Abstract: Syntectonic Sedimentation and Coal Occurrence in the Paleocene Fort Union Formation, Sand Wash Basin, Northwestern Colorado and Southwestern Wyoming

Roger Tyler

An understanding of syntectonic sedimentation processes within Tertiary Rocky Mountain foreland basins facilitates the prediction of coal occurrence and producibility of natural gas. To provide a basis for making such predictions in the Sand Wash basin, the tectonic and stratigraphic settings of the Paleocene Fort Union Formation, a major natural gas exploration target, are described. The principal tectonic and depositional features within the formation resulted from Late Cretaceous to early Oligocene Laramide deformation. Syntectonic sedimentary sequences in the basin include an areally narrow zone of conglomeratic facies adjacent to basement-floored thrusts, thick intermontane fluvial and floodplain sandstone/mudstone/coal facies above the structural and depositional axes, an areall wide zone of distal sandstone/mudstone/coal facies, and a basinal thrustward-thickening mudstone

facies. Lithofacies maps show that intermontane fluvial sandstone sequences in the Fort Union Formation were deposited axially by north-flowing rivers. Coal thickness and coal-seam continuity are greatest in the lower Fort Union Formation where coals are predominantly found associated with bed- and mixed-load channel-fill sandstone sequences. These fluvial sandstones served as stable platforms to optimize subsidence rates and to provide the hydrologic conditions necessary for peat accumulation. Coal-occurrence maps show that maximum coal development corresponds to floodplain deposits above and on the flanks of the north-flowing fluvial systems, confirming a direct relation between the position of the streams that deposited the sandstones and the accumulation of organic matter.

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