Strandline analysis in the southern basin of glacial Lake Agassiz, Minnesota and North and South Dakota, USA

Abstract

Glacial Lake Agassiz, a large Pleistocene lake, left behind strandlines at multiple elevations within its basin. Prior researchers have used these strandlines to identify four relatively stable water level stages, but tracing them over long distances is often problematic. In this study, the elevation of the Tintah strandline is returned to its original designation, and a newly interpreted stable lake level at 1020–1030 ft (311–314 m) is referred to as the Upham level, based on a concentration of ridges and a spit at that elevation. Cores from strandline and lagoon complexes from the Milnor to Tintah beaches reveal a decreasing thickness of strandline sediment associated with shorter aggregate-length strandlines. This relationship is most likely controlled by sediment starvation, greater water level fluctuations and shallower water. Beaches near the head of the southern outlet spillway indicate that shoreline occupation at the Lower Campbell level was either very short-lived or strandline development was hampered by shallow water. The modern-day sill elevation between the southern outlet spillway and the lake basin is found to be 3 km south of White Rock, Minnesota, at an elevation of 974 ft (297 m). All but the Campbell strandlines at the southern outlet are interpreted as the result of a regression driven by incision of the southern outlet spillway.