29 May 2022

Accretionary wedge turbidites from the Lost Coast

Posted by Callan Bentley

My wife just got back from a backpacking trip along the Lost Coast of California, from Mattole to Shelter Cove.

This is along the northern stretch of the San Andreas Fault, just south of the tectonic triple junction at Cape Mendocino.

My wife was kind enough to document some of the geology along the coast there, and to allow me to share the images here…

The local rocks are siliciclastic turbidites, shales and graywackes mainly – with a bit of conglomerate thrown in. These were deposited in deep water due to density currents (underwater avalanches called turbidity currents), which drop their load of grains in order of the particles’ weight, as this boulder of stacked graded beds shows:

A couple of close-ups of laminations and cross-bedding within the sandier turbidites, again showing a fining-upward pattern:

Here’s a muddier package, interspersed with sandy layers:

These deep sea sediments were scraped off the subducted Farallon Plate during the Mesozoic, and were crumpled and deformed as they joined the accretionary wedge. The contrast of the light sands and dark muds shows off the resulting deformation well:

Is it a fold or a fault?


This is my favorite of the photos my wife brought back: Our friend Kristie stands in front of a shale-dominated outcrop, but the handful of graywacke layers show a neat pop-up feature (to her left):

Let’s zoom in:

Annotated, with faults in yellow and kinematic indicator arrows in red:

This outcrop shows a scaly pattern, with a vertical foliation, suggesting it was a more dispersed zone of deformation, a shear zone within the subducted sediments.

Like much of coastal northern California, the Lost Coast has experienced uplift. Once the downward drag of subduction was relaxed, it allowed the flexible edge of California to bob upward. As evidence of that, consider this angular unconformity:

The lower half of that outcrop is tilted, folded, and faulted turbidites like those we have been looking at, but then there is a prominent line of light-colored boulders marking an ancient erosional surface, probably a wave-cut platform. Above that, plenty of beach gravels to a thickness of many meters, much like the modern beach in the foreground. All that was at or below sea level, and now has been lofted well above the reach of the waves.

This recent uplift is preserved in the shape of the land, too, like the two prominent marine terraces seen in this oblique shot along the coast:

Inspiring geology can be found along the Lost Coast.