Abstract

The Willard thrust in the Wasatch Mountains immediately east of Ogden, Utah, is exposed for a distance of 20 miles and has a low dip eastward. It emplaces younger Precambrian strata on Paleozoic strata and older Precambrian crystalline rocks, in which the younger Precambrian beds are missing. Since the discovery of the thrust by Blackwelder in 1910, a number of writers have postulated different connections with other thrusts, opposite directions of translation, and various origins. In the attempt to reproduce the Late Mesozoic paleogeologic outcrop pattern, which would predate the extensive cover of Paleocene and Eocene clastics and volcanics, and eliminate the effects of Basin and Range faulting, a significant picture appears. A north-south uplift, here called the Cache, appears between the Willard thrust on the west and the Woodruff thrust on the east. It extends northward into Idaho and in size and shape is somewhat like the east-west Uinta Mountains uplift. The south end is terminated by a fault that connects the east flank Woodruff thrust with the west flank Willard thrust.

At the southwest corner of the Cache uplift, the Willard thrust has the greatest observable horizontal displacement. Here, the Cache uplift impinges on the Northern Utah uplift, and here also has been the greatest amount of uplift. With the greatest uplift, there has developed a potential for gravity-caused downslope mass movement, and the Willard thrust sheet resulted. Such a hypothesis demands southwestward translation, and, thus, drag phenomena and stratigraphic thicknesses and sedimentary facies are considered. From the writer's point of view, none of these seems to vitiate the concept that the Willard came from the east; in fact, they support the idea.

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