The role of mass transport in the distribution and character of deep-ocean sediments with special reference to the North Atlantic

Abstract

The distribution and character of mass flows, particularly on the North Atlantic margin, are described. On seismic records, mass-flow characteristics range from contorted internal reflectors and a hummocky surface to acoustically transparent lens-shaped deposits. The acoustic character also partly depends on the degree of lithification of the sediment; lithified or semi-lithified material will form more irregular deposits than will unconsolidated muds even though translated over the same slopes and distances. Mass-flow structures found in cores are difficult to interpret, usually because of the small diameter of the sampler. In fact, mass-flow deposits cored with a standard piston corer are sometimes not even recognized. The primary sedimentary structures found in mass-flow deposits range from angular contacts to convoluted folds to clasts of lithified or semi-lithified sediments or sand inclusions. These structures are related to the amount of translation, the lithology of the parent material and the slope over which the flow moved. At least 40% of the continental rise of eastern North America from the New England seamounts to the Blake-Bahama Outer Ridge is covered with a veneer of mass-flow deposits including debris flows. The largest zones are: (1) south of Long Island and Cape Cod; (2) off Maryland and Virginia; (3) off North Carolina opposite Abermale Sound, and (4) off South Carolina between the Blake-Bahama Outer Ridge and the Hatteras Canyon system. In the latter area, the flows onlap the Hatteras abyssal plain to a depth of 5,400 m. On the Maryland-Virginia continental rise, a 10 × 100 km lobe has been discovered which is separated from the primary mass-flow deposit unslope. This may represent either a detached flow or a coalescence of flows triggered by the sudden loading of the rise upslope. Numerous cores taken on the continental rise contain mass-flow structures. These units are usually capped by at least 30 cm of hemipelagic sediment, and radiocarbon dates taken at the base of the hemipelagic units suggest that most of the mass-flow deposits were emplaced in the Late Wisconsin during lowered sea level. Mass-flow deposits are an important passive continental-margin sedimentary facies and are found overlapping or contiguous with turbidites even in very deep water and on very gentle slopes. Determination of the overall importance of this facies awaits detailed examination of CDP records.