Age and Provenance of the Oldest Sediments in the Peninsular Range Forearc Basin, Orange County, California

Late Cretaceous sedimentary rocks exposed along the western flank of the Santa Ana Mountains of Orange County, California record deposition in the forearc basin formed along the active continental margin. This study investigates the extent to which new forearc basins are filled with purely arc-derived sediments versus sediments transported by either longshore drift or arc-traversing fluvial systems. Initial petrographic analysis of three lithologically similar forearc samples reveal differences in mineralogy that are interpreted as reflecting differences in the sediment provenance and transport distance. Based on these petrographic observations, we hypothesize that detrital zircon populations will reveal variations in the fluvial systems delivering sediment to the Late Cretaceous forearc basin.

Detrital zircon grains were separated from samples collected from two locations within the Cleveland National Forest: (1) along Silverado Canyon, from the Baker Canyon Conglomerate member of the Ladd Formation, and (2) within Williams Canyon, from the Shultz Ranch and Pleasant Sandstones members of the Williams Formation. These formations were sampled because they represent the oldest shallow-marine-to-fluvial sediments associated with the Peninsular Ranges batholith (PRB). Use of the LA-ICP-MS at the University of Arizona’s Laserchron Laboratory to measure the U-Pb isotopic composition of ≈100 individual zircon crystals from each sample will provide maximum constraints on the depositional ages of these units. In addition, statistical analyses of any pre-Cretaceous zircons will provide information regarding the provenance of these sediments. Data will be compared with a parallel study underway on the Early Cretaceous Sierra Nevada-Great Valley Group arc-forearc system. The results of this study will contribute to an investigation of the provenance of the forearc deposits associated with the PRB, with the ultimate goal of expanding our understanding of sediment delivery and basin development and interconnectivity within Orange County.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California