Wyoming and adjacent areas (from Oriel and Tracey 1970).

Fig. 3. Physiography of the Fossil Butte National Monument region (from U.S. Department of the Interior 1964).

Hayden’s report (1871:425-437) included a description of the fish fossils from Petrified Fish Cut by E. D. Cope, a vertebrate paleontologist. Several years earlier Leidy (1856), another paleontologist, described a fish from an unknown locality in the Green River Formation.

Hayden divided this survey area into several districts. The Green River district was put in charge of A. C. Peale. Peale’s (1879:535) report contained the first geologic description of Fossil Butte itself, as well as a short discussion of the fish fossils obtained there. The description of the butte is quoted here:

... In the lower part of the bluff from which these specimens are taken, the bright coloured beds of the Wasatch are seen outcropping, although the entire section cannot be seen, as their softness causes them to weather so that the debris conceals the strata. The fossils are found at several horizons in the shales. Near the top of the bluff is a band of hard, bituminous, or oily shale, which burns rather freely with a strong bituminous odor.... It is brownish-black in color and on the weathered surfaces a bluish white....

The next major publication dealing with Fossil Basin was that by Veatch (1907). He mapped the rocks in certain areas of the basin in an attempt to bring a semblance of order to Hayden’s (1869) broad stratigraphic descriptions. His work resulted in the designation of most of the rock units in Fossil Basin from Jurassic to Recent.

Schultz (1914), an associate of Veatch, published a paper dealing with the structures to the north and east of Fossil Basin. This helped to delineate the features surrounding and forming the basin.

Subsequent work on the geology of southwest Wyoming mainly has been concentrated on the Green River Basin just east of Fossil Basin. Significant work on Fossil Basin has been done by Rubey et al. (1968a, b) who mapped the northern part of the basin, and by Oriel and Tracey (1970) who have published results of the latest study on the stratigraphy and age of the rock units in Fossil Basin. This latest work is the culmination of many years of study and incorporates and refines data presented in shorter, earlier papers.

STRATIGRAPHY

The complete geologic history of the Fossil Basin involves many stratigraphic units. Most of these are not exposed within the monument itself but are well known from outcrops outside the boundaries of the monument and from deep wells (Fig. 4).

At the base of the stratigraphic section are rocks of Precambrian age. These have not been penetrated by wells within the Fossil Basin but should occur as much as 25,000 ft beneath the surface. They probably consist of metamorphic and intrusive rocks. Beginning about 600 million years ago, at the beginning of Cambrian time, sedimentary deposits accumulated. These are both marine and continental in origin and range in age from early Cambrian (600 million years old) to early Tertiary (50 million years old). Some 34 geologic formations are recognized in this thick section, a list of which follows:

Within the monument itself the oldest rocks exposed are those of the Thaynes Limestone near Prow Point. As will be seen in the discussion of the structure of the Fossil Basin, all of the rocks beneath the Evanston Formation are folded and faulted and within the monument are buried by deposits of Tertiary age. For this reason detailed discussion of stratigraphic units will be confined to those rocks that can be seen within the boundaries of the monument or closely adjacent to it (Fig. 5). The distribution of the various formations within the monument is illustrated in Figs. 7 and 8.

Fig. 4. Geologic time scale.

Thaynes Limestone

At the very northeastern corner of Fossil Butte National Monument can be seen an outcrop of the Thaynes Limestone of Triassic age, a marine unit deposited some 200 million years ago. This formation is noted for its rich and varied assemblage of marine fossils, mostly forms related to oysters and clams. The Thaynes Limestone consists predominantly of sandy, gray limestone and green-gray limey siltstones. The formation weathers to a dark brown in the lower part and a grayish-yellow in the upper. The fact that the Thaynes Limestone is exposed in the monument at all is because of the faulting that took place just prior to the deposition of the Evanston Formation of latest Cretaceous and