ABSTRACT: Past activity of the NW Laurentide Ice Sheet on southern Melville Island, NWT.

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Abstract

Past advances of the NW Laurentide Ice Sheet have long been recognized across the western Canadian Arctic Archipelago. In this sector, Laurentide ice coalesced with local island-based ice caps that constituted the southwest part of the Innuitian Ice Sheet. The history and dynamics of these different ice masses remains to be clarified and is relevant to the nature of high latitude environmental change, landscape evolution and the sedimentary history of the adjacent Arctic Ocean Basin. During 2004, we extended previous surveys of southern Melville Island to the island’s SW extremity. Surveys included revisiting the three Laurentide till sheets nested along the south coast of Dundas Peninsula. The oldest till sheet (Dundas Till) extends farthest inland, beyond what have been interpreted as successively younger till sheets shoreward (Bolduc Till, Winter Harbour Till). At three localities, Dundas Till was sampled for: AMS dating of ice-transported shells, as well as for geochemical analyses, whole rock (Pb-zircon) and cosmogenic dating of mainland (Shield) erratics. Dundas Till was deposited by grounded Laurentide ice that advanced northward across Dundas Peninsula where it coalesced with local Melville Island ice advancing out Liddon Gulf. Twenty km west of Liddon Gulf, a meltwater channel containing granite erratics records Laurentide onlap to at least 230 m asl (Cape Victoria). Given water depths in M’Clure Strait nearby (>200-<500m today), and accounting for glacioisostatic loading during Laurentide occupation, this ice (>430->730 m thick) would not have floated, placing the grounding line at this time well west of any shown previously. Because M’Cure Strait shallows westward, this trunk ice may have reached Prince Patrick Island (~100 km farther west) where granite erratics are also reported. The age of emplacement for the Cape Victoria erratics is unknown, but the only raised marine deposits observed along the length of M’Clure Strait date in the 11-12 ka BP range, suggesting Late Wisconsinan deglaciation. In outer Liddon Gulf (south shore) meltwater channels record Laurentide ice retreating southward on Dundas Peninsula, separating from trunk ice retreating into Liddon Gulf. All ice-contact deltas fed by these channels date 11-12 ka BP. During this retreat, Laurentide ice floated in M’Clure Strait depositing Liddon Till (former ice shelf deposit). Arrival of marine molluscs by 12 ka BP demonstrates that their entry was predicated by the removal of glacier ice and not by the later reopening of Bering Strait (10.3 ka BP). Bolduc Till was previously attributed to an ice shelf that advanced onshore sometime after deposition of Dundas Till. We now have ten AMS dates (25-30 ka BP) obtained on individual shell fragments collected from Bolduc Till. Two shell dates from Dundas Till are 30 and 35 ka BP. We reinterpret the Bolduc Till to record the initial arrival of a Laurentide ice shelf that transported Stage 3 marine sediments inland (~15 km) after 25 ka BP. This would make Bolduc Till the basal facies of Dundas Till that records overstepping by grounded Laurentide ice that carried far-travelled (mainland) erratics and sparse shell across the peninsula. Widespread marine deposits, as old as 12 ka BP, overly Bolduc Till and record initial deglaciation similar to Laurentide retreat from M’Clure Strait. Laurentide retreat continued back onto northern Victoria Island and was followed by the catastrophic advance of the Viscount Melville Sound Ice Shelf after 10.4 ka BP. This advance deposited Winter Harbour Till over Bolduc Till and its associated postglacial marine sediments (10.4-12 ka BP). We report two concordant dates of 10.4 ka BP obtained on marine shells in silt underlying Winter Harbour Till (Cape Clarendon). Deglaciation from the most inland moraines associated with Winter Harbour Till dates close to 9.6 ka BP. The Viscount Melville Sound Ice Shelf, fed by prominent ice streams to the south, appears to be a Younger Dryas event and likely served to exhaust the M’Clintock Ice Divide, triggering its final collapse with rapid entry of the sea to the mainland coast as early as 9.2 ka BP.

Bibliography of Canadian Geomorphology