Paper No. 93-3
Presentation Time: 8:45 AM
DEPOSITIONAL SYSTEMS ON THE NORTHERN MASSACHUSETTS AND NEW HAMPSHIRE INNER CONTINENTAL SHELF: USE OF HIGH RESOLUTION SEAFLOOR MAPPING TO UNDERSTAND IMPACTS OF GLACIATION, MARINE PROCESSES AND SEA-LEVEL FLUCTUATIONS
Depositional features on the continental shelf off northern Massachusetts and New Hampshire are dominated by remnant glacial features (e.g., drumlins, subaqueous deltas, moraines) that have been significantly modified by marine processes as sea level fluctuated following deglaciation. The sedimentologic, morphologic and controlling processes of many of these features have been studied over the last 15 years using high resolution multibeam echosounder bathymetry and backscatter, subbottom seismics, videography, and bottom sediment sampling. Glacial features on the inner shelf (within 20 km of shore) clearly show the impact of sea-level fluctuations and marine processes. Apparent glacial deposits (e.g., drumlins) have been eroded leaving very coarse lag deposits while supplying sand to develop wave formed features (shoals?). These coarse lag deposits are defined by higher backscatter intensity and surface rugosity index. Many of these features have positive relief standing above the seafloor, lending evidence of their formation by waves and shallow water currents. Of particular interest is a large sand body that is ~3.2 km in length, ~1.3 km in width and has a maximum relief of ~7 m (in comparison to the surrounding seafloor). Subbottom seismics display moderate to intense, parallel to subparallel top reflectors with a mounding geometry. The sand body appears to extend between two eroded drumlins. Further offshore (~50 km) the seaward boundary of the study area is formed by Jeffreys Ledge, a major physiographic feature in the western Gulf of Maine. Jeffreys Ledge rises over 150 m from nearby basins to depths less than 50 m on the ridge surface. The seafloor on Jeffreys Ledge typically ranges from sandy pebble gravel to pebble cobble gravel with boulders. Major glacial features include several boulder ridges ~ 5m in elevation that likely reflect winnowed recessional moraines. On the seaward edge of the platform facing the open Gulf of Maine is an apparent subaqueous delta. Subbottom seismics show evidence of soft sediment faulting and slumping on the flanks of the ridge and in the adjacent muddy basins. The Quaternary geology of the New Hampshire and vicinity shelf exemplifies the interplay between glacial features, sea-level fluctuations and modifications by marine processes.