COMPOSITION AND SIGNIFICANCE OF PLEISTOCENE GRAVEL UNITS IN THE FOOTHILLS SOUTHEAST OF SAN FRANCISCO BAY, CALIFORNIA

Pleistocene nonmarine gravel units exposed in the southeast Bay area foothills consist of three distinct lithofacies distinguished by the composition of clasts in the gravel. We described two of these in a previous abstract, and we describe the third one here for the first time.

Gravel in the Irvington district of Fremont is composed mostly of clasts of Franciscan rocks, which evidently were transported from sources east of the Calaveras fault. To the south, in the Piedmont Creek area east of Milpitas and farther south in eastern San Jose, gravel is composed mostly of clasts of Miocene Claremont Chert and Cretaceous Berryessa Sandstone, which evidently were derived from local sources west of the Calaveras fault. During the 1940's and 50's, the gravel at Irvington yielded thousands of fossils of land vertebrates of Irvingtonian age, and paleomagnetic results by other workers suggest that the age of the gravel there is about 1.5 Ma. The gravel near Piedmont Creek is considered to be Pleistocene, but its specific age is not known.

The third lithofacies was recently exposed in trenches near Mission Boulevard in southeastern Fremont, located geographically between exposures of the two lithofacies described above. Clasts in the gravel at Mission Boulevard are mostly Miocene Briones Sandstone and Cretaceous Berryessa Sandstone, so this gravel, too, consists of sediment derived mostly from local sources west of the Calaveras fault. The Mission Boulevard gravel contains numerous vertebrate fossils, including one identified by other workers as Bison latifrons, so the Mission Boulevard gravel is much younger than that at Irvington, possibly as young as Rancholabrean.

We suggest that the gravel at Irvington represents sediment carried from sources east of the Calaveras fault at a time when the presently elevated areas between the Calaveras and Hayward faults, including Mission Peak, were not an obstacle to westward transport of sediment. The gravel units at Mission Boulevard and Piedmont Creek contain locally derived clasts from those elevated areas, and the bison at Mission Boulevard indicates that uplift of those local sources, at least in the northern part of the area, occurred more recently than 1.5 Ma.