CGRG Bibliography of Canadian Geomorphology
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Author : Fisher, T.G.
Date : 2003.
Title : The chronology and geomorphology of Glacial Lake Agassiz's southern and northwestern outlets.
Publication : Joint Annual Meeting of the Canadian Quaternary Association and the Canadian Geomorphology Research Group. Halifax, Nova Scotia, June 8-12, 2003.
Issue :
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Abstract
A core-based radiocarbon chronology from outlet-scour lakes provides an important temporal constraint upon glacial meltwater delivery from Lake Agassiz. Scour lakes within outlet spillways were chosen for coring because they are located in low positions on the landscape, which increases the opportunity for sediment preservation, and they are directly linked with strandlines and basin hydrology. The types of sediment recovered in cores is variable: gravel directly records outlet occupation; lacustrine mud interbedded with gravel records temporary spillway abandonment; and, the uppermost muck or gyttja overlying sand or gravel records final spillway abandonment. Dated terrestrial macrofossils within these sediments and at contacts are used to reconstruct outlet history. Uncertainty in the number of outlet occupations is introduced by unconformities at the lower contact of gravel units, requiring dependence upon data elsewhere in the spillway or basin to constrain the outlet history. Two radiocarbon-dated cores from lakes in the southern outlet spillway indicate a minimum of two outlet occupations with the first recorded abandonment at 10,800 14C yr BP (end of the Lockhart Phase) and final abandonment by 9400 14C yr BP (end of the Emerson Phase). Because the gravel associated with the first abandonment is located at the deepest portion of the spillway (in its upper end), it is interpreted that the spillway had formed by that time. This implies that the head of the spillway was incised close to the Campbell level by 10,800 14C yr BP. This interpretation is supported by dates on alluvial fans further down the spillway and by forest remains of the Poplar River Formation in the center of the lake basin at Moorhead, MN. Numerous radiocarbon-dated cores from spillway-scour lakes in the northwestern outlet region are characterized by muck or gyttja overlying muddy rhythmites or sand. An AMS date from a core in Klap Lake within the lowest, and presumably youngest, spillway channel, suggests that the northwestern outlet was abandoned by 9450 14C yr BP., which is coeval with abandonment of the southern outlet. This outlet synchronicity strongly confirms that both outlets were occupied at the end of the Emerson Phase in response to the more rapidly rebounding northwestern outlet, and that presumably, the eastern outlets reopened at 9400 14C yr BP.
Bibliography of Canadian Geomorphology