Abstract

Late Holocene stratigraphy, interpreted to represent at least 5 cycles of intertidal sedimentation punctuated by sudden episodes of submergence, have been investigated in the lower part of the Eel River Delta, about 15 km south of Humboldt Bay in northern California. The sediments consist of estuarine muds that grade upward into peats muds, peats containing salt marsh plant fossils, and at two locations weak soils containing rooted stumps of spruce trees. The estuarine muds contain foraminiferid associations dominated by the lower intertidal Miliammina fusca. These muds are in sharp contact with underlying salt marsh peats that contain high marsh foraminiferid associations, dominated by Trochammia macrenscens. The change in foraminiferid associations occurs across a stratigraphic distance of no more than 7 mm at the contact between the peats and overlying muds, reflecting sudden changes from upper intertidal to lower intertidal depositional environments. Convectional C-14 age estimates for the fossil trees and marsh peats indicate ages for the sudden submergence episodes of about 300, 800, 1200, 1500 and 2000 ybp (calendar years before present). We interpret the most likely cause for the sudden changes in sedimentation and foraminiferid association during the late Holocene to be coseismic subsidence resulting from earthquakes on the southern part of the Cascadia subduction zone.