Part 4

Veatch (1907:99) originally considered what is now called the Bullpen Member as being the Bridger Formation, this was based mostly on the Bullpen’s position above the Green River Formation rather than on any lithologic resemblance to the Bridger. The Bullpen is much redder in color than the Bridger.

In early work Tracey and Oriel (1959:729) called these beds an upper tongue of the Wasatch and have since named them the Bullpen Member.

The Bullpen Member is lithologically very like the Main Body of the Wasatch and can be traced into the peripheral units of the Wasatch Formation.

The rocks included within the Bullpen Member are layered sequences of red, pink, gray, and green claystone and mudstone. Bentonite is present in some claystone beds and causes sloughing because of its property of expanding when wet. Some sandstones are present in the northern areas of the member’s distribution. Limestones are also present. They are thin and slabby and have a brown, white, or gray color. Some contain varying amounts of clay. A conglomerate is present in the upper part of the Bullpen toward the basin periphery and merges with the Tunp Member of the Wasatch Formation.

The Bullpen Member is conformable with the underlying Angelo Member of the Green River Formation. The contact is a transitional one reflecting a gradual change from the lake environment of the Green River Formation through a swamp environment to that of a flood plain. The light-colored, fine-grained shales, marlstones, and limestones of the Green River Formation grade upward into drabber, coarser grained clay and mudstones of the Bullpen Member as a reflection of this change of environments.

TUNP MEMBER.

This peripheral unit of the Wasatch Formation was first described by Oriel and Tracey in 1970. It is not exposed in the monument, but forms a belt of outcrops around the edge of Fossil Basin and in channels cut at right angles to the basin edge.

The Tunp Member is seen to intertongue with nearly all of the Wasatch and Green River formations. The member grades laterally basinward from coarse, angular conglomerates to fine mudstone. Two limestone tongues of the Green River Formation are interbedded with the Tunp, indicating that at least twice there was major expansion of the lake.

Lithologically, the Tunp Member is a diamictite. This is a sedimentary rock with a wide range of particle sizes. The Tunp is best described as a red, conglomeratic, sandy mudstone with angular, poorly rounded to smooth, well-rounded clasts with a size range from pebble to boulder. There is no bedding or orientation of clasts.

The Tunp Member probably originated from mudflows and gravity sliding (Tracey et al. 1961). Environmental studies indicate that the area had a warm, humid climate. This would cause deep weathering of the surrounding slopes. This weathered material would then be a prime source to be acted upon by rain, gravity, and possibly earthquakes. These agents of deposition would then cause the material to flow and be deposited with little chance for sorting and rounding of the rock particles. The result was a belt of coarse, unsorted detritus on the basin edges now called the Tunp Member.

AGE OF THE WASATCH FORMATION.

The exact age of the units within the Wasatch Formation can be determined only if fossils are present. None has been found in the basal conglomerates but it is believed to be earliest Eocene. The Lower Member is also not dated with fossils but is believed to be very early Eocene. A number of fossil mammals are known from the Main Body of the Wasatch Formation. The lower part is early, early Eocene as demonstrated by the presence of a very primitive ungulate, Haplomylus speirianus. In the upper part of the Main Body another primitive ungulate, Hyopsodus browni, is found which indicates a mid to early Eocene age. The Mudstone and Sandstone tongues are not dated by means of fossils but the stratigraphic relationships indicate an early Eocene age. Fossil mollusks in the Bullpen Member are not completely diagnostic but suggest either a late early Eocene (Lostcabinian) (Fig. 6) or mid-Eocene age. The Tunp Member interfingering as it does with the entire Wasatch section represents all of early Eocene age.

DEPOSITIONAL ENVIRONMENT.

The sediments that make up the Wasatch Formation in the Fossil Basin were deposited mainly by streams flowing into the basin from the surrounding uplands. Rock types are variable and for the most part individual types cannot be traced over large areas, a condition typical of fluvial environments. The mudstones and siltstones probably were deposited along flood plains, while the lenticular sands and conglomerates were laid down in stream channels. Where streams entered the lake tongues of deltaic deposits, sands or mudstones wedge into the Green River Lake sediments.

The reason for the red color of the Wasatch Formation is not fully known. According to Van Houten (1948), the red is most likely the result of tropical, red lateritic soils forming in the uplands and being deposited on flood plains. The oxidation of organic material in a humid, subtropical environment causes the red color. The bands of purple, gray, orange, and other colors are due to various stages in the reduction of the iron oxides in the rocks.

The presence of palms in the lake sediments and of bones of primates and crocodiles in both the lacustrine and fluvial sediments indicate a heavily forested, tropical environment.

Green River Formation

The Green River Formation is easily recognized by its light color and continuous bedding, in strong contrast to the red, discontinuous, variegated Wasatch Formation below. The Green River Formation can be thought of as a gigantic lens of lacustrine sediments enclosed in the fluvial Wasatch Formation. The name Green River Formation is applied to all of the roughly contemporaneous deposits laid down in lakes of Utah, Colorado, and Wyoming. It is probable that at one time or another all of these lakes were connected.

Oriel and Tracey (1970) have divided the Green River Formation of the Fossil Basin into two members: the Fossil Butte Member and the Angelo Member.

FOSSIL BUTTE MEMBER.

The type section for this member is near the southeastern end of Fossil Butte within the monument boundaries. In the type area the Fossil Butte Member can be seen to consist of four lithologic units.

The lowermost unit is predominantly mudstone. It occurs in a sequence about 45 ft thick and contains light gray, fine-grained, calcareous mudstone and siltstone.

The next overlying unit is about 75 ft thick and is mainly a limestone unit. It consists of tan to gray limestone, shaly limestone, siltstone, and paper shale which weathers into thin, curled flakes. A yellow-brown mudstone tops this unit.

The third unit in the sequence is 45 ft thick and mainly composed of shales. These shales weather a buff color and are calcareous. Oil shale, organic rich paper shale, and marlstone comprise the actual layers in the unit. A few, thin ash beds are also present. This unit is the most significant one for Fossil Butte National Monument. About 10 ft below the top of this unit (about 155 ft above the Wasatch) is a bed of varved shales, one foot thick, that contains the fossil fish for which the monument was established.

The uppermost unit contains a number of beds of oil shale that are brown on a fresh surface but weather a grayish-white. An orange-yellow-weathering limestone caps the 40-ft thick upper unit. Ash beds are common and traceable over a wide area of Fossil Basin.

South of the monument the Sandstone Tongue of the Wasatch Formation wedges in between the lowermost unit and the overlying limestone unit of the Fossil Butte Member.

In the Fossil Butte Member an interesting sequence of facies changes can be seen which reflect lateral changes in the environment of Fossil Lake. In those areas where the member represents a deep-water environment, organically formed limestones and shales are predominant. Shoreward, these rocks grade into ostracodal limestone, gastrapodal limestone, and, closest to shore, algal limestone. Then a muddy, sandy beach facies is encountered where the Green River and Wasatch formations intergrade.

The contact with the underlying Wasatch is conformable and sharp. It is often marked by a bench or by slump blocks of Green River Formation.