Friday, September 14, 2012

Soft Sediment Deformation

"Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." - Steno, 1669.

Steno's basic principle of original horizontality is a key principle in figuring out geologic structures. But sometimes sediment layers loose their original horizontal aspect through soft sediment deformation. Rapid deposition with water being squeezed out of underlying sediments can stir otherwise nice horizontal layers into a bit of a mess.

These particular soft sediment deformed units are within glacial advance outwash on the west side of Marrowstone Island in Jefferson County. The sand and silt and gravel layers were deposited by melt water streams from the advancing glacial ice at the early stages of the last glacial period in the Puget lowlands approximately 18,000 years ago. The sediment may have been deformed by simply very rapid deposition or by the loading of the unit by a couple thousand feet of ice.

Earlier this year I was doing some work on the shore of Bainbridge Island and got a view of some soft sediment deformation from above on a wave cut platform beach.

Haugerud (2005) described this unit at another location on Bainbridge as appearing to have been stirred with a spoon. An apt description that I have used since.

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