Panoramic view of North Peak from the summit of Mt. Diablo.

 

Geology of the South Flank of Mount Diablo

May 2004 NCGS Field Trip

Reported by Richard Cardwell

Photography by Phil Garbutt, California State University, East Bay

 

Looming above the East Bay, Mt. Diablo is the outstanding physiographic feature of the region.  Because of its great height and geographic isolation it forms the basis for the geographic reference system (townships, ranges and sections) used in surveying much of California and Nevada.

It is one of the four major peaks that dominate the landscape of the Bay Area and, at 3849 feet, is third in height behind Mt. Hamilton in the Diablo Range and Mt. St. Helena in the Mayacmas Mountains.  It is reported that the area viewed from the summit is unsurpassed anywhere on earth except for the view from Mt. Kilimanjaro.  From the top it is possible to see across much of California from the Sierra Nevada to the Golden Gate (and some say as far west as the Farallon Islands).

The origin of the Mt. Diablo structure has been argued for quite some time.  Various explanations have included diapiric origins, piercement structures, and volcanic processes.  Since his retirement from Chevron, Ron Crane has undertaken a project to map and understand the geology of this feature and the surrounding region.  In 1995 he proposed that Mt. Diablo consists of thrusted rocks folded into the structure of a broad antiform.  The antiform originated as a west-verging backthrust that was formed by an eastward driving wedge of sediments in front of the Salinian block.  Major compression and uplift of the Mt. Diablo antiform began about 4 to 5 million years ago and is continuing today.

Mt. Diablo has been the focus of several NCGS field trips.  On Saturday, May 1, NCGS members and friends joined Ron on a field trip to investigate the geology of the south flank of the Mt. Diablo structural domain.  Ron led a similar field trip to the north flank in May, 2002.

Our day began at 7:30 a.m. at the Danville Park and Drive lot where Ron gave us an overview of the dayís activities.  The highlights of the trip are discussed below.

Diablo Morning - From Stop 1

 

Stop 1 was on Peters Ranch Road in Danville west of Highway 680.  Here we were in the East Bay Hills domain looking east toward the Mt. Diablo domain.  We could see across San Ramon Valley to a series of three low ridges culminating in the Mt. Diablo massif.  Each of these ridges was uplifted along a west-verging thrust fault.

In Ronís view the Calaveras fault is interpreted as a thrust fault on which the East Bay Hills are being thrust to the east.  This interpretation contrasts with the more conventional view in which the Calaveras fault is primarily a strike-slip fault like the Hayward and San Andreas faults.

 

The Crowd at Stop 1

The San Ramon Valley is being compressed (shortened) between the southwest moving Mt. Diablo domain and the east moving East Bay Hills domain.  Sediments in the San Ramon Valley have subsided over 18,000  feet.

Stop 2 was on Paraiso Drive at the bridge across San Ramon Creek in Danville on the east side of Highway 680.  Here we were in the leading or southernmost edge of the Mt. Diablo domain.  Beneath the bridge over San Ramon Creek we could see nearly vertical dips in the streambed.  This deformation and the presence of a small bump in the topography suggest that we were standing on small thrust anticline that forms the leading edge of the Mt. Diablo domain.

Stop 2:  Outcrop exposure with near vertical dips beneath San Ramon Creek bridge - A small uplifted area in the middle of San Ramon valley suggestive of a possible leading edge of a blind thrust fault (Mt. Diablo Structural Block moving to the SW).

 

Earth flow in hills along Finley Road.  The hills are underlain by the Neroly sandstone.

 

Stop 3 was at the Finley Road entrance to Morgan Territory Regional Park east of Blackhawk.  Here we were on the southeast flank of the Mt. Diablo domain.  We walked down section on the northern flank of an up turned syncline.  We started in the Upper Miocene section and walked through the Diablo formation, the Neroly sandstones, and the massive Briones sandstones.  We continued to the Middle Eocene Domengine sandstone outcrops.  We then crossed the Riggs Canyon Thrust Fault that thrusts Upper Cretaceous over Tertiary and Cretaceous rocks.  North of the fault we were on an overturned anticline in the Campanian section.

Stop 3:  An outcrop of the Briones sandstone along Finley Road.

 

Stop 3:  The group standing at the approximate location of the Riggs Canyon Fault.

 

Stop 3: A view up canyon of the Briones and Domengine sandstones.

 

For Stops 4 and 5 we drove up South Gate Road to the south entrance of Mt. Diablo State Park and had lunch at Rock City.  Here we were again in the up turned northern flank of the San Ramon Valley syncline in the Domengine formation.  These Middle Eocene sands of the Domengine formation were deposited down slope of the shelf edge in deep water.

Stop 5: Rock City picnic area, Mount Diablo State Park.

 

Stop 5:  Looking for the elusive mortar holes in the Domengine sandstone.

 

Continued - The Afternoon

 

 

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