21 December 2010

GoSF3: Chert

Posted by Callan Bentley

“GoSF” = Geology of San Francisco

I’m taking this week to write up my three field trips examining the geology of San Francisco and neighboring areas. My plan is to cover:

Introduction and overview

Seafloor basalt

Deep sea chert (today)

Kirby Cove, Marin Headlands

(wildlife interlude)

Graywacke turbidites

Serpentinite and mélange

Fractures and the chemistry along them

Pleistocene dunes

The San Andreas Fault and Mussel Rock

Here’s a few looks at outcrops of this chert. In most of these photos, my cm-denominated EmRiver mechanical pencil serves as scale:

Here’s an example of where it’s more dominated by chert, with only a minimal amount of shale interbeds:

Closer in to this same outcrop:

In contrast, here’s an outcrop with a lot more of the shale in between the chert layers:

Chert is a microcrystalline sedimentary rock, in this case made of gazillions of little dead radiolaria (silica-shelled plankton). The shale is made of silt and clay blown out to sea from a source on land, and gradually settling down through kilometers of water to drape the seafloor.

You’ll notice that these beds are dipping at a gentle angle, and also weathered out in blocky patterns which are ideal for exploring the difference between apparent dip and true dip:

But much of the chert in Marin Headlands terrane is not horizontal or gently dipping but boldly upright, in a position never imagined by the dead radiolarians raining down on the seafloor:

Here’s Lily against a similar looking outcrop of vertical chert beds… but notice that there are a few pointy folds in there too:

And that brings us to what I really wanted to see at Marin Headlands (on Kim Hannula’s recommendation): chevron folds in chert!

Closer in:

Closer in:

A neighboring exposure, with Wahrhaftig’s (1984) Streetcar to Subduction for scale::

You’ll no doubt have noticed that this chevrons are frequently asymmetric (limbs of different lengths) and/or overturned to the right (which is east in these photos, as most of them are taken looking towards the north at south-facing outcrop surfaces).

This next photo is one of my favorite shots, as it shows the difference in rheology between the relatively competent chert and the relatively incompetent (squishy) shale, as the shale tends to pile up in the hinge area of the fold, while the chert exhibits a more “concentric” geometry. The photo also shows well the resistance of the chert to weathering, while the shale is more easily etched away:

Indurated fault zone (pencil for scale at center lower middle):

And check this out!…

Ka-pow! (Notice that I am holding the cm-marked pencil for scale!)

Close by to the above outcrop, here’s one of the most intensely-folded sections that I observed:

That’s 33 images of chert and shale, folded and not, and probably enough for one day…