--> Abstract: Depositional Setting of Paleocene Coals and Conglomerates in the Southwestern Bighorn Basin, Wyoming, and Implications for Timing of Washakie Range Uplift, by S. B. Roberts; #91017 (1992).

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ABSTRACT: Depositional Setting of Paleocene Coals and Conglomerates in the Southwestern Bighorn Basin, Wyoming, and Implications for Timing of Washakie Range Uplift

ROBERTS, STEPHEN B., U.S. Geological Survey, Denver, CO

Stratigraphic analysis of coal-bearing and conglomeratic facies in the southwestern Bighorn basin has shown that rocks in the lower Ft. Union Formation near the Grass Creek coal mine contain a 115-m-thick quartzite conglomerate that is laterally equivalent to thick middle Paleocene coal deposits (palynomorph biozone P3). The conglomerate may be a record of tectonism in the Washakie Range within 3040 km to the southwest.

Coal beds in the mine are up to 11 m thick, and occur within a 70-m-thick interval composed primarily of flood-plain mudrock, tabular splay sandstones, and scour-based, fluvial-channel sandstones. Discontinuous exposures of conglomerate, including debris-flow deposits of diamictite (conglomeratic mudstone), occur below, within, and above the coal-bearing interval near the mine. Within 10-12 km southeast of the mine, the coal-bearing interval thins and pinches out into a sequence of quartzite conglomerate.

The conglomeratic sequence is characterized by amalgamated braided fluvial and debris-flow deposits. Braided fluvial deposits consist of massive and cross-stratified, framework-supported conglomerate, scour-based, cross-stratified conglomeratic sandstone, and lenses of cross-stratified to parallel-laminated sandstone. Debris-flow deposits are characterized primarily by nonsorted, matrix-supported gravels. Clasts are dominantly well-rounded quartzites, and paleocurrent readings (n = 72) indicate northeastward sediment transport.

The conglomeratic sequence is interpreted as part of an alluvial fan/braid-plain system that developed during Laramide uplift of the Washakie Range. Quartzite clasts reflect reworking of conglomerates of the Upper Cretaceous-Paleocene Pinyon and/or Harebell Conglomerate, deposited in areas west and southwest of Grass Creek prior to uplift. Coals developed lateral to and coeval with braided tributaries on inferred, distal fan reaches. Based on the relationship of conglomerate deposits and coals of P3 age, the Washakie Range must have been positive in the early-middle Paleocene.

 

AAPG Search and Discovery Article #91017©1992 AAPG Rocky Mountain Section Meeting, Casper, Wyoming, September 13-16, 1992 (2009)