Day 4 of the 2021 Coaledo Formation Field Trip

Frank Hladky, a former DOGAMI geologist and geologic educator of the south central Oregon coast, met us on Sunday morning to do the last leg of the trip. We met near the airstrip in the Sixes/Cape Blanco area to hike out to Blacklock Point. Here we were completely out of the Tyee/Coaledo basin and into the Klamath Mountains, right atop the accretionary wedge built up by the subduction zone that has been in action since the Cretaceous. As I mentioned at the beginning of this article, the Klamaths were the source of a lot of the sediment deposited in the Tyee and Coaledo formations.

We were not disappointed by the rocks of Blacklock Point. On the seashore bluff and strewn about the beach were products of the subduction zone, a mélange of basalt, radiolarian chert, mudstone, blueschist, and serpentinite. Most of the rocks were black or green in appearance. These Jurassic age rocks had been sent down into the subduction zone, where they were squeezed, folded and cooked. One of the massive boulders of chert exhibited tight folding and faulting patterns.

Hladky told us that some of the minerals associated with the rocks found in subduction zones are the aluminum silicates kyanite, sillimanite, and andalusite, which all have the same chemical formula, Al2SiO5. Serpentinite is metamorphosed from peridotite or gabbro in the presence of sea water with relatively low temperature and pressure. Blueschist, a high pressure, low temperature product of basalt metamorphism, gets its colors from the two minerals, glaucophane and lawsonite.

Moving north to the terrace above Blacklock Point, the group looked north across a cove to the sandstone mined from the Cretaceous North Point by a Scottish immigrant named Blacklock in the early Twentieth Century. There are several buildings in San Francisco with facades mined from this brown sandstone. At the base of the cove are some fossiliferous layers. Hladky had brought many of his high school classes here to make geologic sections of the north hillside.

After returning from Blacklock Point, the group went to Cape Blanco and descended from the main parking lot to the beach north of the Cape Blanco headland. Boulders of blueschist could be found in the bed of the wash coming down the slope. In the water were outcrops of a black sandstone.

We headed back up the slope to the parking lot at Cape Blanco and said our farewells. It was a fabulous field trip and I don’t think any of the participants were disappointed. We’ll be looking forward to the interesting articles to come out from this research, and also looking forward to another great GSOC field trip!

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Pictures from Day 4:

The group meets with Frank Hladky near the Cape Blanco State Airport for the Day 4 trip to Blacklock Point.

View from the top of the bluff looking south to Cape Blanco.

This area has been classified as part of the Gold Beach Terrane, and is part of the accretionary wedge from the subduction zone.

We gather for a discussion about the Jurassic melange rocks modified by the subduction zone.

A blueschist boulder on the beach in the black sand.

Because they have been through the subduction zone “meat grinder”, different rock types are found on the same boulders. Here we have basalt, radiolarian chert, and mudstone on the boulder to the right.

Closeup of the radiolarian chert boulder.

This boulder of black, greasy texture serpentenite looks gray from its glossy shine.

Frank talks about the various rocks we are finding on the beach.

This conglomerate boulder is dense and completely cemented after being exposed to high pressure.

This large rock of chert shows the intense deformation which happens in the subduction zone.

Denny took this picture of my hat with the Jurassic chert rock for scale.

We climb back up the bluff and walk a bit north to Blacklock Point proper. Note the greenish color of serpentinite.

Frank points out that sandstone was shipped from Blacklock Point in the late 1800’s through a cleft in the point.

The sandstone came from the other side of this cove and it now adorns the facades of several San Francisco buildings.

Frank tells the group that several groups of high school students that he taught came out and mapped the sandstone and shale layers of the cove.

After returning from Blacklock Point, the group heads on down to Cape Blanco for the last stop of the trip.

Looking northward to Blacklock Point from Cape Blanco.

Blueschist boulders spill down a wash in the Cape Blanco hillside.

Frank and Cheryl chat in front of the black sandstone outcrop in the surf at the beach.

Frank examines a small chunk of the sandstone.

Two very different appearing chunks of blueschist exhibiting the minerals lawsonite on the left and glaucophane on the right.