Authors: Andrew V. Zuza, Seth Dee, Christopher D. Henry, Michael W. Ressel, and Charles H. Thorman
Series: Open-File Report 2019-03
Version: partially supersedes Open-File Report 2017-06
Format: plate: 40.5 x 28.5 inches, color; text: 18 pages, color
The Independence Valley NW 7.5-minute quadrangle covers a part of the western Pequop Mountains and adjacent Independence Valley in eastern Elko County. The east-tilted Pequop Mountains have newly recognized Carlin-type gold deposits in a geographic and geologic setting distinct from similar deposits elsewhere in Nevada. Southeast-dipping Cambrian through Ordovician sedimentary rocks are exposed in the range front along the eastern edge of the map area. Eocene rhyolite dikes and sills, and Cretaceous granitic sills and pods locally intrude the oldest Cambrian stratigraphy. The Eocene intrusions may be part of a magmatic system that produced the heat source for the nearby Carlin-type mineralization. The range front is bound on the west by two west-dipping normal fault systems that accommodated late Cenozoic exhumation. Exposed in the hanging wall of the eastern fault system are late Cenozoic basin deposits that uncomfortably overlie Cambrian through Ordovician sedimentary rocks. Logs from three boreholes drilled into the Paleozoic rocks of the hanging wall during mineral exploration were used to help develop cross section A–A”. One of the boreholes encountered an approximately 60-m-thick zone of fault gouge and a fault sliver with repeated Ordovician stratigraphy. This fault zone may be correlative with the enigmatic Pequop fault observed in adjacent quadrangles. Another borehole advanced through the eastern range-front fault constrains its dip to 34° west. Correlation of stratigraphy across the eastern range-front fault suggests approximately 4 km of total dip-slip displacement during Cenozoic exhumation.
The oldest Cenozoic basin deposits exposed between the two range-front fault systems are Miocene tuffaceous sediments with a maximum measured bedding dip of 34° east. New 40Ar/39Ar dates bracket the age of the deposit between approximately 6 Ma and 10.8 Ma. The tuffaceous sediments are overlain by a megabreccia landslide deposit with individual bedrock blocks over 200 m long. The individual blocks have lithologic and textural characteristics similar to rocks exposed along the western flank of the modern Pequop Mountains, which may have been the source of these megabreccia deposits. The megabreccia is overlain by Pliocene fanglomerate deposits with nearly horizontal bedding. New 40Ar/39Ar dates from detrital sanidine grains constrain the age of the fanglomerate to younger than ca. 4.8 Ma. New dating of the Cenozoic basin deposits records the timing of the east-tilting of the range along range-bounding faults.
The western range-front fault, named the Independence Valley fault zone, has evidence for late Quaternary activity. In the footwall of the fault, alluvial-fan deposits of probable middle Pleistocene age are beveled onto the Cenozoic sediments. Late Quaternary displacement along the Independence Valley fault zone has uplifted these fan deposits a minimum of 30 m. The youngest fan deposits offset by the fault zone are of probable latest Pleistocene age, and are displaced by fault scarps up to 3 m high. In Independence Valley, lacustrine gravels are deposited on shorelines, beach bars, and spits recording the highstand and recessional stages of latest Pleistocene Lake Clover. An older lacustrine gravel deposit with a well-developed pedogenic carbonate soil horizon was mapped topographically above the latest Pleistocene shorelines along the western edge of the map area.
This map completes a suite of three new geologic maps in the Pequop Mountains including the Independence Valley NE and Pequop Summit 7.5-minute quadrangles. Together these maps and associated analytical datasets build upon prior research to address basic (characteristics and timing of major contraction, metamorphism, and extension) and applied (origin of Carlin-type gold deposits) geologic research questions. Contraction and metamorphism, which had been attributed to either the Jurassic Elko or Cretaceous Sevier orogenies, is likely Jurassic because of a newly mapped lamprophyre sill that intruded along the major thrust of the range. Although the lamprophyre that intruded the thrust is not yet dated, our dating of similar mafic intrusions across the range all yielded similar ca. 160 Ma ages. Furthermore, a continuous metamorphic gradient from amphibolite-grade Cambrian rocks to non-metamorphosed Permian rocks in the lower plate of the thrust raises questions about previous interpretations of overlying thrust plates that buried rocks to great depths and pressures. New thermochronology reveals three significant overprinting thermal pulses that affected the range—Middle Jurassic, Late Cretaceous, and Eocene—that resulted in the metamorphism and economic mineralization.
This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G16AC00186 (2017) and G18AC00198 (2019).