Using high-resolution Chirp subbottom data to map submerged paleoshorelines: Implications for uplift rates and archaeological sites, Northern Channel Islands, California, USA
Alexander Laws

Alexander Laws
Advisor: Dr. Jillian Maloney

Friday May 4th, 2017
CSL 422 – 8 to 8:45 am

High-resolution Chirp subbottom data were obtained on the shelf offshore the Northern Channel Islands (NCI), California, in June 2016 to image paleoshorelines and refine existing paleoshoreline models around the NCI. The NCI were a unified landmass known as Santarosae during the Last Glacial Maximum (~20 ka), when eustatic sea level was ~120 m lower than it is today. Shorelines and associated marine terraces are recorded and preserved during periods of relatively stable sea level, while a combination of tectonic uplift and sea level fluctuation results in the formation of marine terrace sequences. Today, marine terraces are found both onshore and offshore the modern islands. We identified seven submerged paleoshorelines in the Chirp data, the deepest at ~104 m and the shallowest at ~27 m. Previous studies have attempted to determine the tectonic uplift rate for the NCI by dating marine terraces on the modern islands and shallow submerged paleoshorelines along the shelf. However, these studies yielded conflicting uplift rates, possibly because submerged paleoshorelines were mapped using modern seafloor bathymetry. An isopach map between the transgressive surface and seafloor suggests that the modern bathymetry may not always reflect paleoshorelines, as some submerged paleoshorelines are buried beneath sediment. The paleoshoreline depths were compared to local sea level curves to estimate the paleoshoreline ages.  These results illustrate the importance of subbottom data for mapping submerged paleoshorelines and help to refine uplift rates for the NCI. Furthermore, archaeological sites from Santarosae are found on the modern islands, suggesting that similar sites may be preserved on submerged marine terraces. This research helps to refine models of early coastal human habitation of the islands.