COMPOSITION OF SEDIMENT FROM DEEP WELLS IN QUATERNARY ALLUVIUM, SANTA CLARA VALLEY, CALIFORNIA

The composition of gravel and sand in samples from five wells in San Jose and Campbell helps to constrain patterns of Quaternary sediment dispersal within the Santa Clara Valley. The wells were drilled by the U.S. Geological Survey and the Santa Clara Valley Water District, and samples were collected from depths up to 300 m. Paleomagnetic results obtained by other workers suggest that the sediment sampled ranges in age up to about 800 ka. The gravel contains abundant Mesozoic graywacke and greenstone, with smaller amounts of clasts derived from other Mesozoic rocks and minor amounts of clasts inferred to have sources in Cenozoic rocks. The abundance of greenstone and the near absence of clasts of Miocene Claremont chert suggest that most of the sediment was derived from the Santa Cruz Mountains south and west of the valley and that very little of the sediment in the wells was derived from the east.

The gravel in the wells is compositionally different from any outcrops of the Santa Clara Formation exposed around the margins of the Santa Clara Valley. This difference suggests that the Santa Clara Formation may not be present in the central parts of the valley.

Clasts of blueschist, eclogite, and serpentinite are rare in the gravel, totaling less than 2 percent in most samples but ranging generally from 2 to 8 percent in samples from the deeper parts of each of the wells. In sand samples, sodic amphibole, jadeite, serpentinite, and chromite are rare, but they, too, are more abundant in the deeper than in the shallower parts of the wells. The oldest sediment in the wells thus appears to reflect a source different, at least in part, from that of the younger sediment. We infer that this different source is a buried basement high, indicated by other workers' gravity and magnetic anomalies, that extends northwest through Oak Hill and beneath the central part of the Santa Clara Valley. Our results suggest that, in the subsurface, this basement contains a significant amount of serpentinite and high-rank Franciscan metamorphic rocks. The basement evidently was exposed in the early Pleistocene and contributed sediment to the deposits sampled near the base of the wells. As deposits accumulated and began to cover the basement high, an increasing proportion of the sediment in the wells was derived from the mountains south and west of the valley.