ICE-MARGINAL SLACKWATER IN THE MUSSELSHELL BASIN: IMPLICATIONS FOR LATE WISCONSIN DIVERSION OF THE MISSOURI RIVER IN CENTRAL MONTANA
Cosmogenic 10Be surface exposure ages were obtained for 27 of 125 mapped ice-rafted boulders in the Musselshell basin. All of the ages were ≤21.7 ± 0.6 ka, indicating that the erratics were deposited entirely during the Late Wisconsin glacial stage (MIS 2). In addition, Canadian Shield gravel occurs only in the lowest (probably MIS 2) Pleistocene terrace of the Musselshell River. If the ice sheet was not present to deliver gravel or boulders prior to MIS 2, then the ice dam necessary to divert the Missouri River through the Larb Hills may not have existed until that time. Significant knickpoint reduction is apparent in the gradient of the modern floodplain of the Musselshell River. This incision was probably triggered when the Missouri River was diverted to the south of its former course in the Milk River valley and began to flow in its present channel. As a result of the new, lower base level, the Musselshell River terraces converge upstream and badlands have formed along the lowest 50 km stretch. Upstream convergence of the Musselshell River terraces supports the hypothesis that local displacement of the Missouri River occurred as recently as Late Wisconsin.