Cedar Mountain Formation

Term Cedar Mountain formation used west of Colorado River in east-central UT and northwest CO in Uinta, Piceance, and Paradox basins. Basal contact revised to include the underlying conglomerate previously called Buckhorn conglomerate. Buckhorn reduced in stratigraphic rank to member. A section measured at Buckhorn Reservoir in sec 9, T18S, R10E showed the Buckhorn as 30 ft thick. The member is discontinuous in distribution and is generally too thin to map. Is generally equivalent to Burro Canyon formation. The two formations are in physical continuity near Dewey, Grand Co, UT. Assigned an Early Cretaceous age. Fossil fish scales, freshwater ostracods, pelecypods, and plants found. Is a nonmarine unit. Young, R.G., 1959, Cretaceous deposits of the Grand Junction area, Garfield, Mesa, and Delta Counties, Colorado, IN Haun, J.D., and Weimer, R.J., eds., Symposium on Cretaceous rocks of Colorado and adjacent areas; Washakie, Sand Wash, and Piceance basins: Rocky Mountain Association of Geologists Field Conference Guidebook, no. 11, p. 1-8. Pg. 17-21. Cedar Mountain formation of Dakota group. Described in Grand Junction area, Colorado, where it disconformably underlies Naturita formation (new). Consists of basal conglomeratic sandstone unit and overlying variegated mudstone unit. In most localities, basal sandstone is massive cliff-forming unit with undulatory base and sharply defined upper surface. Farther west in Utah, three such sandstones, separated by mudstone units, are present in formation. These sandstones are termed Lower, Middle, and Upper Cedar Mountain sandstones. Lower, which is probably equivalent to Stokes' Buckhorn is present in deeper scours in Grand Junction area and is directly overlain in these scours by Middle sandstone. Where Lower sandstone is absent, the Middle sandstone constitutes the basal sandstone. Where only Middle sandstone is present basal unit averages about 25 feet thick, but where both sandstones are present it may be as much as 80 feet thick. Mudstone unit averages about 35 feet in Grand Junction area but slightly thicker along western edge. Included in Dakota group. [Age is Early Cretaceous.] [Cullins, H.L., 1969, Geologic map of the Mellen Hill quadrangle, Rio Blanco and Moffat Counties, Colorado: U.S. Geological Survey Geologic Quadrangle Map, GQ-835, 1 sheet, scale 1:24,000 [ Mapped in northern part of quad in Moffat Co, CO in the Greater Green River basin, thus extending the name farther east and north than in many earlier reports. Divided into: Buckhorn Conglomerate Member, adopted at base for a 0-80 ft thick sandstone and conglomerate; and the shale member, a 95 ft thick grayish-green and grayish-red soft, clayey shale interbedded with greenish-gray nodular limestone of fresh-water origin, and some gray medium-grained sandstone. Unconformably overlies Morrison Formation. Unconformably underlies Dakota Sandstone. Of Early Cretaceous age (columnar section). Rowley, P.D., and Hansen, W.R., 1979, Geologic map of the Plug Hat quadrangle, Moffat County, Colorado: U.S. Geological Survey Geologic Quadrangle Map, GQ-1514, 1 sheet, scale 1:24,000 Is Lower Cretaceous formation mapped in southwest Moffat Co, CO (Greater Green River basin). Unconformably overlies Morrison Formation (Upper Jurassic); unconformably underlies Dakota Sandstone (Lower Cretaceous). Contains two members: an upper shale member of soft purple, gray, and greenish-gray shale, mudstone, and minor limestone; and the lower Buckhorn Conglomerate Member of resistant medium-gray, light-gray, and tan medium-bedded, cross-bedded sandy pebble conglomerate, grit, pebbly sandstone, and fine- to coarse-grained sandstone. Overlying shale member rarely exposed. As much as 50 percent by volume of the clasts in the Buckhorn Conglomerate Member consist of gray to black chert. Correlation and usage after Anderman (1961), Cullins (1969), and Hansen (1969a). Unit correlated with the Burro Canyon(?) Formation in the Skull Creek area 15 km (9 m