Cretaceous Stratigraphy of the Coalville and Rockport Areas, Utah

Cretaceous strata exposed in the Coalville and Rockport areas record the deposition and accumulation of terrigenous clastic sediments in alluvial-fan, fluvial, marginal-marine, and nearshore and offshore-marine environments. Marine sediments were deposited in the Coalville and Rockport areas during the Aspen-Mowry, Greenhorn, and Niobrara marine invasions of north-central Utah. Biostratigraphic control in the upper part of the Frontier Formation in the Coalville area is adequate, and the age of this part of the section has been accurately recognized in recent studies. Determining the age of the Aspen Shale and the lower part of the Frontier Formation and, more specifically, the position of the Lower Cretaceous-Upper Cretaceous boundary in north-central Utah has been severely hampered by the paucity of open-marine taxa in the deposits of the Aspen-Mowry marine invasion. Evidence presented here indicates that the Lower Cretaceous Upper Cretaceous boundary should be placed near the middle of the nonmarine Chalk Creek Member of the Frontier Formation, 400 to 500 meters higher than has been suggested previously