Reconstructed ice-flow patterns and ice limits using drift pebble lithology, outer Nachvak Fiord, northern Labrador

Abstract

The geology of outer Nachvak Fiord provides an opportunity to differentiate lithologies originating in the Churchill Structural Province (central and inner fiord) from those in the sedimentary Ramah Group and the Nain Structural Province (outer fiord). As a result, the distribution of glacial erratics from the central and inner fiord depicts the former presence of regional Laurentide ice in the outer fiord, whereas the distribution of glacial deposits characterized by locally derived lithologies delimits the area of local glacier expansion.Based upon these criteria, the suggestion is made that regional ice at some time covered the deeply weathered mountain summits (900 m asl) in outer Nachvak Fiord. A later advance, confined to the fiord and valleys, deposited the highest moraines and till (180–115 m asl) recorded in the area. On the basis of geomorphic relationships, this advance is considered a discrete glacial event, separate from a later glaciation that was responsible for moraines and sediments at lower elevations (130–80 m asl). Two hypotheses are presented to explain the character of glacial features and sediments in the lower valleys.Hypothesis I requires that regional ice advanced through the study area and floated as ice shelves in the outer fiord and adjacent distributary valley. Sea level at this time was approximately 70 m higher than at present. Radiocarbon dates and amino-acid ratios from the shells in associated marine and glaciomarine sediments suggest a Middle Wisconsinan age for this event. During the Late Wisconsinan, regional Laurentide ice was restricted to the inner fiord while the sea (29–40 m above present) occupied the outer fiord area. The expansion of local cirque glaciers in upland areas may have occurred during both regional glaciations.In hypothesis II, the Middle Wisconsinan was characterized by extensive local glacier activity, depositing predominantly local material in the lower valleys, south of the fiord. Related fossiliferous sediments (same as above) provide the dating framework for this event. Late Wisconsinan regional ice advanced to the outer fiord and entered the distributary valley south of the fiord. Till deposited during this event is distinguished from the earlier local glaciation by the predominance of regional lithologies. Both Middle and Late Wisconsinan glaciations resulted in the formation of ice-shelf moraines at similar elevations. This implies similar relative sea-level responses to loading of the crust during both events, and consequently it is suggested that regional Laurentide ice had also advanced during the Middle Wisconsinan.Neither hypothesis conforms to a recently proposed Late Wisconsinan ice model for northern Labrador that requires extensive regional ice coverage in the outer fiord and on the Labrador Shelf.