Cristianitos Fault & Nuclear Generation Plant? EarthCache

As you’re walking to the cache, notice the brown shale that makes up the bottom of the cliffs right next to you. Then look farther down the beach near the power plant and notice that the bottom of the cliffs are white. Somewhere up ahead there is a fault, a crack in the rocks along which the rocks have moved.

These cliffs have been created by a combination of lowering sea levels and the land rising. The effect is a sheer cliff that exposes the Cristianitos Fault. The fault runs diagonally up the cliff until it is cut off by a cobble layer.

On the north side of the fault is the white sandstone of the San Mateo Formation. The San Mateo Formation is about 4 to 5 million years old, within the Pliocene geologic age.

To the south of the fault is the brown shale of the Monterey Formation. This part of the Monterey Formation is about 15 to 20 million years old, within the Miocene geologic age.

Originally, the younger material, the white sandstone of the San Mateo Formation, was physically above the brown shale of the Monterey Formation. Younger sedimentry material must be formed on top of older material. The faulting then caused the white sandstone to drop relative to the brown shale to bring them to the same level.

I am sure many of you have been wondering why put a nuclear generating plant so close to a fault? In this case, it was determined that the Cristianitos Faults is not active and therefore a low risk to the plant.

The California Division of Mines and Geology has defined an active fault as one that has moved in the last 11,000 years. The evidence that the Cristianitos Fault has not moved in the last 11,000 years comes from the boulder layer and brown deposits that cut right across the top of the white sand stone, brown shale, and the fault.

At one time, the white sandstone and brown shale were at sea level allowing the waves to erode a relatively flat surface. As the land rose and the sea level fell, a layer of sand and stones would be left similar to the material you currently see on the beach. That is the layer of rounded rocks that you see directly above the white sandstone and brown shale. Erosion in the hills further inland would deposit material on top of the rounded rocks forming the mass of brown material you see all the way up to the top of the cliff (the flat area you parked on). In this area, that process repeated multiple times creating a series of terraces.

So in this case, the Cristianitos Fault has not moved since the rounded rock layer was deposited since the boulder layer and the material above has not been moved (displaced) by the fault. You can see the fault stops below the rounded rock layer and the rounded rock layer is continuous across the fault. Studies have put the age of the wave-cut platform (the flat horizontal surface below the boulder layer) at about 120,000 to 125,000 years. Well beyond the age defined as active.

However, this does not mean that there are no other geologic hazards which may pose a threat to the San Onofre Nuclear Power Generating Station.

Logging requirements:
Send me a note with :

1. The text "GCP7ZT Cristianitos Fault & Nuclear Generation Plant?" on the first line
2. The number of people in your group.
3. Describe how the cobble layer would look if the fault were to move again (and no, glow in the dark is not an answer)

The following sources were used to generate this cache.

• Geology Underfoot in Southern California, Robert P. Sharp and Allen Glazner, Mountain Press Publishing Company, 1993
• Shlemon. R.J., 1987, The Cristianitos fault and Quaternary Geology, San Onofre State Beach, California: Geological Society of America Centennial Field Guide - Cordilleran Section, pp. 171-174

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