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30 November 2010

Geo Tours in South Orange County: San Onofre State Beach

California is a geologist's paradise. In southern Orange County, opportunities for class field trips less than a half-hour from campus are abundant. A short drive up Silverado Canyon covers a significant fraction of the Mesozoic. We have world-class fossil beds in Mission Viejo with Miocene and Pliocene marine mammals and abundant shark teeth. Granitic mountains, active faulting, beaches backed by multiple marine terraces, Tertiary marine sedimentary rocks criss-crossed with recent andesitic and dacitic dikes, and the beaches are nice too...

For my geotour I present a basic outline of one of my class field trips: San Onofre State Beach, on the border of Orange and San Diego Counties, at the northern edge of the Camp Pendleton Marine Base. This is also the location of the San Onofre Nuclear Generating Station (SONGS), co-operated by Southern California Edison and San Diego Gas and Electric. Just to the south of the plant is the Cristianitos fault, an extensive fault whose proximity to the nuclear plant has in the past represented a source of controversy.

Topographic map showing the location of the trip. Note the significant change in the topography to the west of the Cristianitos fault. To the west of the fault the rock type is mostly the relatively soft San Mateo sandstone (Tsm). To the east of the fault (and northeast of the freeway) the rock is mostly the San Onofre Breccia (Tso). Below the freeway, east of the fault, the rock is the soft Monterey Formation (Tm) -- the softness of the rock is shown by the prevalence of slumping in this area.
A portion of the State of California geologic map of the San Onofre quadrangle. North of the freeway and east of the fault (gray) is the San Onofre Breccia (Tso); the tan is the Monterey Formation (Tm); the pink is the San Mateo Sandstone (Tsm); the pale yellow is the terrace deposits (Qal); and the canary yellow is the landslide deposits (Qls). Note again that the cliffs southeast of the Cristianitos fault are dominated by slumping.

The upper part of the exposed cliff face is alluvial deposits ranging from fine-grained material to coarse, matrix-supported flood gravels. These gravels represent some of the only metamorphic rock exposed in southern Orange County, including Franciscan blueschists and greenschists that have weathered out of the San Onofre Breccia.

The Monterey formation (Tm), exposed here along the beach, is a deep marine diatomaceous shale. To the right (south) is a recently failed block. The preservation of fine layering (below) indicates lack of bioturbation, typical in low-oxygen environments. this makes the Monterey formation an excellent source rock for hydrocarbons. The light-colored layer in the lower image is a volcanic ash deposit, dated by K-Ar to 15 Ma.

The Cristianitos fault (annotated image below). To the left is the 5 Ma San Mateo sandstone (Tsm), a poorly-cemented, coarse-grained shallow marine sandstone. On the right is the Monterey formation (Tm). Note the basal conglomerate (Qbc) stratigraphically below the alluvium but above both the Tsm and Tm.

The Monterey Formation is roughly 10 million years older than the San Mateo sandstone, but it has been brought into contact here by vertical movement of the Cristianitos Fault. Note that there is no offset of the basal conglomerate, indicating that the fault has not moved since the conglomerate was deposited. The conglomerate has been dated at approximately 100 Ka; since it shows no offset we can conclude that it has been at least that long since the Cristianitos Fault has been active.
Wide shot of the Cristianitos fault (annotated version below), showing the San Onofre Nuclear Power Plant 1 km to the north along the beach. If the fault had been active within the past 30,000 years, the plant could note have been built here.


Rockdawg said...

Thank you sending this for Accretionary Wedge 29. It's posted at

Yellow sandstone said...

Great post. Thanks for this post........

Anonymous said...

question - the slumping to the south of the fault, is that the monterrey shale formation or the formations above it? and you said all the slumping is south of the fault, is there any slumping or erosion closer to the san onofre plant? thanks!

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