Paleoenvironments and Petrology of Virgelle Member, Eagle Formation of the Elk Basin (Wyoming and Montana)

Joaquin Owens and Henry Chafetz

The Campanian Eagle Formation in Elk Basin, Wyoming and Montana, and correlative strata representing nearshore sedimentation are relatively well-described in the southern Bighorn Basin where outcrops are nearly continuous along the southern margin. However, detailed stratigraphic and sedimentological studies have not been reported for the Elk Basin deposits. Field observations indicate that the well-exposed Virgelle Member of the Eagle Formation in the Elk Basin can be informally divided into upper and lower members. The paleoenvironment of the lower Virgelle consists primarily of an eastward prograding shoreface. Swaley cross-stratified, middle shoreface facies with thicknesses up to 12 meters are conspicuous cliff-forming units in the Elk Basin. Hummocky cross-stratified, lower shoreface facies are relatively thin (~ 2 meters). Upper shoreface facies are apparently absent. At the southern end of the Elk Basin, tidally influenced channel-fill and bay-fill deposits are exposed within the coastal system facies of the lower Virgelle. Lower Virgelle coastal system facies in the Elk Basin are unconformably overlain by tide-dominated estuarine deposits of the upper Virgelle in an incised valley oriented nearly parallel to the shoreline. Petrographic analyses indicate that samples from both shallow marine and estuarine paleoenvironments are dominantly litharenites or feldspathic litharenites. The relative abundance of feldspar in samples, averages 13% of framework grains, indicates that provenance determinations do not conform well to published reports of foreland basin fill. This abundance of detrital feldspar in a foreland basin fill is likely a reflection of a significant volcanic contribution.

AAPG Search and Discovery Article #90156©2012 AAPG Rocky Mountain Section Meeting, Grand Junction, Colorado, 9-12 September 2012