Monday, May 5, 2008

Oh, Turbidites!

(Sed/stratters near the top of the section)

Saturday we headed to one of my favorite field trip locations, Pebble Beach (NOT the famed golf course) and Pigeon Point. This beach is home to a fantastic set of tilted turbidite deposits, so the students get to walk upsection while walking across the beach. It also represents a nice study in long-shore currents since the pebbles that give the beach its name come from an isolated exposure of Franciscan rocks north of San Francisco Bay.

(beautiful convolute bedding)

For those of you not familiar with turbidite deposits (what??), they are the result of underwater landslides and debris flows. These particular turbidites were deposited during the Cretaceous to Paleocene (circa 65 millions years old). We tried to convince the students that one of the many fantastic things they could see in these rocks were trace fossils made by the rare, long-necked snorkling dinosaur that paddled around, and used its long neck to reach down and scavenge off the bottom. They didn't buy it.

We did all buy into the story about the "doomed pioneers." Apparently this is the term for little shallow marine critters that ride the rocks down the slope in these flows and continue to burrow through their displaced sediment homes, fully unaware of the fact that they are in WAY over their heads (ok, that bad pun was intended) and therefore doomed to die. I'd never thought about the fact that it seemed a little odd that these deposits can be quite burrowed.
(Honey comb weathering adds to the whole picture...)

(Bryn, look! California has turbidites. You should come see the flame structures....)

Pigeon Point is the second stop for the day and is full of jumbled rocks that represent large-scale slumping and folding of the same kind of turbidite rocks. In this case, the landsliding and mass-wasting are of such a huge scale that entire beds are ripped up and folded very shortly after they've been deposited. The students are supposed to wander around and figure out which way is "up." No one leaves fully convinced.

(Wow!)(yes, that is one, single sandstone bed)

And then we all stopped in Davenport for pie and hot chocolate on the way home.

6 comments:

Anonymous said...

Hi, Katie,
Belated happy birthday! I saw your parents in church last week. So good to see them again. They said you had new adventures on your blog so I just caught up with your Dora-like life. Wild! Great pic's. They almost make me understand what you do. Ha!

Take care.
Rita

BrianR said...

oh how I love the Pebble Beach outcrops ... I could go there every weekend ... great pictures!

kes said...

Thanks Brian! Glad to hear there are other Pebble Beach junkies out there.

Anonymous said...

Hi...My professor would be upset with me reasearching in this way, but i'm kinda of stuck. i studied the pebble beach turbidite outcrop. I read a paper by Walker twice, with the understanding that from the paper i would be able to deduct where the deposit was originally ie..levee, channel, etc.. I just don't see that answer in there....Without giving it away, do you have any hints for me?

kes said...

Sorry Anonymous,

It isn't really appropriate for me to help you out, and would probably be misleading for you anyway. The characteristics of the deposits change a lot laterally, so my perspective may not apply to where you went. Good luck!

Asa Deane said...

Katie! It's Asa (you know, one of your old students who drifted off into the real world)! Don't know if you check this section of your blog anymore, but just in case...
I was actually doing some research when I stumbled upon this page. I'm working at San Mateo Outdoor Education right now up in La Honda, and we go to Pebble Beach, Bean Hollow, and Pigeon Point with the kids every week. I'm actually working on a lesson plan for the geology of these locations right now. I'm doing pretty well, just had a few questions regarding the exact nature of Pebble Beach as well as the tafoni. Anyways, good to randomly see your picture out there on the world wide web. If you get this, and have time to email me, my address is asadeane@yahoo.com. Thanks Katie!