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TitreLimits of successive middle and late Pleistocene continental ice sheets, Interior Plains of southern and central Alberta and adjacent areas
AuteurJackson, L E; Andriashek, L D; Phillips, F M
SourceDevelopments in Quaternary Science vol. 15, 2011 p. 575-589,
Séries alt.Secteur des sciences de la Terre, Contribution externe 20100063
Documentpublication en série
Mediapapier; en ligne; numérique
Lat/Long OENS-120.0000 -110.0000 59.0000 47.0000
Sujetsnappes glaciaires; glace; dépôts glaciaires; retrait de la glace; antecedents glaciaires; erratiques; coulées d'erratiques; lithostratigraphie; géologie des dépôts meubles/géomorphologie; stratigraphie; géochronologie; Cénozoïque; Quaternaire
Illustrationslocation maps; tables; stratigraphic columns; photographs; profiles
ProgrammeDiamands, GEM : La géocartographie de l'énergie et des minéraux
Résumé(disponible en anglais seulement)
Cosmogenic exposure dating of glacial erratics has shown that the limit of glaciation by continental ice sheets in Alberta and western Montana dates to the climax of the Late Wisconsinan Glaciation. This glacial limit includes belts of ground moraine and the basins of glacially dammed lakes formerly referred to the Illinoian Glaciation. Lithostratigraphy and chronologic control of glacial sediments preserved in the buried valley systems of Alberta were reviewed to see if they are in accord with this revised surficial geology.
Ice sheets advancing into Alberta from the north, northeast or east must ascend the regional slope in opposition to easterly- or northeasterly-trending drainage systems. Conversely, glacial thinning and retreat progressively descend down the regional slope leaving flights of ice-dammed lakes, drainage channels and their sediments.
Two cycles of advance-retreat lithofacies are present in the Medicine Hat area in southeastern Alberta. These are separated by non-glacial sediments that have been radiocarbon dated to the (non-glacial) Middle Wisconsinan Substage. The Middle Wisconsinan sediments overlie an erosional unconformity that can be traced for kilometres. In east-central Alberta, drilling in the Elk Island area, east of Edmonton, intersected grass in lacustrine sediments that was radiocarbon dated at 26,000+/- 1100 y BP. The lacustrine sediment underlies the uppermost till in the region and overlies an oxidized carbonate-rich till thus making the underlying till older than Late Wisconsinan. In the east-central Alberta, two extended nonglacial intervals between advance-retreat sequences underlie Late Wisconsinan glacial deposits. In total, this stratigraphy may represent three glaciations. The stratigraphy of these deposits is correlated to the well established multiple glaciation stratigraphy in adjacent Saskatchewan. These successions of advance-retreat lithofacies sequences with evidence of intervening non-glacial periods described above are confined to the eastern third of Alberta. Sites limiting their western extents descend in elevation from southeast to northwest over a linear distance of about 800 km. In southeastern Alberta, sediments equivalent to those in the Medicine Hat area cannot be traced farther west than the Taber area (49.9° N, 112.2° W; elevation ca. 750 m a.s.l). Sites farther west and at higher elevations with successions of advance-retreat lithofacies include Kipp in the Lethbridge area (49.7°N, 112.9° W; elevation ca. 900 m a.s.l.) and the Brocket area in the Rocky Mountain Foothills (49.3° N, 113.8° W; ca. 1050 m a.s.l.). These have been shown to lack evidence for older glacial cycles. In Hand Hills at a similar elevation to the Brocket area, but 260 km north-east (area of 51.6° N, 112.3° W), seven Middle Wisconsinan radiocarbon ages have been determined on a fossil prairie dog colony that underlies a single till In west central Alberta, a single advance lithofacies succession is present across the Edmonton area (ca. 53.5° N, 113.5° W; elevation ca. 670 m a.s.l.). It overlies gravel of the Empress Fornmation (EF). EF in this area contains faunal material that has yielded 29 finite radiocarbon ages that fall within the non-glacial Middle Wisconsinan Substage. Gravel and sand in EF are derived from Interior Plains and Rocky Mountain lithologies. They completely lack any source in the Canadian Shield: EF deposition clearly predates incursion of a continental ice sheet into the Edmonton area. A similar stratigraphy has been documented in cliff-bank exposures in the Watino area 360 km to the northwest (area of 55.5° N, 118° W). Eight finite Middle Wisconsinan radiocarbon ages have been determined on wood from fluvial sediments compositionally equivalent to EF (Liverman et al., 1989). These sediments predate a single till containing clasts from the Canadian Shield. The elevation of the prairie surface in this region is no higher than 560 m a.s.l.
The age and distribution of glacial sediments in the buried valley systems of Alberta are in accord with a Late Wisconsinan age for the limits of continental glaciation in the Interior Plains of southern Alberta and adjacent Montana. During the preceding (Illinoian) glaciation of Alberta, continental ice pressed into the eastern third of the province. The progressive growth of successive ice sheets from Illinoian glaciations through the Late Wisconsinan Glaciation fits the model of progressive expansion of the western margins of continental ice sheets through the Pleistocene.
It follows that broad ice-free corridors existed in Alberta during all Pliocene and Pleistocene glaciations except the Late Wisconsinan Glaciation. Currently available evidence suggests that these ice-free corridors extended north to the Arctic Ocean: expansive ice-free regions existed between the Cordillera and the western margins of continental ice sheets during all earlier glaciations. Plants and animals were free to move north and south through this region at the height of pre-Wisconsinan glaciations. The Late Wisconsinan coalescence of montane and continental ice was a unique event. Dramatic morphologic and genetic changes in fauna have been associated with the opening and closing of this corridor during the Late Wisconsinan including the introduction of new species and extinction of many others. The intensity of these changes may have been accentuated by this unique event in contrast to previous glacial terminations.