Late Quaternary events in the Lethbridge area, Alberta

Abstract

New observations in the Lethbridge area permit a more complete reconstruction of the landscape history in Late Wisconsinan and Holocene time. The plain of glacial Lake Macleod, the Lethbridge moraine, and the plain of glacial Lake Lethbridge became exposed in that order and in quick succession. Almost immediately thereafter, a discontinuous mantle of loess began to accumulate on those surfaces. The presence of Glacier Peak layer G or Manyberries tephra near the base of the loess indicates these events occurred just before 11 200 BP. The similarity of this chronology to that established for the older Green Lake end moraine in the Cypress Hills region and the fact that the younger Buffalo Lake moraine was formed before 11 000 BP indicate that deglaciation of southern Alberta proceeded very rapidly.The oldest buried paleosol observed near Lethbridge began to form shortly after 11 200 BP. Subsequently, and throughout the Holocene, intervals of loess deposition alternated with soil-forming intervals. At least six soil–landscape cycles occurred between 11 200 and 6800 BP (before the Mazama tephra was deposited), and at least five cycles occurred subsequently. Occurrences of postglacial loess more than 3 m thick are common. A column of 6.7 m of loess, including 12 paleosols, was observed at one site.