Abstract: Geotechnical Properties of Continental Slope Deposits, Cape Hatteras to Hydrographer Canyon

George H. Keller, Douglas N. Lambert, Richard H. Bennett

Study of the geotechnical properties (sediment, texture, cohesion, water content, unit weight, porosity, and Atterberg Limits) of 73 sediment cores from 21 continental slope transects between Cape Hatteras and Hydrographer Canyon provides insight into the general distribution and variation of these properties within the surficial deposits of this province of the seafloor. The continental slope off the northeast United States commonly displays slopes of 4 to 8° and is heavily dissected by submarine canyons and gullies.

Although a general gradation in sediment texture from coarse to fine prevails in a downslope direction all along the continental slope, fine-grained sediments (silty clay) appear to comprise the predominant sediment type along the slope within the central part of the Mid-Atlantic Bight. Relatively coarse material comprises the slope deposits north of Block Canyon as well as in the general area of Cape Hatteras. The overall depositional pattern appears to account for the generally higher water contents and porosities as well as lower unit weights (bulk densities) in the slope deposits of the central part of the bight. Higher values of cohesion (7 to 15 kPa) are common in the middle to lower slope deposits except in the vicinity of submarine canyons where lower values (2 to 4 kPa) appear to be related to increased concentrations of organic carbon. Sediment sensitivities range from 1 to 12 with a mean of 3.5, giving the indication that these deposits may be "slightly quick" in places, but predominan ly they are classed as a "medium sensitive." Porosities range from 44 to 82% with the higher values occurring in the lower slope deposits. The mean value of 71% for these sediments is slightly higher than that reported for the hemipelagic sediments of the North Atlantic. An analysis of the plasticity characteristics of the slope sediments indicates that they vary little from those of the abyssal plain deposits which are classed as inorganic clays of low, medium, and high plasticity except in the central part of the bight where much of the material is classified as micaceous organic clays of medium to high plasticity.

Distribution of the mean values of the various geotechnical properties in the surface (1 to 290 cm) deposits of the slope indicates that there is a general increase in water content, liquid limit, plastic limit, sensitivity, clay content, and porosity from off New England toward Cape Hatteras. On the other hand, unit weight, sand content, and silt content tend to decrease from north to south.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma