Paleotsunami Research at the Seal Beach Wetlands, Seal Beach, California
Authors:Ohara Creager, Robert Leeper
- Brady Rhodes, Professor of Geological Sciences, CSUF
- Matthew Kirby, Professor of Geological Sciences , CSUF
The Seal Beach Marsh is located inside the Seal Beach Naval Weapons Station in north Orange County, CA. The wetland formed as a result of flooding and infilling of topographic lowlands during early Holocene sea-level rise. The Seal Beach marsh may contain a record of prehistoric tsunami and other paleoseismic data because it is a low-energy depositional environment and historic anthropogenic disturbance is limited. To test for a record of tsunami, sixteen reconnaissance gouge cores between 150 and 240cm in length were collected and described. These cores showed peaty organic layers interbedded with mud and sand. To investigate the stratigraphy at greater depths, a 377cm vibracore was collected. Preliminary analyses show the top 15cm is modern marsh. From 15-107cm below land surface (bls), peaty mud and mud of varying thicknesses are interbedded. At 118cm bls, a 10-cm thick sand layer covers mud at a sharp irregular contact. A 10-cm sand layer with an irregular basal contact at 137cm bls covers peaty mud consisting of 50% organic matter. Alternating mud and muddy sand layers of varying thickness continue to 246cm bls. At 250cm bls, a 2-cm thick mud layer caps a muddy peat layer. Mud at 270cm bls extends down to a sharp irregular contact with a 21-cm thick sand layer at 356cm marking the base of sediment recovery. The core was analyzed for loss on ignition at 550°C (% total organic matter) and 950°C (% total carbonate), also magnetic susceptibility at 1-cm intervals. These analyses confirm the existence of organic-rich zones alternating with organic-poor mud. Our working hypothesis is that these peaty layers represent repeated subsidence, perhaps related to seismic activity on the Newport-Inglewood Fault zone. Microfossil identification will be used to confirm that these alternating layers represent abrupt environmental changes, and C-14 ages will document the chronology of subsidence.