EXAMINING CROSS-CUTTING RELATIONSHIPS OF VEINING/BRECCIATION EPISODES IN THE MIOCENE POINT ARENA FORMATION USING CATHODOLUMINESCENCE, EDS, AND EBSD

The Point Arena area is located approximately 165km north of San Francisco and consists of a Mesozoic Salinian block granitic basement overlain predominantly by marine shales and mudstones, which are bounded on the east by the San Andreas Fault (SAF). Of particular interest for this study, the Miocene, Point Arena Formation, a Monterey Formation equivalent, has been extensively folded and faulted owing to its proximity to the SAF in this area.

Within the relatively localized area of Moat Creek Beach, ca. 3 km south of Point Arena, the Point Arena Fm. is folded by a NW-SE trending, shallowly plunging fold forming at the tip of a blind thrust, subparallel to the SAF. Within this exposure of calcite-rich, asphaltic mudstone, the Point Arena Fm. records multiple episodes of veining and brecciation. Samples exhibiting high densities of vein material collected from these outcrops were analyzed under cathodoluminescence (CL), electron dispersive spectrometry (EDS), and electron backscatter diffraction (EBSD). The results of these analyses indicate that at least three different episodes of fracturing and mineral precipitation can be documented in this area manifesting in two cross-cutting sets of calcite veins and one set of quartz veins.

The use of CL expedited mapping of the gross geometries of the veins structure and identifying particular areas of interest with respect to evaluating cross-cutting relationships. Additionally, the chemical mapping performed via EDS was particularly useful at mapping the thin (5-30 micron) quartz veins. Because the grain size of many of the calcite veins is so small, individual grains were difficult to distinguish via EBSD and produced low hit rates. Regardless, EBSD was the most useful technique for determining cross-cutting relationships for these samples. We were able to use the variable hit rate of the EBSD analysis along with the band contrast image to differentiate subtle variations in grain size of the vein material, which aided in the determination of cross-cutting relationships. A combination of these three analytical tools was valuable in documenting the off-fault fracturing history adjacent to the SAF.