This Special Paper is the product of nearly 25 years of geologic investigations. It is an exposition of two small areas, both less than 25 km east of the Mississippian Roberts Mountains allochthon, but each displaying a different, unique geologic terrane, previously undocumented in Nevada and perhaps in North America. One area, the Bisoni-McKay, at the south end of the Fish Creek Range, displays an olistostrome, shed eastward during the late Late Devonian (early Famennian) from a migrating Antler orogenic forebulge. The other, the Warm Springs–Milk Spring, at the south end of the Hot Creek Range, displays a deeper marine terrane affected by the early Late Devonian (middle Frasnian) Alamo impact.
Both areas are readily accessible to visitors driving passenger cars. The Bisoni-McKay area borders a well-maintained gravel county road that extends southward into Nye County from a junction with U.S. Highway 50, about ten miles south of Eureka, the Eureka County seat. The Warm Springs–Milk Spring area borders U.S. Highway 6, about 50 miles east of Tonopah, at the road junction with Nevada Highway 375, the Extraterrestrial Highway, which passes just north of Area 51. Both areas contain many dirt roads and trails that can be traversed by all-wheel-drive vehicles.
Chapter 1 on the Bison-McKay area provides details of upper Lower Devonian to lower Upper Devonian elongate gray limestone olistoliths, which were encased in yellow silty argillaceous sediments, shed from the forebulge on listric faults early in the Famennian, and then were covered by younger Famennian sediments of the synorogenic Woodruff Formation. The chapter presents a geologic history and, combined with data from the authors’ many years of regional studies in Nevada and Utah, redates the Antler orogeny as having been initiated in latest Middle Devonian time. Dating of this and two other Devonian Antler orogenic pulses is related to regional unconformities. This chapter also documents and evaluates Devonian deep-marine syngenetic vanadium deposits.
Chapter 2 on the Warm Springs–Milk Spring area not only reinforces the interpretation of geologic history, but also documents two large olistoliths that were dislodged from the seaward rim of, or the seafloor beyond, the Alamo impact crater. Using differences in Alamo impact effects, the chapter divides the area into four fault blocks separated by younger left-slip faults and demonstrates that the report area was offset eastward from the main part of the range and that the structural blocks were offset progressively farther eastward by each fault. This chapter also documents a Mississippian deep seafloor that was dominated by barite-depositing white smokers, and redates Oligocene volcanic rocks north of the Kawich caldera. New dates on Paleogene volcanic-rock units in both chapters are determined by the 40Ar/39Ar method.
Detailed geologic maps at scales of 1:8,000 and 1:10,000 document the conclusions, interpretations, and hypotheses presented in Chapters 1 and 2, respectively. Identification and dating of Paleozoic rock units are accomplished by means of nearly a hundred acid-dissolved carbonate conodont samples and at least 50 collections of conodonts on siltstone bedding planes that were identified either in the field or later in the office.