Tectonic, Depositional, and Diagenetic History of Monterey Formation (Miocene), Central San Joaquin Basin, California

ABSTRACT The Miocene Monterey Formation in the central San Joaquin basin of California consists of biogenic siliceous rocks with a significant terrigenous clastic component. Correlation of electric logs in conjunction with analysis of outcrop sections, core samples, and paleontologic data suggests that deposition of the Monterey was controlled by an interplay of basin geometry, San Andreas-related tectonics, fluctuations in sea level, and changes in climate. The basin is bounded on the east by the Sierran magmatic arc complex and on the west by the San Andreas fault. During the late Saucesian through early Mohnian (20-10 Ma), the basin was reshaped by tectonism related to the migrating Mendocino triple junction, creating a broad, gently sloping shelf to the east that cut off the supply of clastics to the central basin. With the subsequent evolution of the San Andreas fault, the Salinian block was emplaced along the western basin margin, restricting the seaway and permitting low-oxygen, nutrient-rich waters to occupy the basin. As a result, laminated, organic-rich, fine-grained pelagic to hemipelagic sediments of the Monterey Formation were deposited. A warmer climate during Saucesian through Luisian time, with less intensive upwelling, led to more calcareous, less siliceous sedimentation. A cooling trend during late Miocene time (10-5 Ma) increased upwelling activity, diatom productivity in the water column, and deposition of diatomaceous debris. Although turbidites accumulated on the basin floor and terrigenous clastics were deposited on the eastern shelf, relatively undiluted diatomaceous deposits developed on outer shelves and slopes, atop anticlinal banks generated by wrench tectonism, and in some areas of the basin floor. A mid-Mohnian eustatic sea level rise caused fine-grained deposition to extend onto the eastern shelf. Slightly later a mid-late Mohnian period of exceptionally cool temperatures resulted in the most extensive development of upwelling-related siliceous sediments throughout the basin. With diagenesis, these deposits formed the siliceous shales and porcelanites of the Monterey Formation. High sedimentation rates, coincident with a eustatic drop in sea level, led to clastic progradation in latest Miocene time, as the basin filled. Analysis of organic matter from cores of the Monterey Formation shows that kerogen type reflects overall sedimentation trends. Samples from northern and eastern areas contain abundant terrestrial organic matter, whereas samples from western areas mostly contain marine kerogens. Source potential is thus strongly influenced by geographic locality, as well as lithology. Maturity of kerogen was assessed using vitrinite reflectance, thermal alteration indices, and pyrolysis techniques. The latter two appear to be the more reliable indicators of organic matter maturity in the Monterey Formation of the San Joaquin Valley. Although much of the Monterey Formation in the study area may be thermally immature, portions are probably generating hydrocarbons today in deep synclinal areas.