Accreted terranes: a slow-motion pileup on the Pacific Coast

Oregon became a US State in 1859, so you would think the underlying rock would at least be North American. It turns out that like the modern population of the state, though, southwest Oregon’s bedrock is an international melting pot.

Let’s take a step back and deconstruct that piece of jargon I threw out there in the post title.

Accreted = added on, and terrane = small bit of independent continent.

If you’re familiar with the theory of plate tectonics, it’s often simplified into a huge shifted puzzle of large plates that either collide violently or slide under each other neatly. However there are actually some smaller pieces that get swept up in the cycle of creation and subduction. These could be pieces of oceanic crust that got scraped off of a subducting plate or a volcanic arc like Japan, for instance.

When the supercontinent Pangaea was torn apart by rifts starting around 200 million years ago it started a planet-wide game of bumper cars. The Mid-Atlantic rift separated North America from Europe and it pushed North America westward; this sped up activity along the subduction zones on the continent’s western coast. The Oregon coast shows evidence of the odds and ends of lithosphere that the newly liberated North America plowed into on its journey west.

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Figure showing how accretion works, from Miller 2014

The southwest coast of Oregon where my family vacationed is a giant 11-car pileup of accreted terranes ranging in age from 180 million to 100 million years old. When we hiked on Cape Sebastian we were standing on the Gold Beach Terrane, which took an unconventional path to Oregon. Unlike the neighboring terranes that were scraped onto the continent by converged plates from similar latitudes, studies of the rocks found in the Gold Beach Terrane show that they originated near southern California and were transported north on a transform fault similar to the San Andreas!

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Closeup of southern Oregon and northern California from Miller 2014 showing the sequence of terranes plastered onto the coast

The most prevalent family of rocks in the Gold Beach terrane is the Otter Point Formation. This melange formation (melange being geology-ese for “ungodly mess of rock types”) contains mostly sandstone with dashes of conglomerate, mudstone, bits of interleafed sandstone and shale, and blocks of misplaced metamorphosed oceanic rock. The sandstone from this formation creates many of the dramatic sea stacks that we saw at Secret Beach, Arch Rock, and near Port Orford.

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Arches, mussels, and starfish on the Otter Point Sandstone at Secret Beach

On Myers beach we saw another Otter Point rock that, although softer than the sea stack sandstone, still had a great story to tell.

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Turbidite on Myers Beach

These rocks are called turbidite, and were originally deposited in a deep, quiet environment underwater on the continental shelf. The water was so calm at that depth that any storm deposits that rushed out of a delta upslope sorted themselves gently into larger, heavier particles on the bottom and lighter particles on top. Eventually they formed parallel layers of sorted sediment, one per storm event, and were cemented together by pressure and mineralization.

And then their quiet neatness was ruined when the rocks were scraped onto another continent a few million years later.

Now that turbidite has been bent at 90 degree angles, faulted, hoisted above sea level, and is eroding into nice fine beach sand. Not the retirement it was hoping for, I think.

Source:

Miller, Marli B. Roadside Geology of Oregon. 2nd ed., Mountain Press Publishing Company, 2014.

 

 

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