T164 Geological Records of Earthquakes and Tsunamis on Passive and Active Margins Coasts (Posters)

Tuesday, 6 November 2012
9:00 AM - 6:00 PM
Hall B

GRAIN SIZE CHARACTERISTICS OF TIDAL MARSHES DURING A SIMULATED MEGATHRUST EARTHQUAKE AT BANDON, OREGON

Paper number: T164-3
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Estuaries on the western coast of North America preserve a geologic record of great plate-boundary earthquakes on the Cascadia subduction zone. These are archived stratigraphically as peat-mud couplets that reflect the subsidence of these estuaries from high salt marsh/uplands to lower intertidal environments. Using techniques adapted from the study of sea-level rise on passive margins, namely lithostratigraphy, microfossil analysis and stable carbon isotope geochemistry, the amount of vertical crustal deformation can be estimated, which can then provide estimates of rupture width and magnitude of prehistoric earthquakes. These techniques require comparison with modern marsh environments to determine accurate pre- and post-seismic elevations. However, the redevelopment of marsh environments after a large earthquake and the subsequent colonization by vegetation and microfossil groups remains poorly understood.

In August 2011, the US Fish and Wildlife Service conducted a restoration project at Bandon Marsh National Wildlife Refuge in the Coquille River estuary in southern Oregon. Dikes which had previously isolated the 582-acre Ni-les'tun unit of the Refuge from tidal flow were removed, flooding the marsh and simulating the abrupt rise in relative sea level that accompanies a plate-boundary earthquake. Commencing one week prior to the restoration of tidal flow, regular sediment samples have been taken from ten sites forming a transect across the Ni-les'tun unit. The sediment samples have been analyzed for grain size distribution. The time series of grain size data indicate an abrupt transition from an underlying peaty soil to a new sediment layer that thins and fines from a grain size of a fine sand to a fine silt moving away from the edge of the marsh indicating the new sediment layer is deposited by tidal flow from the estuary. This suggests that sedimentation representative of a new environment occurs rapidly and therefore, that mud layers in the stratigraphic record are accurate recorders of the magnitude of coseismic subsidence. Further investigation of grain size will allow the reconstruction of hydrodynamic parameters throughout the redevelopment of the marsh.