Stratigraphy (Sunnyside and Cottonwood-Jack Canyon tar sand deposits)

During the earliest 40% of Tertiary time, from about 65 to 40 million years ago, much of the Uinta Basin of northeast Utah was occupied by a large lake termed Lake Uinta. For most of its existence, this lake was a shallow body of fresh or slightly brackish water. Its remarkable, 15 million-year history spans middle Paleocene to middle or late Eocene time. The highly organic sediments of this lake are generally termed the Green River Formation. The Green River Formation spans almost the entire 15 million years and interfingers vertically and laterally with savanna, floodplain and shoreline sediments on all sides of the basin. These are variously termed Wasatch (or Colton) Formation in the lower part of the overall sequence and Uinta (or Duchesne River) Formation in the upper part. The Green River Formation also varies greatly in thickness from more than 12,000 feet in the depositional "deep" of the basin (northwest) to less than 2,000 feet around the basin edges. In the Nine Mile Ranch area, thickness is about 2,400 to 3,000 feet (Finure 1). The top has been eroded off in all of the study area, and the complete section north of Nine Mile Canyon is about 3,300 to 3,500 feet thick. Figure 1 depicts the stratigraphic section of the study area and vicinity and particularly the relationship of the Parachute Creek Member which underlies the entire study area to the observed oil saturation. This member is in turn divided into three numbered subdivisions, 1, 2, and 3, in ascending order shown on geologic maps as: Tgp]_, Tgp2 and Tgp3. This designation is derived as follows: Tertiary, Green River, Parachute Creek, and the subdivision number. This nomenclature and subdivision is taken from the 1961 geologic map of Utah (Stokes and Madsen, 1961) and was developed originally from field mapping and aerial photography by Humble Oil (see index to sources of data, Stokes and Madsen, 1961). Loren 0. Gray, consulting photogeologist, who worked on this project in 1983, was one of the original geologic team that worked out the subdivisions for Humble Oil in the 1950's.