DIAGENETIC TRENDS IN SILICEOUS LITHOFACIES OF THE MONTEREY FORMATION IN THE SOUTHWESTERN CASMALIA HILLS: SANTA MARIA REGION, CALIFORNIA

Kenneth A. Pisciotto, who measured the d₁₀₁-spacing of several rocks to determine the silica phase zones, did the last major study of the Casmalia Hills in 1981. Thus, this area was chosen for this study, for it houses one of the finest surface exposures that show the opal-A (biogenic amorphous opal) to opal-CT (interlayered cristobalite and tridymite) to quartz (equant mosaic of microquartz crystals) transition in the highly siliceous Monterey Formation. Silica diagenesis of the highly reactive biogenic silica is a very important factor in the lithification of the Monterey sediments.The purpose of this study is to test predictions of previous diagenetic models and to test the natural variability in crystallographic ordering of opal-CT (stratigraphic and lateral consistency) at high spatial resolution. The results from a preliminary study of eight samples helped to identify broad transition zones. The three main questions that will be answered from this study are: (1) how much variability is in the diagenetic state at a single maximum burial depth for a specific compositional range; (2) how much compositional variability is along a single distinct bed; and (3) is crystallographic ordering a linear progression. Over 150 samples were later collected that span the entire section. The collected data combines both field observations of individual beds and chemical and mineralogical analysis of the samples by XRD and XRF. This information will be valuable for depiction of depositional, post-depositional, burial, and diagenetic histories of the area, which could potentially be applied to the subsurface or similar surface areas where the Monterey Formation exposures are poor.