Abstract

Six sedimentary facies have been identified from vibracores of sand ridges and medium- to large-scale bedforms on the tide-dominated high-energy continental shelf of western Canada. In northern Hecate Strait, a nearshore sand ridge field in less than 20 m of water depth displays sharply contrasting sequences between cores that are on, rather than between, ridges. A Crossbedded and Graded Sand and Shell Facies is likely a product of deposition from wave- and current-generated bedform migration over the crest and flanks of sand ridges. Between and adjacent to the sand ridges, a Laminated Sand Facies constitutes the majority of cored sequences. This facies is characterized by inclined parallel laminations, wavy bedding, clay drapes and mixed, low diversity, trace fossil assemblages that suggest deposition by tidal processes. A Graded Sand and Gravel Facies in ridge cores is interpreted as a product of storm-generated density currents.

Sand ridges from central Hecate Strait and Laskeek Bank in 65 – 105 m of water, contain facies of a similar sedimentary nature and origin to those in the north. However, these sand ridges are capped by a Burrowed Sand and Interbedded Sand and Silt Facies that is in sharp or erosional contact with underlying deposits. The presence of glauconite, mixed textures and lithologies, silicified polychaete worm tubes and density of Thalassanoides burrow traces, has led to the interpretation of these deposits as relict shelf sands. The sand ridges in these areas, therefore, are not in equilibrium with the present day hydrodynamic regime.