Composition of sediment records late Quaternary paleogeographic evolution of Santa Clara Valley, California

Abstract

Gravel and sand samples from alluvium in five wells in the Santa Clara Valley, California (USA), have particle compositions that help constrain patterns of Quaternary basin evolution and sediment dispersal within the valley. Samples were collected from depths as great as 307 m, and paleomagnetic results obtained by other workers suggest that the sediment sampled ranges in age to ca. 800 ka. The gravel contains common to abundant Mesozoic graywacke and metavolcanic rocks; less common clast types indicate derivation from other Mesozoic rocks. Clasts having lithologies that match Cenozoic source rocks are minor in amount. The abundance of metavolcanic rocks, as well as serpentinite, blueschist, and eclogite, among the less common gravel clasts, generally increases with depth in each well. Sand samples contain a variety of mostly metamorphic heavy minerals, including some distinctive blueschist facies minerals. Blue amphibole and chrome spinel are rare to common in the sand, and their abundance also increases with depth. The most useful clasts for determining the location of the sediment source are the metavolcanic rocks and some of the less common clasts. Metavolcanic rocks, common in modern outcrops only in the Santa Cruz Mountains south and west of the Santa Clara Valley, also are common in the gravel. In contrast, with only a few exceptions at shallow levels in the northeasternmost wells, distinctive gravel clasts with sources east of the valley are nearly absent. These observations suggest that most of the sediment was derived from the south and west. Thus, the topographic asymmetry of the valley, with its modern axial drainage east of the valley center, evidently has persisted for at least the past 800 k.y. The increased abundance of pebbles of serpentinite, blueschist, and eclogite and of sand-sized particles of blue amphibole and chrome spinel with depth suggests a change in source through time. The source for these clasts in the deeper levels, and perhaps also some of the metavolcanic clasts, evidently was a basement high within the valley that was gradually buried as the basin filled with sediment. The gravel in the wells has a composition different from that of any previously described outcrops of Pliocene and Pleistocene nonmarine gravels, including the Santa Clara Formation, exposed around the margins of the Santa Clara Valley. This difference suggests that the Santa Clara Formation and similar units are not present in the parts of the valley penetrated by the wells.