Fieldtrip Stops

 

The San Onofre area is located on the Pacific Plate and separated from the North American Plate by the San Andreas Fault. This region is dominated by right-lateral slip faults that run sub-parallel to the San Andreas Fault.  This style of faulting is apparent onshore as well as offshore.

 

 

The Cristianitos Fault is located between the Elsinore Fault to the east and the Newport-Inglewood Fault to the west.  Both the Newport-Inglewood and the Elsinore Faults are right-lateral slip faults, are known to be geologically active, and capable of producing large earthquakes.

 

The Cristianitos Fault is a normal-slip fault that runs sub-parallel to the shoreline and extends inland for a little over twenty miles.  It is exposed in the sea-cliffs along San Onofre State Beach where it offsets the San Mateo (5 MYO) and Monterey (15 MYO) Formations. These formations are overlain by much younger marine terrace and alluvial deposits. The fault comes within 1 mile of the San Onofre Powerplant and because of this, the activity of this fault has come under close scrutiny. 

 

In addition, this fault controls the stratigraphy that is exposed in the sea-cliffs.  To the south-east of the fault, the Monterey Formation is exposed and soft-sediment slumping is the dominant feature in the sea-cliffs. To the north-west the San Mateo Formation is exposed.  This formation is well cemented and near vertical in the area.  The style of mass wasting in the San Mateo Formation is called rock/cliff fall.