High-Resolution Foraminiferid Stratigraphy as Evidence for Rapid, Coseismic Subsidence: Preliminary Results

Abstract

Abstract A series of three Holocene peat horizons, one of which is a closely spaced couplet, are found at several locations along the margins of Humboldt Bay, northwestern California. These peats are found at the Mad River slough on the north edge of the northern compartment of the bay. These peats have radiocarbon dates of approximately 1700 ybp (years before 1950), 1300 and 1100 ybp (for each surface of the couplet of peat horizons), and 800 ybp (corrected radiocarbon dates). The modern and stratigraphic distributions of salt marsh foraminiferids are being studied to determine changes in relative sea level caused by tectonic events on the Cascadia subduction zone. Preliminary studies of modern salt marsh foraminiferid distribution on the east coast of Canada and New England define two to three narrow zones in the marsh relative to sea level. The studies indicate that small scale changes in paleo-sea level could be determined by identifying modern associations, determining the zone in the marsh that association represents, and showing the changes in the marsh zones over time. West coast studies, however, have encountered problems that will limit the utility of using modern salt marsh foraminiferid associations to determine paleo-sea level to more than the order of several tens of centimeters. The technique is also hampered by what appears to be preferential degradation of a species, which may be introducing a taphonomic bias in the longer term (>1000 yr) record. In the short-term, however, the use of salt marsh foraminiferid stratigraphy to indicate sudden sea level changes works fairly well. The upper surface of the first buried peat at Mad River slough shows a foraminiferid association indicative of a position near the higher high water boundary. Three centimeters above this horizon, within a massive bed of intertidal muds, an association was discovered that lived between the mean tidal level and mean high water. Considering the elevation of these tidal datums at Mad River slough, this change in foraminiferid association corresponds to a potential magnitude of subsidence of between 0.4 and 1.3 m within the span of time necessary to deposit 3 cm of sediment.