Sedimentary processes on the continental slope off New England
MacIlvaine, Joseph Chad
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LocationCape Cod, MA
KeywordContinental slope; Sedimentary processes; Quaternary sedimentation; Gosnold (Ship : 1962-1973) Cruise 177; Gosnold (Ship : 1962-1973) Cruise 183; Gosnold (Ship : 1962-1973) Cruise 189; Gosnold (Ship : 1962-1973) Cruise 191; Atlantis II (Ship : 1963-) Cruise AII72; Alcoa Seaprobe (Ship) Cruise 1; Alcoa Seaprobe (Ship) Cruise 2; Lulu (Ship) Cruise 66
A detailed study of a small (5000 km2) area of the continental slope south of Cape Cod, Massachusetts, was conducted. Bathymetry, 3.5 kHz profiles, seismic profiles, suspended sediment analysis, bottom photographs, television laboratory flume experiments, studies of surface sediments, and piston cores were combined to form the basis for understanding the sedimentary processes which control transportation, deposition, and erosion of sediments, and the geomorphic features of the continental slope. Gravitational processes (slumping, creep, and turbidity currents) are apparently the most effective erosional processes on the continental slope. Massive large-scale failure occurs where the slope steepens from a gradient of 1.50 to 7.60, producing scarps hundreds of meters in height. Upslope propagation of slumping on the upper continental slope has formed steep-sided gullies with layers of disturbed residual material and humocky floors. On the steep lower continental slope small slump scars on the order of 100 m in horizontal extent and several meters high are common. Material removed by slumping is emplaced at the foot of the continental slope as intact and disrupted blocks 1 to 100 m thick. Turbidity currents generated by slumping have apparently eroded V-shaped gullies in the lower continental slope. Bottom currents are most influential at the shelfbreak, where they produce sorting of surface sediments and suspension of fine material by erosion of the bottom. Internal waves may be a significant source of high velocity bottom currents and turbulence. Laboratory flume experiments and observation of the bottom indicate that the sediments of most of the continental slope are not normally affected by bottom currents. Sediments at the foot of the continental slope on the upper continental rise are reworked by bottom currents. Biological activity causes both roughening and smoothing of the sediment surface. Tracking of the bottom produces small-scale roughness, and reworking of the bottom reduces larger roughness elements. Biological production of fibrous structures helps render the sediment surface extremely resistant to erosion by bottom currents. Biological eros ion of rock outcrops produces rubble slopes locally at the bases of scarps. Conditions have varied markedly during the Pleistocene and Holocene. During glacial periods rapid deposition increased the activity of gravitational processes, while during interglacial periods of slow deposition biological and hydrodynamic processes became relatively more important.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August, 1973
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