MEASURING LATE QUATERNARY UPLIFT RATES OF MARINE TERRACES USING HIGH-RESOLUTION CHIRP SUB-BOTTOM DATA, NORTHERN CHANNEL ISLANDS, CALIFORNIA, USA

High-resolution Chirp sub-bottom data were obtained on the shelf offshore the Northern Channel Islands (NCI), California, in June 2016 to image paleoshorelines and reconcile the rate of late Quaternary tectonic uplift for the islands. 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. Previous studies 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. In the Chirp data, a strong acoustic reflector was observed and interpreted as the transgressive surface created by eustatic sea level rise since the Last Glacial Maximum. An isopach map between the transgressive surface and seafloor suggests that the modern bathymetry may not reflect paleoshorelines, as some submerged paleoshorelines are buried ~20-100 m beneath modern sediment. The ages of paleoshorelines were estimated using a local sea level curve and uplift rates were determined. These results illustrate the importance of sub-bottom 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 submerged on the shelf. This research has the potential to contribute significantly to our understanding of early coastal human adaptations in the Americas, as archaeological sites may be found within preserved submerged marine terraces.