Tuesday, May 20, 2008

Perspectives on coastal tectonics

In December of 2005, I went out to The Sea Ranch, California, for Christmas. (The Sea Ranch is one of those towns that is officially called "The" something, kinda like The Plains, Virginia. Sorta weird, but there it is.) I want to share an experience I had there, because it gave me an important perspective on my own 'native' geology back in the mid-Atlantic region. It was a significant moment of understanding for me. Let me walk you through it...

The following collection of images are what I saw walking a mere 1 mile up and down the coast from the house where we were staying. I hope you will be struck by the incredible diversity of rock types seen here (as I was):



Siltstone and shale interbedded (vertical bedding):

Siltstone and shale interbedded (anticline):

Siltstone and shale interbedded (syncline):

Mudchip conglomerate (mud chips are "rip-up" clasts due to scouring of a muddy location by a sudden intense current, which carries much larger particles like the sand that now surrounds the darker, finer-grained mud chips):

Quartz-rich sandstone:

Graywacke (showing mouthwateringly beautiful graded bedding):

A zoomed-out shot of that graded bed:

Various sedimentary layers (sandstone, silstones, shale partings):

And a close-up of a few small faults that cut through them:

And it's not just sedimentary rocks. Here's some greenstone (metamorphosed basalt). Note the cluster of amygdules (infilled vesicles) in the center:

The greenstone is green due to a lot of chlorite, but it also shows some nice epidote:


Looking north up the coast from our rental house, you could see greenstone and conglomerate intermingled on the 10m-scale:

This is in the small cove directly in front of our rental. There are three different rock units seen here (greenstone, conglomerate, clayey sand), all indicating different things. Note the big clast of greenstone "hovering" in the clayey sand part:


So after taking a walk along the lovely coast there, and seeing all this stuff, I thought "Wow."

The tremendous diversity of rock types along this section of the Sonoma County coast was due to tectonic shuffling of rock types at a subduction zone. In the Mesozoic, this part of California was at a trench where the Farallon Plate was being subducted to the east underneath North America. Melting at depth produced magma, which resulted in the Sierra Nevada continental volcanic arc (excellently reviewed by Geotripper in his "Under the Volcano" series examining the Sierras). But at the trench itself, all the sediments at the edge of North America were being compressed and squeezed and mixed up with the sediments being scraped off the subducted oceanic slab. Some knobs and bumps of basalt even got scraped off the Farallon Plate and added into this jumbled mess. Altogether, this big pile of debris from the convergent boundary is referred to as an accretionary wedge. "Accretionary" because it got accreted, or added, onto the western edge of North America. "Wedge" because that's its overall shape in cross-section.

When subduction ceased (due to the subduction of the East Pacific Rise), the Farallon Plate was gone at this latitude, and the Pacific Plate and the North American Plate were now in direct contact for the first time. As time went by, the accretionary wedge reacted to now longer being dragged downward, and it began to isostatically rebound. It bobbed upward, and brought its 'melange' (French for mixture) to the surface. The uplifted accretionary wedge is the California Coast Ranges, a fantastic place for varied geology mainly because of the tectonic "shuffling" that happened here during the Mesozoic.

So, I mentioned that seeing all this diversity in so short a hike really impressed me. But the insight it gave me is that the same thing happened on the east coast. Where I live and work, in DC and Virginia, an accretionary wedge developed during the early Paleozoic, just like in California, with the exception that ours got subsequently squeezed and metamorphosed in a series of mountain-building events. It's a bit more difficult to recognize, partially due to that metamorphism and partially due to all the @#$%ing vegetation obscuring the underlying bedrock. But it's there: we have metagraywacke, with relict graded beds, metabasalt, quartzite, schist ("meta-shale") and metaconglomerate: it's everything I saw in California with a metamorphic overprinting!

"Wow," I thought again.

Here's some shots of DC-area rocks that are analogues for the ones I've already showed you in California:

Metamorphosed mud-chip conglomerate (near Chain Bridge, DC):

Metamorphosed quartz-rich sandstone (the Sugarloaf Mountain quartzite, MD):


Metagraywacke showing metamorphic chlorite, garnet, and pyrite (both from DC):

Graded bed preserved in metagraywacke (Billy Goat Trail, MD):

Metabasalt (amphibolite, again from the Billy Goat Trail, MD):

Metaconglomerate (Klingle Road, DC):

The experience comparing the two coasts greatly enriched my understanding of tectonics and subduction, and gave me perspective on DC's geologic history. Two different accretionary wedges, two coasts, two eras... but one underlying process. That's what really hit home. Geology repeats itself. It gave me a renewed interest in my local geology. Everyone always hears about what great geology California has (and it does), but doggone it, DC pulled that same trick millions of years earlier, and experienced a series of orogenies immediately afterwards (which California can't claim!).

If it's true that "the best geologist is the one who has seen the most geology," then I became a better geologist that day on the Sonoma coast.

PS - I think it's funny to note that I didn't put a sense of scale in any of the California pictures, but that most of the DC area pictures do have one. I think that says something about my development as a geologist and educator too...

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Blogger Silver Fox said...

This is a very interesting comparison of areas - east and west - and some great photos of metamorphic textures.

May 20, 2008 9:46 PM  
Anonymous BrianR said...

excellent post, Callan ... my mouth did indeed water over the graded bedding ... but, then again, it always does :)

May 20, 2008 11:36 PM  
Blogger andrew said...

Most excellent set of photos and interpretation! Ron Schott will surely add his bit, since Sea Ranch is in his master's project research area.

May 23, 2008 12:33 PM  
Anonymous Mrs. Clark said...

This is awesome! Would you please consider teaching in California Public Schools for Grade 6? Many of our Learning Standards for Grade 6 deal with teaching Geology.

If not, would you please consider making a link "For Kids"? What I'd be looking for is something that has a readability of a Lexile Level of 500 or less, which of course would be hard to do.

The reason for a "readability" level of K-4 is because there are a significant number of students who read below grade level. Such students are enrolled in a phonics class which REALLY works, but meanwhile, I need to be able to deliver content in a way that a reluctant reader is willing to read. I know this may sound like spoonfeeding or catering, but it is not about that. It is about making the content standards *accessible* to the students who read at a lower level.

July 1, 2008 7:09 PM  

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