MAYWOOD, PAUL S., Atlantic Environmental Services, St. Louis, MO

ABSTRACT: Stratigraphic and Depositional Setting of Fort Union Formation on the Northeast Flank of the Rock Springs Uplift, Wyoming

The Rock Springs uplift is a 100-km-long, doubly plunging, north-south-trending, asymmetrical anticline located in southwestern Wyoming. A Laramide angular unconformity/paleosol horizon separates the uppermost Cretaceous Lance Formation from the overlying Paleocene Fort Union Formation on the northeast flank. Coal deposits have been mapped in both formations and attract conventional strip mining and methane development interests.

The lower Fort Union contains approximately 60% sandstone, 17% claystone, 13% siltstone, 8% coal, and 2% limestone, which exhibit a high degree of lateral and vertical variability. High sandstone percentages and a noticeable lack of lateral accretion surfaces along with mapped ancient channel occurrences in active mine highwalls are interpreted to define sedimentation in low-sinuosity, perhaps braided, stream environments. Significant muscovite grains in sandstones and strong southerly paleocurrents infer a source area in the Wind River Mountains, the only sediment source in the region during the Paleocene with requisite regional metamorphic rocks.

Raised peat swamps and high annual precipitation contributed to an elevated water table, which inhibited clastic influx and promoted prolific floral development. Temperate to near subtropical conditions were ideal for Sequoia, dogwood, palm, birch, walnut, a katsura relative, and several different ferns.

Stream gradients decreased and meander belts stabilized as the clay-dominated (up to 95%) upper Fort Union was deposited. Clays tended to compact and contribute to stable, high-sinuosity flood-plain environments.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.