REPORT OF INVESTIGATION NO. 84 SUMMARY OF STRATIGRAPHY IN THE MINERAL RANGE, BEAVER AND MILLARD COUNTIES, UTAH November, 1973 Introduction Many persons, who are currently interested in the geology of the Mineral Range because lands that may have value as geothennal resources are nearby, have requested stratigraphic data on the range from UGMS. This paper has been prepared to make available unpublished stratigraphic data that deals directly with the Mineral Range, and also to apply current stratigraphic nomenclature from nearby areas to the Mineral Range. It is apparent from the following text that problems still exist with stratigraphy of the Mineral Range; however, additional field work is planned by the author. The Mineral Range lies in west-central Utah, within the Basin and Range physiographic region. The town of Milford is about 10 miles west of the range, and Beaver is about 10 miles to the southeast. The range is approxi­ mately 30 miles long, with a maximum width of about 6 miles. The bulk of the range is made up of granitic intrusive rocks, but volcanic, metamorphic, and sedimentary rocks are also present. Metamorphic Rocks An irregular belt of biotite gneiss, schist, phyllite and migmatite is exposed on the west side of the Mineral Range. The area of outcrop is about 5 miles long and a half-mile wide, and Earll (I 957, p. 9) computes a stratigraphic thickness of approximately 3,000 feet. The rocks are assigned a Precambrian age on the basis of lithologic Similarity to other rocks of known Precambrian age in Utah, such as those of the Beaver Mountains mentioned by Woodward (I973, p. 5). Sedimentary Rocks Several late Precambrian formations that are present 15 miles to the west in the Beaver Mountains do not outcrop in the Mineral Range. Instead, the Lower Cambrian Prospect Mountain Quartzite is the oldest sedimentary unit exposed. Uese (I957) noted it as a massive, thick-bedded, gray to pink and darker purple to medium gray quartzite. He measured the thickness as 1,006 feet, which might include 228 feet of beds repeated by faulting. The Cambrian Pioche Shale overlies the Prospect Mountain Quartzite. Uese (I957) described the shale as an olive drab to green, micaceous rock, with the exposed thicknesses ranging from 25 feet to more than 100 feet. Uese (1957) mapped a medium to dark gray, massive and thick-bedded limestone that conformably overlies the Pioche Shale as Undifferentiated Cambrian limestones. He measured the exposed thickness of this unit as 188 feet, and noted that a metamorphosed section of the unit was about 500 feet thick. This unit might correspond to the Middle Cambrian Lyndon limestone in the Beaver Lake Mountains, as described by Welsh (1973, p. 10). 2 Liese (1957) mapped another limestone unit in the Mineral Range as Cambrian (1) Limestone. This unit is dark gray, massive, fmely crystalline, has abundant chert lenses, is unfossiliferous, and is 1,285 feet thick. All of the Cambrian rocks occur at the northern end of the Mineral Range. They have been domed, broken by normal faults both parallel to and transverse to the trend of the range, and the Prospect Mountain Quartzite-Pioche Shale-Undifferentiated limestone sequence has been repeated in the northern Mineral Range by overthrusting. Middle Paleozoic through Mesozoic sedimentary rocks are largely restricted to the southern end of the Mineral Range, although these rocks are included in a few inselbergen on the west and east flanks of the range. This area was originally mapped by Earll (1957), but his nomenclature was revised on the State Geologie Map (Hintze, 1963). Further revisions are made in this report in order to bring the Mineral Range nomenclature into conformity with current usage for southwestern Utah. Earll (1957) mapped a thick sequence of thin-bedded to massive, unfossiliferous, tan to white dolomite and dolomitic limestones with a minimum thickness of 2,712 feet as undifferentiated Paleozoic rocks. This unit is labelled Mississippian Redwall Limestone on the State Geologic Map. Earll (1957, p. 13-17) does not mention any chert in the unit, and he emphasizes its unfossiliferous nature. This description does not accord well with the descriptions of Mississippian Redwall Limestone by Baer (1973) or the equivalent Mississippian Monte Cristo of Baetcke (1969); however, the stratigraphic position and thickness of the unit do lend support to the Mississippian age. Similarly, Earll mapped a thin-bedded to massive, tan to gray limestone unit, which was 917 feet thick and had poorly preserved fossils. He assigned this unit to the Upper Mississippian (1) Topache Limestone; but the unit was labelled Pennsylvanian Callville on the State Geologic Map. Subsequent studies on the Callville Formation in the Star Range (Baetcke, 1969), which is 10 miles to the west of the Mineral Range, have indicated that the upper portion of the Callville