Abstract: Sedimentologic Framework of the Lower Fort Union Formation (Paleocene), Eastern Great Divide Basin, Wyoming, and Implications for Tectonic Influence on Coal-Forming Environments

James G. Honey, Stephen B. Roberts

The lower Fort Union Formation (Paleocene) is coal-bearing in a 490 meter-thick interval along the eastern flank of the Great Divide basin. Stratigraphic sections measured over 50 km from Hadsell north to Bairoil record a decrease in number and thickness of coal beds, thinning of the coal-bearing interval, and changes in fluvial sandstone morphology. Near Hadsell, coals up to 6 m thick are interbedded with amalgamated sandstones up to 20 m thick; paleocurrent data indicate an upsection change from easterly to southerly paleoflow. Equivalent strata near Bairoil contain coals less than 1 m thick interbedded with 2 to 4 meter-thick ribbon sandstones; limited paleocurrent data suggest persistent southerly paleoflow. The coal-bearing interval thins northward due to facies variations and er sion below an angular unconformity. In all measured sections there is an upward decrease in coals and an increase in coarse clastics.

Paleocene coal formation in the eastern Great Divide basin was constrained by changing tectonic influences. Early Paleocene coal development proximal to the Granite Mountains (Balroil) was disrupted by rapid subsidence and sediment influx associated with thrust uplift; thick coals were restricted to intrabasinal settings (Hadsell) distal to Granite Mountains influence. Subsequent rise of the Rawlins uplift in the early to late Paleocene, recorded by a change in paleocurrents and development of an angular unconformity, formed uplands proximal to coal depocenters near Hadsell; increased subsidence and sediment influx associated with this uplift terminated coal development in the southeastern Great Divide basin until latest Paleocene time.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994