Abstract: Lithofacies Characteristics of the Upper Cenozoic Irvington Gravels (Santa Clara Formation) Between Fremont and Milpitas, CA

HOLLAND, PETER, San Jose State University, San Jose, CA; JAMES ALLEN, San Jose State University, San Jose, CA

Preliminary work has revealed that the Upper Cenozoic Irvington Gravels (Santa Clara Formation) are composed of at least two distinct lithofacies between Fremont and Milpitas, California. The northern outcrops (in Fremont) are typically clast-supported conglomerates with only minor amounts of fine-grained material. These beds are composed predominantly of Franciscan-derived clasts. Historically, this lithofacies has been the reference for other Irvington outcrops.

In exposures around Toroges Creek and Calaveras Road near Milpitas, the Irvington is distinctly different. Beds are mostly sandstone to mudstone with minor amounts conglomerate. The conglomerate clasts are relatively smaller than in the north and are composed of a significant amount of locally derived chert from the Monterey Formation. This lithofacies also contains abundant small gastropods and bivalves. In some zones, these fossils constitute the majority of the rock.

Previous researchers have assumed that the Irvington is homogenous in composition, lithology and depositional environment. This preliminary work reveals that the Irvington Gravels are more diverse than previously thought. Both fluvial and lacustrine environments are represented in this unit, and sediment was derived from at least two distinct source areas. The relationship between these two lithofacies will aid in the understanding of the paleogeographic evolution of this part of the Coast Ranges and the Santa Clara Valley.

AAPG Search and Discovery Article #90911©2000 AAPG Pacific Section and Western Region Society of Petroleum Engineers, Long Beach, California