The oxygen and hydrogen isotope compositions of volcanic ash layers from coastal outcrops of the Miocene Monterey Formation elucidate the progressive burial diagenesis of glass to bentonite and metabentonite. Volcanic glass that texturally appears unaltered is found to have δl8O and δD values that are significantly higher than fresh volcanic glasses. The positive shift in δ18O and δD values is proportional to the amount of hydration and chemical alteration of the glass samples. Initial hydration of the glass results in the exchange of Na for H; later hydration results in the loss of K and Fe in exchange for H and Na. The δ18O values of the most hydrated glass, and clay minerals from bentonite and metabentonite layers are approximately in equilibrium with slightly modified seawater. The δD values of the hydrated glass, and clay minerals from bentonite and metabentonite layers, are significantly depleted in D relative to seawater and suggest meteoric exchange associated with tectonic uplift and erosion in the Pliocene and Pleistocene. Smectite from bentonite layers has δ18O and δD values similar to the most hydrated glass samples, suggesting similar glass-water and smectite-water fractionation factors. Kaolinite and mixed-layered illite-smectite (I-S) altered from smectite have lower δ18O and higher δD values than their precursor. The δ18O and δD of nonvolcanic siliceous mudstones from the Pt. Arguello oil field show an unusual decrease in δD with increasing burial depth that probably reflects the presence of organic hydrogen in the analyzed samples as well as possible D-depleted formation waters from detrital illite-water and hydrocarbon-water exchange.

The Miocene Monterey Formation constitutes a fracture-controlled petroleum reservoir, with intercalated calcareous and fine-grained siliceous rocks serving as both the source and reservoir for oil accumulations. Petroleum is produced from macroscopic fractures, and numerous tar and asphalt seeps at the surface attest to the present-day movement of hydrocarbons through fractures in the Monterey Formation. Many fractures are filled with carbonate (mostly calcite and dolomite), quartz, baryte and anhydrite. These same fractures often contain tar or oil filling openings, and occasionally a thin layer of oil can be seen coating growth surfaces between two generations of vein-filling minerals.

Evidence for migration of fluids through these fractures in the geological past is provided by aqueous and petroleum fluid inclusions contained within vein-filling minerals. Vein-filling dolomite from Jalama Beach contains three different types of primary petroleum inclusions (based on fluorescence characteristics)—indicating that oils with significantly different API gravities flowed through the fractures. Petrographic and microthermometric analyses of oil and coexisting aqueous inclusions indicate that the fracture-filling minerals precipitated from aqueous solutions of seawater salinity at ∼75–100°C, and that oil was introduced into the fracture system episodically during mineral growth. A sample from the Lion's Head area consists of early calcite and late quartz, both of which contain aqueous inclusions with seawater salinity. Inclusions in quartz homogenize at slightly higher temperatures than those in calcite. These data are consistent with calcite deposition during an early heating event, followed by quartz deposition during cooling. No petroleum inclusions were observed in the Lion's Head sample.