ABSTRACT: Holocene geomorphological evolution of southern Nastapoka Sound, Eastern Hudson Bay (Northern Quebec).

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Abstract

Geophysical surveys conducted in southern Nastapoka Sound (eastern Hudson Bay) provide new information on the local bathymetry and seafloordeposits in order to reconstruct the Holocene evolution of this coastal zone. This study allows us to locate the major sedimentary basin and sedimenttransport pathways, to describe the complete Holocene stratigraphic sequence and to provide a new dataset for mapping marine habitats. More than 350 km of survey lines were collected on two small boats using the Hypack® software at an interval of 200 m for north-south transects and 500 m for east-west transects. The surveys were carried out using a single beam echosounder for bathymetry (200 kHz) and a chirp-sonar for subbottom acoustic profiles (12 kHz). Side-scan sonar data (100 & 500 kHz) provided information of sea floor morphology and surficial deposits. Surface sediments were also sampled using a grab sampler. Bathymetric data (data est pluriel en Anglais) show that the terrestrial basaltic and metasedimentary cuesta ridges also occur offshore in a parallel-to-coast direction, forming deep sediment filled basins (15-20 m) enclosed between bedrock ridges (20-30 m). Many bedrock escarpments are also visible on the cuesta fronts and sides. The occurrence of this high submarine relief separates two sedimentary basins where glaciomarine (Unit 1), paraglacial (Unit 2) and postglacial sediments (Unit 3) have been trapped, filling the bedrock depressions. Submarine slope failures have been observed in different sectors of the study area and include numerous submarine gravity flows in Unit 1 and submarine slides in Unit 3. As reported in previous studies in eastern Hudson Bay, submarine mass movements appear to have acted as a major reworking process during the Holocene. Earthquakes might have played an important role for triggering mass-movements during the rapid postglacial uplift, although no record of past seismic activity exists for the area. However, forced regression and reducing accommodation space might also be responsible for generating these mass-movements by enhancing bottom wave erosion of unconsolidated sediments deposited very rapidly during deglaciation. Local submarine morphology (erosion/sedimentation features) reveals strong south-north currents occurring in both basins located between the coast and the island. Along with currents, the relatively low modern sedimentation rate seems to allow important bioturbation on the seafloor as observed in surface and sediment cores.

Bibliography of Canadian Geomorphology