Deep-Sea Channel/Submarine-Yazoo System of the Labrador Sea: A New Deep-Water Facies Model (1)

Abstract

The deep-sea channel/submarine-yazoo in the Labrador Sea consists of a major basin-draining trunk channel, the Northwest Atlantic Mid-Ocean Channel (NAMOC), with an interconnected basin-wide network of tributary and satellite channels that are linked up with slope canyons. The 3800-km-long and up to 200-m-deep NAMOC has asymmetrical levees rising up to 80 m above the adjacent flood plains and braid plains, whereas the channel floor lies up to 100 m below the plains. This limits major mid-reach channel avulsion and permits the main channel to retain a relatively stable position over long periods of time. Tributary channels are denied entrance to the main channel by the high natural levees and are forced to flow parallel to the NAMOC for hundreds of kilometers in a fashion similar to yazoo-type rivers on land. The satellite channels east of the NAMOC form a submarine braid plain. Grain-size fractionation into coarse to very coarse-grained channel-fill deposits and muddy spillover deposits on the levees is extreme for the main channel. Fractionation is min r for the shallow yazoo-type and other tributary channels on the basin plain whose levees tend to be sandy. Nine depositional facies have been differentiated on the basis of piston cores and echo character in seismic profiles: (1) deep-channel fill, (2) small-scale slide-slump facies on the walls of deep channels or canyons, (3) spillover facies on the levees of the main channel, (4) channel fill of shallow second-order channels including yazoo-type channels, (5) levee facies of second-order channels, (6) submarine braid-plain facies, (7) debris-flow deposits, (8) contourite facies, and (9) pelagic and hemipelagic sediments. In contrast to the centrifugally diverging pattern of distributary channels of classical deep-sea fan depositional systems, the submarine drainage system of the NAMOC forms a centripetally converging channel pattern. The submarine yazoos and other tributaries join a single trunk channel. The is multisourced by definition. Progressive proximal-to-distal merger of tributaries with the main trunk channel focuses the coarse-grained facies along the basin axis. This contrasts sharply with classical deep-sea fans whose sands are being diffused distally in depositional lobes. Development of deep-sea channel/submarine-yazoo systems is controlled by basin geometry and the number and intensity of sediment sources along the basin margins. Marginal seas or small ocean basins in the vicinity of waxing and waning major continental ice caps or actively rising mountain belts are ideal hosts for this kind of deep-water turbidite system.