1:24,000 SCALE GEOLOGIC MAPPING OF CENOZOIC UNITS IN HUNTINGTON VALLEY AND THE EASTERN PIÑON RANGE, ELKO COUNTY, NEVADA

New 1:24,000 scale geologic mapping and dating of Cenozoic units in central Huntington Valley and the eastern Piñon Range, northeast Nevada, reveals much greater Middle Miocene and younger deposition, and much less Eocene–Oligocene deposition, than previously thought.

Geologic mapping, supported by geochronology and geochemistry, was employed to better describe the stratigraphy in the study area and to investigate in detail the history of deposition, erosion, faulting, and volcanism during and preceding regional extension. The oldest mapped units are likely Late Cretaceous to Eocene in age (though the youngest detrital zircons are Jurassic) and include basal conglomerate, sandstone, siltstone, and limestone redbeds and overlying limestone. The basal units unconformably overlie Paleozoic strata and lack Tertiary volcanic clasts characteristic of younger sedimentary rocks. They are locally up to 600 m thick but are not present above the basal unconformity in all exposures. The fluvio-lacustrine Eocene Elko Formation overlies the basal units and has a maximum thickness of 180 m in the study area, significantly thinner than observed elsewhere.

Our mapping, together with U-Pb detrital zircon and ⁴⁰Ar-³⁹Ar geochronology of interbedded tephra, reveals scant deposition of sedimentary strata between the Eocene Elko Formation and the overlying Middle–Late Miocene Humboldt Formation. The only sedimentary exposures within this age range are minor, discontinuous horizons of fluvio-lacustrine rocks within the Eocene–Oligocene Robinson Mountain Volcanic Field. Nearly all the sedimentary strata previously mapped as Eocene–Oligocene Indian Well Formation have been reassigned to the Miocene Humboldt Formation, and we therefore abandon the Indian Well Formation nomenclature.

The remaining Eocene and Oligocene volcanic and minor sedimentary rocks have been subdivided into four distinct packages. These packages record progressive westward tilting as evidenced by angular unconformities between ~36.8–31.1 Ma (10–15º) and again between ~31.1­–16 Ma (~30º), which were likely caused by normal faulting in the Piñon Range to the west. Westward tilting was followed by gentle eastward tilting that was synchronous with, and continued after, deposition of ~2130 m of Humboldt Formation strata between ~16 Ma and ~8.2 Ma.