Mass-physical and geotechnical properties of surficial sediments and dense nearbed sediment suspensions on the Amazon continental shelf

Abstract

Mass-physical and geotechnical properties of the surficial (to 3 m) sediments on the Amazon continental shelf were analyzed from 36 short (0.5 m) and longer (3m) cores obtained during a June–July 1983 cruise which coincided with the high discharge period of the Amazon River. Data included undrained shear strength (measured by a hand-held shear vane) and water content measurements, and were used to evaluate the state of consolidation of the uppermost 3 m of the continental shelf. The Amazon continental shelf, within the 30-m isobath, was divided into three groups, based upon these measurements. A southern group (Group 1), between 0 and 2°N, revealed low water contents, significant undrained shear strength, and was normally consolidated to overconsolidated. A northern group-(Group 2), extending from 2 to 3.5°N, showed significantly greater water content, low undrained shear strengths, and was generally normally consolidated. A third group, possessing properties intermediate between those of Groups 1 and 2 is located north of 4°N. Mass-physical and geotechnical properties of Group 1 and Group 2 sediments correspond very well with observed sediment accumulation rates. Viscous properties of dense nearbed sediment suspensions which occur in this environment indicate that they exhibit non-Newtonian flow behaviour, varying from shear thinning (pseudoplastic) at densities <1.20 Mg m −3 to shear thickening (dilatant) at greater densities. Yield stress and apparent viscosity increases linearly with density. These characteristics interact to inhibit resuspension where dense suspensions overlie the seabed, in spite of the energetic continental shelf environment. Such inhibition would be most effective in muds deposited during slack water intervals (associated with tidal currents). Nearbed sediment transport by mass movement of the dense suspension may be an effective transport process, as observed in other macrotidal environments.