Sudden, probably coseismic submergence of Holocene trees and grass in coastal Washington State

Abstract

Growth-position plant fossils in coastal Washington State imply a suddenness of Holocene submergence that is better explained by coseismic lowering of the land than by decade- or century-long rise of the sea. These fossils include western red cedar and Sitka spruce whose death probably resulted from estuarine submergence close to 300 yr ago. Rings in eroded, bark-free trunks of the red cedar show that growth remained normal within decades of death. Rings in buried, bark-bearing stumps of the spruce further show normal growth continuing until the year of death. Other growth-position fossils implying sudden submergence include the stems and leaves of salt-marsh grass entombed in tide-flat mud close to 300 yr ago and roughly 1700 and 3100 yr ago. The preservation of these stems and leaves shows that submergence and initial burial outpaced decomposition, which appears to take just a few years in modern salt marshes. In some places the stems and leaves close to 300 yr old are surrounded by sand left by an extraordinary, landward-directed surge—probably a tsunami from a great thrust earth-quake on the Cascadia subduction zone.