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

Abstract

The continental slope off the northeastern United States commonly displays gradients ranging from 3 to 10 is heavily dissected by submarine canyons and valleys and is an area of considerable slumping activity A study of the geotechinal properties sediment texture shear strength water content wet bulk density porosity and Atterberg Limits of 73 sediment cores from 21 transects across the continental slope from Cape Hatteras to Hydrographer Canyon provides insight into the general distribution and variation of these properties within the near surface deposits of this province of the seafloor Although a general gradation in sediment texture from coarse to fine prevails in a down slope direction all along the continental slope fine grained sediments silty clay appear to comprise the predominant sediment type along the slope particularly within the central portion of the Middle Atlantic Bight This depositional pattern appears to account for the occurrence of generally higher water contents and porosities as well as the lower wet bulk densities found in the slope deposits of the Middle Atlantic Bight Relatively coarse grained sediments of low water content and porosity and high bulk density make up the slope deposits to the north of Block Canyon as well as in the general area of Cape Hatteras Higher values of shear strength 7 to 14 kPa 1 2 psi are commonly found in the lower mid to lower slope deposits except in the vicinity of submarine canyons where lower values 2 to 4 kPa 0 3 0 6 psi appear to be related to a combination of increaspd concentrations of organic matter and fine grained sediments Sediment sensitivities range from I to 12 with a mean of 3 giving the indication that these deposits may be slightly quick in places but they are predominantly classed as medium sensitive Porosities vary from 4482 with the higher values occurring along the lower slope 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 mid and lower slope sediments indicates that they vary little from those of abyssal plain deposits which are classed as inorganic clays of low medium and high plasticity An exception is found in the central part of the Middle Atlantic Bight where a large proportion of the sediments are classified as organic clays of medium to high plasticity and micaceous Distribution of the mean values of the various geotechnical properties in the near surface 1 290 cm deposits of the slope indicates that there is a general increase in water content liquid limit plastic limit clay content and porosity from off New England towards Cape Hatteras Analyses of slope deposits using the infinite slope analysis method to determine the effects of overburden pressure on their stability revealed no indication of instability Factors other than overburden however undoubtedly playa role in the slumping of slope deposits