Document Type

Thesis

Date of Degree Completion

Spring 2016

Degree Name

Master of Science (MS)

Department

Geological Sciences

Committee Chair

Dr. Breanyn MacInnes

Second Committee Member

Dr. Joanne Bourgeois

Third Committee Member

Dr. Lisa Ely

Abstract

Crescent Harbor marsh, on northeastern Whidbey Island, records evidence of co-seismic land-level change 1825 to 1925 cal. yrs. BP. The lithostratigraphy and diatom microfossil assemblages reveal a marsh peat abruptly overlain by intertidal mud, indicating rapid subsidence. Analysis of the modern-day position of depositional facies indicates subsidence from a high marsh to a tidal-flat environment representing an estimated 1.7 m elevation change. The timing of subsidence fits within the dates of a rupture found on the nearby Utsalady Point fault between 1,100 and 2,200 years BP (Johnson et al. 2004). Likely, the stratigraphy at Crescent Harbor records the same event and refines the age of rupture to ~2,000 yrs BP. Crescent Harbor stratigraphy supports evidence that the Utsalady Point fault is an active feature in northern Puget Sound and poses a seismic hazard to northern Whidbey Island. In addition to the paleo-seismic interpretation, stratigraphy also indicates that tidal exchange in the marsh was restricted or non-existent for the last ~1,000 years BP, up until AD 2009 when the barrier was intentionally breached and the majority of the marsh became intertidal.

Available for download on Tuesday, June 20, 2017

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