Stratigraphy of the upper Triassic Chinle Group. Four Corners region
— Spencer G. Lucas, Andrew B. Heckert, Estep John W., and Orin J. Anderson

Upper Triassic strata exposed in the Four Corners region belong to the Chinle Group of late Carnian–Rhaetian age. Chinle Group strata can be divided into eight lithostratigraphic intervals: (1) mottled strata/Temple Mountain Formation—as much as 31 m of mostly color mottled, deeply pedoturbated siltstone, sandstone and conglomerate; (2) Shinarump Formation—up to 76 m of trough-crossbedded sandstone and siliceous extrabasinal conglomerate; (3) Monitor Butte/Cameron/Bluewater Creek Formations— up to 84 m of varied lithofacies ranging from green bentonitic mudstones (Monitor Butte) to sandstones (Cameron) to red-bed mudstones (Bluewater Creek); (4) Blue Mesa Member of Petrified Forest Formation—up to 100 m of blue, gray, purple and red variegated bentonitic mudstone; (5) Moss Back Formation/Sonsela Member of Petrified Forest Formation—up to 50 m of trough-crossbedded sandstone and intrabasinal conglomerate; (6) Painted Desert Member of Petrified Forest Formation—up to 150 m of mostly red-bed bentonitic mudstone and siltstone; (7) Owl Rock Formation—up to 150 m of pale red and orange siltstone interbedded with ledges of pedogenic calcrete limestone; (8) Rock Point Formation—up to 300 m of reddish brown, cyclically-bedded sandstone and non-bentonitic siltstone. In southwestern Colorado, the base of the Chinle Group is the Moss Back Formation resting on Lower Permian strata. We abandon the term Dolores Formation and correlate its informal members as follows: (1) lower member = Moss Back Formation; (2) middle member = Painted Desert Member of Petrified Forest Formation; and (3) upper member = Rock Point Formation. The informal term "Kane Springs strata," applied to some Chinle Group coarse-grained strata in southeastern Utah, is also abandoned. Church Rock Member (Formation) is a synonym of Rock Point Formation, and the term Church Rock should not be applied to nearly all the Chinle Group section in southeastern Utah. Palynomorphs, megafossil plants and fossil vertebrates support the following age assignments for Chinle Group strata in the Four Corners region: late Carnian = mottled strata/Temple Mountain Formation, Shinarump Formation, Monitor Butte/Cameron/Bluewater Creek Formations and Blue Mesa Member of Petrified Forest Formation; early-middle Norian = Moss Back Formation/ Sonsela Member of Petrified Forest Formation, Painted Desert Member of Petrified Forest Formation and Owl Rock Formation; and Rhaetian = Rock Point Formation. The Chinle Group consists of three unconformity-bounded sequences: Shinarump–Blue Mesa sequence of late Carnian age; Moss Back–Owl Rock sequence of early-middle Norian age; and Rock Point sequence of Rhaetian age. Facies architecture and biostratigraphy support a genetic relationship between Chinle Group strata on the Colorado Plateau and shallow marine strata of the Mesozoic marine province of western Nevada. This relationship suggests that eustasy was the primary allochthonous control on Chinle Group sedimentation. At Big Indian Rock in the Lisbon Valley of southeastern Utah, a skull of the phytosaur Redondasaurus is in a thin, discontinuous mud-pebble conglomerate near the base of the Wingate Sandstone. Redondasaurus is an index fossil of the Late Triassic Apachean (Rhaetian) land-vertebrate faunachron. Unabraded surface texture, large size and preservation of thin, fragile bone suggest that the phytosaur skull is not reworked, so the Triassic-Jurassic boundary is stratigraphically above it. No unconformity surface is present in the lower Wingate Sandstone above the skull. Thus, at Big Indian Rock, the J-0 unconformity is not at the base of the Wingate Sandstone. If the basal Wingate is of Late Triassic age, then the Moenave Formation, with which it intertongues laterally, must also include Triassic strata. This suggests the Triassic-Jurassic boundary on the Colorado Plateau is relatively transitional—not a profound unconformity—within the Wingate–Moenave lithosome.