Abstract

Middle Cretaceous rocks (Aptian to Coniacian) on the south side of the Uinta basin include the nonmarine Cedar Mountain Formation and Dakota Sandstone and the lower part of the overlying marine Mancos Shale. The lower part of the Mancos comprises the Tununk, Ferron Sandstone, and Juana Lopez Members and part of the main body of the Mancos Shale. The aggregate thickness of these rocks is 800–1000 ft (240–300 m) over most of the area but increases to as much as 2400 ft (730 m) under the Wasatch Plateau to the west. Detailed outcrop studies augmented with drill hole data indicate that (1) the Mancos Shale unconformably overlies the Cedar Mountain Formation over most of the area, (2) the Dakota Sandstone, where present, probably overlies and onlaps the unconformity, (3) the middle Turonian Ferron Sandstone Member grades southeastward into siltstone of the Tununk Member between Price and Green River, Utah, and (4) southeast of where the Ferron grades into the Tununk, the cuesta-forming upper Turonian Juana Lopez Member, which was previously mapped as Ferron Sandstone Member in the study area, overlies the Tununk Member.

A shelf sandstone unit as thick as 75 ft (23 m) occurs in the middle of the Tununk Shale Member. This unit, the Coon Spring Sandstone Bed of the Tununk Member, forms prominent outcrops between Cedar and Green River and can be traced eastward almost to Grand Junction, Colorado. To the west, however, the sandstone grades to undifferentiated siltstone of the Tununk, apparently because of slightly increased water depths toward the foredeep.