ALBERT, KEIL A.
San Jose State University, San Jose, CA, 95192 (kalbert@ geosunl.sjsu.edu)

Abstract: The Geologic and Tectonic Significance of the Pleistocene Irvington Gravels, East Bay Area Foothills

The Pleistocene Irvington Gravels exposed in a small outcrop area in the east Bay Area foothills may reveal clues as to the tectonic and sedimentary history of the region during the Pleistocene.

The Irvington Gravels were deposited as braided fluvial stream deposits. The coarsegrained portions of the Irvington Gravels bear a striking resemblance to the coarse-grained portions of the Upper Livermore Gravels, which are age correlative and are exposed in the Livermore Valley some 23 kilometers to the east of the Irvington outcrop. Clast compositions were examined in the Irvington Gravels and compared to previous work completed on clast compositions and provenance studies of the Upper Livermore Gravels. Both units are composed primarily of Franciscan Assemblage clasts from the central Diablo Range, with material from Tertiary sedimentary units being subordinate.

The recognition that these deposits may be related has tectonic implications for the region. The two units are now separated by the Diablo Range, which rises to an elevation of at least 400 meters in the area between the outcrop exposures. This would suggest Middle to Late Pleistocene through recent uplift of this region by at least this amount, and probably much more. Uplift was accomplished by movement along the Hayward and Calaveras fault systems, and associated contractional stepover faults.

AAPG Search and Discovery Article #90920©1999 AAPG Pacific Section Meeting, Monterey, California