New Geologic Map—Robinson Mountain Volcanic Field / NW Huntington Valley

of2015-2_plate1

Title: Preliminary geologic map of Cenozoic units of the central Robinson Mountain volcanic field and northwestern Huntington Valley, Elko County, Nevada
Author: Jens-Erik Lund Snee[earth.stanford.edu] and Elizabeth L. Miller[earth.stanford.edu]
Year: 2015
Series: Open-File Report 15-2
Format: two color plates and color text; plate 1 (geologic map): 41 x 54 inches, plate 2 (cross sections): 43 x 21 inches, text: 42 pages
Scale: 1:24,000

Huntington Valley is situated east of the Piñon Range, in the hanging wall of a shallowly west-dipping detachment system bounding the west side of the Ruby Mountains–East Humboldt Range (RMEH) metamorphic core complex. This geologic map of Cenozoic sedimentary, volcanic, and subvolcanic rocks of the central Robinson Mountain volcanic field and northwestern Huntington Valley provides important information about the history of Cenozoic crustal extension, magmatism, sedimentation, and paleogeography near the RMEH.

Depositional rates in the Elko Basin were minor from Cretaceous to Oligocene time, and became rapid in the middle Miocene. Late Cretaceous(?)–Eocene(?) conglomerate, sandstone, siltstone, and limestone “redbeds” (TKcs) and limestone (TKl) are exposed at the base of the Cenozoic section in places, where they each reach thicknesses of ~600 m, but they are not exposed at all in other locations. The overlying Eocene Elko Formation is only ~180 m thick at its greatest in the map area. Detrital zircon geochronology conducted on two samples collected near its base yields a maximum depositional age of ~45.9 ± 1.0 Ma, and a third sample collected near the top of this unit yields a maximum depositional age of 37.9 ± 0.5 Ma.

The calcic to calc-alkalic Robinson Mountain volcanic field records early peraluminous to weakly metaluminous “ignimbrite flare-up” volcanism of basaltic andesite to trachydacite and rhyolite composition, which occurred mostly between about 38.5 and 36.8 Ma. Early eruptions were roughly synchronous with the end of deposition of the Elko Formation, and no significant unconformity is observed beneath the volcanic units. The only Eocene–Oligocene sedimentary rocks exposed above the Eocene Elko Formation are thin, fluviolacustrine deposits interbedded within the volcanic rocks. For this reason, the “Indian Well Formation” name for Eocene–Oligocene volcanic and sedimentary rocks has been abandoned. The rhyolitic, Oligocene-age tuff of Hackwood Ranch is significantly younger than the Eocene deposits of the Robinson Mountain volcanic field. This tuff was erupted at ~31.1 Ma, which coincides with a lull in regional volcanism, but could represent far-traveled deposits from a distant volcanic center.

Significant, approximately westward tilting developed a shallow angular unconformity between ~37.3 and 33.9 Ma (10–15°). Subsequently, between ~31.1 and 24.4 Ma or later, 10–15° additional westward tilting occurred. These westward tilting events were likely associated with slip on east-dipping normal faults in the Piñon Range. Following the second episode of westward tilting, a basin rapidly developed near the present-day RMEH, as recorded by thick deposits of the mostly Miocene Humboldt Formation that dip gently eastward toward the RMEH. Greater than 1 km of the sedimentary strata previously mapped as Eocene–Oligocene age has been reassigned to the Miocene Humboldt Formation as a result of our geologic mapping and supporting geochronology.

Detrital zircon geochronology yielded a coherent age group at ~24.4 Ma for one tuffaceous pebble conglomerate and sandstone sample at the base of the Humboldt Formation, but it is unlikely that this maximum depositional age constrains the timing of the start of basin sedimentation. Deposition accelerated at ~16–15 Ma, when most of Humboldt Formation pebble conglomerate, sandstone, siltstone, marl, and air-fall tuff were deposited in Huntington Valley. Locally, pre-Tertiary rocks were exposed by faulting by ~16 Ma (although this depositional age is not well constrained), and RMEH provenance is not detected until ~14 Ma, suggesting that the metamorphic core complex was not exposed until about this time. Greater than 2100 m of Humboldt Formation strata were deposited in the study area alone (thickening further eastward toward the RMEH), and deposition continued until at least ~8.2 Ma, but the rate apparently decreased before ~12 Ma.

Miocene or later fault slip occurred along a well preserved, east-dipping normal fault system exposed at the east side of the Piñon Range, partially synchronous with faulting at the RMEH. Uplift and erosion of Eocene- to Quaternary-age sedimentary and volcanic deposits on the west side of Huntington Valley support a significant magnitude of slip on west-dipping normal faults west of the study area during or after Miocene time. Open folding of the Humboldt Formation occurred during or after the middle–late Miocene, perhaps due to normal fault slip offsetting underlying Paleozoic basement.

This geologic mapping study thus supports the hypothesis that surface-breaking extensional faulting in and near the mapped area was minor from the Late Cretaceous(?) through the early Miocene, and that most surface-breaking extension and sediment deposition occurred in middle Miocene time.

This map covers all of the following 1:24,000-scale quadrangles: Robinson Mountain, Cedar Ridge, and Red Spring and a portion of the following quadrangles: Bailey Mountain, Bullion, and West of Lee.

This map was prepared as part of the EDMAP component of the National Cooperative Geologic Mapping Program in cooperation with the U.S. Geological Survey and with support from the Geological Society of Nevada.

View or purchase Open-File Report 15-2 here: http://pubs.nbmg.unr.edu/product-p/of2015-02.htm

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