ABSTRACT: Morphological evidence for late glacial invasion of the western flanks of the Northern Labrador Peninsula by Hudson Strait and Ungava Bay Ice and 14C dating of the Sheppard Moraines.

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Abstract

Striae and fluting forms on the western flanks of the Torngat Mountains between the Koroksoak estuary and the northern tip of Labrador show a contrast between a dominant northwesterly ice flow pattern from the Torngat Mountains into Ungava Bay for the southern sector, and a ENE to ESE flow pattern for the northern sector, north of Sheppard Lake (Gray et al., 1996; Gray, in press). This contrast in flow patterns coincides with a similar contrast in erratic distribution and carbonate rich drift - the southern zone displaying only local Torngat lithologies and nocarbonates in till, the northern zone showing limestone erratics derived from the floor of Ungava Bay/Hudson Strait, and occasional ironstone conglomeratic clasts transported from the Fenimore iron formations on the western and southwestern coasts of Ungava Bay. Only the northernmost part of the Labrador Peninsula, with relief up to 500 m was overrun during the Last Glacial Maximum by continental ice from the west and southwest - the southern sector harbouring important local centres of ice dispersion into both Ungava Bay and the Labrador Sea. Coinciding with this northerly zone of easterly flow is a linearly contiguous end moraine, outwash plain and outwash delta complex, mapped and described by Løken (1962, 1964) as the Sheppard moraines. This feature runs NNE - SSW along the peninsula for a distance of 100 kmbetween Killinek Island and Sheppard Lake, at an altitude of 250 - 350 m. It represents the latest recognisable eastern margin of the continental ice sheet in northern Labrador. There has been much uncertainty as to whether this moraine complex represents a still-stand during westward retreat of the front of the continental ice sheet, or a renewed and very late thrust eastward of ice in eastern Hudson Strait. The unusually high carbonate values obtained from the moraine crest, and the existence of moraine lobes cutting across pre-existing WNW-ESE fluting features near Ikudlayiuk Lake, along with burial of marine sediments beneath glacial outwash at the latter locality suggest a readvance, however. At the Ikudlayiuk Lake site, shell fragments principally of Balanus balanus and Balanus crenatus spp, were collected from stratified silty clays, beneath 8 m of deltaic sands. The stratigraphic context suggests a sparse mollusc population in an infra-littoral situation, near the head of a Labrador coastal fjord, into which a large proglacial delta sequence of sands was constructed by meltwaters flowing east from a prominent terminal moraine, 2 km to the west. Kettle-hole lake filled depressions were noted on the highest level of the delta at 34m, which is considered to represent the postglacial marine limit at this locality, and also on a lower level of 15m, indicating the former presence of blocks of buried glacial ice detached and floated forwardduring the early stages of the outwash delta construction. Ages of 8.3, 8.4 and 8.7 ka, with low error ranges, were obtained for 3 of 4 mollusc fragments at the University of Arizona AMSfacility. A fourth sample containing only .05 mg carbon was dated at 9.4 ka with a quoted error range of 0.4 ka. A reasonable inference from these dates is that they immediately pre-date the Sheppard re-advance or ice front position. Thus ice moving east onto the northern Labrador coast across the limestone basin of northern Ungava Bay and/or Hudson Strait subsequent to 8.7 ka invaded the tip of the Labrador Peninsula to a minimum elevation of ca 350 m. That this must have been a short-lived readvance is indicated by the following morphological and chronological data. No retreat phase moraines or outwash deposits were noted between the Sheppard moraine and the west coast of the Labrador Peninsula, and 14C ages of molluscs from the base of amarine core in eastern Ungava Bay (Manley and Jennings, 1996), and from onshore marine deposits on NE Akpatok Island (Gray et al., 1993) indicate re-establishment of marine conditions towards 8.2 ka. South of the sector characterised by the Sheppard moraine complex, glacial lakes dammed up in NW trending valleys may also be associated with the last readvance phase of Ungava Bay ice, after retreat up-valley of local glaciers. A complex of glacio-lacustrine shorelines, at elevations of 200 - 300m, with northerly draining spillways have been identified for the valleys between Keglo Bay and Abloviak Fjord. A well-defined glacio-lacustrine terrace at 190 - 208 m in the Abloviak and Vent de l'Ouest valleys, first identified by Ives (1957), mark the shoreline of Glacial Lake Abloviak which drained eastward by a low pass to the Labrador coast. Its western ice dam margin is characterised by locally developed De Geer moraines. As yet it has not been possible to clearly establish the direction of the ice flow causing the ice dam in SE Ungava Bay, but the evidence for N to NNE trending ice flow into Ungava Bay from Central Qu/bec-Labrador, coupled with the absence of limestone erratics or carbonates in till, suggest that the glacial lakes were associated with a northerly flow, impinging on the coast, between the Korok estuary and Abloviak Fjord. This ice may or may not have converged with the easterly flow responsible for the Sheppard moraines. Drainage of the glacial lakes can be chronologically associated with the earliest postglacial marine conditions in southeastern Ungava Bay, prior to 7.4 ka (Allard et al., 1989) for the George River sector, and probably prior to the above mentioned basal marine core age of 8.2 ka in eastern Ungava Bay.

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