Geology and geochronology of Cenozoic units in the Piñon Range and Huntington Valley, Nevada

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 extensional 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 Valley, 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 history 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 Cretaceous–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 documented 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.

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 peraluminous to weakly metaluminous “ignimbrite flare-up” volcanism of basaltic andesite to trachydacite 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 plagioclase) 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 coincides with a lull in regional volcanism, but could represent far-traveled deposits from a distant volcanic center. Significant ~westward tilting developed angular unconformities 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 reclassification 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.

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 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).