ABSTRACT: Laramide Basin Subsidence and Fluvial Architecture of the Fort Union and Wasatch Formations in the Southern Greater Green River Basin

Philip L. Johnson

The late Paleocene Fort Union Formation and early Eocene Wasatch Formation exposed around the Rock Springs uplift demonstrate subsidence variations in the southern greater Green River basin. Total unit thickness and distribution of channel sandstones within overbank deposits record differences in subsidence rate across the basin. On the west flank of the Rock Springs uplift, west of the bounding fault, channels have close spacing and thickness is low. On the south flank within the uplift, the thickness values are intermediary but channels are very closely spaced. Away from the uplift on the southeast flank, the thickness is greatest and channels are very widely spaced.

Paleocurrents indicate that rivers flowed southward across the central basin to an eastward-flowing axis trunk river at the southern end of the basin. Both the south and southeast flank area were within the basin axis, but the west flank areas was within the central basin.

Thickness trends represent subsidence variations across the basin. Subsidence was slowest at the west flank area. On the south flank, subsidence was greater, and the highest subsidence rate was on the southeast flank. Generally, thickness indicates increasing subsidence toward the Uinta uplift, but the south flank area is an exception.

Basin subsidence occurred by flexure of the lithosphere under a tectonic load from the Uinta uplift to the south. Thickened lithosphere at the Rock Springs uplift bounding fault was resistant to flexure. Thus, on the south flank near the fault, subsidence was slower than on the southeast flank where the lithosphere was not thickened.

The closely spaced fluvial architecture on the south flank resulted from a narrow basin axis flood plain. A narrow flood plain possibly resulted from the subsidence resistance of thickened lithosphere at the Rock Springs uplift bounding fault or from topographic expression of the uplift itself. A widely spaced channel architecture on the southeast flank reflects widening of the flood plain eastward. Because the central basin rivers had a wider flood plain than the basin axis river, a comparison of architecture across the basin reflects differences in both subsidence and flood-plain width.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990