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Geologic map of the Parran quadrangle, Churchill County, Nevada [MAP AND TEXT]
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Title: Geologic map of the Parran quadrangle, Churchill County, Nevada

Author: James E. Faulds, Richard D. Koehler, Nolan P. Dellerman, and Heather L. Green
Year: 2022
Series: Open-File Report 2022-02
Format: 1 sheet: 38.5 x 28 inches, cross sections, color; text: 6 pages, b/w
Scale: 1:24,000

NBMG geologists James Faulds, Richard Koehler, Nolan Dellerman, and Heather Green completed a detailed geologic map of the Parran 7.5ꞌ quadrangle. The quadrangle is located in the eastern Hot Springs Mountains of Churchill County, Nevada about 80 km northeast of Reno. This area contains the Desert Queen geothermal system, which is a hidden geothermal system (no surface expressions such as hot springs) with temperatures >140°C at depths as shallow as ~150 m. The map area includes the western part of a large composite basin, the Desert Queen basin, and adjacent parts of the Hot Springs Mountains. The detailed mapping has revealed the stratigraphic and structural framework of the area, which is critical to developing a conceptual model for the geothermal system.

In the Desert Queen area, Tertiary volcanic and sedimentary strata rest nonconformably on Mesozoic metasedimentary-metavolcanic and plutonic basement. Quaternary sediments onlap the bedrock units. The Mesozoic basement is exposed directly west of the quadrangle in the footwall of the east-dipping Desert Queen fault zone. An erosional surface of moderate relief (i.e., hundreds of meters) is developed on the Mesozoic rocks, above which rests the Tertiary section. The Tertiary section is a heterogeneous mix of volcanic and sedimentary rocks, with a total of thickness of ~2–3 km. Only middle to late Miocene rocks are exposed in the quadrangle, but Oligocene units have been observed in deep wells in the area and altered sedimentary rocks of probable Eocene age crop out directly west of the map area. Although significant lateral variations exist, the Tertiary section exposed within the quadrangle can be grouped into several discrete packages. In ascending order, these include:
  • A lower sequence of middle Miocene andesite, dacite, and rhyolite lavas and subordinate rhyolitic to dacitic ash-flow tuffs, ranging from ~16 to 13 Ma.
  • A complex section of dominantly aphanitic basalt lavas, but with intercalated sequences of white siltstone, diatomite, and tuffaceous sedimentary rocks, ranging from ~13 to 10 Ma.
  • A sequence of dominantly sedimentary rocks (mainly diatomite and tuffaceous siltstone) and lesser olivine basalt flows, ranging from ~12 to 10 Ma.
  • An ~9–10 Ma limestone unit that pinches out eastward and interfingers with siltstone and diatomite units.
  • Olivine basalt flows and porphyritic dacite domes and flows, which dominate the southern part of the map area.
  • A capping 7.46±0.31 Ma aphyric dacitic ash-flow tuff.
  • Late Miocene to Pleistocene basin-fill sediments primarily in the Desert Queen basin.

Tephras (i.e., ash-fall deposits) are common in the late Miocene section and facilitate dating and correlation of stratigraphic units through geochemical fingerprinting. The capping ~7.5 Ma ash-flow tuff is one of the youngest pyroclastic deposits in the region. It forms a resistant cap on the ridges and mesas to the south of the Desert Queen basin and appears to underlie the basin itself, as evidenced by several small exposures along the western margin of the basin. It provides an important marker from which to gauge the magnitude of late Miocene to recent extension.

The Quaternary sediments in the area are dominated by lacustrine deposits of Pleistocene Lake Lahontan. Most of the closed basins in the Great Basin contained numerous pluvial lakes during the Pleistocene. However, lacustrine deposits older than those of the late Pleistocene (Sehoo) cycle were not observed in the Desert Queen area. During the Sehoo highstand, Lake Lahontan reached a maximum altitude of about 1330 m submerging much of the map area. Lake Lahontan deposits include beach gravels and sands and lacustrine silt and mud. Quaternary alluvial deposits include alluvial fans of different ages. In the central part of the map area, two small areas contain tufa mounds aligned ~NNE, parallel to the predominant structural grain in the area. These tufa mounds may mark the sites of late Pleistocene hot springs that issued along NNE-striking normal faults.

The structural framework of the Parran quadrangle is dominated by NNE-trending gently to moderately tilted (<10° to ~60°) fault blocks bounded by moderately to steeply dipping normal faults. Most of the faults accommodated relatively minor offset (i.e., hundreds of meters or less), but the east-dipping Desert Queen fault zone in the westernmost part of the quadrangle accommodated 2–3 km of normal displacement. West-tilted fault blocks dominate the area within and proximal to the Desert Queen basin. Bedding and layering show a distinct NNE-trending structural grain in accord with tilting along the NNE-striking normal faults. The Dessert Queen geothermal system is probably controlled by both a step-over (or relay ramp) within and horse-tailing southern termination of the Desert Queen fault zone.

NBMG open-file reports have not been formally peer reviewed. This geologic map was funded in part by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G20AC00390, 2020 and partly by Magma Energy Corporation and Raser Power Systems, LLC (Cyrq Energy).

Suggested Citation:
Faulds, J.E., Koehler, R.D., Dellerman, N.P., and Green, H.L., 2022, Geologic map of the Parran quadrangle, Churchill County, Nevada: Nevada Bureau of Mines and Geology Open-File Report 2022-02, scale 1:24,000, 6 p.

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