Tuesday, August 27, 2013

Ripple Cross Sections Near Port Townsend

The shore bluff facing the Strait of Juan de Fuca in Port Townsend provides some great cross sections of a very wide variety of sediment deposits (Click on location at bottom of post). I came across this cross section of a set of ripples.

Ripples in sand deposit with U.S. quarter for scale

Same set of ripples from a bit back (note quarter) with other mostly sandy layers topped with a layer that has been highly deformed by soft sediment deformation

The soft sediment deformation may indicate sediments deformed by very rapid deposition before they became very compacted. The ripple deposits may mark a bit of a transition from preglacial sediments to early glacial advanced outwash. Besides the nice form of the preserved ripples, the ripple unit contained bits of charcoal.

Bits of charcoal within deposit

More charcoal within sand deposits

The presence of charcoal hints at a preglacial deposit. Mapping by Gayer (1976) and generally adopted by Schasse and Slaughter (2005) do not distinguish the bluff units lumping them into a broad category of "undifferentiated glacial and non glacial deposits" due to the scale of the maps. The top of the bluff is mapped by Schasse and Slaughter as glacial marine drift. That is the sediment at the top of the bluff was deposited while the area was submerged during the late stages of the last glacial period.  The mass of glacial ice had pressed the local land surface downward hundreds of feet and hence the area was submerged for a brief time before the land rebounded. My observations of the upper bluff are consistent with Schasse and Slaughters interpretation although the marine drift at this location is a bit thin and there may be patches of till - a tough call.

The ripples at this location almost looked like tire tracks

More tire tracks. Note the ripples grade upward from silt dominated to sand dominated

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