Petrographic evidence from the eastern Canadian margin of shelf-crossing glaciations

Abstract

Listen

Translate article

It is difficult to evaluate when and where glaciers extended across the outer continental shelf off eastern Canada. The outer shelf and upper slope are commonly hard bottomed or have a great thickness of post-glacial sediment, in either case making direct sampling and dating of ice-margin sediment impossible. The timing and extent of glacier ice is important for understanding continental slope sedimentation and ice sheet dynamics. We have therefore determined the lithologic components of the 1–4 mm granule fraction of over 100 well-dated samples from 40 cores from the more distal parts of the deep-water margin off Nova Scotia and Newfoundland. This fraction is demonstrably ice-rafted in these cores and geographic variation in component results from input of icebergs at different calving margins. The samples show that Heinrich layers have a distinctive granule fraction that shows some areal variation as a result of dilution by sources other than Hudson Strait. Heinrich event 2 (ca. 21 ka) is widespread as far southwest as the West Scotian Slope, but by the last glacial maximum (LGM), numerous discrete sources of ice-rafted granules are recognised, implying that calving ice margins were present in several places off Newfoundland and Nova Scotia, including off Banquereau. A distinctive purplish LGM gravelly mud is recognised on the eastern Newfoundland margin, derived from a calving margin on the NE Newfoundland Shelf. At the time of Heinrich event 1 (ca. 14 ka), there was a calving ice outlet somewhere on the Grand Banks and another in Halibut Channel. Retreat of ice from the Gulf of St. Lawrence can be tracked by changing granule petrology from the LGM to 13 ka. These data suggest that final ice retreat from many parts of the outer shelf was later than is shown in many recent ice-margin reconstructions.