Fowkes Formation
This is the middle formation of Veatch’s (1907) tripartite division of the Wasatch Group. It is now found to be the youngest formation in Fossil Basin. It is not exposed in Fossil Butte National Monument.
Oriel and Tracey (1970) have formally divided and named three members of the Fowkes Formation: a lower Sillem Member, a middle Bulldog Hollow Member, and an upper Gooseberry Member.
SILLEM MEMBER.
Like most of the Fowkes Formation, this sequence of rocks has been eroded extensively and is preserved as erosional remnants, where protected by faulting, in the western part of Fossil Basin.
The Sillem Member consists of a lower conglomeratic sequence with some sandstone and mudstone. The conglomerate contains well-rounded clasts of gray quartzite, chert, and Paleozoic limestone. The sandstone is light gray, calcareous to muddy, and coarse to medium-grained. The mudstone is pink, gray, or tan in color.
The upper part of the Sillem Member is a mudstone and claystone unit. It ranges in color from pink and yellow to gray and green. Some volcanic debris is found. There are also interbedded layers of marlstone and limestone. Some sandstone is present.
The Sillem Member is between 100 and 400 ft thick and most probably rests unconformably on the Bullpen Member of the Wasatch.
BULLDOG HOLLOW MEMBER.
This middle member of the Fowkes Formation has the thickest and most extensive outcrops. The Bulldog Hollow Member is exposed along the west side of the basin.
Included rocks are green, white, and blue-green mudstone with ash beds, green and buff claystone, and tuffaceous, limy sandstone. A high percentage of the iron mineral, magnetite, occurs in the sandstone. Conglomerate occurs as lenses.
The Bulldog Hollow Member has a gradational contact with the underlying Sillem Member. The amount of volcanic material increases upward from the Sillem, indicating an increase in volcanic activity during the deposition of the Bulldog Hollow Member.
GOOSEBERRY MEMBER.
Oriel and Tracey (1970:55) place this uppermost member provisionally within the Fowkes Formation. Most of the Gooseberry Member is a puddingstone, a lithology with well-rounded, spherical pebbles in a marlstone, sandstone, or sandy limestone matrix. The pebbles are too rounded for the rock to be a diamictite, and too separated from each other to be called a conglomerate.
The nature of the Gooseberry-Bulldog Hollow contact is not completely known. It appears to be gradational in some areas and to be an angular unconformity in others.
AGE OF FOWKES FORMATION.
Fossils date the Sillem and Bulldog Hollow members as middle Eocene in age. These fossils consist of ostracodes, gastropods, leaves, and vertebrates from the Bulldog Hollow Member (Nelson 1973). The Gooseberry Member has yielded a few vertebrate remains and is late Miocene or early Pliocene in age (Oriel and Tracey 1970).
DEPOSITIONAL ENVIRONMENT.
The Fowkes Formation is an alluvial deposit, much like the Wasatch Formation. The chemical and climatic conditions of deposition were different from those of the Wasatch, and the extensive red-beds are not developed.
Small lakes were present in which limestone and marlstone accumulated. The puddingstone may be a mudflow. Volcanic activity left its record in the ash found in the Fowkes Formation.
In the past, the Fowkes Formation had a greater distribution. Postdepositional faulting down dropped parts of the Fowkes protecting them from subsequent erosion.