15 - Large Submarine Sand Waves and Gravel Lag Substrates on Georges Bank Off Atlantic Canada

Abstract

Georges Bank is a large, shallow, continental shelf feature offshore of New England and Atlantic Canada. The bank is mantled with a veneer of glacial debris transported during the late Pleistocene from continental areas lying to the north. These sediments were reworked by marine processes during postglacial sea-level transgression and continue to be modified by the modern oceanic regime. The surficial geology of the Canadian portion of the bank is a widespread gravel lag overlain in places by well-sorted sand occurring as bedforms. Georges Bank is dominated by energetic tidal currents associated with the M2 semidiurnal tidal resonance in the Bay of Fundy–Gulf of Maine system. The major axis of the semidiurnal tidal current ellipse is oriented northwest–southeast across the bank, and the amplitude of this current at the seabed increases from <10 cm/s along the deeper southern flank to >100 cm/s on the relatively shallow northern edge of the bank. The most widespread bedforms are large, mobile, asymmetrical sand waves up to 19 m in height formed through sediment transport by strong tidal-driven and possibly storm-driven currents. Well-defined curvilinear bedform crests up to 15 km long form a complex bifurcating pattern having an overall southwest–northeast strike, which is normal to the direction of the major axis of the semidiurnal tidal current ellipse. Minor fields of immobile, symmetrical sand waves are situated in bathymetric lows. Rare mobile, asymmetrical barchan dunes are lying on the gravel lag in areas of low sand supply. On Georges Bank, the management of resources and habitats requires an understanding of the distribution of substrate types, their surface dynamics and susceptibility to movement, and their associated fauna.