Abstract

The Tertiary Green River Formation occupies three large areas in the Piceance Creek basin. This basin and its surrounding uplifts have a well-documented history beginning with basin inception during the Laramide orogeny that began 65 to 62 Ma. The first deposits formed in the basin were Paleocene and early Eocene fluvial and paludal sediments of the Ft. Union and lower Wasatch Formations that rest unconformably on folded and erosionally-truncated Late Cretaceous rocks. Low-grade oil shale and other lacustrine deposits of the Green River Formation began to form in Lake Uinta that came into existence in the deeper northern portion of the basin somewhat later in the early Eocene. Fluvial Wasatch sediments continued to form around the lake margins. In late early to early middle Eocene time, the lake deepened and expanded to cover the entre basin. High-grade oil shale began to form throughout the basin and, as salinity increased, saline deposits accumulated in the deeper northern portion. These high-grade oil shales and related rocks have been the subject of many geologic investigations dealing with depositional facies, sedimentary structures, lithologic character, correlations, subdivisions, and origins. Many of these facets are discussed briefly herein.

In middle to late Eocene time, a large delta of volcaniclastic debris (Uinta Formation) prograded southward across the basin and finally obliterated Lake Uinta and the basin it occupied, about 43 Ma.