Prosenance, Dispersal, and Tectonic Significance of the Evanston Formation and Sublette Range Conglomerate, Idaho-Wyoming-Utah Thrust Belt

Tectogenic sediments of the latest Cretaceous-Paleocene Evanston Formation were deposited in proximal braided streams in northeastern Utah, and in distal gravelly rivers in the Fossil basin of southwestern Wyoming. Paleocurrent data provide evidence for a north-to-south axial drainage system in both areas. The ubiquitous presence of Precambrian-Cambrian clast types throughout the Evanston Formation indicates a source area in the present day Bear River Range (Paris and Willard thrust plates). The Sublette Range Conglomerate is a crudely stratified, clast-supported, proximal braided stream deposit. Paleocurrent data and clast lithology also indicate a north-westerly source area on the Paris and Willard plates. It is proposed that the undated Sublette Range Conglomerate is a remnant of a proximal deposit which linked distal Evanston sediments in the Fossil basin with their source area on the Paris and Willard plates. Early Eocene reactivation of the Crawford thrust resulted in the present elevated position of the Sublette Range Conglomerate.