Part 2

Hayden divided this survey area into several districts. The Green River district was put in charge of A. C. Peale. Peale’s (1879:535) report contained the first geologic description of Fossil Butte itself, as well as a short discussion of the fish fossils obtained there. The description of the butte is quoted here:

... In the lower part of the bluff from which these specimens are taken, the bright coloured beds of the Wasatch are seen outcropping, although the entire section cannot be seen, as their softness causes them to weather so that the debris conceals the strata. The fossils are found at several horizons in the shales. Near the top of the bluff is a band of hard, bituminous, or oily shale, which burns rather freely with a strong bituminous odor.... It is brownish-black in color and on the weathered surfaces a bluish white....

The next major publication dealing with Fossil Basin was that by Veatch (1907). He mapped the rocks in certain areas of the basin in an attempt to bring a semblance of order to Hayden’s (1869) broad stratigraphic descriptions. His work resulted in the designation of most of the rock units in Fossil Basin from Jurassic to Recent.

Schultz (1914), an associate of Veatch, published a paper dealing with the structures to the north and east of Fossil Basin. This helped to delineate the features surrounding and forming the basin.

Subsequent work on the geology of southwest Wyoming mainly has been concentrated on the Green River Basin just east of Fossil Basin. Significant work on Fossil Basin has been done by Rubey et al. (1968a, b) who mapped the northern part of the basin, and by Oriel and Tracey (1970) who have published results of the latest study on the stratigraphy and age of the rock units in Fossil Basin. This latest work is the culmination of many years of study and incorporates and refines data presented in shorter, earlier papers.

STRATIGRAPHY

The complete geologic history of the Fossil Basin involves many stratigraphic units. Most of these are not exposed within the monument itself but are well known from outcrops outside the boundaries of the monument and from deep wells (Fig. 4).

At the base of the stratigraphic section are rocks of Precambrian age. These have not been penetrated by wells within the Fossil Basin but should occur as much as 25,000 ft beneath the surface. They probably consist of metamorphic and intrusive rocks. Beginning about 600 million years ago, at the beginning of Cambrian time, sedimentary deposits accumulated. These are both marine and continental in origin and range in age from early Cambrian (600 million years old) to early Tertiary (50 million years old). Some 34 geologic formations are recognized in this thick section, a list of which follows:

Eocene Fowkes Formation Wasatch-Green River Formations Paleocene Evanston Formation (upper part) Upper Cretaceous Evanston Formation (lower part) Adaville Formation Lazeart Sandstone Hilliard Shale Frontier Formation Lower Cretaceous Aspen Shale Bear River Formation Gannett Group Jurassic Stump Sandstone Preuss Sandstone Twin Creek Limestone Nugget Sandstone (upper part) Triassic Nugget Sandstone (lower part) Thaynes Limestone Woodside Formation Dinwoody Formation Permian Phosphoria Formation Pennsylvanian Wells Formation Mississippian Brazer Limestone Madison Limestone (upper part) Devonian Madison Limestone (lower part) Darby Formation Silurian Not Present Ordovician Leigh Dolomite Bighorn Dolomite Cambrian Gallatin Limestone Gros Ventre Formation Precambrian Complex of metamorphic and intrusive rocks

Within the monument itself the oldest rocks exposed are those of the Thaynes Limestone near Prow Point. As will be seen in the discussion of the structure of the Fossil Basin, all of the rocks beneath the Evanston Formation are folded and faulted and within the monument are buried by deposits of Tertiary age. For this reason detailed discussion of stratigraphic units will be confined to those rocks that can be seen within the boundaries of the monument or closely adjacent to it (Fig. 5 ). The distribution of the various formations within the monument is illustrated in Figs. 7 and 8.

Era/Period/Epoch Age Major Biological & Geologic Events (Millions of years) CENOZOIC Quaternary Ice Age begins and evolution of modern mammals; man reaches N. A.; general uplift of continent. Holocene (Recent) .01 Pleistocene 3 Tertiary Evolution and Development of major mammal groups; Cascadian Orogeny in northwestern United States; in latest Cenozoic, Eocene lake system. Pliocene 5.5 Miocene 25.5 Oligocene 36 Eocene 54 Paleocene 65 MESOZOIC Cretaceous 130 Extinction of dinosaurs; flowering plants flourish; height of Cordilleran Orogeny. Jurassic 185 First birds, dinosaurs and mammals continue to evolve; beginning of Cordilleran Orogeny which formed the Rocky Mountains. Triassic 230 Conifers evolved; first dinosaurs; first mammals; Appalachian Orogeny; some orogeny in Cordillera. PALEOZOIC Permian 265 Archaic reptiles and mammal-like reptiles evolve. Appalachian Orogeny. Pennsylvanian 310 First reptiles, coal swamps formed; beginning of the Appalachian Orogeny. Mississippian 355 Expansion of amphibians; much carbonate deposition; Antler Orogeny in Cordilleran Mobile Belt (Rocky Mountain area). Devonian 413 First land plants; first jawed fishes, sharks and bony fish; amphibians appear; Acadian Orogeny in the Appalachian belt. Silurian 425 Jawless fishes evolved; much reef development; evaporite mineral development. Ordovician 475 First vertebrate remains known from this time; Taconic Orogeny in Appalachian belt. Cambrian 600 First shelled invertebrates; major flooding of North America.

