Work by the United States Geological Survey in the Confusion Range of west-central Utah during the past few years has resulted in the accumulation of considerable data regarding the stratigraphy of rocks ranging in age from Late Mississippian through Early Triassic. These rocks are here discussed as six formations, whose physical limits more or less coincide with the boundaries selected by earlier workers in the area, although considerable changes in stratigraphic nomenclature have been made.
The Ely limestone of Late Mississippian, Early and Middle Pennsylvanian, and Permian age consists of 1,850–2,000 feet of alternating resistant thick-bedded limestone and slabby slope-forming limestone. The upper 100–350 feet of the formation contains an early Permian fauna and is generally more massive and less cherty than the remainder of the formation, which contains a fauna characteristic of the lower half of the Pennsylvanian and locally Late Mississippian. A hiatus representing Late Pennsylvanian time separates Middle Pennsylvanian and Permian faunas.
The Arcturus formation, which conformably overlies the Ely limestone, consists of poorly indurated yellowish gray calcareous sandstone with thin, ledge-forming interbeds of dark gray to tan limestone and dolomite. The few fossils found in the Arcturus formation, and its stratigraphic position, suggest that the formation is early to middle Permian in age. The total thickness of the formation is uncertain, but a minimum thickness of 2,700 feet has been measured.
The succeeding three formational units, the Kaibab limestone, the Plympton formation (new name), and the Gerster limestone, all of Permian age, have been included in the Park City group.
The Kaibab limestone overlies the Arcturus formation and consists of 480 feet of somewhat cherty bioclastic limestone. The lithologic character of the Kaibab is in sharp contrast to the succeeding unit which is here named the Plympton formation and is predominantly dolomite and chert. The Plympton formation is 690 feet thick and contains phosphatic beds believed to be lithologic correlatives with parts of the Park City formation of northwestern Utah. The overlying Gerster limestone consists of 1,100 feet of very fossiliferous cherty bioclastic limestone and argillaceous limestone. The Gerster limestone is the youngest Permian unit in the Confusion Range. Like the Kaibab, it is differentiated from the Plympton formation by its high limestone content and lack of dolomite. The contact between the Plympton and the Gerster hmestone is conformable throughout the area. The Kaibab limestone, the Plympton formation, and the Gerster limestone are believed to be correlative with units within the Park City and related formations of nearby areas in northern and northwestern Utah and northeastern Nevada. In areas south of the Confusion Range, the younger Permian units, the Plympton formation, and the Gerster limestone, are absent and only the Kaibab limestone is present in the Grand Canyon area.
The Thaynes formation of late Early Triassic age consists of thick beds of predominantly greenish gray calcareous clay shale interbedded with thin beds of gray to chocolate brown limestone, and is more than 1,900 feet thick in the Confusion Range. It is unconformably overlain by Tertiary and Quaternary strata, and no younger Mesozoic rocks crop out in the area. The contact of the Thaynes formation with the underlying Gerster limestone appears conformable; however, faunal evidence indicates a considerable hiatus between the two formations.