27 September 2019
Last week saw me out in the San Francisco area, working on our revision to Clyde Wahrhaftig’s classic 1984 field guide, Streetcar to Subduction. One fold-o-rific locale I got to see for the first time was the west side of O’Shaughnessy Boulevard, west of Glen Canyon Park. It’s a great place to observe deformation in chert+shale interbeds of the Marin Headlands Terrane.
Now for some close-ups, including this one which shows the interbedding of the chert and shale quite well. These beds are much easier to crumple up by virtue of this mechanical layering.
And here’s a return to the same spot about an hour and a half later, when the outcrop was completely in shadow, so the lighting’s more consistent, but the rocks don’t “pop” quite as much.
The bedding there is overturned on the short limb, and the fold has become a fault!
If you want to see these outcrops for yourself, here’s a Google map link, and a link to the best outcrop in Street View. It’s walking distance from the Glen Park BART station. If driving, you might consider parking on Malta Street. That’s what we did. It would take only a couple of hours away from your AGU Fall Meeting experience to go and check out these folds. An alternative would be to participate in our pre-AGU field trip on “highlights of the Franciscan Complex,” which will be available for sign-up in October. That trip will showcase similar folds in the same rocks, seen at Marin Headlands itself.
Happy Friday; enjoy your weekend!