ABSTRACT: Time constraints for the earliest records of glaciation in the Canadian Interior Plains and American Midwest.

CGRG Bibliography of Canadian Geomorphology
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Abstract

The earliest dateable evidence of Laurentide glaciation on the Canadian Prairies occurs in western Saskatchewan/eastern Alberta, and in the Hudson Bay Lowland of Manitoba. Absolute ages for the sedimentary record immediately predating the first arrival of ice (sections near Wellsch Valley, SK and Galt Island, AB), as well as the age of glacial lake sedimentsassociated with the first glaciation (Wascana Creek Section, SK), are obtained from fission track ages on three tephras, which range from 0.7-0.4 Ma. These tephras and their enclosing sediments are normally magnetized, and based on their fission track ages, are assigned to the Brunhes Chron (<0.78 Ma). Fossil vertebrate assemblages contained within preglacial sediments immediately predating the arrival of the first ice at the Wellsch Valley, Galt Island and Medicine Hat sections indicate an early-middle Pleistocene fauna, and further strengthen a Brunhes age assignment for the first Laurentide glaciation of this region. Preglacial sediments at Wascana Creek and Wellsch Valley are reversely magnetized, and based on the late Blancan and Early Irvingtonian fossil assemblages at Wellsch Valley, can be assigned to the upper Matuyama Chron. At Medicine Hat and Galt Island, the preglacial sediments are normally magnetized, suggesting that preglacial sedimentation continued on into a portion of the Brunhes Chron in the valleys to the west of Swift Current. At least two major till sheets, referred to as the Labuma and Buffalo Lake tills, are present throughout large portions of southern Alberta and can be correlated to the Sutherland Group and Upper Floral Formation respectively, in Saskatchewan. Based on the 0.43 Ma tephra age and late Irvingtonian faunal assemblages of the underlying sediments, and a Sangamon age for overlying sediments, the Labuma till is Illinoian (most probably stage 6), and the Buffalo Lake till is Late Wisconsinan based on Chlorine 36 dates on the associated erratics and the radiocarbon ages on underlying wood and bone. abuma and Buffalo Lake tills, are present throughout large portions of southern Alberta and can be correlated to the Sutherland Group and Upper Floral Formation respectively, in Saskatchewan. Based on the 0.43 Ma tephra age and late Irvingtonian faunal assemblages of the underlying sediments, and a Sangamon age for overlying sediments, the Labuma till is Illinoian (most probably stage 6), and the Buffalo Lake tillis Late Wisconsinan based on Chlorine 36 dates on the associated erratics and the radiocarbon ages on underlying wood and bone. In Manitoba, the most extensive Quaternary record occurs in the Gillam area, where all sediments are normally magnetized. The lowermost (Sundance) till has a well-developed paleosol, and is overlain by the Amery till (most probably stage 6) which can be traced throughout the region. It is overlain by Nelson River sediments which yield amino acid ratios on Hiatella arctica shells that indicate a Sangamon age. The overlying tills are Wisconsinan in age. Recent detailed paleomagnetic studies carried out at or near the "classic" sections of the American midwest and in the general vicinity of the Missouri River, in the states of Iowa, Nebraska, Missouri and Kansas indicate that the earliest glaciation(s) there occurred during the Matuyama Chron. Twelve sections reveal reversely magnetized lower tills and associated intertill beds, indicating that the so called "A" and "B" tills of the region predate the Brunhes/Matuyama boundary (0.78 Ma). Tephra dates at the Afton, David City, and County Line sites corroborate a late Pliocene and early Pleistocene age for these tills. Clearly, the extent and timing of the earliest Laurentide glaciations in the Interior Plains of Canada are different from those in the American midwest. Canadian sites reveal only normally magnetized sediments while in the US both normal andreversed sediments occur. The intriguing question is: Why?

Bibliography of Canadian Geomorphology