Part 3

Eocene Late Eocene Uintan Mid Eocene Bridgerian Early Eocene Wasatchian Lostcabinian Lysitean Graybullian Paleocene Late Paleocene Tiffanian Mid Paleocene Torrejonian Early Paleocene Dragonian Puercan

The Almy and Knight, as defined by Veatch, are probably different facies of Wasatch, the Almy being a more peripheral facies and the Knight a more basinal facies (Oriel and Tracey 1970). These authors proposed that the terms Almy and Knight be dropped and that the name Wasatch be applied to all these rocks to avoid confusion.

The latest subdivision of the Wasatch Formation in the Fossil Basin is that of Oriel and Tracey. The members of the Wasatch Formation they propose are described in order from oldest to youngest.

BASAL CONGLOMERATE MEMBER.

This member has only local development in Fossil Basin. Where found, it is a lenticular conglomerate with pebbles and cobbles of buff and tan sandstone from the Nugget Sandstone and limestone fragments from the Thaynes and Twin Creek formations.

The basal conglomerate is essentially a channel fill in ancient stream beds cut into Mesozoic rocks.

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).

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

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.

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.

The most remarkable feature of the Main Body is its color. On the lower slopes of Fossil Butte are bands of bright to dull red, pink, purple, yellow, and gray color arranged in various patterns. This unit is best observed at a distance, especially after a rain. The bright hues of the Wasatch contrast markedly with the whites and tans of the overlying Green River Formation.

Individual bands of color range from 1 to 10 ft thick (Oriel and Tracey 1970:78). The colors are brightest in the upper part of the member and drabber in the coarser-grained lower part.

The predominant rock types in the Main Body are banded, variegated mudstone with interlayered sandstone, conglomerate, marlstone, siltstone, and claystone.

The upper part of the Main Body is mainly mudstone composed of fine silt and very fine, bedded sand with a clay binder. Conglomerates occur as channel fills and contain calcium carbonate as cement as do a number of sandstone and siltstone layers in the upper Main Body.

Conglomerates and sandstone are more common in the lower part of the Main Body. Some are part of Veatch’s old Almy Formation. They are best developed along the edge of Fossil Basin. The Main Body overlaps the Evanston Formation in some places and may rest directly on Mesozoic or Paleozoic rocks.

SANDSTONE TONGUE.

This is a tongue of cross-bedded sandstone. It is brown in color and is composed mostly of quartz with some black chert grains. This unit is limited in distribution to the south of Fossil Butte National Monument. The Sandstone Tongue thins and pinches out to the north and is not present within the monument. The area of pinchout represents the shoreline at one stage of Fossil Lake. The Sandstone Tongue wedges into the Fossil Butte Member of the Green River Formation. The sediment sources were probably the Uinta Mountains at the south edge of Fossil Basin. Tectonic events caused an uplift and erosion of Mesozoic and Paleozoic rocks in the Uintas and debris was deposited in Fossil Lake as an encroaching delta of sand. When deposition of the sand halted, the lake expanded and covered the sand, encasing it within the shales of the Green River Formation.

The Sandstone Tongue can be traced into the Main Body of the Wasatch, hence its assignment to the Wasatch Formation.

MUDSTONE TONGUE.

This tongue of the Wasatch Formation can be seen in the northwestern part of Fossil Butte National Monument and over most of the northern part of Fossil Basin. In the area of its distribution, the Mudstone Tongue separates the underlying Fossil Butte Member of the Green River Formation from the overlying Angelo Member of the Green River Formation. The tongue thins and disappears to the south, that is, basinward. As with the Sandstone Tongue, the edges of the Mudstone Tongue where it pinches out represent an ancient shoreline of Fossil Lake. To the north and west the Mudstone Tongue merges with the Tunp Member of the Wasatch Formation.

The Mudstone Tongue is a composite of dark-red mudstone which becomes lighter in color basinward, changing to light red, pink, or greenish-gray claystone. The unit is a mixture of silt and clay derived from the north and west, and was deposited as a large delta in Fossil Lake.

An interesting aspect of the Mudstone Tongue is the presence of algal logs. These are cylinders of limestone that apparently formed as an encrustation of calcium carbonate around logs and branches that fell into the edge of the lake. The calcium carbonate resulted from the action of algae which grew around the log. Successive growths of algae resulted in successive layers of calcium carbonate being deposited.

BULLPEN MEMBER.

This uppermost member of the Wasatch Formation is found mostly to the west and south of the monument. A few, small, isolated caps on the top of the Green River Formation east of Prow Point occur within the monument. These low hills, mere bumps, are remnants of a once more extensive distribution of the Bullpen Member.