Mapping and characterization of the Upper Lake Agassiz beaches along the Manitoba Escarpment between the international border and the Assiniboine River

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McMillan, Kyle
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The paleo-shorelines of glacial Lake Agassiz are inferred from beach ridges and wave-formed scarps. Shoreline features of the oldest, uppermost beaches, the Herman, Norcross, and Tintah groups of beaches, were mapped from aerial photographs in southern Manitoba between about the Canada - U.S. border and the Assiniboine River (-49.0" - 49.5" N latitude). Aerial photo maps were initially made on transparent overlays and later digitized. This mapping was supplemented by fieldwork, including ground penetrating radar surveys, power augering, and by data from existing boreholes, pit surveys, and soil surveys. The Herman, Norcross, and Tintah beach sets each contain several low-relief beach ridges that extend discontinuously over tens of kilometres; these ridges are generally less than 3 m in relief, and a few tens of metres wide. In places, beach ridges are replaced by erosional scarps, metres to tens of metres in relief and continuous over distances of tens of kilometres. Sediments in the Herman, Norcross, and Tintah beaches range from units of well sorted sand to sand-gravel diamictons; sediments between beaches include clayey to sandy silts, and gravel lags. These three beach sets predate the well-developed Upper Campbell Beach, which was deposited about 9.4 14C ka, and are distinct from it because of their lateral discontinuity and much poorer sorting. Based on their small size, poor sorting of sediments, discontinuous development, and multiplicity, the Herman, Norcross, and Tintah beaches, are concluded to have formed by storms during shoreline regression, probably in sequence over only a few centuries, as there is no pronounced break in the continuum of beach ridges nor major differences in their size or characterisitics. The effects of lake ice may also help explain the discontinuous nature and sedimentological immaturity of these beach ridges. The Herman beach is thought to have formed about 11 14C ka, based on other dated events in the Lake Agassiz basin. It is concluded therefore that the Herman, Norcross, and Tintah bench sets represent a continuous period of episodic formation between about 11.0 - 10.8 14C ka. The Upper Campbell beach is well dated at 9.3 - 9.4 14C ka, and is too well-developed to be a storm beach; this beach was deposited following a transgression from a low-water lake phase that was initiated after the depostion of the lowest Tintah beach.-=