SILICA PHASES AND DIAGENETIC TRENDS FOUND WITHIN THE MONTEREY FORMATION OF COASTAL ORANGE COUNTY, CALIFORNIA, REFLECT THE TECTONIC AND BURIAL HISTORY

The middle-to-upper Miocene (17-5 Ma) Monterey Formation was deposited along the California margin in a range of marine basins, slope, bank top, and shelf settings. The formation consists of highly siliceous sedimentary rocks derived from the tests of diatoms, radiolarians, and fine-grain detritus. The Monterey Formation outcrops in the San Joaquin Hills area between Newport and Laguna Beach and consists of laterally variable exposures of thinly interbedded shale, sandstone, diatomite, porcelanite, and minor chert and dolomite. Two studies were completed in order to resolve the diagenetic, burial, and uplift history of the Monterey Formation. An initial x-ray diffraction study (Singley et al.) of 152 samples discovered that all three silica phases (opal-A, opal-CT, and quartz) were present in the area and found a NW to SE lateral progression in silica phase in a strip along the shore of increased burial from opal-A in Newport Back Bay (NBB) to opal-CT in Corona del Mar to quartz in Crystal Cove State Park (CCSP). The second study (David et al.) expanded the first by mapping the distribution inland into the hills and refined the diagenetic stage of these rocks by determining the lattice d-spacing of opal-CT phase rocks that reflects crystallographic ordering with increased temperature. The distribution of silica phases and maximum burial depth is complex, but provides information on the tectonic structure of the uplifting San Joaquin Hills. Assuming a high geothermal gradient (~60ºC/km), we interpret that CCSP region was buried at least 0.60 km deeper then NBB and thus has been uplifted by post-depositional shortening from a minimal depth of ~1.2-1.8 km. Future studies will integrate compositional data with silica phase data to refine the estimates of maximum burial temperatures and subsequent uplift.