining-upward trends dominate.

Spectacular soft-sediment deformation involves Sm, Fm, Fl, Sr, and C. Funnel-like structures consisting of synformal folds attain depths of 2 m. One tight antiformal fold 2+ m in height exists in Fl+C.