Comparison of trends of iceberg scour marks with iceberg trajectories and evidence of paleocurrent trends on Saglek Bank, northern Labrador Shelf

Abstract

High-resolution seismic and sidescan sonar surveys on Saglek Bank indicate extensive areas of iceberg-scoured sea bottom and provide data for an analysis of 6075 iceberg scour marks in 537 samples, each of 0.25 km by 2.0 km area.The general trends of iceberg scours, measured on sidescan sonographs, are comparable to iceberg trajectories, which are mostly controlled by currents. On Saglek Bank, iceberg-scour and iceberg-drift directions are generally northwest–southeast.Straight scours predominate on the shelf edge, reflecting the influence of unidirectional oceanic (Labrador) currents. Bank-top scours are commonly curved, reflecting the influence of slower moving water more subject to the effects of rotary tidal currents and storms. This difference in dispersion of scour trends between the bank top and shelf edge is due to the relative strengths of variable tidal and storm-driven currents and unidirectional oceanic currents. Pack-ice forces may also influence iceberg movement.Modern icebergs scour the shelf to about 220 m water depth, locally to 280 m, and can change draft by about 25 m. Scarps in excess of 25 m vertical relief appear to redirect icebergs offslope or along slope.Relict scours were observed in the 220–300 m depth range and were differentiated from recent scours on the basis of greater width, discordant orientation, or advanced degree of degradation. Relict scours near water depths of 220 m are superimposed by modern scours and in shallower water are completely obliterated. Those scours remaining below 220 m show evidence of paleoceanographic conditions, which, in places, are similar to present conditions such as at northeast Saglek Bank. In other places, currents flowed in a different direction from those at present, as indicated by east–west scour trends in Okak Trough. Thus paleoscour directions may be useful in the reconstruction of paleocurrent trends.