DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF THE MODERN SACRAMENTO-SAN JOAQUIN RIVER DELTA, CALIFORNIA AND IMPLICATIONS FOR INTERPRETATION OF LATE CRETACEOUS GREAT VALLEY GROUP SANDSTONE PROVENANCE

The Sacramento-San Joaquin river system drainage basin is dominated by the Sierra Nevada mountain range, and the deltaic sediments in this system thus provide an outstanding integrated sampling record of the Mesozoic batholith history of California. Detrital zircon U-Pb ages reported here from modern Sacramento-San Joaquin delta and river sand define dominant Cretaceous and Jurassic peaks on age probability plots that closely reflect the batholith crystallization history as determined from extensive in situ zircon U-Pb dating of batholith samples

In strong contrast, Late Cretaceous forearc strata of the Great Valley Group have previously published detrital zircon U-Pb age distributions that look nothing like age distributions determined from the in situ Sierra Nevada batholith or detrital zircon from modern sediments of the Sacramento-San Joaquin river system. This observation, and particularly the paucity of Late Cretaceous age zircon (~105-84 Ma) in Great Valley Group forearc strata, presents a strong challenge to the prevailing view that the late Mesozoic Great Valley forearc basin sequence was eroded off of the Sierra Nevada batholith. The evidence suggests that either (1) the Late Cretaceous bedrock exposure across the Sierra Nevada was very different than that represented by the modern erosional level, or (2) that the late Cretaceous forearc basin was sourced by a different magmatic arc segment and subsequently translated along the margin by strike-slip faulting following its deposition, or (3) that sediment pathways into the forearc basin involved substantial axial transport or other unrecognized complexities such as input from extra-regional sources.