ABSTRACT: Cosmogenic exposure ages of boulders indicate extensive Late Wisconsinan ice on the Aston Lowlands, Baffin Island.

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Abstract

Aston Delta (~69o54'N, 67o34'W, >25 sq km), a raised glaciomarine feature ~80 m asl, lies on the extensive forelands between Clyde Inlet and McBeth Fiord on the NE coast of Baffin Island. In situ mollusks that are radiocarbon dead (>54,000 yr BP, Linken, 1966) led to the interpretation of the delta as an early Wisconsinan or pre-Wisconsinan ice-marginal feature. Hence, the delta became the cornerstone of the minimal model for Laurentide ice extent in the eastern Canadian Arctic during the Last Glacial Maximum (LGM). We used cosmogenic nuclides to date eight boulders and one cobble lying on the surface of Aston Delta; all but one dated ~26 to ~13.5 10Be kyr (one outlier at 95 10Be kyr includes isotope inheritance). Two boulders and one cobble that were sampled beyond the delta lip at 37 to 47 m asl date ~14 to ~17 10Be kyr. We also collected three felsenmeer samples and one cobble from a bedrock knoll at 125 m asl near the delta apex; the cobble dated ~35 10Be kyr, whereas the felsenmeer (bedrock) samples ranged from ~33 to ~140 10Be kyr, suggesting that ice was not erosive enough to remove inherited isotopes from previous exposure. Finally, three boulders, three cobbles, and one felsenmeer sample were collected from 340 to 465 m asl on summits that rise above the Aston Lowlands; five of the boulder and cobble samples dated ~10 to ~23 10Be kyr, whereas one boulder dated ~100 10Be kyr and the felsenmeer sample dated ~133 10Be kyr. We interpret some of the older LGM 10Be ages to reflect pre-LGM deposition and exposure followed by LGM burial under cold-based ice. Re-dating of LNken's in situ shells and related detrital organics yielded >48,800 and >47,800 AMS 14C ages, respectively (Coulthard et al., 2003). We propose that the Aston Lowlands, and perhaps other forelands along the NE Baffin Island coast, was glaciated by non-erosive cold-based ice during the LGM, leaving the underlying in situ shells and pre-LGM delta undisturbed. Taken together, these 23 cosmogenic nuclide ages suggest that LGM ice extended across the Aston Lowlands, and was a minimum thickness of almost 500 meters. This scenario is compatible with recent exposure dating evidence for cold-based ice on the Clyde Foreland to the north and vigorous warm-based ice streams that occupied Baffin fiords during the Late Wisconsinan (Briner et al., 2003).

Bibliography of Canadian Geomorphology