Tectonic mélange: A mélange produced by tectonic processes.
|The Franciscan Melange at Shell Beach, Sonoma County CA|
It was not strictly serendipity that found me at Shell Beach on the Sonoma Coast of California, a 45-mile drive north of Point Reyes National Seashore on Route 1, but the internet. I had originally planned to visit Mount Diablo State Park east of San Francisco, where rocks of the Late Jurassic Franciscan Complex mélange present themselves for examination by anyone who is interested. I had heard of these rocks for years but had not been able to wrap my mind around them. Mount Diablo, it appears, is a park dedicated to the Franciscan rocks – just what I was looking for!
However, an excursion from Point Reyes to Mount Diablo seemed less than appealing after an 800-mile marathon drive from southern Utah. For that reason, before leaving home I googled “Franciscan Complex” and hoped for something closer. What I discovered was a field trip to Shell Beach, complete with photos of what to look for. Geology on a beach! It does not get much better than that.
Serendipitously, it was not even raining on that particular day of my mission to investigate these remarkable coastal rocks. The geology gods and goddesses were certainly watching out for me.
After parking my car, I tell everyone who happens to walk by that I have driven nearly 900 miles from southern Utah to come to this particular beach.
Why? They ask with smiles, not a little perplexed.
I explain that the rocks exposed here are from a subduction zone, and that they are around 150 million years old. I smile gleefully as if I know what I am talking about. In my positive uncertainty, I hope that the age is at least nearly correct and that this is a good enough answer to satisfy these friendly folk. Frankly, that was pretty much all I (barely!) understand of the Franciscan Complex at the moment.
They have no idea what I am talking about and are duly astounded, taking my word for it all. We smile our goodbyes and then, with camera around my neck and field guide printout in hand, I stumble down the trail steps to peer at the jumbled eroding edge of a continent.
|Overlooking the Franciscan Melange at Shell Beach|
During the Late Jurassic Period, nearly 145 million years ago, the eastward moving Farallon plate commenced its collision with the North American plate. For the next 100 million years, the two plates would continue colliding as the Farallon was subducted beneath the North American plate.
Over time, yet another island arc terrane was “rafted in” as a geologic passenger on the Farallon plate. This island arc came in from an unknown somewhere, docking itself against earlier accreted terranes already attached to this western edge of a future California. As the subducting Farallon plate melted many miles beneath the surface, large magma chambers formed and rose to become a chain of volcanoes in what is today eastern California. Subduction processes changed around 90 million years ago and volcanism ceased; the magma chambers slowly cooled and crystallized to form the granitic batholith of the Sierra Nevada Mountains.
Meanwhile, where seafloor of the Farallon plate was subducted beneath the North American plate, a trench formed and top layers of the seafloor were scraped or sheared off as an accretionary wedge or prism. These slivers of seafloor and arc ended up attached to the North American plate, emplaced among the formations of the continent.
|Image courtesy of US Geological Survey|
A large sort of “berm” formed from these slivers and became the California Coast Range. This berm includes large slices of the seafloor and a jumble of oceanic and continental materials now known to geologists as the Franciscan Mélange.
Extensive field studies reveal that there is no stratigraphy in the mélange, no consecutive story of deposition – as John McPhee so succinctly described it, the Franciscan mélange is “just mountains of bulldozed hash.” Sandwiched as it was between the incessant grinding of the Farallon and North American plates, the Franciscan mélange appears today as a pervasively sheared, multihued 145-million year old fruitcake smorgasbord of pillow basalt, sedimentary rocks such as shale, chert and greywacke sandstone, and assorted metamorphic rocks.
So as I slowly move down the sea cliff at Shell Beach, I gaze thoughtfully at this smorgasbord of jumbled hash tumbling out of the eroding gully.
|Eroding cliffs of the Franciscan Melange|
The first boulder I investigate (and sadly take bad blurry close-up photos of) is a hard block containing eclogite, an oceanic crustal rock derived from lava and basaltic tuff (ash) or from gabbro; it had been metamorphosed in a continental plate subduction zone at relatively low temperature. There are some other conditions under which an eclogite will form, but this simple definition will suffice well enough for now.
