ICE-MARGINAL SLACKWATER IN THE MUSSELSHELL BASIN: IMPLICATIONS FOR LATE WISCONSIN DIVERSION OF THE MISSOURI RIVER IN CENTRAL MONTANA

During the Pleistocene, the Laurentide ice sheet transported hundreds of boulders from the Canadian Shield to the Montana plains. Most of these erratics are north of the Missouri River where till is also found, but some are exposed in basins south of the farthest mapped ice margin. Near the city of Great Falls, boulders south of the Missouri are found in association with varved silt and clay. This supports the hypothesis proposed by previous workers that erratics north of the Missouri were deposited directly by the ice sheet while the southernmost erratics were ice-rafted in a chain of large ice-marginal lakes. However, scattered erratic boulders in the Musselshell basin are associated with neither till nor varves. Instead, these erratics are found on top of and within thick (>10 m) deposits of bedded silt and fine sand interpreted as slackwater sediment. We propose that these boulders were rafted into ice-marginal slackwater that built up in the Missouri and Musselshell River valleys. The slackwater apparently drained eastward down the modern Missouri River channel through the Larb Hills, either underneath or in front of the ice sheet.

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.