Isostatic rebound in the Lake Agassiz Basin since the late Wisconsinan

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Brevik, Eric C.
Ice on rivers; lakes; etc--North Dakota--Agassiz; Lake; Geology; Stratigraphic--Pleistocene; Isostasy; Geology
thesis / dissertation description
This study addressed three main questions: I) how thick was the ice that covered the southern Lake Agassiz basin during the Wisconsinan and how much that ice depressed the crust, 2) how much rebound has occurred since deglaciation and whether or not rebound is complete, and 3) what were the effects of this rebound on the basin.The most direct method of measuring rebound in the Lake Agassiz basin is from strandlines left by glacial Lake Agassiz. The oldest complete strandline, the Herman, presumably rebounded, with the northern end rebounding more because the ice was thicker there and had melted from that end later. The difference in elevation of this strandline represents absolute minimum rebound, 54.5 meters. Up to 73% of rebound was restrained; the initial depression may have been as much as 200 meters. However, restrained rebound may have been regarded as ice was replaced by Lake Agassiz water and sediments. The average depth of Lake Agassiz at Grand Forks, ND, was as much as 100 meters, and the average thickness of sediments as much as 46 meters. These masses would cause crustal depression of 38 meters and 40 meters, respectively. The sediments are still in place in the Lake Agassiz Basin, causing 40 meters of depression. When added to the 54.5meters of minimum depression, a total of 94.5 meters of depression is indicated. Minimum ice thickness would have been approximately 280 meters. Using a slope profile method, former ice thickness in the Grand Forks, ND area was about 390 meters, with approximately 424 meters at the international border. Basal shear stress methods indicate ice thicknesses between 313 and 986 m. Maximum ice thickness indicated by the strandlines is 1040 meters.Ice thickness must have exceeded the minimum. Several beach and scarp remnants are as much as 30 meters above the Herman strandline. On the other hand, the water and sediments of Lake Agassiz slowed rebound. Ice thickness, therefore, most likely was between 435 and 986 meters, causing a depression of 140 to 330 m.Results of the rebound include decreased river gradients, changing river courses, and more frequent and intense flooding in the Lake Agassiz basin. Rebound definitely continues north of Lake Winnipeg, and may still be occurring in the southern Lake Agassiz basin, although the strandlines indicate that rebound in the southern Lake Agassiz basin is complete. In either case, the potential for increased flooding exists.