24 December 2010

GoSF6: Graywacke turbidites

Posted by Callan Bentley

“GoSF” = Geology of San Francisco

Regular readers are by now aware that I’m taking this week to write up the three field trips I took last week to examine the geology of San Francisco and neighboring areas. My plan is to cover:

Introduction and overview

Seafloor basalt

Deep sea chert

Kirby Cove, Marin Headlands

(wildlife interlude)

Graywacke turbidites (today)

Serpentinite and mélange

Fractures and the chemistry along them

Pleistocene dunes

The San Andreas Fault and Mussel Rock

Graywacke is an immature sandstone: mostly sand, but with a fair amount of mud in it too. Similar sediments get deposited today in the deep sea on the edges of continents (the source of all the sediment), frequently dumped in big billowing pulses called turbidity currents. Where graywacke deposits show graded bedding, we may interpret them as turbidites. That’s the case with the graywackes of San Francisco: geologists interpret them as the deposits of turbidity currents coming off of Mesozoic North America’s western edge.

Like this:

Later, of course, they were scraped up in a big pile (along with the offshore chert and the pillow basalts beneath that) called an accretionary wedge. This rocky mess got slathered onto the edge of North America like chunky peanut butter being smeared onto the edge of a slab of artisinal multigrain bread.

I got good looks at graywacke on two of my three field trips. The first was on the two-year college field trip run by Katryn Wiese of San Francisco City College. On the way down to Point Bonita, we rounded a bend in the path with graywacke on one side and greenstone on the other, and a presumed fault marking the contact between the two, since a pronounced gully ran down to the sea betwixt them. Here’s Katryn walking from the greenstone towards the graywacke:

A look at the graywacke, with my super duper awesome cm-scale pencil for scale (you know you want one, too):

Closer in to the same outcrop:

The greenstone next door is rather homely compared to the sheer sexiness of kinkily-folded chert and bulbous throbbing pillows of basalt, but I felt obligated to document it regardless:

Sheesh, really we should be turning basalt to serpentinite if we’re going to be metamorphosing it along San Francisco’s coast… but that can wait for tomorrow’s post. If you’re good, the Santa will bring you some serpentinite for Saturnalia.

Back to the graywacke…

I had much better looks at graywacke along Baker’s Beach on the west coast of the San Francisco Peninsula, when I rode out there on bikes supplied by my former student Alan. If you want a good guide, I recommend trip #3 in Streetcar to Subduction by Clyde Wahrhaftig (1984).

Here’s a few shots of the turbidites at Baker’s Beach:

I thought this looked like a pretty good graded bed showing that this particular bed was right-side-up, but Wahrhaftig (1984) interprets the strata of Baker’s Beach to have been tectonically inverted on the basis of up-side-down graded bedding and upside-down crossbeds and upside-down scour marks. I didn’t see anything that I considered to be unambiguously flipped, but then again, I was only there for 20 minutes or so.

And, after all, I suppose the two aren’t necessarily mutually exclusive in a puréed lithologic hodgepodge like the Franciscan Complex.

One neat thing about Baker’s Beach that I’ve never seen before was plant fragments in the turbidites, turned to coal:

Wahrhaftig interprets these as “floating to the top” of the turbidity current slurry, which is an interesting notion. Certainly we would expect plant fragments to be lower in their density than the rock fragments and mineral grains they are jostling along with. I suppose some might even have shapes that keep them lofted up in the current regardless of their density, like a kite or a parachute.

This was a nice thick bed of coalified plant remains:

But it ended rather abruptly at a fault,

A final plant fragment — the biggest one I saw — revealed in a boulder split open by the surf:

You see similar turbidites at Rodeo Beach, on the western part of Marin Headlands National Recreation Area, as we see here, though they are a bit sheared-out in the muddier (weaker) sections:

Just to complete that tangential thought, here’s a more zoomed-out view of that same outcrop, showing a nice colluvium horizon capping off the eroded sandstone strata:

Back to Baker’s Beach now, to finish off our post and offer you a bonus. For there you can find a nice “colony” of tafoni developing:

I first heard there was tafoni at this location thanks to a blog post by Michael Welland. It’s not the most spectacular tafoni I’ve ever seen, but it’s always a treat to run into this lovely weathering phenomenon in different places around the world.

Hasta la mañana, and enjoy your long winter’s nap.