1. GEOLOGY AND GEOCHRONOLOGY OF CENOZOIC UNITS IN THE
PIÑON RANGE AND HUNTINGTON VALLEY, NEVADA
A THESIS
SUBMITTED TO THE DEPARTMENT OF GEOLOGICAL AND
ENVIRONMENTAL SCIENCES
AND THE COMMITTEE ON GRADUATE STUDIES
OF STANFORD UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS
FOR THE DEGREE OF
MASTER OF SCIENCE
Jens-Erik Lund Snee
June 2013

2. Abstract
The Ruby Mountains–East Humboldt Range (RMEH), NE Nevada, is a classic example of
a metamorphic core complex (MCC), which exposes deep crustal levels, providing insight
into the complex Tertiary and Mesozoic geologic history of the Cordillera. The exten-
sional history of the RMEH has been controversial for decades but comparatively little
study has focused on the surrounding sedimentary basins, which record an impressively
complete Cenozoic sedimentary and volcanic history. Cenozoic rocks in Huntington Val-
ley, which separates the RMEH and the Piñon Range to its west, and is situated within the
geographic extent of the Elko Basin, were mapped at 1:24,000 scale to constrain the his-
tory of sedimentation, volcanism, and upper crustal deformation in the hanging wall above
the west-dipping detachment bounding the west side of the RMEH. Geologic mapping and
interpretations were supported by geochronology of igneous and sedimentary rocks and
geochemical analysis of igneous rocks (including trace element geochemistry of zircon).
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 Tertiary section in places, where they each reach estimated thicknesses of
~600 m, but they are not exposed at all in other locations. One sample of the Late Creta-
ceous–Eocene(?) redbeds was analyzed by U-Pb detrital zircon geochronology and yielded
no grains younger than Triassic age. Zircon populations in this sample match those doc-
umented in the Lower Chinle-Dockum Triassic paleodrainage system (e.g. Dickinson &
Gehrels, 2008) and suggest that sediment deposited in these redbeds was recycled from
Triassic rocks.
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3. 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. A small number of Jurassic
zircons in the Elko Formation and overlying units are likely derived from plutons in the
Cortez Range to the west, and a ~46 Ma population may represent air fall from the Challis
volcanic field in Idaho. No Idaho Batholith detrital zircon signature is observed in Tertiary
units, indicating that the Elko Basin probably had no northern sediment sources, consistent
with the recent work showing a network of east- and west-draining paleorivers in the Great
Basin during Eocene–Oligocene time (e.g. Henry, 2008).
The calcic to calc-alkalic Robinson Mountain volcanic field records early peralumi-
nous to weakly metaluminous “ignimbrite flare-up” volcanism of basaltic andesite to tra-
chydacite and rhyolite composition, which occurred mostly between 38.5–36.8 Ma, based
on 4 new U-Pb SHRIMP (zircon) ages and 4 unpublished 40Ar-39Ar (sanidine and plagio-
clase) ages by C. Henry and D. John. Early eruptions were roughly synchronous with the
end of deposition of the Elko Formation and no significant unconformity is observed at
the top of that unit. The rhyolitic Tuffs of Hackwood Ranch were erupted at ~31.1 Ma,
based on 2 new SHRIMP U-Pb (zircon) and 2 new 40Ar-39Ar (sanidine) dates, which co-
incides with a lull in regional volcanism, but could represent far-traveled deposits from
a distant volcanic center. Significant ~westward tilting developed angular unconformi-
ties between ~36.8–31.1 Ma (10–15º) and again between ~31.1 Ma and perhaps as late
as ~16 Ma (~30º additional), and was likely associated with slip on normal faults in the
Piñon Range. Westward tilting appears to have been confined mostly to the study area and
its immediate surroundings. It is proposed that the Indian Well Formation nomenclature
for Eocene–Oligocene volcanic and sedimentary rocks be abandoned due to the discovery
(this study) that nearly all (> 1 km) of the sedimentary strata previously mapped as part of
that unit are actually Miocene in age and should be reassigned to the overlying Humboldt
Formation. The remaining Eocene and Oligocene volcanic rocks and minor sedimentary
horizons have been subdivided into four packages of similar volcanic rocks. This reclassi-
fication is made on the basis of map relations, stratigraphic correlation, and high-resolution
detrital zircon (5 ages) and 40Ar-39Ar (4 ages) geochronology throughout the section.
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4. Detrital zircon geochronology yielded a coherent age group at ~24.4 Ma for one tuffa-
ceous 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 For-
mation 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 (al-
though this depositional age is not well constrained), and RMEH provenance is not detected
until ~14 Ma, suggesting that the MCC was not exposed until about this time. Deposition of
the Humboldt Formation 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, imbricated, east-dipping
normal fault system exposed at the east side of the Piñon Range, synchronous with faulting
at the RMEH. However, uplift and erosion of Eocene–Quaternary sedimentary and volcanic
deposits on the west side of Huntington Valley suggest a significant component 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. The findings
of this study are consistent with recent work showing that surface-breaking extensional
faulting in the vicinity of the RMEH was minor and local in the Eocene–Early Miocene
and that the bulk of Cenozoic extension occurred in the Middle Miocene (e.g. Colgan et
al., 2010).
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