Mount Diablo Field Trip Explores Complex East Bay Regional Geology

Ron Crane, Consultant, and Craig Lyon, Retired Chevron—May 18, 2002

By Michelle McMahon, Cal State University Hayward, from the June 2002 NCGS newsletter

Download the field trip guidebook here.

On May 18th NCGS sponsored a field trip to look at the geology of the Central California Mount Diablo Region, led by Ron Crane and Craig Lyon. The group enjoyed hot coffee and donuts as Ron introduced the tectonic history of the region, proposing that the uplift of Mount Diablo was the product of an underthrust from the west, which is passively uplifting the range. Ron continues to postulate that the strike-slip movement of the faults in the region is a secondary byproduct of the SW to NE thrusts. Ron’s interpretation was supported by an impressive series of cross-sections. Craig, in a heroic attempt to contain the resulting discussion, introduced the regional stratigraphy and we loaded into the vehicles, knowing we were spending the day with two regional experts.

The first stop was at the Hamilton Sandstone (Meganos D), which is associated with Eocene Meganos Canyon fill. This stop included a discussion of the Brentwood gas fields, important because these fields had oil production in what otherwise is considered a gas province. Driving back downsection, the next stop was a well-exposed Upper Cretaceous turbidite sequence. Craig noted that these turbidite units and many others in northern California do not contain complete Bouma sequences.

Mining related stops included the tailing pile for the now abandoned Mount Diablo Mercury Mine; runoff from this mine empties directly into Marsh Creek and the Marsh Creek Reservoir. The diabase, part of the Mount Diablo pillow basalts in an ophiolite terrain, is currently being quarried at the RMC Pacific Material quarry on the east and west flank of Mount Zion. We also passed the now abandoned Cowell Cement Company, which mined the 20-foot-thick travertine deposit that overlies the Domengine Sandstone on Lime Ridge.

After a picnic lunch and a visit to the Mount Diablo Interpretive Center, we headed into the State Park. A stop at Rock City–Wind Caves introduced the group to the Eocene Domengine Formation. This stop provided the group our first overview of the San Ramon Valley beginning with the Miocene Briones Sandstone on Fossil Ridge just to the west. A quick cultural history stop was included at the Wind Caves, at a spot where Native Americans had ground holes in the Domengine Formation while pulverizing acorns.

As the group moved up the flank of Mount Diablo through roadcut exposures of the Franciscan cherts and graywackes, a stop at Curry Point allowed for an opportunity to overlook the Franciscan Complex exposed at the summit of Mount Diablo, and the Tertiary ridges of the south flank. Discussion and debate of the structural interpretation for the region continued. The trip was concluded at the top of Mount Diablo with a summit walk through exposures of the Franciscan graywacke, sandstones, cherts, and greenstone as well as excellent views to the west, north and east. This was the perfect location to reflect on a great day and the vast complexities of the geology of the Mount Diablo region.