SAND DEPOSITION MECHANISMS IN HOLOCENE MARSH STRATIGRAPHY FROM A TSUNAMI PRONE LOCALITY, CRESCENT CITY, CALIFORNIA, SOUTHERN CASCADIAN MARGIN

The objectives of the study were to describe the stratigraphy of South Crescent Marsh and to identify the candidate processes responsible for anomalous sand sheets in this coastal freshwater marsh. Understanding the depositional mechanisms for the emplacement of anomalous sands in coastal marshes will aid in the interpretation of marsh stratigraphy. To investigate the stratigraphy at South Crescent marsh, 18 gouge cores were extracted along a transect perpendicular to Crescent Beach that bisected the adjacent beach berm and South Crescent Marsh. The subsurface stratigraphy was mapped approximately 200 meters east from the western most edge of the marsh. All one-meter-long gouge cores were extracted at closely spaced intervals to better asses the continuity of the subsurface stratigraphy and to facilitate in layer correlations across the marsh. Four laterally continuous sand sheets were documented within the stratigraphy of South Crescent Marsh. Because the marsh is a prime wash-over setting located at MHHW behind a relatively small beach berm, elevated ocean levels from both tsunamis and storms have an equal chance of inundating this area. Therefore the candidate mechanisms responsible for the deposition of these four sand sheets are (1) near-field tsunamis generated from the CSZ, (2) far-field tsunamis generated from within the Pacific Rim, and (3) storm-waves caused by extreme climatic conditions.