Abstract

Recent geological-geophysical studies of the Labrador Shelf demonstrate very dynamic bottom conditions over 70% of the shelf area and extending to depths of 230 m. A variety of sediment reworking processes that affect the sea floor are described, but the most important agent — iceberg scouring — is emphasized. A series of oriented bottom photographs (colour) taken every 14 m along a 6 km transect show a clearly defined “fresh” keel mark cut through a lag gravel and into the underlying gravelly, silty-clay substrate of Makkovik Bank. Transects taken from Saglek and Nain Banks in similar water depths ranging between 140 and 160 m also showed one “fresh” keel mark per 6 km of sea floor photographed. On Saglek Bank, a detailed synthesis of high resolution acoustic data, side-scan sonar and age determinations from piston core samples suggest complete iceberg reworking of all shelf areas shallower than 152 m within the last 10 000 years. Another important dynamic process on the shallow (< 165 m) areas of the shelf is winnowing by bottom currents. This process has produced a well developed lag gravel overlying the gravelly, silty-clay substrate. The presence of sharp-crested sand waves and stringers migrating across the sea bed in some places indicates a contemporary current regime strong enough to develop the lag surface. Photographic evidence also indicates rapid winnowing of the “fresh” scour marks. The evidence for strong current winnowing and the clearly defined keel marks suggest that “fresh” may represent days or months. The recurrence interval of modern iceberg scouring on the Labrador Shelf could be determined by measuring the rate of scour degradation by current winnowing for a given area and relating the number of partially degraded scours of a known age to the number of fresh scours within the same area. It appears that in shallow areas there is an erosional setting with only a limited supply of mobile sediment available due to the effective armouring by the lag deposit. However, dissection of the lag surface by modern grounded icebergs does expose the substrate to the winnowing process, thereby generating some additional sediment. Contemporary deposition is mainly limited to depths >300 m in the transverse saddles, the marginal trough and shelf edge. Additional, but relatively minor, processes affecting the sea bottom arc: ice-rafting, reworking by macrobenthos and slumping.