Abstract

The Arapien Shale of Jurassic age, locally more than 2700 m thick, has received little study since it was defined more than 30 years ago. In particular, the origin of the extensive evaporite deposits has received scant attention. There is now considerable interest in the formation because it contains possible source and reservoir beds, and there have been minor shows of petroleum. It may seal reservoir beds in the underlying Navajo Sandstone (Jurassic) which are productive in the Overthrust Belt of northeast Utah, western Wyoming, and adjacent Idaho.

Gypsum and salt have been mined from the Arapien for many years. The thicker evaporite and claystone sequences are the result of flowage into diapiric masses and smaller salt cells.

Claystone, siltstone, and micrite constitute more than 50 percent of the formation. There is 5 to 20 percent sandstone and nearly the same amount of carbonate rock exclusive of micrite. Anhydrite (gypsum at the surface) and salt are the other major rock types and, locally, can make up nearly 40 percent of the Arapien sequence. Porosity is low (less than 12 percent) in most of the sandstone and carbonate beds, but there is minor oil staining.

During the Middle Jurassic, the Sevier orogenic belt in western Utah was elevated as a broad arch, and the Jurassic marginal basin was downwarped and filled with the thick sequence of evaporites, fine-grained clastics, carbonates, and sandstone which make up the Arapien Shale. Present evidence indicates that the evaporites are shallow water marine in origin. The formation is correlative with the Carmel Formation and the Twin Creek Limestone.