LOCAL RECORDS OF FORMER ICE-SHEET MARGINS: GEOMORPHOLOGICAL DYNAMICS AND SEA-LEVEL EVOLUTION IN THE NAIN ARCHIPELAGO (LABRADOR, ATLANTIC CANADA)

Nain archipelago (Labrador, Canada) is located along the eastern edge of the former Laurentide Ice Sheet (LIS). The geomorphological dynamics during deglaciation stand at the confluence of eustatic, isostatic and topographic driving factors. Following the post-glacial marine transgression, glacio-isostatic adjustment led to the formation of successive raised marine beaches and deltas in addition to other formations.

Our approach consists in developing a multiscale analysis that includes: 1) geomorphological mapping and morphometrical analysis based on satellite imagery derived DEM and bathymetry to document the history of the archipelago in larger context and 2) local studies, to compare and relate the dynamics of multiple independent toposequences, based on sequential stratigraphy and cross-dating methods (cosmonuclides, OSL, 14C) along two E/W and N/S transects.

Several types of submarine and inland moraines (De Geer, ribbed, frontal moraine), glacial lineation, and quaternary surficial deposits have been mapped. Submarine glacial valleys in outer structural troughs associated with a palaeo-grounding line (-200 m) and a concomitant lateral extension limit of till cover on shoal have also been identified. On South Aulatsivik Island, a stratigraphic section revealed a deglacial sequence of glacio-marine rythmites at the outlet of a kettle-like lake embedded between two former grounding lines, deposited between 9438 and 9244 cal. BP at +35 m, while the marine limit was observed uphill at +76 m.

The orientation of the lineation and the moraine suggests diverging glacial flows at the outlet of the Fraser River fjord during the glacial maximum and a possible topographic obstacle on Dog Island. The presence of important frontal moraines parallel from the continent suggests a stillstand during deglaciation. The availability of glacio-fluvial sedimentary stocks, the persistence of stagnant-ice masses and delayed deglaciation seems to have controlled the local configuration of raised beaches, along with shore-specific dynamics. The reconstruction of former shorelines and sea-level evolution will ultimately allow us to discuss the potential role of post-glacial tilting and neotectonics and will form the basis of a geoarchaeological framework for the analysis of prehistoric settlement patterns.