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ABSTRACT

Tertiary strata of the La Honda basin are exposed in the Santa Cruz Mountains along the central California coast south of San Francisco. The basin fill has a composite thickness of more than 14,500 m and consists of sedimentary and volcanic rocks that in places rest on granitic basement rocks of the Salinia terrane. Paleogene strata are mainly turbidite sandstone and hemipelagic mudstone that accumulated in deep-sea fan and basin plain environments at lower bathyal to abyssal depths. Neogene rocks are mainly shallow-marine shelf sandstone and upper to middle bathyal siliceous mudstone. Both Paleogene and Neogene strata exhibit rapid lateral variations in thickness and facies, several local and regional unconformities, numerous folds, and ubiquitous faults.

The complicated geology and geologic history of the La Honda basin reflect the fact that, throughout its history, the basin has been located at or near the tectonically active plate boundary between the North American continent and various oceanic plates of the Pacific basin. The La Honda basin originated during the Paleocene, perhaps during an episode of wrench tectonism associated with oblique subduction and arrival of the Salinia terrane. Major restructuring of the basin during the Oligocene—including uplift and erosion of the basin margins, movement along the Zayante-Vergeles fault, and deposition of two sand-rich deep-sea fans—apparently resulted from the approach of the Farallon-Pacific spreading ridge and its collision with the California continental margin. During the late Oligocene and early Miocene, widespread volcanism and marine transgression accompanied an episode of regional transtension along the San Andreas fault system. Deposition of shallow-marine sandstones and deeper-water siliceous mudstones occurred during much of the Miocene and Pliocene but was interrupted at least three times by brief episodes of uplift and erosion associated with transpressional wrench tectonism along the San Andreas fault. Marine deposition ended and uplift of the modern Santa Cruz Mountains began during the late Pliocene in response to the most-recent episode of regional transpression.

Five small oil fields in the La Honda basin have produced a total of 1.7 million barrels of oil and 300 million cubic feet of gas, mostly from reservoirs in Eocene turbidite sandstone and Miocene limestone.

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