HR: 1330h
AN: H42C-1086    [PDF]
TI: Cosmogenic $^{36}$Cl dating of the all time limit of glaciation, Del Bonita upland Alberta/Montana border and insights into changing extents and ice-flow patterns in successive continental ice sheets
AU: * Jackson, L E
EM: lijackso@nrcan.gc.ca
AF: Geological Survey of Canada, Terrain Sciences Division, 101-605 Robson Street, Vancouver, BC V6B5J3 Canada
AU: Phillips, F M
EM: phillips@mailhost.nmt.edu
AF: Department of Earth and Environmental Sciences, New Mexico Tech, Socorro, NM 87801 United States
AB: The Del Bonita upland is a nearly horizontal, unglaciated interfluve along the Alberta, Canada/Montana, USA border (centered on $49\deg$N and $113\deg$W). It separates the basins of the Milk and St. Mary rivers. It is capped by the distinctive maroon Flaxville gravel of late Tertiary age. It is bordered to the west, north and east by a belt of moraine and glaciofluvial channels. The moraine represents the local limit of glaciation. It contains clasts derived from the Canadian Shield, Interior Plains and Rocky Mountains. Whether it is Late Wisconsinan in age or older has been disputed, with proponents on both sides of the international border arguing at various times for Wisconsinan and pre-Wisconsinan ages. Ages of emplacement were determined for eight large glacial erratics (2-30 m$^{3}$) using the cosmogenic $^{36}$Cl exposure dating method. These included carbonate and quartzite erratics from the Rocky Mountains and plutonic and metamorphic erratics from the Canadian Shield. Erratics were situated between $112\deg$ 02' W to $112\deg$ 51' W and $49\deg$ 1'N to $49\deg$ 6'N. Zero erosion ages are, in increasing age (in years): 11,500$\pm$420,14,040$\pm$510, 15,500$\pm$630,19,600$\pm$780, 20,800$\pm$780, 21,050$\pm$670, 22,100$\pm$800, 30,300$\pm$1160. All, save the last age, indicate emplacement during the Late Wisconsinan Stage. The ca. 30 ka age was determined on one of the quartzite 'Foothills erratics'. It indicates some pre-exposure to cosmic radiation. That is consistent with the rockfall-on-mountain-glacier origin of the Foothills erratics. The mixture of Rocky Mountain and Canadian Shield erratics in this area is consistent with southeast flow of coalescent Laurentide Ice Sheet and Rocky Mountain piedmont glaciers along the eastern margin of the Rocky Mountain Foothills during the climax of the Late Wisconsinan Stage. The clearly Late Wisconsinan age of the erratics further supports the view that the Laurentide Ice Sheet of the LGM was the most extensive of all Pleistocene ice sheets in southwestern Alberta. Furthermore, when combined with the regional distribution of tills predating the Late Wisconsinan in southeastern Alberta, southwestern Saskatchewan and northern Montana, the southeastern flow pattern in the southwestern margin of the Laurentide Ice Sheet during LGM is shown to be unique: previous ice sheets had north to south or northeast to southwest flow patterns in the same region.
DE: 1035 Geochronology
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 1827 Glaciology (1863)
DE: 9350 North America
DE: 9604 Cenozoic
SC: Hydrology [H]
MN: 2003 Fall Meeting