-2- Silver Point

Driving on the beach, we can see three famous Oregon coast sea stacks- Jockey’s Cap to the south; Haystack Rock and Tillamook Rock behind to the north. Each is composed of invasive Columbia River Basalt. We were left wonder about their geologic stories…

Getting concerned about the tide, Al quickly guided our group to the next story stop. His enthusiasm increased with each step until we heard the now famous summons, “Over here, rock hounds!

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Turbidity Currents-

The theme of the following story depended on recalling the importance grains of rock play in a bed of sedimentary rock. Just where the large and small grains rest can reveal very precise clues in unravelling geologic mysteries. Graded beds of sedimentary rock display a change in grain size from bottom to top…

Graded pebbly sandstone beds and mudstone.

Al’s illustration  allows us to imagine that much of the action in this story occurred underwater in 1000’s of feet of sea. Some have called these geologic catastrophic events “hurricanes of the deep sea.”

Envision a tectonic event (caused by the Cascadia Subduction Zone) on the continental shelf that triggered mixtures of sand, mud, and water to tumble down the continental slope at speeds of 60 mph or more. While the mixture flowed, it incorporated more and more seawater into the slurry. As the mixture was diluted, a turbidity current was unleashed that could cut 5,000 feet deep, mile-wide canyons on the continental shelf and slope. The conclusion of this story resulted in the formation of parallel or horizontal layering on the seafloor as graded beds of gravel came to a stop. Turbidite laminates were deposited. I marvel that we stood beside hard evidence of ancient sedimentary deep-water rocks formed by a hurricane of the deep

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Base of Silver Point Sea Cliff-

Landslides are not uncommon in the dynamic landscapes of the Oregon coast. In 1974, one was so large, it caused US highway 101 to drop 20 feet vertically and moved it westward a few hundred feet. Why did this story occur? Hint- the main characters in this tale are the ocean and Cannon Beach mudstone.

The landslide was triggered when waves eroded the base, or toe, of the a water-saturated mudstone, sandstone, and conglomerate layered cliff. Although attempts were made by Oregon Department of Transportation to stabilize conditions, they proved to be in vain. Nature’s plan prevailed. Several beach cottages and homes were lost. In an ensuing lawsuit, ODOT, not a natural act of nature, was blamed. Moral of the story- (from ODOT’s perspective) it’s not always wise to mess with Mother Nature.

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Time to Move on… the tide was coming in

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-1-  Hug Point
-2-  Silver Point
-3-  Ecola State Park
-4-  Necanicum River Estuary
Thoughts about the Cascadian Subduction Zone

2 comments

  1. Regarding turbidity currents: I’m thinking of the home test for garden soil, putting a handful in a mason jar of water, giving it a good shake, and letting it settle. Did something similar happen with these “underwater cyclones”, particles of like size (and mass) settling together in layers? And are the bigger particles lower down, as they are in the mason jar after the slurry settles?

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    1. I think this sounds like a very good mini-representation of the process. Thank you for sharing the “aha” moment. I’m going to pass this along to one of the professors that facilitated the class. It seems like a great in-front-of-the class demo that he would enjoy. (He used Oreo cookies with us to demonstrate plate tectonics!) ~Jane

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