Depositional Environments of Jeffreys Ledge, Western Gulf of Maine: Impacts of Glaciation, Sea-Level Fluctuations and Marine Processes Assessed Using High Resolution Multibeam Bathymetry, Subbottom Seismics, Videography, and Direct Sampling

Abstract:

Jeffreys Ledge, located ~50 km off the coast of New Hampshire, USA, is a major physiographic feature in the western Gulf of Maine (WGOM), extending over 100 km along its axis, while generally only being 5 to 10 km in width (~20 km maximum). The feature rises over 200 m from nearby basins to depths less than 50 m on the ridge surface. A major field campaign was undertaken at Jeffreys Ledge and adjacent areas in order to assess the roles of glaciation, sea-level fluctuations, and marine processes on the development of the major depositional environments. The study focused on an ~500 sq km area that extended from the shallowest reaches to the deep adjacent basins utilizing high resolution multibeam echosounder bathymetry (synthesized and gridded at the highest possible resolution), subbottom seismics, videography, and bottom sediment sampling. Due to the coarseness of the sediments composing the surface of Jeffreys Ledge (which frequently included cobbles and boulders), direct sampling was largely limited. Therefore, a descriptive seafloor classification utilizing video was developed and applied. 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 extend kms in length. These ridges, which run roughly parallel to the axis of Jeffreys Ledge, likely reflect winnowed recessional moraines. Smaller, asymmetric linear ridges with varying orientations are found on the western (landward) boundary. Although the origin of these features are unknown, they are believed to be relict glacial features as well. On the seaward (east) edge of the platform facing the open Gulf of Maine is a relatively smooth, sandy bathymetric high that likely was formed by wave processes during the last sea-level low stand. Subbottom seismics show evidence of soft sediment faulting and slumping on the flanks of the ridge and in the adjacent muddy basins. The use of high resolution bathymetry, along with the subbottom seismics, video, and direct sampling, reveal features not previously identified aiding our understanding of the development and controls on the major depositional environments.