29 March 2019

Friday fold: Plunging Purisima

Posted by Callan Bentley

That photo shows a crack team of geologists at Fitzgerald Marine Reserve near San Mateo, California. Standing on the seaside outcrop from left to right are: Jamie Kirkpatrick (McGill University), Christie Rowe (McGill University), and John Wakabayashi (California State University at Fresno). Together with Kim Blisniuk (San Jose State University) and me, they comprise a team of geologists updating the beloved “Streetcar to Subduction” field guide to San Francisco geology in time for AGU’s Centennial year and the return of the AGU Fall Meeting to San Francisco. This guide, by Clyde Wahrhaftig of U.C. Berkeley, was published (by AGU) in 1984, a good long 35 years ago. Some things have stayed the same, but other things have changed since then, and we’re preparing an exciting new version of the tour for digitally-enabled modern geologists.

A few weeks ago, the five of us did essential field work, travelling around to various sites both in Wahrhaftig’s original field guide and several others besides, checking on outcrop quality and capturing imagery for our update. The Fitzgerald Marine Reserve wasn’t in the original guide, but it has some great geology exposed among the gulls, sea lions, and octopuses:

That’s the trace of bedding, planed off by the waves, showing the outcrop pattern of a particularly coarse conglomerate within the (late Miocene to early Pleistocene) Purisima Formation. The fold in question is a plunging syncline. The Purisima is a pretty young package of sediments to show this kind of deformation, and the interpretation is that the adjacent San Gregorio Fault’s motion is responsible for the deformation.

And finally, we’ll zoom in here to where the axis of the fold crops out:

I look forward to sharing more of my images from this trip with you as the year unspools. San Francisco has awesome geology, distinct and exemplary, worth experiencing and celebrating alongside AGU’s Centennial year. You should attend the 2019 AGU Fall Meeting, and try some of the revised trips out for yourself when you’re next in the San Francisco Bay area.