Notes: Reformatted table to combine ‘Era’, ‘Period’, and ‘Epoch’ columns into one column in outline form. Beneath heading ‘Epoch’: “(Cenozoic only)”; beside other periods, “Epochs for these periods are not definitely established” As the last line, “Precambrian time began 5 billion years ago.”

Thaynes Limestone

At the very northeastern corner of Fossil Butte National Monument can be seen an outcrop of the Thaynes Limestone of Triassic age, a marine unit deposited some 200 million years ago. This formation is noted for its rich and varied assemblage of marine fossils, mostly forms related to oysters and clams. The Thaynes Limestone consists predominantly of sandy, gray limestone and green-gray limey siltstones. The formation weathers to a dark brown in the lower part and a grayish-yellow in the upper. The fact that the Thaynes Limestone is exposed in the monument at all is because of the faulting that took place just prior to the deposition of the Evanston Formation of latest Cretaceous and earliest Tertiary age (refer to section on structure) and the erosion of the Tertiary rocks that once covered it.

Evanston Formation

Although not seen within the boundaries of the Fossil Butte National Monument, the Evanston Formation is exposed just south of Highway 30N, 1.5 miles southeast of the southeast corner of the monument. The Evanston was not involved in the complex folding and faulting but it is somewhat disturbed and rests under the Wasatch Formation with angular uncomformity. The Evanston Formation bridges the time boundary between the Cretaceous and Tertiary periods. In the lower part of the unit are found many fossil leaves, pollen, and spores and a jaw of the horned dinosaur Triceratops that prove its Cretaceous age, and in the upper part are found fossil mammals of Paleocene age.

The Evanston Formation has been studied in detail by Oriel and Tracey (1970). These authors divide the formation into three members. The lowest, which they called the Lower Member, is predominantly “gray to very dark gray mudstone, siltstone, claystone and gray carbonaceous sandstone.” The Lower Member reaches a thickness of 500 ft in some places. Above and in part interfingering with the Lower Member is a 1000 ft thick unit that was named the Hams Fork Conglomerate Member. This unit consists of beds of boulder conglomerate interstratified with thick beds of coarse, partly conglomeratic brown sandstone and gray mudstone. Where the Lower Member of the Evanston is missing, the Hams Fork Conglomerate forms the base of the formation.

The Upper Member of the Evanston Formation is termed by Oriel and Tracey (1970) the Main Body. It is more than a thousand feet thick and the lower part intertongues with the Hams Fork Conglomerate. The Main Body is “light to dark gray carbonaceous sandy to clayey siltstone interbedded with gray, tan, yellow and brown sandstone and conglomerate and carbonaceous to lignitic claystone.” It is this Main Body that can be seen along the highway just southeast of the monument.

The types of sediments and fossils found in the Evanston indicate that the formation was deposited by streams on flood plains and in marshes and ponds. A subtropical climate is indicated and the area was heavily wooded.

Wasatch Formation

The term Wasatch was first used by Hayden (1869:91) as follows:

Immediately west of Fort Bridger commences one of the most remarkable and extensive groups of Tertiary beds seen in the West. They are wonderfully variegated, some shade of red predominating. This group, to which I have given the name of Wasatch group, is composed of variegated sands and clays. Very little calcareous matter is found in these beds.

In Echo and Weber Canyons are wonderful displays of conglomerates, fifteen hundred to two thousand feet in thickness. Although this group occupies a vast area, and attains a thickness of three to five thousand feet, yet I have never known any remains of animals to be found in it. I regard it, however, as of middle Tertiary age.

The Wasatch is well exposed in Fossil Basin. There the unit was regarded by Veatch (1907) as a group and divided by him into three formations: the Almy, Fowkes, and Knight. He wrote (1907:88):

In the Wasatch group as thus defined by Hayden the field work of the season 1905 showed three divisions: 1) a basal member composed of reddish-yellow sandy clays, in many places containing pronounced conglomerate beds, which has been named the Almy Formation; 2) a great thickness of light-colored rhyolitic ash beds containing intercalated lenses of white limestones with fresh-water shells and leaves—the Fowkes Formation; and 3) a group of reddish-yellow sandy clays with irregular sandstone beds (the Knight Formation) closely resembling 1) lithologically and separated from 1) & 2) by a pronounced period of folding and erosion.

Veatch, however, erred in his field work and did not realize that the Fowkes Formation had been downfaulted into the position in which he saw it (Tracey and Oriel 1959; Eardley 1959). The Fowkes is actually the youngest of the three formations of Veatch and is considerably later in age than the true Wasatch.

The Almy and Knight formations are not separable (Oriel and Tracey 1970:16), and can be seen to grade into each other at the basin edges.

Recent, Pleistocene Pleistocene and Recent gravel and alluvium Pliocene Gooseberry Member of Fowkes Formation Oligocene to Pliocene Hiatus Eocene Bridgerian Fowkes Fm. Bulldog Hollow Member Sillem Member Unconformity Wasatchian: Wasatch Formation, Tunp Member Bullpen Member Green River Formation Angelo Member Mudstone Tongue Fossil Butte Member Sandstone Tongue Main Body Lower Member Basal Conglomerate Member Paleocene Possible hiatus or unconformity Evanston Formation Main Body Hams Fork Conglomerate Member Latest Cretaceous Lower Member UNCONFORMITY Late Cretaceous Adaville and older formations