SEA-LEVEL LOWSTAND AND TRANSGRESSIVE EVOLUTION OF OUTER SACO BAY, MAINE

Bedrock geology, glacial deposits, postglacial sediments, and both glacial and littoral processes constrain the morphology and sedimentology of Saco Bay, Maine. Rapid relative sea-level change was the primary process forcing migration of littoral environments in Saco Bay during the last 14,000 years. Earlier work suggested a lowstand at about 60 m below present sea level at approximately 10,800 years ago, followed by the on-going transgression. To better understand coastal responses to changing sea level, we created a base map of the outer bay with a 45 km² mosaic of narrow swath bathymetry and backscatter coupled with side-scan sonar in the 40 to 80 m depth range. Based on this we gathered 120 km of seismic reflection profiles to link the subsurface with bathymetric landforms. We used these data to take 20 vibracores of potential lowstand deposits. Sand and gravel have been identified in the core-catchers and initial opened cores, indicating remnants of beach environments. Complete analysis of the cores is underway at the time of this writing. This pairing of bathymetric and surfical geology with stratigraphy allowed the interpretation of the study area, including beach, river and tidal environments, which have continued to be modified by modern processes. Placing these environments into a sequence stratigraphic framework and series of paleogeographic reconstructions have aided in our knowledge of the evolution of the bay. This understanding of past continental shelf submergence will also provide insight to the modern environment’s response to a changing sea level and consequent environmental stress.