Formation is Permian in age and should be included in the Permian Pakoon Limestone. Because the Pennsylvanian-Permian division is difficult to locate in the field, current practice is to map Penns ylvanian -Pe rmian Callville-Pakoon Undifferen tia ted. Again, the lithology of Earll's measured section (1957, p. 19-20) of '"Topache Limestone" does not accord particularly well with Baetcke's measured section (1969, p. 37-40) of Callville-Pakoon Undifferentiated; but the thickness and stratigraphic position are similar. A thick unit of medium-bedded, buff to pink, cross-bedded orth<XJ.uartzite was referred to the Permian Coconino Formation by Earll (1957). The measured thickness is 1,181 feet, which agrees very well with 1,212 feet of Talisman Quartzite measured by Baetcke in the Star Range (1969, p. 48). Baetcke (1969, p. 44) points out that upper beds of the Talisman Quartzite may be Coconino equivalents, but that recent workers have tended to restrict the name HCoconino" to Permian sandstones that were deposited by 3 eolian processes. In the absence of direct evidence for eolian deposition of Permian sandstones in the Star Range, Baetcke preferred the name "Talisman," which is equivalent to the Permian Queantoweap Formation. The Permian sandstones of the Mineral Range are labelled Talisman Formation on the State Geologic Map. [Note: The State Geologic Map places the Talisman Quartzite within the Pennsylvanian System. However, the work of Brill (1963, p. 323) indicates that the Talisman is Permian.] Earll (1957) mapped a medium-bedded to massive, tan to light gray limestone with an abundance of chert as Permian Kaibab Formation. Earll's measured thickness of 698 feet (I 957, p. 26) accords well with the total thickness of 660 feet of Kaibab-Plympton Undifferentiated measured by Baetcke (1969, p. 66) in the Star Range. Earll noted that the Kaibab Formation was deposited disconformably on the underlying Coconino (Talisman) Formation, which indicates that the Toroweap Formation is absent. Earll (1957, p. 27-28) reported that the Triassic System is represented in the Mineral Range by three rock units. The lower unit has a local basal conglomerate, but mostly consists of red, brown, and yellow calcareous sandstone and sandy shale. The thickness is 141 feet. The middle unit is medium-bedded to massive gray limestone interbedded with thin-bedded, gray and brown limestone and calcareous shale. This unit is 567 feet thick and contains a Meekoceras fauna. The upper unit consists of red-brown to maroon, ripplemarked sandy shale, with minor interbedded limestone. Beds of the upper unit are repeated by faults, but thickness of the unit is estimated to range from 574 to 1,000 feet. Earll included all three rock units in the Moenkopi Group, and they are so labelled on the State Geologic Map. However, Stewart, Poole, and Wilson (1972, p. 16) state that strata which contain the Meekoceras fauna and outcrop near Minersville are correlative with the Timpoweap Member of the Moenkopi Formation. The Timpoweap is the lowest member of the Moenkopi formation recognized in southwestern Utah, so the lower red bed unit and the middle limestone unit of Earll (1957, p. 28) should be collectively correlated with the the Timpoweap Member of Stewart, Poole, and Wilson (1972). Such a correlation would make the Timpoweap Member 708 feet thick in the Mineral Range, whereas Stewart, Poole, and Wilson (I972, p. 17) report a maximum thickness of 450 feet for the Timpoweap Member. The upper red bed unit of Earll (1957, p. 28) might be correlated with the lower red bed member of Stewart, Poole, and Wilson (I972, p. 18). Earll reported the thickness of this unit as 574 feet to 1,000 feet, which also is much greater than any thickness reported by Stewart, Poole, and Wilson for the correlative unit. Baetcke (I 969) also noted an anomalously thick section of "Moenkopi" in the Star Range, 10 miles to the west. Two formations of the Jurassic System outcrop in the southern Mineral Range. One is the Navajo Sandstone, a medium- to thick-bedded, buff to pink, cross-bedded sandstone with a measured thickness of 1,538 feet (Earll, 1957, p. 33). The other is the Carmel Formation, a sequence of limestones and calcareous shales with a measured thickness of 574 feet (Earll, 1957, p. 35). Two conglomerate units in the Mineral Range have tentatively been assigned to the Cretaceous System. Uese (1957) mapped a 112 foot thick conglomerate which rests unconformably on the undifferentiated known Cambrian 4 limestones near the north end of the Mineral Range. He correlated this unit with the Cretaceous (?) Indianola Conglomerate of the Canyon Range. Earll mapped a conglomerate of similar lithology near the south end of the Mineral Range, which he correlated with the aaron Formation of Iron Springs. The relationship between the two conglomerate units of the Mineral Range is uncertain.