Cosmogenic 36 Cl dating of the maximum limit of the Laurentide Ice Sheet in southwestern Alberta 1 Lionel E. Jackson, Jr., Fred M. Phillips, and Edward C. Little Abstract: Cosmogenic 36 Cl ages were determined on 11 glacial erratics from the summits of Porcupine Hills and Cloudy Ridge, Waterton valley, and the Foothills south of Cardston, Alberta. These erratics were derived from the Canadian Shield and the Rocky Mountains of the Waterton area. They were laid down by (1) the most extensive advance of a Canadian Shield centred continental ice sheet into this region (stratigraphically oldest glacial deposits); (2) a montane glacial advance from the Waterton valley (stratigraphically intermediate glacial deposits); and (3) an advance of continental glacial ice that overrode deposits of the intermediate-age montane advance. Zero erosion rate 36 Cl ages of the erratics, uncorrected for snow cover, range between about 12 and 18 ka. They support the hypothesis that the Laurentide Ice Sheet reached farther into the southwestern Foothills than did all the previous continental ice sheets. Résumé : Les âges 36 Cl cosmogéniques ont été déterminés sur onze erratiques glaciaires des sommets des collines de Porcupine et de la crête de Cloudy, dans la vallée de Waterton et des Foothills au sud de Cardston, en Alberta. Ces erratiques proviennent du Bouclier canadien et des montagnes Rocheuses de la région de Waterton. Ils ont été déposés par (1) l’avancée culminante de la calotte glaciaire continentale centrée sur le Bouclier canadien et qui pénétra cette région (dépôts glaciaires les plus anciens dans la stratigraphie); (2) l’avancée glaciaire en montagne issue de la vallée de Waterton (dépôts glaciaires intermédiaires dans la stratigraphie); et (3) l’avancée de glace de glacier continentale qui a transgressé les dépôts d’âge intermédiaire de l’avancée en montagne. Les âges 36 Cl remis à érosion zéro, non corrigés pour la couverture de neige, varient d’environ 12 à 18 ka. Ils plaident en faveur de l’hypothèse évoquant que l’Inlandsis laurentidien s’est étendu plus loin dans le sud-ouest des Foothills que n’importe laquelle des autres calottes glaciaires continentales précédentes. [Traduit par la Rédaction] Jackson et al. 1356 Introduction Two long-standing and interlinked controversies in the in- terpretation of the Quaternary geology of southern Alberta are (1) whether or not the region had been glaciated by con- tinental ice sheets (the Laurentide Ice Sheet and its prede- cessors) prior to the Late Wisconsinan substage, and (2) the extent of the Laurentide Ice Sheet in this region during the Late Wisconsinan substage (especially in the vicinity of the “ice-free corridor”). These issues have grown from funda- mentally differing interpretations of the Pleistocene stratig- raphy of west-central and southwestern Alberta. The hypothesis of a single, Late Wisconsinan substage Pleistocene continental 3 glaciation of southern Alberta (Horberg 1954; Wagner 1966; Bayrock 1969) has contended with a multiple-glaciation hypothesis which proposes at least three continental glaciations for southwestern Alberta (Day 1971; Alley 1973; Stene 1976; Stalker 1977; Stalker and Harrison 1977; Jackson 1980; Jackson et al. 1989). It was argued that these events extended back to the Illinoian or earlier. Both schools based their views largely upon differing interpretations of the same stratigraphic successions in southwestern Alberta. The single-glaciation hypothesis has been recently supported by a variety of findings. Finite radiocarbon ages have been determined on samples taken from preglacial deposits underlying single continental tills in central, west-central, and south-central Alberta (Liverman et al. 1989; Burns and McGillivray 1989; Young 1991; Young et al. 1994). These dated successions have led these authors to conclude that continental ice sheets never extended to the elevation of Edmonton, Alberta, area prior to the Late Wisconsinan substage. In the Foothills of southwestern Alberta, reexamination of stratigraphic successions was carried out in the Oldman River and Highwood River basins as a part a Geological Survey of Canada NATMAP Project (Fig. 1, shaded area). Can. J. Earth Sci. 36: 1347–1356 (1999) © 1999 NRC Canada 1347 Received November 23, 1998. Accepted March 16, 1999. L.E. Jackson. 2 Terrain Sciences Division, Geological Survey of Canada, 605 Robson Street, Vancouver, BC V6B 1R8, Canada. F.M. Phillips. Department of Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801, U.S.A. E.C. Little. Department of Earth and Atmospheric Sciences, University of Alberta, Earth Sciences Building, Edmonton, AB T6G 2E3, Canada. 1 Geological Survey of Canada Contribution 1997085. 2 Corresponding author (e-mail: ljackson@gsc.nrcan.gc.ca). 3 The term continental refers to any past Canadian Shield- centred ice sheet, including the Laurentide Ice Sheet (the most recent such ice sheet), without reference to age. Montane refers to glaciers with a source in the Rocky Mountains.