40 MILLION YEARS OF MISSING LOWER CRETACEOUS GREAT VALLEY FOREARC STRATIGRAPHY WITHIN THE SAN JOAQUIN BASIN: FOREARC BASIN EVOLUTION AND THE NORTH AMERICA CORDILLERA MARGIN

The Great Valley forearc (GVf) basin preserves >13 km of stratigraphy deposited during latest Jurassic-earliest Cretaceous to Miocene time, preserving one of the most complete sedimentary archives of the North American Cordillera. However, several major (>5 myr) unconformities occur within this record, and interpretations of their origin include mechanisms such as exhumation of the Franciscan subduction zone complex or regional incision during sea level regression. This study integrates new geochronologic data from Del Puerto Canyon, in the San Joaquin basin (SJB) of the southern GVf, with basin-wide stratigraphic correlations to investigate the duration, spatial extent, and potential mechanisms driving a documented Lower Cretaceous to Upper Cretaceous regional unconformity within the southern GVf. At Del Puerto Canyon, ~145-139 Ma strata of the Knoxville Formation are coeval with strata of the Sacramento basin in the northern GVf, and record the onset of sedimentation within the SJB. The Cenomanian-Turonian (~96-85 Ma) Panoche Formation depositionally overlies the Knoxville Formation, forming a ~ 40 myr unconformity. One interpretation of this unconformity is that low erosion rates during a magmatic lull within the primary source region of the Sierra Nevada-Klamath arcs, combined with no mechanism to capture sediment at the latitude of the SJB, resulted in a period of sediment bypass. Alternatively, Lower Cretaceous sediments were deposited in the SJB, but subsequently eroded. In the Sacramento basin, deposition of the Platina and Lodoga Formations occurred during the interval of the unconformity at Del Puerto Canyon, along with a record of accretionary processes seaward of the Sacramento basin. In contrast, there is no evidence of an accretionary subduction complex immediately adjacent to the SJB until Cenomanian time. Additionally, successions of Upper Cretaceous rocks are thickest in the SJB compared to the Sacramento basin, reflecting a spatial shift in the locus of deposition during the interval of the unconformity. This suggests the forearc may have been segmented during its early stages of development, with the SJB transitioning from a sloped forearc basin with sediment bypass during the Early Cretaceous to a ridged system, with an established outer forearc high behind which to trap sediment, during the earliest Late Cretaceous.