CONCEALED SEDIMENTARY BASINS BENEATH THE SANTA CLARA VALLEY, CALIFORNIA

Beneath the flat, urbanized floor of the Santa Clara Valley are the deposits of a Quaternary alluvial basin, the Santa Clara basin, and two hidden Tertiary sedimentary basins that formed in contrasting tectonic settings, the Cupertino and Evergreen basins. Quaternary strata of the Santa Clara basin are generally flat-lying and up to 600 m thick, and were deposited on a regional unconformity that was created by erosion about 1 Ma. Below this unconformity, flat-lying to gently-deformed strata of the NW-trending Cupertino and Evergreen basins are separated from each other by a NW-striking structural high composed of Mesozoic rocks.

Along the southwestern margin of the valley, the Cupertino basin is marked by a NW-striking, isostatic-gravity low about 35 km long. Gravity modeling suggests that the basin fill is 2,500-3,000 m thick and that the basin shape is markedly asymmetric, with a gently-sloping northeastern margin and a steep southwestern margin that may have been controlled by Miocene normal faults. A borehole sample from immediately below the sub-Quaternary unconformity yielded fossil diatoms of late Miocene age. The chemistry of oil from the southern basin margin suggests that Miocene strata extend to depths of at least 2,100 m. The age of the oldest strata in the Cupertino basin is unknown, but the basin may have originated about 17-14 Ma during an episode of normal faulting, volcanism, and crustal extension related to passage of the northwestward-migrating Mendocino triple junction.

The Evergreen basin is located on the northeastern side of the Santa Clara Valley and is marked by a NW-striking, isostatic-gravity low about 40 km long. The southwestern and northeastern margins of the basin are steep and fault-bounded. Gravity modeling suggests that the basin fill is 4,000-6,000 m thick. Evergreen basin deposits from below the sub-Quaternary unconformity are correlative with nearby outcrops of Pliocene strata. No deep wells have penetrated the Evergreen basin and its pre-Pliocene stratigraphy is therefore unknown, but regional considerations suggest that the basin fill may include Miocene marine rocks. Subsidence of the basin is thought to have occurred from about 12 Ma to about 2.5 Ma in association with crustal extension in a right-step between two right-lateral faults, the Silver Creek and proto-Hayward faults.