TRIASSIC-JURASSIC STRATIGRAPHIC RELATIONS IN THE EASTERN JACKSON MOUNTAINS, NORTHWEST NEVADA: INSIGHTS FROM NEW U-PB ZIRCON GEOCHRONOLOGY

New U-Pb zircon geochronology calls for a modification of the Early Mesozoic stratigraphy of the eastern Jackson Mountains. The Jackson Mountains form a prominent range in northwest Nevada that contains a suite of late Paleozoic through Mesozoic arc-related volcanic, plutonic, and sedimentary rocks. In previous studies, the volcanogenic, marine strata (unit Tr₃ of the Boulder Creek Beds) flanking the eastern side of the Jackson Mountains had been assigned a Carnian to Norian (Late Triassic) age based on the youngest age of identified fossils and the interpretation that it is intruded by the Happy Creek Igneous Complex, which was thought to be of latest Triassic-Early Jurassic age. However, new U-Pb detrital zircon geochronology of samples of medium- to coarse-grained sandstone from the upper (BCB-2) and lower portions (BCB-4) of unit Tr₃ in addition to a sample of fine-grained andesite (HC-JM-7) from the Happy Creek Igneous Complex require revision of the stratigraphic relationships. When plotted on a relative probability diagram, BCB-4 (n=170) displays a dominate ²⁰⁶Pb/²³⁸U age peak at ~204 Ma with grains as young as ~190 Ma. Similarly, BCB-2 (n=117) shows a dominate ²⁰⁶Pb/²³⁸U age peak at ~205 Ma with grains as young as ~189 Ma. The sample from the Happy Creek Igneous Complex reveals a ²⁰⁶Pb/²³⁸U age of 191.9 ± 3.0 Ma (n=17). Incorporation of Early Jurassic (~190-195 Ma) grains into units of the “eastern” Boulder Creek Beds requires 2 things: 1) unit Tr₃ of the Boulder Creek Beds is younger than previously thought and at least as young as the Early Jurassic; and 2) unit Tr₃ is younger than the Happy Creek Igneous Complex. Therefore, we propose that the Happy Creek Igneous Complex represents the depositional basement for the “eastern” Boulder Creek Beds, as originally suggested by Willden (1963), rather than intruding into the Boulder Creek Beds. Future work using CA-ID-TIMS will be done to further constrain the timing of these relationships.