Saturday, March 22, 2008

San Onofre Breccia

I've pretty much managed to describe the entire field trip over the course of a few other entries by now, but I've been saying I'd post pictures for three weeks now, and I'm only delivering now. Eesh!

The weather looked pretty threatening the entire time, but there fortunately was no actual rain.

This is a section of the breccia from Laguna Beach; the clay matrix is redder at Dana Point. Only reason I'm not posting one of those shots is I'm not sure whether or not my classmates would appreciate being in my blog, particularly if I'm only using them as scale.

The San Onofre formation is riddled with a bunch of little faults, like this one. We also saw two or three buildings - one of which is a popular restaurant - that are directly on top of some of said faults. They manifest pretty clearly, so one would think builders would shift stuff a few meters out of the way, but nope. Perhaps The O.C. is too cool for avoiding faults...

This fault had one of its sliding surfaces partly exposed, complete with weathered slickensides. Though my finger is ostensibly for scale, I also could not pass up the opportunity to take "fault poking" (my term for merely visiting faults) to a whole new level.

Some of these faults, as well as other cracks/fractures were healed up with calcite.

This is a narrow crevasse/groove that is developing a druse of calcite. (The dead bug is not for scale. The groove in question is about two inches wide and several feet long.) Apparently the mineralogy class goes on this trip every year, and this encrusting of crystals was not there last year.

Seeing as all of the stuff in this breccia was spit out of a subduction zone, one would expect a bunch of spectacularly deformed metamorphic rocks. I thought this one was particularly awesome, since it has clearly been kicked back and forth between brittle and ductile zones several times. There are places where twisted and folded bands have been displaced across a clear break, but also places where a break with displacement across it has been twisted and folded in turn. It really is amazing how much can happen to one rock without its past history getting distorted beyond a point where it can be deciphered.

And that is it for this quarter's field trips. But I'm going on a week long fault-poking road trip next week (the Carrizo Plain is the longest stop on the itinerary), and I've been invited to go on some field trips for a class I'm not officially taking next quarter, so I am not doomed to spending too much time indoors!

1 comment:

Silver Fox said...

That's a very neat breccia, and the faults cutting it add to the interest. Thanks for posting such great photos. I'll have to go there sometime!