Red garnets pockmark the boulder, and flaky white mica crystals glimmer in the winter sunshine. I cannot distinguish any green omphacite pyroxene crystals, perhaps because the blue stripes of spray-painted graffiti are so distracting. I sigh. If people only knew what they were defacing…would they still do it?
|Eclogite with garnets and mica|
The next boulder I stop at is a blueschist, again formed in the regional metamorphic environment of our low temperature, high pressure subduction zone.
|Blueschist of the Franciscan Melange|
I learn that the blue mineral is glaucophane, found only in these low temperature, high pressure metamorphic rocks. Here, the alternating streaks are blue (glaucophane) and green (eclogite, which I can actually see in this rock). The sheared streaks are evidence of flow in the rocks as they are intensely squeezed through the subduction zone.
Next, I climb about in the eroding gully with its gray-green clay matrix containing sheared serpentine boulders.
|Serpentine boulders and clay matrix eroding in gully|
Serpentine occurs from the transformation of olivine and pyroxene found in peridotite, the dominant rock found in the upper part of Earth’s mantle. The iron-rich rocks combine with hot water to change the olivine to serpentine.
|Blocks of serpentine in fine-grained clay matrix|
The slick, sheared appearance of these rocks is evidence of their being caught up in a fault zone.
|Sheared serpentine with digits for scale|
My mountain of references say that serpentine is generally green, but it looks blue to me.
|Sheared serpentine - hand lens for scale|
The entire north slope of the gully is composed of shimmering, sheared serpentine blocks in a finer-grained clay matrix. These clays of the south slope are eroding into a badlands topography.
|Eroding badlands of fine-grained clay matrix|
Descending to the beach, I putter around in the wreckage of a subduction zone.
|Blocks of Franciscan Melange at Shell Beach|
|Huge block of Franciscan Melange|
I am thinking this possibly may be a chunk of mantle peridotite, with its black surface on a light-green rock containing greenish-white veins…
|Possible mantle peridotite?|
…and this looks like pillow basalt next to greywacke sandstone. Pillow basalts form when molten lava erupts underwater. The greywacke sandstone is a “dirty” sandstone, originating in an underwater landslide off an ancient continental shelf.
|(L) Graywacke sandstone with (R) possible pillow basalt|
|Conglomerate of Franciscan Melange|
Conglomerates would have formed from stream gravels or the turbidity currents of underwater landslides.
As I wander about, I think that I could surely spend a lifetime in this microcosm of the larger Franciscan Complex. However, I must soon leave this amazing geology even though I have barely begun to see what there is to see. Low tide is starting to turn as the sun sinks incrementally lower towards the Pacific horizon.
I take a few minutes to leisurely walk along the top of the sea cliffs, fervently hoping that the sandy prominence on which I pause does not slump off into the ocean because the San Andreas Fault suddenly takes a notion to shift its britches.
|Trail along sea cliffs at Shell Beach|
Evens, J.G., 2008, Natural History of the Point Reyes Peninsula, second edition, California Natural History Guides: University of California Press.
McPhee, J., 2000, Assembling California, in McPhee, J., Annals of the Former World: Farrar, Straus, and Giroux, p. 429-621.
Minerals of the World, Johnson, O., Princeton Field Guides, Princeton University Press
Simon and Schuster’s Guide to Rocks and Minerals (my well-thumbed copy dates from the early 1980s).
I am particularly indebted to Terry Wright at http://terrywrightgeology.com/shellbeach.html (accessed 2/18/2011) for his field guide to Sonoma County Geology at Shell Beach. It was this website which ultimately led me to drive those 900 miles to a better understanding of the Franciscan Mélange. For this I am grateful.