Part 7

While most of this complex structural history took place before the Tertiary sediments were deposited, some deformation continued into the early Eocene. Near Prow Point, for example, is a fault that developed during early Eocene time. This can be noted in the very northwest corner of the monument, just to the west of Prow Point (Fig. 6). Here, the Lower Member of the Wasatch Formation on the west is faulted against the Main Body on the east (USDI 1964). This fault also is responsible for elevating the Thaynes Limestone so it is exposed (Fig. 10).

PALEONTOLOGY

Fossils of plants, invertebrates, and vertebrates are abundant in rocks exposed in the Fossil Basin. For example, Oyster Ridge received its name because of the well-preserved and abundant fossil oysters found there. Of course, Fossil Butte and Fossil Butte National Monument were so named because of the beautifully preserved fossil fish. A brief review of the main groups of fossils to be found in the basin follows.

Flora

Many of the sediments in the Fossil Basin contain an abundance of fossil plants. These consist of pollen, spores, wood, and leaf impressions. Those of the Green River Formation are best preserved and have received the most study.

Brown (1929, 1934) and Lesquereux (1883) have presented the best information on the flora of the Green River Formation. Much of the present knowledge is based on their work.

Brown (1929, 1934) concluded that the Green River flora is a mixed forest type. It contains some plants common to warm, wet lowlands and others adapted to cooler, drier uplands. Pollen studies support Brown’s original concept that several plant communities, including mountain communities, existed around Fossil Lake.

The flora was situated in an inland, mountain basin. Fossil Lake was in the middle of the basin. Swamps and flood plains bordered the lake, while the flanking ridges and mountains provided altitudinal variations in the plant assemblage.

Spruce, fir, and pine are indicated by fossil pollen. The presence of these conifers suggest that at least some of the nearby mountains rose to heights of 6000-8000 ft.

Leaf imprints indicate that deciduous trees occurred on the lower slopes. Oak, elm, maple, and beech were common forms.

Closer to the lake where it was wetter and warmer there were willows, laurel, and fig trees. Palms grew in the sandier soils (Fig. 11). Cypress grew in quiet embayments of Fossil Lake. The shoreline plant assemblage had a rather subtropical appearance.

Ground cover was provided by a mold of dead leaves, holly, liverworts, mosses, and ferns (Fig. 12). Prairie-type grasses are not present and large savanna areas probably were not developed.

In the lake itself small, one-celled algae floated about in the waves and currents. Bacteria grew on the lake bottom and in the water and helped to contribute to the organic ooze that built up on the bottom. Fungi, wonderfully preserved in oil shales, are further indicators that Fossil Lake was a fresh-water lake and deep enough to have depths below the zone of sunlight penetration (Bradley 1964a). Reeds, rushes, and similar lake-shore plants grew in shallow, near-shore waters.

The large amount of organic material in the rocks is evidence of the prolific microscopic plant growth in the lake.

Occasional dry periods in the Fossil Basin are indicated by fossils of a plant called Ephedra. This plant had leaves with a thick, waxy cuticle to prevent desiccation during drought. The absence of saline minerals in the Green River Formation in Fossil Basin suggests that these arid periods were never of the intensity that occurred in the Green River Basin to the east.

The flora of Fossil Basin, especially that from the Green River Formation, is usually compared to that of the present-day Gulf Coast. Seaward from the Appalachians, the composition of the flora is very similar to that of the Green River. Spruce, pine, and fir grow in the Appalachians at elevations of 6000 or more feet, supporting the idea that similar elevations existed near Fossil Lake and that a general Gulf Coast climate prevailed in Fossil Basin.

Invertebrates

Molluscs, ostracodes, and insects comprise the invertebrate fauna of Fossil Basin.

A large number of marine forms are found in the earlier marine deposits, while fresh-water and terrestrial forms are common in the Tertiary units. It is these later forms which are considered here.

The fresh-water and terrestrial molluscs from the Wasatch and Green River formations include bivalves (clams) and gastropods (snails).

Aquatic clams from the Green River Formation include Unio and Plesiellipta. Pisidium and Sphaerium are common in the Wasatch Formation.

Land snails from the Wasatch include such genera as Grangerella, Discus, Oreoconus, and Glypterpres. Oreoconus is also known from the Green River Formation.

Fresh-water snails are very abundant from both the Wasatch and Green River formations. Physa, Planorbis, Elimia, Bioniphalaria, Virpains, and Omalodiscus are from the Wasatch. Physa, Elimia, Diomphalaria, and Goniobasis are recorded from the lacustrine Green River sediments. In some areas of the Green River Formation, these snail fossils are so abundant that they form a major constituent of the limestones. The presence of land snails in the Green River and fresh-water molluscs in the Wasatch is evidence that the two formations are not totally representative of a single environment, but represent several related ones.

Most of these fresh-water snails were herbivorous and frequented the shallow, well-lighted areas of Fossil Lake which had a good growth of plants and algae. This naturally occurred close to shore, hence the gastropodal limestones reported by Bradley (1926) and Oriel and Tracey (1970) are interpreted as being near-shore facies.

Ostracodes are extremely small arthropods that produce a two-piece, hinged shell within which they live. Pseudocypus is the most common form. Like the snails, the ostracodes were so abundant in areas that they, too, formed a large part of some Green River Formation limestones. And like the snails they lived in a near-shore environment.

Within the laminated shales of the Green River Formation is preserved an abundant and diverse assemblage of fossil insects (Fig. 13).

These insects are important because they demonstrate that many modern families and even genera were in existence during the Eocene. In contrast to this, knowledge of insect evolution prior to the Eocene is rather poorly known. Scudder (1890) and Cockerell (1920) have described most of the Green River insects.

Beetles are the most common forms, followed by dragonflies. Maggots and larvae of flies are commonly preserved.