Abstract: Geology of the Mount Diablo Region

Ronald C. Crane

Mount Diablo is a Late Pleistocene-Holocene backthrust structure, formed by a deep, underlying wedge of unknown composition which is driving to the northeast on a blind thrust at basement level. The eastern flank of Mt. Diablo has been detached and passively tilted at a 45 degree angle, exposing a section of Jurassic to Holocene rocks.

The tilt of the flank exposes a Stratigraphic and structural cross-section of the southern Sacramento Basin, including listric normal faults, grabens, submarine canyons, and reservoir sands. One of the major grabens includes the eastward bounding master Midland Fault, the Brushy Creek sole, and the western bounding Kirby Hills-Kirker Pass Fault. The wedge-backthrust reoccupies the same fault plane over part of its length.

The Jurassic ophiolite and "Franciscan" lithologies originated as a massive gravity slide in Early Cretaceous time and formed a subsea topographic high in a deep trough. Cretaceous and early Tertiary prograding fans from the N&E thinned and lapped out on the high. The high was buried in Miocene time, but remained a shelf edge, with deeper water to the west. Stratigraphy on either side is fundamentally different.

In Late Pleistocene, the Mt. Diablo structure started growing passively as a response to the NE moving wedge and a series of westerly directed backthrusts formed. The N & E structural boundaries of the Mount Diablo Region are at the axis of a syncline which extends around the structure under the Great Valley. Tears and thrusts subdivide the structural domain, including faults such as the Concord, Diablo, Riggs Canyon, Morgan Territory, Kirker Pass, Kellogg Creek, Brushy Creek and Greenville Faults. The frontal boundary is a subsurface tear following Highway 242 which turns into a exposed high-angle thrust, placing west-vergent Castle Hill against the east-vergent East Bay Hills domain at Tice Valley. A buried blind thrust follows the east side of the San Ramon Valley, the base of he Dougherty hills and forms the north flank of the Livermore Valley. The Green Valley Tear divides this plate into 2 domains.

AAPG Search and Discovery Article #90958©1995 AAPG Pacific Section Meeting, San Francisco, California