SEDIMENTARY ENVIRONMENTS AND PROCESSES OF CROSS RIP CHANNEL, OFFSHORE SOUTHEASTERN MASSACHUSETTS

Gridded multibeam bathymetry covers approximately 10.4 sq. km of seafloor in the vicinity of Cross Rip Channel in Nantucket Sound, offshore southeastern Massachusetts. These acoustic data, originally collected for charting purposes, and the sediment samples and bottom photography subsequently collected to verify them: 1) show seabed topography and composition, 2) provide information on sediment transport pathways and benthic habitat, and 3) are part of an expanding series of cooperative studies by the USGS, NOAA, and MA CZM, providing a fundamental framework for research and resource-management activities (e.g., windfarms and cables). The Holocene section is thin or absent in Cross Rip Channel and in the study area’s southeastern part, where high-energy processes of erosion and nondeposition prevail. In the channel, boulders are present on the exposed Pleistocene surface, scour depressions occur around obstructions, and pavements of gravelly sediment and dense shell beds armor the seafloor. The shell beds are ecologically important because they form seafloor habitats of rough, relatively hard substrates that contribute to the overall benthic compositional complexity. Bottom photography shows that infauna construct burrows in the shell deposits, juvenile finfish shelter between shells, and sessile fauna and flora attach themselves to the hard substrate provided by shells. Small-scale erosional bedforms present on relatively flat areas in the study area’s southeastern part give the seafloor a current-swept appearance. Sand waves and megaripples, suggestive of processes associated with coarse bedload transport, cover the seafloor in more than 70% of the study area. Transverse morphologies dominate crests and upper flanks of Horseshoe, Cross Rip, and Halfmoon Shoals, where sand supply is abundant; barchanoid morphologies dominate along the lower flanks of shoals, where sand supply is limited. Megaripples are prevalent throughout the study area’s southeastern part and, along with current ripples, are also present on the stoss slopes of the sand waves, indicating active transport. Sand-wave and scour-mark asymmetry shows that net sediment transport is to the east and flood-tide dominated throughout most of the study area.