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Department of Geology and Geophysics, University of Wyoming, Laramie, WY

Abstract: Architecture and Facies Changes Within the Wave-Dominated Deltaic Twentymile Sandstone, Upper Cretaceous Mesaverde Group, NW Colorado

The Twentymile Sandstone (TS) has an anomalous character: unusual thickness (45 in, compared to 15 in for a typical shoreface) and rapid thinning (39 in in 2.7 km). Stacked parasequences, composed of deltaic shoreface sands, account for the TS?s thickness. Two to 3 parasequences (8-22 m thick) form a regressive set, with a typical outcrop succession composed of a transitional facies at the base (interbedded siltstones and thin wave-rippled hummocky cross-stratified fine sands) with medium-grained, cross-bedded sands at the top. The thickest exposure of this sandstone body (45 in) thins to 6 in in 2.7 km to the SE, then pinches out entirely within a kilometer. Another pinch-out occurs 7 km due east, with the thick sand body exposed as two small sands (each ~ 1 m), which disappear within a few hundred meters. A slightly oblique exposure (of the thick 45 in sand body) shows clinoforms dipping at ~2 degrees indicating deltaic progradation to the SE. The internal facies of the TS generally consists of a lower section (4-26 in), characterized by wave rippled hummocky/swaley cross-stratification, and horizontal beds containing the clinoforms, and an upper section (2-19 in) recognized by an increase in grain size, decrease in bioturbation, and (dominantly) trough and planar cross bedding. Trough cross beds are associated with upper shoreface long shore drift (generally in proximal sections), with trough cross bedding found in distributary channels frequently near the deltaic pinch-out. Planar cross bedding (beach deposits?) is seen occasionally. The rapid thinning of this sandstone tongue produced distinctive internal architectural and facies changes characterized in this study.

AAPG Search and Discovery Article #90919©1999 AAPG Rocky Mountain Section Meeting, Bozeman, Montana