LATE QUATERNARY SEA-LEVEL LOWSTAND ENVIRONMENTS AND CHRONOLOGY OF OUTER SACO BAY, MAINE

Rapid late Quaternary relative sea-level change, driven by both isostasy and eustasy, is the primary process that drove migration of littoral environments in the northwestern Gulf of Maine over the last 14,000 years. Earlier work suggested a lowstand at about 60 m below present sea level at approximately 10,500 ¹⁴ C years ago, followed by the on-going transgression, but the lowstand chronology is poorly constrained. To test the lowstand hypothesis, we created a base map of outer Saco Bay with a 10 km² mosaic of multibeam bathymetry and backscatter coupled with 45 km² of side-scan sonar. We surveyed stratigraphy and bathymetric landforms with 120 km of seismic reflection profiles in the 40 to 80 m depth range and collected 17 vibracores of potential lowstand deposits. The area mapped by side-scan sonar is bedrock dominated with extensive gravel talus. Sand bodies are present at -60 meters and give way to mud below -65 m water depth. Much of the seismic data shows erosional shoreline features cut into glacial sediment. Cores also revealed up to 3 m of sand with sections of low-angled laminae suggesting remnants of constructional beach environments. Radiocarbon dates of cored intertidal Mya arenaria at -64 m and Mytilus edulis beds at -36 m better constrain timing of the lowstand and rapid transgression. The SWAN wave model run in Saco Bay at the lowstand water depth of -60 m will suggest areas of likely wave erosion and longshore sand transport that are tested by the seismic and core data. By testing the -60 m lowstand hypothesis and wave model with direct geophysical observations, cores and radiocarbon dates, a strong argument is made for the lowstand position and timing of sea-level changes in the northwestern Gulf of Maine. This area contains critical evidence that suggests rapid and small-scale isostatic effects not reproduced by existing numerical models.