Year: 1990
Type: report
Abstract: The Frontier Formation of the Mancos Group in northeastern Utah and northwesternmost Colorado (proposed new rank designations; formerly known as the Frontier Sandstone Member of the Mancos Shale) consists of several facies of marine and nonmarine rocks of Late Cretaceous (Turanian) age that grade eastward into totally marine rocks in easternmost Utah and northwestern Colorado.The Mancos Group in the report area is divided into the Mowry Shale, an unnamed shale unit, the Frontier Formation, and the main body.Six major facies, some of which are only in parts of the report area, are recognized in the Frontier.In ascending order, these are: (1) a basal, transgressive marine sandstone;(2) a marine shale tongue that correlates with the Tununk Member of the Mancos Shale in central Utah and is here recognized as a member of the Frontier; (3) a prograding coastal sandstone; (4) a sequence of nonmarine sandstone, shale, and coal; (5) an upper, transgressive coastal sandstone; and (6) an offshore-bar sandstone.Along the south flank and along the north flank of the eastern part of the Uinta Mountains, these rocks range in thickness from 760 ft (232 m) on the west to 140 ft (43 m) on the east.A lower Turanian transgressive sandstone at the base of the Frontier unconformably overlies an unnamed Cenomanian shale unit in the southwestern part of the Uinta Mountains area.Elsewhere around the Uinta Mountains, an upper middle Turanian shale member of the Frontier unconformably overlies the lower Cenomanian Mowry Shale (proposed new rank designation; formerly known as Mowry Member of Mancos Shale in all but the Utah part of the north flank of the Uinta Mountains).Apparently the intervening Turanian rocks onlap from west to east.After deposition of the basal shale member of the Frontier, a deltaic wedge prograded eastward and southeastward to the vicinity of Split Mountain east of ^.
DOI: 10.3133/b1787m
Authors: Cornelius M. Molenaar, B.W. Wilson
Institution: United States Geological Survey
Language: English
Subfield: Earth-Surface Processes
Field: Earth and Planetary Sciences
Domain: Physical Sciences
Open Access status: gold
OpenAlex ID: W138794233