BEDROCK-CONTROLLED INNER SHELF OFF WEST CENTRAL MAINE

The coast of Maine is well known as being shaped by bedrock control. Muscongus Bay, in the west-central coastal region, lies between Pemaquid Point and Monhegan Island. On land, this region was mapped as high-grade metasedimentary rocks of the Devonian-Ordovician Bucksport Fm and unnamed Ordovician units. The Late Silurian – Early Devonian granitic Waldoboro Pluto underlies inner Muscongus Bay, while Monhegan Island sits on a similar-aged late Acadian gabbroic pluton. These rocks extend well offshore. The metasedimentary units in particular define linear shoals and intervening basins striking generally SSW to NNE. This inner shelf was glaciated and partially infilled with till and glaciomarine mud during deglaciation between 24 and 14 ka. Isostatically driven relative sea-level fell to -60 m ca 12.3 ka and exposed the inner reaches, but outer Muscongus Bay remained below wave base in many places. High-resolution seismic reflection profiling and sidescan sonar surveys since 1987 have identified the seismic stratigraphy, and allowed a general mapping of the seafloor. This remote sensing was supported by coring, grab sampling, and manned submersible dives. These data were brought together in a broad-brush map of the seafloor in 1996 that identified Inner Basin, Shelf Valley, Bedrock Ridge, and Offshore Basin sectors. Recent focused mapping has allowed us to identify the extent and variability of substrate within and adjacent to these basins and ridges. Newly acquired multibeam sonar surveying equipment at UMaine and Maine DMR is starting to allow a more comprehensive mapping than was previously available. The combination of geophysical approaches may help with extension of bedrock mapping to the offshore. These new approaches will become increasingly important as new industries such as offshore wind power extend into areas previously dominated by fishing, shipping and recreational activities, and geological and geoarchaeological research continues.