|Résumé||(disponible en anglais seulement)|
Multibeam bathymetric data were recently collected, compiled, and processed by Fisheries and Oceans Canada (Canadian Hydrographic Service) in a proposed Marine
Protected Area near St Anns Bank. They are supplemented with video transects, photographs, existing seismic reflection profiles, and seabed samples to map geologic features and seabed texture to provide a basis for future biological habitat
classification. The mapped area ranges in water depth from 25 to 275 m, the deeper parts flanking the glacially formed Laurentian Channel and northern flank of St Anns Basin at the southern map boundary. A wide range of bedrock and overburden types,
features, and seabed textures are present. Rugged-relief outcrops (primarily Precambrian) in topographically elevated questas contrast with expansive bedrock plateaus exhibiting broadly folded Carboniferous strata. Thick stacked sequences of tills
flanking the Laurentian Channel transition to thin and patchy ground moraine in the shallow areas, generally with a gravelly iceberg scoured seabed. The till surface is locally fluted, drumlinized or exhibits delicate patterns in small and mid-size
moraines only metres high. Generally these are thin deposits with intervening bedrock or thin gravels on bedrock. Large, more continuous moraines/drumlins mark glacial margin still-stands, commonly where bedrock scarps, basin flanks or questas govern
their trace. The variety, spatial distribution, and pattern of these forms suggest superposition of more than one glacial event but the sequence is not immediately clear. Some elements may indicate a landward direction, lending some credence to the
late offshore ice cap concept. The finer, smaller elements suggest a thin, locally lobate ice margin on retreat.
Local, glacially-carved basins in the bedrock preserve a thin basal till and overlying sequence of glaciomarine and postglacial muds,
in addition to the continuous mud cover of the Laurentian Channel. These muds generally display basin center to margin textural facies transitions reflecting wave and current energy magnitude. With decreasing water depth, basinal muds give way to
sandy muds, muddy sands, muddy sands with patch gravel, then sands and gravels, and finally bedrock that is washed of most overburden. Above 60 to 70 m water depth most overburden and glacial features are largely modified or removed, indicating a
post-glacial sea-level low-stand at this depth. Rare littoral and sub-littoral elements, including beach and washover sands and gravels, are also present. In low current and basinal areas, post-glacial mud with relatively smooth surfaces drapes
glacial deposits and features. Non-deposition around iceberg pits and base of escarpments, current-parallel elongation of pockmarks, sandwave formation, and various forms of current-parallel furrowing blur the distinction between modern, Holocene and
low-stand relict (palimpsest) facies.