First Comprehensive Lithofacies and Stratigraphic Architecture Analysis of the Ash Hollow Formation, Ogallala Group: A Model for Modern Fluvial Reservoirs

Fluvial sediments of the Ash Hollow Formation around Lake McConaughy, Nebraska offer the opportunity to examine a modern analogue to petroleum producing fluvial reservoirs. The fluvial sediments in the Ash Hollow Formation are dominated by massive fine to very fine sandstone (Sm) and massive coarse siltstone to sandy siltstone (Fm). These lithofacies exist in sheet-like bodies generally less than 3 meters thick and, in the case of the fine sandstones, as channel fills. Both fine sandstone (Sm) and coarse to sandy siltstone (Fm) lithofacies contain fine siliceous rhizoliths and insect burrows. Sm exhibits filled vertical and horizontal desiccation cracks and columnar jointing. Fm exhibits strong platy to moderate subangular blocky soil structure and reddening. Low-angle lateral accretion surfaces extending several meters are prominent in some Sm and Fm units. Bar forms exist in Sm units. 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 sheet-like bodies and in one lateral-accretion unit. Intraformational clasts in Gt reach maximum diameters of ~ 90 cm. 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 sheet like bodies generally less than 0.5 m in thickness. Lithofacies Fl (laminated clay to medium siltstone) and C (calcareous siltstone to silty carbonate) occupy the least volume overall, but they appear in many sections.

Lithofacies association Fl+C appears in thin sheets or fills of very shallow (~30-60 cm) lenses: the terminal fills of abandoned stream channels. Lateral accretion surfaces (LAS) appear within an association of St+Sm±Sh, overlying St±Gt. LAS indicate common deposition on point bars in laterally migrating stream channels. Fine to very coarse pebbly sandstone (St) and granule to boulder conglomerate (Gt) record the deposition of coarse bedload on channel beds and point bars. Spectacular soft-sediment deformation exists and chiefly involves Sm, Fm, Fl, Sr, and C. Multiple, funnel-shaped structures consisting of synformal folds attain depths of 2 m. One tight isoclinal antiformal fold 2+ m in height exists in Fl+C. Long, low-angle vertebrate burrows as much as 40 cm in diameter are common and they sometimes contain laminated “megageopetal” sediment fills.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California