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.
QUATERNARY
Rubey et al. (1968a, b) have mapped several forms of Quaternary deposits in Fossil Basin. These include stream alluvium, rock and landslide debris, river terraces, and gravels, all derived from local formations. These deposits are the work of water, wind, and ice acting in relatively Recent time.
THE GEOLOGIC STRUCTURE OF FOSSIL BASIN
The Fossil Basin is a small, linear and structurally controlled basin in the southeastern part of the Wyoming overthrust belt. This “overthrust belt” is represented by a number of small mountain ranges and high ridges formed by the “thrusting” of sedimentary rocks over other sedimentary rocks. Topographically, the Fossil Basin is bounded by the Crawford Mountains and Tunp Range on the west, by Oyster Ridge on the east, and by the Uinta Mountains on the south. The Crawford Mountains, Tunp Range, and Oyster Ridge ([Fig. 2]) are areas of high relief developed upon southerly extended salient ridges of deformed Paleozoic and Mesozoic strata. In the center of the Fossil Basin, these earlier rocks are covered by a veneer of early Tertiary sediments. Superficially, the Fossil Basin appears to be a broad syncline with tilted beds dipping sharply or gently basinward from the basin margins. The Tertiary sedimentary cover, however, partially obscures what is a more complex structural history.
Following deposition of the Late Cretaceous Adaville Formation, the Fossil Basin was included in a period of intense structural deformation. This deformation was the result of compressional forces acting along a more or less east-west alignment. The strain, or the resolution of these forces, was developed along a north-south alignment or perpendicular to the compressional forces. As the stress level became too great, the rocks were first folded and then faulted. Initial faulting of the rocks relieved some of the stress; however, with continued application of compressive forces many stages of folding and faulting were generated. Because the alignment of the compressive forces remained about the same throughout deformation (i.e., east-west) and because the strongest relative compressive forces were from the west, successively younger folds and faults were generated in an eastward direction.