The Santa Cruz basin is a highly compressed structural and stratigraphic basin extending from Half Moon Bay, through the Santa Cruz Mountains, and may extend across the San Andreas fault to include the Hollister basin on the southeast. It is bounded on the southwest by the Ben Lomond-Gabilan granitic shelf, and on the northeast, by the Montara granite mass and the Gilroy Franciscan shelf.

The Tertiary rocks of the region are divided into two sequences by an angular unconformity. The

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lower sequence includes rocks of Eocene to older Miocene age. It is more than 10,000 feet thick in the Santa Cruz basin, but thins southwest. Several thousand feet of the lower sequence are present in the Hollister basin but are absent on the Gilroy shelf.

The upper sequence includes rocks of younger Miocene to Pliocene age. The sequence is thickest in the area of Half Moon Bay (10,000+ feet), and in the Hollister basin (8,000+ feet). It is 1,000-3,000 feet thick over the Ben Lomond-Gabilan shelf and less than 1,000 feet thick on the Gilroy shelf.

The primary feature of structural importance is the right lateral San Andreas fault. The Santa Cruz basin proper is a highly mobile belt between two moving buttresses--the Ben Lomond granite mass and the Gilroy Franciscan shelf. There were two episodes of intense structural deformation--one at the end of lower sequence deposition, and one during the Pleistocene and Recent epochs.

The region has been prospected for oil since 1886. A period of intense exploration since World War II has resulted in several oil and gas discoveries of some importance in the last 5 years. Because of the structural history of the region, it is believed that prospecting will be most successful where searching for structural and stratigraphic traps formed prior to the more recent deformation.

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