THE TRIASSIC SINBAD FORMATION
AND CORRELATION OF THE MOENKOPI GROUP,
CANYONLANDS NATIONAL PARK, UTAH
Spencer G. Lucas
New Mexico Museum of Natural History and Science
1801 Mountain Road, N.W.
Albuquerque, New Mexico 87104
The eastern limit of the Lower Triassic Sinbad Formation is a 0.2-0.3-m-thick fossiliferous limestone bed in the Holeman Spring Basin (T28S, R18E) of Canyonlands National Park, Utah. The Sinbad Formation here intertongues with uppermost strata of the Ali Baba Formation of the Moenkopi Group.
Early-Middle Triassic strata exposed in Canyonlands National Park belong to the Moenkopi Group (sensu Poborski, 1954) and are a sequence of mostly marine red beds as much as 160 m thick. McKnight (1941) identified a very distinctive, thin bed of limestone in the Stillwater Canyon region of the Canyonlands as part of the Sinbad Formation. My purpose here is to (1) document the stratigraphic position of the Sinbad Formation in Canyonlands; (2) describe the eastward extent of the Sinbad Formation in eastern Utah; and (3) correlate Moenkopi strata in the Canyonlands and Moenkopi strata to the west in the San Rafael Swell.
McKnight (1941, p. 57-58) accurately described the Sinbad Formation in the Canyonlands as an "Impure limestone, 15 inches in thickness, that lies near the base of the greenish-gray shale series on the Green River, 220 feet above the base of the [Moenkopi] formation." He noted that it is "muddy and also carbonaceous" and "gray in color, though it weathers to a buffy yellow." McKnight reported that Girty identified the following taxa from fossil collections made from the Sinbad Formation in the Canyonlands: Lingula sp., Monotis ? aff. M. thaynesiana, viviparoid gastropods and Meekoceras ? sp. Gastropods (Figure 2D) dominate the fossil assemblage and are poorly preserved steinkerns and impressions that resemble the gastropods Batten and Stokes (1986) described from the Sinbad Formation in the San Rafael Swell.
Moenkopi Group strata exposed in the Canyonlands along the flank of Steer Mesa (sec. 6, T28S, R18E) include the Sinbad Formation and are presented here as a characteristic section (Figs. 1-2). These strata can be assigned to the Moenkopi Group formations named by Shoemaker and Newman (1959). The lower 46 m of red-bed, fine-grained sandstones and siltstones belong to the Tenderfoot Formation of Permian? or Early Triassic? age. About 25 m of reddish-brown to brown, coarser-grained sandstones, conglomerates and siltstones overlie these strata and are assigned to the Ali Baba Formation. The Sinbad Formation is 0.2-0.3 m of limestone near the top of the Ali Baba Formation. A definite change in color, grain size and lithology to yellow-gray and pale green, gypsiferous siltstones and fine sandstones marks the base of the 52-m-thick Sewemup Formation. Overlying reddish-brown sandstones and siltstones are 12 m thick and are assigned to the Pariott Formation. Siliceous conglomerate of the Shinarump Formation of the Chinle Group (Upper Triassic) disconformably overlies the Pariott Formation. Previous claims that the Sinbad Formation is laterally equivalent to the Sewemup Formation (Shoemaker and Newman, 1959; Stewart et al., 1972) thus are incorrect. The Sinbad intertongues with uppermost Ali Baba Formation.
EASTERN LIMIT OF THE SINBAD
The eastern limit of the Sinbad Formation has long been placed in the Salt Valley northeast of Moab in T22S, R20E, Grand County, Utah (e.g., Blakey et al., 1993, Fig. 8). Shoemaker and Newman (1959, p. 1849) identified a thin limestone in supposed Sewemup strata here as Sinbad based on the putative presence of silicified juvenile specimens of the Smithian ammonoid Meekoceras. Recent examination of this limestone (especially in the SE1/4 NE1/4 SE1/4 sec. 31, T22S, R20E) reveals it contains a diverse silicified fossil assemblage of Early Cretaceous gastropods, unionid bivalves, charophytes and ostracods; the supposed juvenile Meekoceras are specimens of the planorbid gastropod Gyraulus veternus (Goodspeed et al., 1992). Doelling (1985) correctly mapped this limestone and the adjacent "Sewemup" strata as Lower Cretaceous Cedar Mountain Formation. The eastern limit of the Sinbad Formation thus is in the Canyonlands in the Holeman Spring Basin (T28S, R18E), as originally documented by McKnight (1941).
