NEW IGNEOUS AND DETRITAL ZIRCON DATA FROM MOUNT JURA, TAYLORSVILLE, CALIFORNIA: IMPLICATIONS FOR THE LATEST TRIASSIC THROUGH EARLIEST MIDDLE JURASSIC EVOLUTION OF A SHALLOW MARINE BASIN AFFECTED BY ARC AND CRATON-SOURCED SEDIMENT

The Jurassic clastic rock on Mt. Jura remains an enigma in the northern Sierra Nevada of California. U/Pb age data indicate that the basement underlying the Lower Jurassic section consists of mid-Rhaetian (ca. 206 Ma) rhyolite. Fossil control (Echioceras) indicates Jurassic deposition did not begin until ca. 193 Ma. U/Pb data on interbedded felsic pyroclastic rock indicates volcanism was active by early Pliensbachian (ca. 190 Ma). A hypabyssal basaltic andesite (ca. 187 Ma) cuts the unconsolidated Lower Jurassic sediment and pyroclastics.

An unconformity separates the Lower Jurassic section on Mt. Jura from conglomeratic Middle Jurassic rock containing impressions of Euhoploceras n. sp. Aff. E. modestum Buckman (equiv. to the Graphoceras concavum Tethyan ammonite zone). These fossils indicate deposition had resumed late in the Aalenian. Detrital zircon (DZ) sampling just above the fossil-bearing horizon included two youngest single grains with U/Pb ages of 171.0 ±1.7 Ma and 171.9 ±2.5 Ma, respectively.

Abundant Mesozoic zircon and volcanic lithics in DZ samples from the bases of the Lower and Middle Jurassic sections on Mt. Jura indicate the depositional site was tied to an arc active from the late Norian through late Aalenian. Each sample also contains significant amounts of Paleozoic and Precambrian DZ interpreted as being originally sourced from the Appalachian, Grenville, and Yavapati-Mazatzal provinces and accreted Peri-Gondwanan terranes before being recycled from Mesozoic erg deposits on the Colorado Plateau.

The depositional ages of each sampled DZ location on Mt. Jura suggest age equivalency with sediment of the Kayenta Formation and San Rafael deposystem, respectively. The presence of this erg sourced DZ in the Mt. Jura depositional site indicates it is unlikely to have been located in its current position (northeastern Sierra Nevada) during the Early and Middle Jurassic. More likely, it was positioned as a basin near the southwestern edge of the Mojave block California, in a location which could have received wind-blown dust sourced from the craton interior in the early Pliensbachian through latest Aalenian. Its current location is likely the result of Cretaceous, margin-parallel strike-slip displacement.