ABSTRACT: Late Wisconsinan glaciation of Prince Patrick Island, NWT: integratinggeomorphological mapping and radiocarbon, U-Pb zircon and terrestrialcosmogenic nuclide dating.

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Abstract

During the Late Wisconsinan, Prince Patrick Island (PPI) was glaciated by a local island based ice cap or the Laurentide Ice Sheet (LIS). Evidence of Laurentide glaciation reported by previous investigators includes far travelledgranite erratics from mainland Canada up to 190 m asl. Observations by the authors during 2004 and 2006 indicate that mainland erratics occur up to the summit of the island (280 m asl). Furthermore, the distribution of erratics coincides with the distribution of granite-bearing diamicton (=1 m thick, probably a till) which caps the late Tertiary Beaufort Frm. (Pliocene, underlying 2/3 of PPI), and bedrock plateau surfaces correlative to it. However, in dissected bedrock regions, both granite erratics and diamicton are rare but never absent, suggesting that the far-travelled clasts were deposited before the incision of the modern valleys. The age of delivery of granites to PPI remains unknown, however, recent observations to the south and east (England et al., this volume) indicate that the Late Wisconsinan LIS was more extensive than previously thought, and may have reached PPI. Terrestrial Cosmogenic Nuclide (TCN) dating of erratic boulders is presently underway at the Cosmogenic Isotope Extraction Laboratory, University of Alberta (CIEL-UA) with dates expected in summer 2007. These results should differentiate between Late Wisconsinan vs. pre-late Wisconsinan deposition of erratics. A furtherconstraint can be placed on the erratic transport history using U-Pb dating of zircon crystals within the granites, which can be tied to unique source terranes on the Canadian mainland. Preliminary results indicate that all dated zircons may originate from the same terrane, an unexpected result unless the LIS sourced granites at the edge of the mainland shield, and did not entrain any other granitic rocks en route (= 1500 km). Such a pattern might be expected only if granite was carried from the mainland across a previously unglaciated landscape, conceivably as long ago as the late Pliocene. The pattern of Late Wisconsinan glacier retreat, mapped from the complex network of glacial meltwater channels, indicates the sequential retreat of a local cold-based ice cap, rather than an integral part of the regional LIS. However the LIS may have advanced over PPI from the mainland, and after separation from down-wasting Laurentide trunk ice in M’Clure Strait (to the south) retreated as this local ice cap. If this occurred during the Late Wisconsinan, a more random distribution of U-Pb dates would be expected. Radiocarbon dates on deglacialmarine limit show earliest ice retreat in the south, with successively younger dates of deglaciation to the north. The ice retreat pattern and radiocarbon data, in combination with the new U-Pb data, indicate it is likely that Late Wisconsinan ice cover of PPI was of local rather than regional (LIS) origin, and granite-bearing diamictons and erratics were deposited during an earlier glaciation, however this conclusion remains tentative pending new TCN dates.

Bibliography of Canadian Geomorphology