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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 and Elizabeth L. Miller
Year: 2015
Series: Open-File Report 15-2a
Version:
Format: plate 1 of 2 only (geologic map): 41 x 54 inches, does not include cross sections or text
Scale: 1:24,000
For complete product including both plates and text, order OF2015-02 (see related items).
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 38.5–36.8 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 ~37.3–33.9 Ma (10–15°).
Subsequently, between ~31.1 Ma 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 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.
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.
Original Product Code: OF152a dox
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