Geotechnical Properties of Continental Slope Deposits—Cape Hatteras to Hydrographer Canyon
Published:January 01, 1979
George H. Keller, Douglas N. Lambert, Richard H. Bennett, 1979. "Geotechnical Properties of Continental Slope Deposits—Cape Hatteras to Hydrographer Canyon", Geology of Continental Slopes, Larry J. Doyle, Orrin H. Pilkey
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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 increased concentrations of organic matter and fine-grained sediments. Sediment sensitivities range from 1 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 44-82% 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 play a role in the slumping of slope deposits.
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Geology of Continental Slopes
Continental slopes are the edges of continental blocks, the zones of change from continental crust to oceanic crust. They are critical links in the chain of sedimentary processes that eventually carry sediment to the true ocean basin floor. In spite of their importance, until recently continental slopes have been largely ignored when compared with research focused on other provinces of the continental margins and deep sea. Spurred by the recognition that a key portion of the margin has been overlooked and by the extension of hydrocarbon exploration into ever deeper waters, interest in continental slopes has burgeoned. In response a special symposium was convened sponsored by SEPM and AAPG at the 1978 meeting in Oklahoma City. This volume, a result of that meeting, is comprised of papers presented at that time.