LOWER MEMBER.
This is an irregular sequence of flood-plain and stream-channel deposits. It is exposed along the southern part of the Tunp Range and extends into the far western section of Fossil Butte National Monument just below Prow Point.
Mudstone is the main rock type. It can be tan, brown, pink, red, or gray in color. Black, carbonaceous siltstones are also present. Gray sandstone that weathers yellow or brown and coarse-grained, cross-bedded conglomerate and sandstone are also prominent. Limestone occurs as thin lenses and is often brown, platy, and carbonaceous.
It is interesting to note that the Lower Member is intermediate not only in stratigraphic position but also in color and composition between the underlying Evanston Formation and overlying Main Body of the Wasatch Formation. The Lower Member thus appears to represent a gradual change in either climatic and/or sedimentary conditions in Fossil Basin (Oriel and Tracey 1970).
Fig. 7. Geologic map of Fossil Butte National Monument (after Rubey et al. 1968).
Fig. 8. NW-SE section across Fossil Butte National Monument, Sage and Kemmerer quadrangles (mapped by Rubey et al. 1968).
MAIN BODY.
This unit of the Wasatch Formation is that part of the formation which produces the spectacular red-colored badlands in Fossil Butte National Monument. Particularly typical exposures can be seen in the south-facing scarp of Fossil Butte where the Main Body makes up the lower portion of the butte.