Paleocene-Early Eocene Sedimentation and Depositional Environment in the Washakie Basin, Southwestern Wyoming, USA: Implications for Tectonic Evolution During the Overlapped Sevier and Laramide Orogenies

Abstract

The central Rocky Mountain area in Wyoming was situated in the broad foreland of the Sevier fold-and-thrust belt during the Cretaceous-Paleocene, and was partitioned into a group of intermontane basins and mountain ranges by the Laramide orogeny during the latest Cretaceous-early Eocene. How the overlapped Sevier and Laramide orogenies influenced paleogeography, drainage patterns, and thereby sedimentation in the intermontane basins require detailed sedimentology and provenance studies. In addition, hydrocarbon production in the lower Paleogene strata in some of the intermontane basins highlights the importance of sedimentologic characterization of the strata. Presented here are detailed lithofacies description and interpretation, and sandstone petrography and paleocurrent direction data collected from a measured ∼1200 m thick continuous stratigraphic section covering the Paleocene-early Eocene in the Washakie Basin, southwestern Wyoming. The Paleocene Fort Union Formation consists of multiple fining-upward sequences of fine- to medium-grained sandstone, siltstone, and carbonaceous shale and coal deposited in low-energy fluvial environments. The lower Eocene Wasatch Formation contains massive granule-pebble conglomerate and thick trough cross-stratified sandstone that were deposited in high-energy fluvial channels and red mudrock and paleosols formed in floodplain environments. Paleocurrent direction inferred from trough limb orientations was northward during the early Eocene, suggesting the rivers most likely drained the Uinta Mountains to the south of the basin. Preliminary interpretation of depositional environment seems to show that the Laramide exhumation of the Uinta Mountains caused the change of paleogeography and regional drainage patterns during the early Eocene, resulting in a more proximal depositional setting in the study area and thereby caused the high-energy style sedimentation. Sandstone petrography data will assist the interpretation of drainage patterns and paleogeography. This study allows for a better understanding of the depositional history of the Washakie Basin during the transition from the Sevier orogeny to Laramide orogeny.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016