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Significant Ammonite Finds in Uppermost Mancos Shale and Overlying Formations Between Barker Dome, New Mexico, and Grand Junction, Colorado
The ammonite record of the Campanian part of the Upper Cretaceous Series was examined in three areas — north side of the San Juan Basin, an area southeast of Montrose, and the Book Cliffs area near Grand Junction.
On the north flank of the San Juan Basin, in the general area of Barker dome in San Juan County, N. Mex., in Mesa Verde National Park, Colo., and near Durango, Colo., rocks of Campanian age comprise the uppermost part of the Mancos Shale and the overlying Mesaverde Group, Lewis Shale, Pictured Cliffs Sandstone, and possibly the Fruitland and Kirtland Formations. Eight of the 23 Campanian ammonite zones known from the western interior region have been found in these rocks. The oldest zone is represented by the earliest form of Scaphites hippocrepis (DeKay) in the uppermost part of the Mancos Shale. The youngest ammonite record is in the lower part of the Pictured Cliffs Sandstone where poorly preserved fragments of Didymoceras suggest the zone of D. cheyennense (Meek and Hayden).
The Mesaverde Group grades northward into shale, and in the Montrose, Colo. area, the Mancos Shale includes marine strata up to the base of the Pictured Cliffs Sandstone. Near Montrose, nine ammonite zones are known in the Campanian part of the Mancos Shale including three that have not been found on the north flank of the San Juan Basin. The top of the Mancos seems to lie in the zone of Didymoceras cheyennense.
Northeast and east of Grand Junction, Colo. the Mancos Shale is overlain by the Mount Garfield Formation, which has the Rollins Sandstone Member at its base. The Rollins has not been closely dated, but it probably lies in the zone of D. cheyennense inasmuch as Exiteloceras jenneyi (Whitfield), the next older index ammonite, has been found near the top of the Mancos Shale in areas farther east. Eleven ammonite zones are known from the Campanian part of the Mancos Shale in the Grand Junction area.
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