Sunday, August 25, 2013

Lummi Formation Notes

Had a chance to look at some Lummi Formation. The Lummi Formation crops out on the east side of the San Juan Islands and of course Lummi Island where its named is derived. The formation is a chunk of Cretaceous to Jurassic ocean floor. It is one of numerous blocks and slices of terrains that have been accreted onto the North American margin. Where the story of this unit and others gets complicated is just what happened after the initial accretion. 

The particular portion of the formation I was observed was primarily a portion that is a turbidite sequence. 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 or the upper part of the sequence might get eroded by later density currents. 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 along the near shore by streams and rivers. 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.
For the most part the section I was looking at consisted of alternating silt and sand layers suggesting a fairly distal location out away from the high energy flows or at least on a quieter part of the turbidite fans that were built out onto the sea floor.

The bedding orientation was very easy to measure at this site with alternating beds of siltstone and sandstone.

The Lummi Formation has been metamorphosed and on some of the bedding plain surfaces faint lineations were present.

A common feature at this particular spot was rip up clasts of dark silt embedded within the sand stone. In this case the density current ripped up chunks of previously deposited silt that was then deposited within the sand.

When the only units are alternating siltstone and sandstone determining which was is up in the original formation can be tricky. But a layer of conglomerate grading into sandstone allows for that determination.

Conglomerate grading upward with pen pointing towards the original up direction.
In this case up is slightly downward and to the right.
The dark 'pebbles' are small rip of clasts of siltstone that were eroded by the density current that carried the gravel.

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