GEOLOGIC FRAMEWORK OF THE MASSACHUSETTS INNER CONTINENTAL SHELF: PRELIMINARY RESULTS OF SEAFLOOR MAPPING

A seafloor-mapping effort that began in 2003 is investigating the geologic framework of approximately 900 km² of the Massachusetts inner continental shelf. Data acquired with bathymetric sonars, sidescan sonar, chirp seismic-reflection profiling, sediment sampling and bottom photography are being used to produce high-resolution geologic maps. Water depths range from 4-90 m in the study area, which generally lies inside the 3-mile limit of State waters and extends along the coast from the N.H. border to western Cape Cod Bay. The main objective of the study is to understand the impact of sea-level change and sediment supply on coastal evolution in the region.

Late Quaternary glaciation, deglaciation, and sea-level change have strongly modified the coast and inner shelf. High-relief bedrock is overlain by Pleistocene glacial and deglacial sediments that have been reworked by shoreline regression/transgression over the last 14-15 ka. Glacial isostasy dominates relative sea level, which fell to a lowstand of ~50 m below present at 12 ka B.P., and rose at varying rates to its present position. The timing and magnitude of these fluctuations are poorly constrained, but recent mapping has identified submerged deltaic and estuarine deposits that potentially contain sea-level records. A large sandy paleodelta formed seaward of the Merrimack River mouth when abundant sediment was delivered to the lowstand shoreline (Oldale et al., 1983). As the Holocene transgression progressed, shoreface erosion exhumed underlying glacial, fluvial, and littoral units and exposed a variety of textural facies on the upper surface of the paleodelta. Reworking of this relict deposit probably supplies sand to Plum Island, the largest barrier island in the Gulf of Maine, but the relative contribution of sediment from the shelf vs. fluvial sources is unknown. Although no major rivers enter Cape Cod Bay, subbottom profiling has revealed a large paleochannel trending seaward from Plymouth Bay that probably represents the outlet of glacial Lake Taunton. Thin lenses of estuarine sediment are preserved offshore of Scituate in depths of 25-30 m; these deposits unconformably overlie glacial-marine sediment and exhibit low-backscatter where exposed on the seafloor. Additional geophysical surveys, sampling, and coring are planned for 2007.