SEDIMENTARY FACIES AND INTERTONGUING IN THE UPPER CRETACEOUS OF THE BOOK CLIFFS, UTAH-COLORADO

Abstract

The Upper Cretaceous of the Book Cliffs in central Utah and western Colorado exhibits intricate lateral and vertical intertonguing of marine and nonmarine facies. Excellent exposures permit detailed observations and facilitate tracing of individual units. Dominantly continental deposits of the Star Point, Blackhawk, and Price River formations pass eastward into lagoonal deposits formed behind offshore bar sandstones, which interfinger eastward with the marine Mancos shale. The Star Point sandstone consists of two littoral marine sandstone tongues. Overlying the Star Point is the Blackhawk formation consisting of six members separated by thin tongues of Mancos shale, which grade upward into the overlying littoral marine sandstones. Each marine shale tongue rests with a slight disconformity on the underlying member. Where the littoral marine sandstones are absent the rocks cannot be so subdivided. The Price River formation which rests disconformably on the Blackhawk consists of two facies—the Farrer or barren facies and the Neslen or coal-bearing facies which is divided into five cyclic members similar to those of the Blackhawk. The intricate intertonguing of these deposits is interpreted to be the result of deposition in a shallow basin in which there were long periods of relative stability separated by sharp pulses of subsidence. Thick peat beds formed during periods of quiet coincide with the tops of offshore bars. Sharp subsidences preceded formation of the basal sandstone tongues; lesser pulses preceded formation of the offshore bars. A generalized cycle of four units can be recognized in these deposits: (1) basal marine shale, (2) littoral marine sandstone, (3) lagoonal rocks, (4) coal.