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Evolution of Monterey Formation Strike-Slip Basins, Santa Lucia Range, California


The Miocene Monterey Formation is the principle petroleum source rock in California. The unit is characterized by fine-grained carbonate, phosphatic, and siliceous rocks of diatomaceous origin, deposited in silled, oxygen-poor basins. It is generally bounded both below and above by coarse-grained clastic rocks, preserving the history of basin formation, fill, and uplift. Extensional tectonics related to migration of the present-day Mendocino Triple Junction and development of the strike-slip San Andreas Fault is thought to be the primary tectonic driver of basin formation. In many basins, releasing bends or stepovers on strike-slip fault systems appear to play a large role in controlling sedimentation, while near the classic Santa Barbara outcrops, there is the additional influence of vertical-axis block rotation. Although the Monterey Formation is widely thought of as a single unit, previous work suggests variations to this framework. The unit is diachronous, with the timing of the transition from calcareous to siliceous facies varying from basin to basin. In addition, the unit thickness and lithological facies vary over short distances. It is likely that tectonics has a strong influence on these sedimentary characteristics, but it is not clear to what degree due to the limited understanding of the evolution of the geometry of basin-bounding strike-slip faults. The Monterey Formation exposed along the eastern flank of Santa Lucia Range, west of the Salinas Valley, may be a good location to test models of strike-slip basin formation. Along this trend, the Monterey Formation can be mapped as a continuous unit extending from Monterey southward toward Santa Maria. However, previous workers have noted that the base of the formation is diachronous along strike, there is evidence of rapid subsidence and uplift, and the unit contains large facies variations over short distances. These features are not consistent with simple models of releasing bend evolution. Instead, we suggest that instead of static releasing bends or step-overs, a more dynamic model involving migrating releasing bends, or releasing bends that evolve into restraining bends, may be able to better explain the sedimentary record of these basins. Migrating releasing bends allow for depocenter migration and extinction that may better explain the sedimentary variability within the Monterey Formation of the Santa Lucia Range.