Regional Implications of an Extensive Linear Sediment-Dispersal System Along Western Margin of Cretaceous Interior Seaway: Second Wall Creek Sand, Powder River Basin, Wyoming

Carl F. Vondra, Nazrul I. Khandaker

The Second Wall Creek sand in the Powder River basin in Johnson and Natrona Counties is similar in clast lithology, primary sedimentary structures, and facies association to the Torchlight Sandstone at the top of the Frontier Formation in the northern Big Horn basin. The Second Wall Creek sand is predominantly composed of medium to coarse-grained, moderately sorted massive to cross-bedded quartz-lithic wacke with a minor amount of carbonaceous shale and siltstone. The unit is conglomeratic at the top and contains abundant granule to cobble-size clasts of andesite, quartzite, chert, granite, and sandstone. The largest clasts are concentrated in the Kaycee-Mayworth area in Johnson County and progressively decrease in size southward toward Arminto in Natrona Country. Paleocu rent directions obtained from the cross-bedded unit indicate a general south-southeast trend. The Second Wall Creek sand is thickest in the Kaycee-Mayworth area and thins southward toward Arminto. The presence of a unique petrologic suite places a constraint on provenance and sediment-dispersal patterns in a tectonically active rapidly evolving foreland basin. Of particular interest is the peculiar lensoid distribution of andesite clasts, which follow a general northwest-southeast trend for more than 150 mi from Cody to the Kaycee-Mayworth area, Wyoming. Noticeable absence of andesite clasts on either side of this observed trend suggests a strong dependence of the ultimate sediment-dispersal system on several physical constraints, including local morphotectonic setting, paleohydraulics, nd provenance. A large high-energy distributary complex is invoked for the deposition of this linear conglomeratic facies. This dispersal system extended east-southeastward from the orogenic fold-and-thrust-belt into the adjoining foreland basin.

The observed field relationships and petrographic data have important tectonostratigraphic implications concerning Late Cretaceous sedimentation in the Western Interior. In addition to its great potential as an oil-bearing horizon in both Big Horn and Powder River basins, this sediment-dispersal pattern will aid in reevaluating the regional sandbody architecture and reservoir characteristics of the upper Frontier Formation.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.