New Geologic Map – Independence Valley NW

Preliminary Geologic Map of the North Half of the Independence Valley NW Quadrangle and the Adjacent Part of the Independence Valley NE Quadrangle, Elko County, Nevada
Author: Seth Dee, Christopher D. Henry, Michael W. Ressel, and Andrew V. Zuza
Year: 2017
Series: Open-File Report 2017-06
Version: first edition, September 2017
Format: plate: 35 x 30.5 inches, color; text: 4 pages, b/w
Scale: 1:24,000

The north half of 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. Mapping in the quadrangle was done in the summer of 2017.

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 is interpreted to place Permian Pequop Formation above Ordovician Fish Haven Dolomite and may be correlative with the enigmatic Pequop fault observed in the adjacent Pequop Summit and Independence Valley NE quadrangles and variably interpreted as a thrust (Thorman, 1970) or a low-angle normal fault (Camilleri, 2010). Another borehole was advanced through the eastern range-front fault and constrains the dip of the fault 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 tuffaceous sediments with a maximum measured bedding dip of 34° east. 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.

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 high-stand and recessional stages of latest Pleistocene Lake Clover (Munroe and Laabs, 2013). 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 geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G16AC00186, 2017.

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