Publication Date

Fall 2011

Degree Type


Degree Name

Master of Science (MS)




David Andersen


paleodrainage, sand provenance, Santa Clara Valley, Well cores

Subject Areas

Sedimentary geology


Medium sand samples from well cores taken in the Santa Clara Valley, California, were studied to determine their composition and, if possible, their provenance. Sand samples were taken from various depths from five wells distributed over the western and central parts of the valley. The oldest of these samples is known to date to about 800 ka.

Thin sections of sand samples were point counted to determine quartz, feldspar, and lithic percentages. The samples are very lithic, with some quartz and very little feldspar. Common lithic grains include argillite and graywacke. Less common are metavolcanics, serpentinite, and chert or metachert. Siltstone, sandstone, volcanics and volcanic porphyry are rare or absent in the samples. All of these grain types are represented in the mountains surrounding the Santa Clara Valley.

The composition of the well sand as well as the previously known composition of well gravel samples provided indicators of well sediment provenance. At lower depths, the well sands and gravels in the western wells came predominantly from south of the valley, although the modern drainage that feeds those locations is from the southwest, suggesting a change in drainage patterns over time.

A bedrock high ran down the center of the valley ca. 800 ka. Compositions of the well sand and gravel indicate that this high provided serpentinite to the eastern wells and Franciscan chert and metavolcanics to several wells. The abundance of chert and metavolcanics in some middle and upper well samples indicates that the bedrock high was a significant source of these rock types until at least 400 ka.