MAXIMUM DEPOSITIONAL AGES AND PROVENANCE FROM DETRITAL ZIRCONS COMPARED TO PALEONTOLOGIC AGES AND IMPLICATIONS FOR DEFORMATION TIMING, LOWER GREAT VALLEY SEQUENCE, KNOXVILLE TO LAKE BERRYESSA, WEST OF SACRAMENTO VALLEY, CA

To compare the reliability of fossil vs detrital zircon-constrained ages in the lower Great Valley Sequence (GVS) and explore the nature and timing of its deformation, we collected samples for detrital zircon (DZ) U-Pb dating and Buchias from 4 lower GVS localities near Knoxville, CA. Along Pope Ck, NW of Lake Berryessa, we also collected sandstone and a thin tuff from strata overlying mafic breccia at the base of the GVS, that in turn, is unconformable on the Coast Range Ophiolite (CRO). Low yield zircon separates from the tuff at Pope Ck and the stratigraphically lowest sandstone at Knoxville (n=10 and 24, respectively) were analyzed at the Stanford/USGS SHRIMP. Zircons from 3 localities at Knoxville (each with n>100) and 1 locality at Pope Ck (n=60), were analyzed at the USGS Denver ICPMS lab. A Knoxville maximum depositional age (MDA) was based on the youngest cluster of 3 or more ages that overlap at 2 sigma (Dickinson and Gehrels, EPSL, 2009); which was compared with the age indicated by the Buchias. The stratigraphically lower 3 localities at Knoxville all contain B. piochii and (or) B. elderensis (Tithonian) and have MDAs of 154.1 ± 9.9 Ma, 142.0 ± 13 Ma and 153.6 ± 2.0 Ma. A 4th locality (stratigraphically highest) contains B. pacifica(Valanginian), with a MDA of 139.6 ± 2.8 Ma. At Pope Creek, the mean age at, of 4 youngest zircons in the tuff from shale above the basal breccia, is 151.1 ± 2.5 Ma (1 sigma). The MDA of sandstone downfaulted onto the basal breccia is 142 ± 1.8 Ma (Berriasian).

DZ-derived MDAs of lower GVS strata reasonably match fossil ages. A major change in zircon provenance occurs in our samples in the Early Cretaceous (~145 -143 Ma), based largely on ≥145 Ma GVS samples having significant Precambrian zircons likely sourced from south of the Sierran arc, that contrast with <145 Ma GVS samples having mostly Late Jurassic - Early Cretaceous zircons sourced in the Sierran–Klamath arc. Wright and Wyld (2007, GSA Special Paper 419) invoked ≥ 500 km of Early Cretaceous dextral displacement of the lower GVS from S of the Sierran arc to explain such a change. If our observed provenance change at 145-143 Ma was due to dextral translation, it was bracketed by major CRO unroofing recorded by the basal breccia (≥ 151 Ma), sedimentary serpentinite (134-131 Ma) and later by mixing of the CRO and lower GVS into mélange.