オレゴン州サウススルーの沖積低地の掘削調査に基づく完新世後期の古地震の復元

Abstract

A partial history of large earthquakes can be inferred from the stratigraphy, radiocarbon ages, fossil diatom assemblages, and FeS2-S contents of late Holocene intertidal sediments beneath South Slough, a narrow, protected arm of the Coos Bay estuary on the central Oregon coast. A total of 51 25-mm-diameter gouge cores collected to 2-7m depth showed sequences of interbedded peaty and muddy sediments beneath fringing marshes at ten small inlets around South Slough. Radiocarbon, diatom, and FeS2-S samples were obtained from two additional 70-mm-diameter cores at the Talbot Creek and Middle Creek sites. As many as nine peaty horizons, most interpreted to be former marsh soils, were encountered at some sites. Many of the buried soils have abrupt to sharp upper contacts with overlying mud suggesting that upper intertidal marsh soils were suddenly lowered into the lower part of the intertidal zone. In contrast, the gradual lower contacts of the peaty soils suggest gradual shoaling from a mud flat to the higher parts of a marsh. Repeated times of very rapid submergence inferred from the abrupt lithologic changes in the cores is supported by our interpretations of changes in diatom assemblages and FeS2-S contents in the two large-diameter cores. Peaty soils contain diatom assemblages of Group III, dominated by fresh to brackish species, whereas muddy sediments contain assemblages of Groups I (marine species) or II (brackish species). FeS2-S contents increase abruptly across the upper contact of peaty soils, also indicating a sudden change to a more strongly marine environment. Accelerator mass spectrometer radiocarbon dating indicates times of sudden submergence at about 0.4, 1.0, 1.3, 1.7, and 2.5ka.South Slough lies near the distal part of the active fold and thrust b elt of the North America plate, about 75km east of the inferred trace of the pl ate-bounday thrust of the Cascadia subduction zone. Pleistocene marine terraces near the slough record deformation on northwest-trending folds and flexure-slip and high-angle faults due to crustal shortening in the overriding North America plate. Thus, the repeated sudden submergence inferred from the sediments of the slough may record localized subsidence during shallow earthquakes on local structures in the overriding plate. However, given its distance arcward of the trace of the Cascadia thrust, South Slough may also fall within the western part of regional zones of coseismic subsidence during great plate-boundary earthquakes. If so, the interbedded peat and mud stratigraphy probably records a partial history of great subduction -zone earthquakes. Because many localized deformation events may be coincident with plate-boundary earthquakes, distinguishing local from regional coseismic subsidence is difficult at Sough Slough.