Sunday, July 29, 2012

Distal Lummi Formation Turbidites and Skipping Stones

The Lummi Formation is named for Lummi Island where there are excellent exposures on the cliffs and along the shore of the south, mountainous half of the island and at the very northern tip of the island. The formation also forms the cliff knob on the north end of Cypress Island and is exposed along the southeast shore of Orcas Island and north shore of Blakely Island as well as Obstruction,Vendovi, Eliza, Trump, Peapod, Jack, Fidalgo Sinclair Islands and a few small rock islands in between.

A significant portion of the Lummi Formation consists of distal turbidite that has been metamorphosed. Turbidites are sediments that have been deposited on the sea floor via density currents resulting from under water landslides. The sediment creates high density fluid in the water and flows down and out across the slope of the ocean floor. The sequence of events starts with sediment piling up on the shallow sea floor near the coast from rivers and streams. The loose saturated sediment then collapses as an under water landslide. As the density current propagates across the deep ocean floor, the heavy sediment is deposited first and then the sediment fines upward. If the site of deposition is at just the right spot a sequence of gravel fining up to fine silt will be deposited, but more often only part of the range of sediment is deposited in a given spot. Sites far from the landslide will be dominantly fine grained silts and clays. Sites that are closer may be dominated by gravel and sand. Repeated slides will take place as sediment gets piled up again just off shore of streams. The slides might be correlated with repeated earthquakes along the coast that will readily trigger under water slides, but the slides can take place without quakes as well.

On the southeast shore of Orcas Island I had a chance to assess some of the Lummi Formation turbidite with alternating layers of metamorphosed siltstone and sandstone. The metamorphic fabric at the site was generally parallel to the original bedding and overall had formed a nice dip slope down to the shore.

Lummi Formation and measuring strike and dip of turbidite beds
Metamorphosed fine sandstone with thin beds of silt stone

The distal thin bedded sandstone layers eroding onto the beach created a shingle like beach of flat pebbles. The beach was covered with fantastic skipping stones.

A grab sampling of skipping stones

I skipped a few stones towards Cypress Island and Lummi Island
Fog bank and Cypress Island

Cliffs on Lummi Island with Mountain Baker a bit hidden in clouds on right.

1 comment:

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