Uplift-based limits to the thickness of ice in the Lake Agassiz basin of North Dakota during the Late Wisconsinan

Abstract

Upper and lower limits to ice thickness in the southern Lake Agassiz basin (northeast North Dakota) during the Late Wisconsinan have been calculated. The oldest well-preserved tilted strandline of glacial Lake Agassiz, the Herman, was used to determine how much the basin was depressed by the ice sheet and, from that, how much ice was necessary to cause the given depression. Based upon the difference in elevation between the southernmost part of the strandline near Dumont, MN, and where it crosses the US–Canadian border, the absolute minimum rebound (and thus, earlier depression) in the southern Lake Agassiz basin is 54.5 m. The rebound that occurred over the 300 years between deglaciation and the Herman level of Lake Agassiz was added, yielding a total initial depression of 70 m. The added effects of about 46 m of accumulated lake sediments has caused an unrecovered crustal depression of approximately 23 m. Total minimum depression, therefore, was about 93 m. Assuming that up to 73% of rebound was “restrained”, the initial depression may have been as much as 340 m. These values (93 to 340 m) were used to calculate minimum and maximum ice thicknesses in the basin of 250 to 920 m, respectively. These thicknesses correspond to basal shear stress values of 0.32 to 4.4 kPa, respectively.