DERIVATION OF A PLEISTOCENE TOPOGRAPHIC SURFACE MAP AND A HOLOCENE DEPOSIT THICKNESS MAP FOR SANTA CLARA VALLEY, CALIFORNIA

A surface representing the top of the Pleistocene, and a derivative Holocene deposit thickness map, were created for Santa Clara Valley. The top of the Pleistocene is an erosional surface that lies below Holocene deposits in the center of the basin and is exposed on the flanks of the basin. The Holocene deposits in the basin include estuarine bay mud immediately adjacent to San Francisco Bay, and lap onto the latest Pleistocene fans around the valley margin. The top of the Pleistocene surface was optimally modeled using well data and the current topographic surface. The challenge was to characterize the buried Pleistocene surface, which is an old topographic surface, over a large areal extent, from sparse, unevenly-spaced scattered point data.

The Pleistocene topographic surface was modeled using data from 481 wells provided by the Seismic Hazard Mapping Program of the California Geological Survey (Clahan and others, 2002), combined with data derived from the topographic surface of valley-bounding Pleistocene fans and contacts between Pleistocene and Holocene units around the valley margin. A number of different surface-generation algorithms, including Arc/Info TOPOGRID, ArcGIS Spatial Analyst Spline, and ArcGIS Geostatistical Analyst Kriging were applied to determine which would be the most accurate method. Each of these methods was used to recreate the modern day topographic surface, sampled at the same point locations used in the generation of the Pleistocene surface. TOPOGRID yielded the best result, most accurately recreating the modern day topographic surface. The thickness of Holocene sediments was derived by subtracting the top of the Pleistocene surface from the modern day topological surface, using a Digital Elevation Model (DEM).

The resulting model shows that Holocene sediments are thickest in the axial part of the basin and thin to the east and west where they lap up against Pleistocene deposits extending into the basin from the surrounding uplands. Thicknesses range from 0 to 90 ft. within the basin, with a mean value of 22 ft. The areas with the thickest Holocene deposits are just to the southeast of the Coyote Hills, and in the proximity of the Silver Creek Fault, southwest of the Evergreen basin.