Oregon Coast Notebook: Melange

From www.searchanddiscovery.com ‘Deepwater Fold-Thrust Belts: Not All the Beasts Are Equal’, Ana Krueger and Ed Gilbert, 2009

The Southern Oregon Coast geology is a distinctly different beast than its more northerly neighbor. The southern coast is dominated by the Franciscan Melange. It’s the detritus scraped off the top of the subducting Juan de Fuca and Pacific Plates. Geologists call it an accretionary wedge or prism. It’s not a good place to be if you’re a rock.

It’s rock that has been put through the geologic meat grinder, what John MacPhee, in his classic Assembling California called “rock gunk.” In a sandstone matrix – well, technically, graywacke – you can find embedded pretty much everything that could have been laying on ocean floor, in no order, with no structure.

WC spent some time with the rocks at Harris Beach State Park. They demonstrate vividly just what a jumble the Franciscan Mélange is. Here’s a looks at one fragment of a sea stack just above tideline.

Sea stack fragment, Harris Beach State Park, Oregon

The white stuff is limestone. The roundish blobs in the lower left are pillow basalts, magma that erupted underwater. The small, brownish oval in the upper right is a mussel shell fossil; WC isn’t good enough at this game to tell you what species. It’s embedded is a small chunk of sandstone, where the grains, viewed in a loupe, are sorted, like beach sand. The rest of it is graywacke. Just one face of one outcropping, a jumble of geology mushed together. The remarkable distortion of the limestone alone tells you this rock has been through a lot of stresses.

Second sea stack fragment, Harris Beach State Park, Oregon

Remember that the white limestone was originally laid down in a horizontal layer. When that layer of limestone reached the North American Plate it was literally shattered, with chunks left embedded in the rock above and below it. In places, it was subjected to enough heat and pressure that some metamorphism occurred, the bluish-gray areas in the center of the photo are blue schist, likewise shattered and mixed as it was scraped off the subducting ocean bottom.

The Franciscan Melange is most famously found in the hills of San Francisco. The stuff in southern coastal Oregon is the northernmost extension of the formation. The sea stacks that extend photogenically out into the Pacific at Harris Beach are all Franciscan Melange, and represent the former coastline.

Sea Stacks composed of Franciscan Melange, Harris Beach State Park, Oregon

All those sea stacks, big and small, are composed of the same melange. The closeups in the first two photos were taken from the two small stacks in the right middle foreground. Note the people for scale. Under that sand? Still more melange. There’s other rock formations in the area, but the melange dominates.

The process will repeat again and again. The Pacific and Juan de Fuca Plates continue to slip under the North American Plate. The accretionary prism accrete. The compressed wedge gets lifted up and the ocean erodes it away. Non-geologists see the sea stacks out in the Pacific and assume it has always been like that. Not even close. Human time is a single frame in a extremely long movie. Because we live in that single frame we think of it as permanent. It isn’t.

Real Geologists will justifiably accuse WC of simplifying an extremely complex set of geologic formations. And it’s true. But not, WC hopes, oversimplifying.

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