Diagenesis of the Miocene Obispo Formation, Coast Range, California

Abstract The pyroclastic Miocene Obispo Formation (15-17 m.y.b.p.) was deposited in a continental submarine margin along the “leaky” San Luis Obispo transform (SLOT). These bimodal volcanic rocks (rhyolitic pyroclastics and basaltic flows and sills) occur along a linear trend that extends for 140 to 200 km in the southern Coast Range of California. In the area of investigation the Obispo Formation consists of a prism of volcanic rocks 500 to 1000 meters maximum thickness. Much of the pryo-clastic material, which commonly consists of over 80% of the formation, has been diagenetically altered. Early in the diagenetic history of the Obispo Formation much of the pyroclastic material, particularly the fine-grained glassy fragments, has been altered to the zeolites clinoptilolite and morden-ite. These early diagenetic products resulted from hydration reactions. Besides the addition of H2O, the reactions involved only minor mass transfer. Early diagenesis coincided with the injection of intrusive bodies shortly after deposition of the tuffaceous material. Intrusive bodies acted as heat sources which heated the interstitial fluids in the wet tuffaceous sediments causing the convection of the fluids and acceleration of hydration reactions (zeolitization). The distribution of the mordenite and clinoptilolite is controlled by the proximity of the intrusive bodies; with the mordenite being formed closest to the intrusives (higher temperature) and the clinoptilolite forming more distant from the intrusives (lower temperature). A second generation of diagenesis of the Obispo Formation occurs late in the burial history after the deposition of the overlying Monterey Formation. Reactions characterizing late diagenesis resulted in larger scale mass transfer. Dolomitization of many of the tuff beds took place during this stage of diagenesis. CO2-charged fluids were largely responsible for the later stage of diagenesis. These waters were derived from the dewatering and diagenesis of organic-rich sediments in adjacent basins.