Monday, September 12, 2011

Cristianitos Fault-San Diego County, CA

On days whenever I didn't feel quite up to hiking long distances in the desert, we would stick close to home and just check out Geo-places I had read about in books. One place in particular that attracted a raised-in-the-Missouri-gumbo girl like myself was the Cristianitos Fault; being born on the New Madrid Fault Zone, unable to see what lay so deep below us, naturally, I was utterly fascinated with the opportunity to actually see a clear cut fault.

The book that affected me most while I lived in SoCal was "Geology Underfoot in Southern California," by Robert Sharp and Allen Glazner. I was determined to go to every single site these authors described; however, I only went to about half of the places. I'll be posting more on those trips, too, so stay tuned.

After paying $10 for parking, we made our way down the steep trail to the beach. The day was a typical, on-shore flow, with the sunlight deflected from millions of suspended micro-droplets of mist in the air. There was a softness to the day that was in direct contrast to the noise of the restless sea.

And this is me, taking in the gorgeous scenery: 

The walk was fairly long, but the tide was low and the exposed accretioned sections of seafloor lay slanted, pointing at the direction from whence they originated. The Farallon Plate had long ago ceased to subduct the coast, but its strength and longevity was scattered along the shores of southern California via marine terraces.

We made our way to Echo Arch Camp and the Cristianitos Fault, which lay just beyond it. Here's a map of the area courtesy of the same book that inspired my trip:

The following image is the entrance to the Echo Arch Gully Trail:

To my right, the bluffs underscore the fact that ancient sea levels were higher and by its tilt that the land has risen by some radical force.

And then, suddenly we are in a wide open area, the Echo Arch Camp, which is absolutely stunning:

Thanks to millennia of erosion by the sea undercut this cliff, thereby eliminating layer after layer of deposits a few feet at a time. Otherwise, we would not be able to enjoy the scenery within this arena.

Rounding the corner from Echo Arch, we begin to see signs of the fault.

The white sandstone below a line of cobbles is an old wave cut platform about 30-40' in height known as the San Mateo Sandstone. About 20' of that layer was shaved off and drilled into so that the San Onofre Nuclear Power Plant could be founded safely. Above the line of cobbles, 60-90' of a dark brown collection of detritus and material sits that took around 100,000 years to accumulate. Think of the sea moving inward covering the original wave-cut platform and leaving its particulate traces, such as sand and shells, then the wash of small stones, boulders, soil, and other materials move down to land on the leeward side of the platform, making a nice sloping apron. Sea level changes, allowing for more accumulation. Thousands of years later, sea level finally stabilizes and once more wave cutting occurs, removing enough material so that we are now able to see these two bedding layers with a fault running through them.

In case you think that hmm, San Onofre will be undercut, thankfully, solid concrete walls with a berm of large wedges of rocks buttress the plant and so time will be long coming before this nuclear plant is at risk.You can see the plant below and the layer of San Mateo Sandstone overlaid by the Alluvial Deposits (also, take care to see the nude sunbather, too ;-)

Next, let's look at the fault. It extends inland on a bearing of WNW for about 26 miles, crossing Ortega Highway, all the way through the valley of CaƱada Chiquita. The Monterey Shale was laid down during the Miocene (20-15 million years ago) and the San Mateo Sandstone, during the Pliocene (4-5 million years ago. The Monterey Shale? Hey, where is that? Take a look below:

It's behind those bushes.

The San Mateo formation has moved over the Monterey Shale, which dropped allowing the situation to occur. Also, notice that banding has taken place near the fault in the San Mateo formation, while the Monterey Shale is crumpled and broken. But look at the layer of cobbles above the fault--they are level, which signifies no movement in the fault since they were laid down. The Alluvial Deposits are at roughly 90 degrees, which also means that they have not been displaced. Studies show that the wave-cut platform is about 120,000 to 125,000 years old; faults are considered active if they have moved within the last 11,000 to 35,000 years. Therefore, the fault is dead, and no worries over this one affecting San Onofre.

What's really neat, though, is that you can place your hand in a genuine fault. Way cool.

And that's it. Hope you had as much fun as I did. Gotta run! See you later!

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