CGRG Bibliography of Canadian Geomorphology
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Author : Gray, J.T.; Gosse, J.C.; Kinnard, C.; and Marquette, G.
Date : 2003.
Title : A re-interpretation of weathering zones and glacial history in Nakvak Valley, Torngat Mountains using exposure dating.
Publication : Joint Annual Meeting of the Canadian Quaternary Association and the Canadian Geomorphology Research Group. Halifax, Nova Scotia, June 8-12, 2003.
Issue :
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Abstract
Geomorphological mapping and terrestrial cosmic nuclide (TCN) exposure dating in the Nakvak Valley has prompted a re-interpretation of the concept of altitudinally differentiated weathering zones for the Torngat Mountains, introduced by Ives (1958). A parallel sequence of sharp-crested lateral moraines form an upper boundary for the lowest, or Saglek zone; attributable to outlet glaciers flowing eastwards to the Labrador coast. 10Be exposure ages on the most distal moraine segments suggest an initial phase of glacier stabilization in the main valley between 14.6 and 15.7 ka, and a second stabilization or re-advance, responsible for damming Square Lake towards 10.7 ka. Paired 10Be and 26Al ages of 11.4 and 11.7 ka, respectively, for a glacially molded bedrock surface close to the Saglek moraine limit, also date initial thinning of the outlet glacier, which finally liberated the col at the head of Nakvak Valley towards 9 ka. Above the Saglek zone on the north side of Nakvak Valley, is a narrow, frost shattered bedrock surface, interpreted by Ives (1958) as an incipiently weathered zone (Koroksoak zone) eroded during an earlier glaciation. This zone is, however, very discontinuous, and is closely associated with meltwater channels. It is, in fact, a zone of meltwater washing, with exposure of isolated large boulders from a till cover, as late as ca 10 ka. According to Ives' scenario, there is an abrupt upward transition to a thick felsenmeer covered surface with occasional erratics (his Komaktorvik zone). Perched boulder ages allow us to re-interpret this transition as a Late Wisconsinan meltwater washing limit, and the thick sediment cover above as being at least partly glacial till deposited during the same glaciation. Komaktorvik and Koroksoak weathering zones are no longer suitable terms for the wide, altitudinal swath of glacially molded bedrock and glacial till, between the Saglek zone and the high summit felsenmeers. Three 10Be ages of 71, 87 and 94 ka from a large incipiently frost shattered bedrock outcrop in a 1170 m high col, may indicate that high level glacial scouring of bedrock by basally erosive ice, took place during a pre-Wisconsinan glaciation. Summit surfaces above 1200 m display angular felsenmeer, tor-like outcrops and chemically advanced weathering of soils. 10Be ages for isolated bedrock outcrops range from 88 - 157 ka, and these surfaces have therefore not been subjected to glacial erosive processes since the Sangamon interglacial period. At least one period of burial beneath an ice cover is suggested, however, by differential 10Be and 26Al ages. Occasional erratics indicate that frozen-based ice was eventually overridden by basally mobile ice. Only the two highest summits, above 1350 m were erratic free, and they also display the oldest exposure ages: 156 and 157 ka for duplicate samples from one site and 123 ka for the second. They may be considered representative of Ives' fourth and highest zone - the Torngat surface, not necessarily nunataks, but overlain by thin frozen-based ice caps.
Bibliography of Canadian Geomorphology