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).
[Illustration: Fig. 7. Geologic map of Fossil Butte National
Monument (after Rubey et al. 1968).]
LEGEND
Quaternary—Qg-Qaf—Gravel & stream deposits
Eocene
Green River Fm.
Tga—Angelo Member
Tgf—Fossil Butte Member
Wasatch Fm.
Twb—Bullpen Member
Twm—Mudstone Tongue
Tw—Main Body
Twl—Lower Member
Triassic TR†—Thaynes Limestone
[Illustration: Fig. 8. NW-SE section across Fossil Butte National
Monument, Sage and Kemmerer quadrangles (mapped by Rubey et al.
1968).]
Eocene
Twb—Bullpen Member of Wasatch Fm.
Tga—Angelo Member of Green River Fm.
Twm—Mudstone tongue of Wasatch Fm.
Tgf—Fossil Butte Member of Green River Fm.
Tw—Main body of Wasatch Fm.