A REVIEW OF THE INNER SHELF PHYSIOGRAPHY OF THE WESTERN GULF OF MAINE: PRESENT UNDERSTANDING AND FUTURE WORK

The seafloor of the inner continental shelf of the western Gulf of Maine (0m-70m depth) is organized into six physiographic zones. The most extensive, the Rocky Zone (RZ), is a bathymetrically irregular region of exposed bedrock and gravel with a thin veneer of sediment, and is widespread in the 20m-40m depth range. Both washboard moraines and more isolated, larger landforms are found throughout the RZ, though no eskers are known to exist. Shelf Valleys (SV) are rock, sand and gravel floored channels cut through the bedrock in more than 40 locations, and extend from the coast into >70 m depth. With an unknown origin and possibly important ecological role, SV are regions in need of multidisciplinary study. There are too many of these 10->50 m deep features to have been cut by existing rivers; many are not oriented in the direction of ice advance and some branch and split. These valleys connect deeper Gulf of Maine Intermediate Water to modern estuaries, and may represent sediment and water conduits between shallow and deep water. Some bedrock-framed channels are filled with glacial-marine sediment. SV commonly extend landward, where they are buried by mud in Nearshore Basins (NB) and sand in Nearshore Ramps (NR). NB dominate all muddy embayments, and are distinguished by deposits of natural gas and occasionally pockmarks (pm’s). The 69 gas and 10 pm fields identified in the Gulf of Maine have been studied, but still pose questions about the source of the gas, its age, and the mechanism(s) of formation and maintenance. NR are seaward dipping extensions of beaches. NR off the Saco and Kennebec Rivers represent reworked paleodeltas formed at the time of sea-level lowstand. These are well studied for their sand resources, but need more examination of sand dynamics with respect to modern beaches. The remaining Outer Basins (OB) and Gravel Plain (GP) zones are poorly studied. The OB are muddy and extend from the sea-level lowstand at 60 m into the Gulf of Maine. The GP is located at the entrance to the Bay of Fundy, and has been eroded deeply and planed by tidal currents. Discreet grain size analyses distinguish each of these zones, and it is likely that organisms are similarly partitioned. More work will occur as these zones are eyed by the offshore wind and tidal power communities.