AGE AND PROVENANCE OF THE OLDEST SEDIMENTS IN THE NORTHERN PENINSULAR RANGES FOREARC BASIN, ORANGE CO., CA

Late Cretaceous sedimentary rocks of the Ladd and Williams Formations record the initial stages of deposition in the Peninsular Ranges (PR) forearc basin, as well as the transition from solely arc-derived deposits to those containing extra-regional sediments. In this study, detrital zircon grains were separated from sandstone samples collected along 2 traverses exposed along the northwestern flank of the Santa Ana Mountains: (1) a northern traverse, along Silverado-Williams Canyon, and (2) a southern traverse, along the Ortega Highway. Collection of U-Pb zircon data by LA-ICP-MS was conducted at the University of Arizona’s Laserchron Center. Maximum depositional ages estimated for the lowest and highest stratigraphic units from the northern transect correspond fairly well with their respective stratigraphic ages: 97.9±2.7 Ma compared to a stratigraphic age of 91-89 Ma; and 75.3±1.6 Ma compared to a minimum stratigraphic age of 70 Ma. In contrast, detrital zircon dates from the stratigraphically intermediate sample (Williams Formation, Shultz Ranch Sandstone member) provides a maximum depositional age of 95.8±1.6 Ma; this date is ~10 Ma older than the accepted maximum stratigraphic age of 83 Ma, but approximates the time that local arc magmatism ceased. Statistical analysis also provides information about sedimentary provenance: the stratigraphically lowest samples are purely arc derived, while the stratigraphically highest sample contains a significant population of extra-regional zircons. Possible origins of these exotic zircons include ≈83 Ma plutons present in the northeastern PR and ≈75 Ma plutons exposed in western Arizona. Petrographic analysis reveals the appearance of metamorphic clasts in this youngest sample (Williams Formation, Pleasants Sandstone member), consistent with arc dissection and a significant change in the provenance of forearc sediments. Taken together, the detrital zircon data record the initial filling and breaching of the PR forearc basin as arc magmatism ceased. The degree of lateral variation of sedimentary provenance will be assessed by comparing data from the more southern, Ortega Highway section, to the northern, Silverado-Williams section. The results of this study will form a baseline for studying sediment delivery to early Cenozoic basins within Orange County.