Mancos-Mesaverde (Upper Cretaceous) Intertonguing Relations Southeast Piceance Basin, Colorado

Abstract

In the southeastern Piceance basin, Delta, Garfield, Mesa, and Pitkin Counties, Colorado, gas has been discovered in sandstones of the Mesaverde Formation (Upper Cretaceous) that interfinger with the Mancos Shale. Electric log correlation from within the basin to the outcrop along the Book Cliffs, Colorado, shows that the producing sands belong to the Corcoran and Cozzette Members of the Mesaverde. These two members and the Rollins Sandstone can be traced in wells across the basin from the outcrop in the type area, near Grand Junction, to the outcrop along the southern Grand Hogback, thus demonstrating their continuity across the basin. By the use of eight measured surface sections, correlations of these members northward along the Grand Hogback has shown that the Corcoran and Cozzette are correlative with the non-marine part of the Iles Formation, near Meeker, and that the Rollins Sandstone is the lithogenetic equivalent of the Trout Creek Sandstone of northwestern Colorado. Fossils from below the Corcoran Member and from above the Cozzette Member date these two units as Campanian and place them stratigraphically between the base of the Baculities scotti faunal zone and the top of the Exiteloceras jenneyi faunal zone. Shoreline trends of the Corcoran and Cozzette Members are generally east-northeast in the southeastern Piceance basin. This east-northeast trend is divergent from a broad north-south trend in the Western Interior as a whole. Shoreline movements during the deposition of these members were transgressive toward the northwest or landward direction and regressive toward the southeast or seaward direction. These northwest-southeast shoreline movements probably did not proceed beyond the limits of the present structural Piceance basin. In the southernmost part of the Piceance basin, where most Corcoran-Cozzette gas discoveries have been made, these sandstones were deposited in an offshore marine environment of low to moderate wave and current action and are therefore silty and have low poros ty and permeability. The total thickness of the Cozzette Member ranges from 0 to 220 feet and isopach trends of the Cozzette conform generally to the east-northeast shoreline trend of this unit. The total Corcoran Member ranges from 0 to about 180 feet in thickness, but it can not everywhere be separated from the upper sandstones of the Sego Member; the combined maximum thickness of the Corcoran and upper Sego reaches 300 feet.