EPISODIC TECTONIC SUBSIDENCE OF LATE HOLOCENE SALT MARSHES, NORTHERN OREGON CENTRAL CASCADIA MARGIN

Salt marsh subsurface deposits (0–4 m depth) in Netarts Bay, a small coastal lagoon of northern Oregon, record six events of marsh burial in the last several thousand years. Five of the buried marsh surfaces show sharp, nonerosional upper contacts with either anomalous sand layers (tsunami deposits) or tidal flat mud deposits. These sequences indicate episodic, abrupt subsidence of the marsh surfaces to low intertidal levels. In contrast, lower marsh contacts with underlying intertidal muds are gradational, indicating gradual uplift and development of the marsh. Three independent measures of deposit elevation relative to mean tidal level (percent organics, diatom assemblages, and percent eolian sand) have been used to estimate vertical displacements of marsh surfaces. Abrupt subsidence displacements of 1–1.5 m alternate with gradual uplift displacements of the order of 0.5–1.0 m. The vertical tectonic movements are interpreted to reflect coseismic strain release (abrupt subsidence) following interseismic strain accumulation (gradual uplift), associated with interplate coupling between the Juan de Fuca Plate and the North American plate in the Cascadia subduction zone. Recurrence intervals between subsidence events range from possibly less than 300 years to at least 1000 years, with the last dated event likely occurring 300–400 radiocarbon years before present (RCYBP). Significant 14C age overlaps of at least four subsidence events recorded at Netarts and reported for southern Washington and other northern Oregon bays (at 300–500, 1000–1300, 1400–1800, and 3000–3300 RCYBP) suggest the potential for event synchroneity over at least 200 km of the central part of the Cascadia subduction zone. Additional work is needed to test the synchroneity of these episodic events of coseismic subsidence. Copyright 1990 by the American Geophysical Union.