YOUNGER THAN ANTICIPATED MAXIMUM DEPOSITIONAL AGES OF SANDSTONES FOUND IN THE GREAT VALLEY FOREARC BASIN REVEAL LENGTHENED UNCONFORMITY BETWEEN THE COAST RANGE OPHIOLITE AND BASAL GREAT VALLEY GROUP, DEL PUERTO CANYON, CALIFORNIA, USA

The Great Valley Forearc (GVF) of California is a site for geo-economic-exploration and subduction-zone research, yet details about its evolution and initial tectonic setting remain actively debated. During initiation of the basin in the latest Jurassic-earliest Cretaceous, the sedimentary succession comprising most of the GVF, the Great Valley Group (GVG), was deposited atop the Coast Range Ophiolite (CRO) along its eastern margin. To better understand the temporal evolution of the GVF and the CRO, this study examines the maximum depositional age (MDA) of GVG rocks and crystallization ages of ultramafic and volcanic rocks of the CRO collected from Del Puerto Canyon, central California. We sampled the Knoxville Formation (traditionally assigned a Jurassic depositional age), including at its contact with the CRO. We also sampled the GVG in the subsurface east of the outcrop to target the CRO-GVG contact at depth. Two sandstones at the CRO-GVG contact in outcrop analyzed for detrital zircon U-Pb geochronology by LA-ICP-MS yield unimodal age peaks with MDAs of ~142-144 Ma, with 2-sigma overlap amongst the youngest statistical peak, maximum likelihood age, and youngest cluster of 3 or more grains overlapping at 2-sigma error. One additional surface sample shares a similar young age peak centered at ~148 Ma, but due to a tail of younger zircons, yields an MDA of ~102 Ma; CA-ID-TIMS analysis is in progress to analyze this youngest age cluster to achieve a more precise and potentially more accurate MDA. Detrital zircons from four subsurface sandstones yield MDAs of ~84-94 Ma with the same methods as above. The age spectra of these samples include additional older age peaks at ~95 Ma, ~125 Ma, and ~150 Ma. Integration of sedimentary facies analysis and field mapping with these detrital zircon MDAs suggest the three surface samples are part of the most basal forearc strata in the area, the Knoxville Formation. The results indicate the Knoxville at Del Puerto Canyon may have been deposited during the earliest Cretaceous. By contrast, the subsurface samples are consistent with deposition of the Late Cretaceous Panoche Formation, highlighting the regional-wide unconformity as seen in the lack of strata present between the Knoxville and Panoche Formations. Ongoing work seeks to establish the temporal relationships between the GVG and CRO.