High-resolution seismic transects of the upper continental slope off southeastern Canada

Abstract

Hard unstratified sediment underlies the upper continental slope off Nova Scotia and Newfoundland and terminates seaward in "till tongues" imaged in high-resolution seismic-reflection profiles. Sparse piston cores confirm that these units consist of sandy diamict. Shear strength measurements indicate that the diamict is overconsolidated. Most piston cores fail to penetrate this lithology and the abundance of iceberg scour pits, rather than linear furrows, confirn1s the strength of this sediment. Some diamict deposits are less highly consolidated and may be melt-out tills from the buoyancy line. Tills that reach beyond the shelf pass into downslope thinning wedge shaped bodies (till tongues) on the upper slope that interfinger with proglacial muddy sediment deposited from pro glacial sediment plumes. The temlination of the till tongue also passes laterally into stratified proglacial sediment, commonly with more abundant thin sands and local erosional channels. In many cases, shallow failure on the slope has retrogressed to the limit of till, thereby obscuring this transition. The limit of the till tongue is interpreted as corresponding approximately to the ice-sheet buoyancy line. Evidence for deposition of "flow tills" or glacigenic debris-flows downslope from the buoyancy line is limited to a few local areas - off Trinity Trough, off Laurentian Channel, and on the central Scotian Slope. We infer that most mass flows accelerated downslope and evolved into turbidity currents. The morphology of upper slope till varies considerably along the southeastern Canadian margin. The till limit is at maximum water depths of 250 to 700 m below present sea level, with deeper limits found seaward of ice streams. The geometry of stacked till tongues is a consequence of the volume of glacial supply and the rate of subsidence of different parts of the margin, with strongly aggradational sequences in areas of subsidence due to active salt tectonics and strongly progradational sequences in stable areas with major glacial supply. The most recent till tongue stacks appear to be retrogressive up-slope. Chronology of stacked till tongue successions were interpreted by interpolation between regional seismic stratigraphic markers and comparison with dated sediment flux to distal offshore settings. On the Scotian Slope, the most recent till-tongue dates from between 18.5 and 20.3 ka; off Laurentian Channel there was a maximal ice advance at about 16.5 ka; whereas off the eastern and northern Grand Banks the maximum ice advance was at about 25 ka. Stacked till tongue successions appear to date back to MIS 12 at 0.45 Ma.