PROVENANCE OF CENOZOIC DETRITAL ZIRCONS IN THE MIOCENE HUMBOLDT FORMATION IN THE NORTHERN WOOD HILLS, NORTHEASTERN NEVADA: IMPLICATIONS FOR LOCAL SEDIMENTATION IN THE NEVADAPLANO

Basin and Range physiography in northeastern Nevada is largely a product of the development of rift grabens that evolved between 16 and 8 million years ago. These grabens are filled with sediment derived from 1) erosion of Mesozoic to Archean basement rocks in the uplifted footwalls bounding the graben, and 2) volcanic material derived from ~ 16 to 8 Ma rhyolitic eruptions of the migrating Yellowstone hotspot. We investigated a sequence of the Humboldt Formation at the northern end of the Wood Hills that lacked the characteristic hotspot-derived, white vitric ash typical of the Humboldt Formation. This sequence contains conglomerate and sandstone with texturally and compositionally immature lithic clasts indicating local derivation. Six samples of sandstone and conglomerate were collected from the upper part of the Humboldt Formation for detrital zircon analysis. In aggregate, these samples contain a near continuous age distribution of detrital zircons ranging from 42 to 16 Ma. Northeastern Nevada contains eruptive volcanic rocks ranging from 42 to 28 Ma that could be local sources of the older zircons. However, there are no local sources for zircons in the range of 28 to 16 Ma. Consequently, they most likely were deposited locally by air-fall from distal eruptions in central and southern Nevada and possibly southern Rocky Mountains. Some age distribution peaks of the samples match the age of voluminous eruptions at 25 Ma, 28 Ma, 30 Ma, and 36 Ma from these distal sources and hence provide additional support for a dominantly air-fall source for the zircons. Our near continuous spectrum of Cenozoic detrital zircon ages is significantly different from previous work on the Humboldt Formation in another location 100 km to the southwest, which has gaps in the age distribution. Overall, our analysis of the detrital zircon data indicates that there must have been a local pre-Humboldt-Formation basin capturing air-fall zircons between ca. 42 to18 Ma that subsequently made them available for transport and deposition in the upper part of the Humboldt Formation between 18 and 16 Ma. The preservation of any sediment during 42 to 16 Ma is unusual because this area is interpreted to be part of a broad paleogeographic highland, the Nevadaplano, that underwent extensive erosion during this time interval.