Episodic tectonic subsidence of Late Holocene salt marshes, northern Oregon Central Cascadia Margin

Abstract

Salt marsh subsurface deposits (0-4 rn 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 fiat 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 gradafional, indicating gradual uplift and development of the marsh. Three in&pendent 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 rn 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 radiocgbon years before present (RCYBP). Significant x'•C age overlaps of at least four subsidence events recorded at Netarts and reported for southern Washington and other northern

Preliminary estimates of the strength of prehistoric shaking in the Columbia River valley and the southern half of coastal Washington, with emphasis for a Cascadia subduction zone earthquake about 300 years ago

“…The thickness is controlled by the level of high tide, with there being no deposition of sediment above high tide. This 1-m thickness lies well within the estimates of coastal tectonic submergence (0.5 to 2 m) by Atwater (1987;, and Darienzo and Peterson (1990). Interpretations on the basis of the five factors above, with the exception of the ages of the living trees, were made in 1992 by Curt Peterson (Portland State Univ.…”

“…The submerged marshes most likely reflect flexure of the crust by tectonic thrusting along the interface where the Juan de Fuca plate is subducting beneath the north American plate (Atwater, 1987(Atwater, , 1992Darienzo and Peterson, 1990). Inferred earthquakes originated by subduction of the oceanic (Juan de Fuca) plate beneath the continental (North America) plate ~ the Cascadia subduction zone (Fig.…”

“…1). Great (M 8 to 9) subduction earthquakes have been inferred partly on the basis of the exceptional length along the coast that appears to have downdropped at about the same time, and perhaps simultaneously (Atwater, 1987(Atwater, , 1992Darienzo and Peterson, 1990;Nelson and Atwater, 1993). Radiocarbon dating of coastal downdropping allow that possibly more than 700 km of coastline was tectonically warped simultaneously in the latest downdropping event (Nelson and Atwater, 1993).…”

Turbidite event history—Methods and implications for Holocene paleoseismicity of the Cascadia subduction zone

“…First thought to be aseismic owing to the lack of historical seismicity, great thickness of subducted sediments, and low uplift rates of marine terraces (Ando and Balazs, 1979;West and McCrumb, 1988), Cascadia is now thought capable of producing great subduction earthquakes on the basis of paleoseismic and tsunami evidence (for example, Atwater, 1987;Atwater and others, 1995;Darienzo and Peterson, 1990;Nelson, A.R., and others, 1995;others, 1996, 2003), geodetic evidence of elastic strain accumulation (for example, Mitchell and others, 1994;Savage and Lisowski, 1991;Hyndman and Wang, 1995;Mazotti and others, 2003;McCaffrey and others, 2000), and comparisons with other subduction zones (for example, Atwater, 1987;Heaton and Kanamori, 1984). Despite the presence of abundant paleoseismic evidence for rapid coastal subsidence and tsunamis, the Cascadia plate boundary remains the quietest of all subduction zones, with only one significant interplate thrust event ever recorded instrumentally (Oppenheimer and others, 1993).…”

“…Coseismic subsidence in the last few thousand years has been well documented in coastal bays and estuaries in the form of rapidly subsided marsh deposits and tsunami sand sheets along the Cascadia coastline (for example, Atwater, 1987Atwater, , 1992Clague and Bobrowsky, 1994a,b;Williams and others, 2005;Darienzo and Peterson, 1990;Atwater and others, 1995;Nelson, A.R., and others, 1995Nelson, A.R., and others, , 2006Nelson, A.R., and others, , 2008Kelsey and others, 2005;Witter and others, 2003). These events indicate sudden coseismic submergence, inundation of coastal lowlands, and burial of the former land surface.…”