Correlation of Moenkopi strata between the San Rafael Swell and the Canyonlands-Arches area (Fig. 1) can be achieved through lithostratigraphy, very limited biostratigraphy and sequence stratigraphy. The Sinbad Formation provides an early Smithian datum upon which to hang this correlation.
Like most previous workers (e.g., Shoemaker and Newman, 1959; Stewart et al., 1972), I consider the Tenderfoot Formation to be older than Moenkopi Group strata exposed in the San Rafael Swell. Black Dragon Formation strata in the San Rafael Swell represent a transgressive systems tract equivalent to the lower-middle Ali Baba Formation. The highstand systems tract represented by the Sinbad Formation overlies the Black Dragon in the San Rafael Swell and is at the approximate top of the Ali Baba in the Canyonlands. Overlying Sewemup Formation strata are generally correlative with the Torrey Formation. Correlation of the Moody Canyon and Pariott Formations is tentative.
The superintendent of Canyonlands National Park permitted fieldwork in the park. The National Geographic Society supported this research. Tom Goodspeed and Adrian Hunt assisted in the field, and Orin Anderson and Adrian Hunt reviewed an earlier version of this paper.
Batten, R.L. and Stokes, W.L., 1986. Early Triassic gastropods from the Sinbad Member of the Moenkopi Formation, San Rafael Swell, Utah: American Museum Novitates, no. 2864, 33 p.
Blakey, R.C., Basham, E.L. and Cook, M.J., 1993. Early and Middle Triassic paleogeography of the Colorado Plateau and vicinity: Museum of Northern Arizona Bulletin, v. 59, p. 13-26.
Doelling, H.H., 1985. Geologic map of Arches National Park and vicinity, Grand County, Utah: Utah Geological and Mineralogical Survey, Map 74, scale 1:50,000.
Goodspeed, T.H., Lucas, S.G. and Kietzke, K.K., 1992. Supposed Lower Triassic marine strata in the Salt anticline of eastern Utah are Lower Cretaceous nonmarine strata: Geological Society of America Abstracts with Programs, v. 24(6), p. 15.
McKnight, E.T., 1940. Geology of area between Green and Colorado Rivers, Grand and San Juan Counties, Utah: U.S. Geological Survey, Bulletin 908, 147 p.
Poborski, S.G., 1954. Virgin Formation (Triassic) of the St. George, Utah area: Geological Society of America Bulletin, v. 65, p. 971-1006.
Shoemaker, E.M. and Newman, W.L., 1959. Moenkopi Formation (Triassic? and Triassic) in Salt Anticline region, Colorado and Utah: American Association of Petroleum Geologists Bulletin, v. 43, p. 1835-1851.
Stewart, J.H., Poole, F.G. and Wilson, R.F., 1972. Stratigraphy and origin of the Triassic Moenkopi Formation and related strata in Colorado Plateau region: U.S. Geological Survey, Professional Paper 691, 195 p.
Figure 1. Index map, measured stratigraphic section of Moenkopi Group strata a Steer Mesa in the Canyonlands (sec. 6, T28S, R18E) and correlation of Moenkopi Group formations between the San Rafael Swell and Canyonlands.
Figure 2. Moenkopi Group strata at the measured section (Fig. 1) along the flank of Steer Mesa in the Canyonlands (sec. 6, T28S, R18E). A, Overview of section; units are: 1 = Tenderfoot Formation, 2 = Ali Baba Formation, 3 = Sewemup Formation, 4 = Pariott Formation, 5 = Shinarump Formation. B-C, Close-ups of Sinbad Formation limestone bed(s). D, Gastropod steinkerns in Sinbad Formation limestone at this section.
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