NEW EVIDENCE FOR LAURENTIDE GLACIAL LAKE MUSSELSHELL, CENTRAL MONTANA

Glacial Lake Musselshell is one of six large ice-dammed lakes in Montana suggested nearly a century ago (Calhoun, 1906) to explain hundreds of exotic boulders found in basins south of the farthest mapped glacial margin. The ages of these lakes are based on the limits of continental glaciation and, where available, the stratigraphic position of lake sediments relative to till. The timing of till deposition has been interpreted by Fullerton and Colton (1986) largely from field characteristics of the till because reliable radiocarbon dates are sparse.

Lake Musselshell spanned over 100 km of the Laurentide ice front and inundated about 5,000 km² of the Montana plains. The major lake stage at 1,011 m (3,150’) has been identified solely on the distribution of boulders and is depicted as Illinoian; a lower late-Wisconsinan lake has been proposed at 835 m (2,600’) (Fullerton and Colton, 1986, pers. comm.). Detailed fieldwork revealed neither shorelines nor varves, however deltaic deposits and “bath-tub rings” of boulders were found that support the existence of the lake.

In the Cat Creek drainage, 30 m of thickly-bedded, steeply dipping gravel with a maximum altitude of 937 m is probably a delta from an intermediate lake stage. The 0.85 km by 0.25 km deposit is elongated in the direction of N10^oW. The strike and dip of the beds is N30^oE 28^oSE. A 0.42 km² deposit at 802 m was mapped in Deep Coulee. The exposure is 10 m thick, and consists of gently dipping laminated silty fine sand with lenses of imbricate gravel and exotic boulders. This could be a mass movement deposit or a delta from the lowest lake stage, composed of local Cretaceous source material and lenses of dropstones reworked slightly by the tributary stream. Regardless of its origin, numerous exotic clasts found on its surface indicate that the deposit is either last glacial or pre-last glacial in age. Several other similar deposits exist in the basin.

Over 100 exotic boulders mapped in the basin are clustered by elevation, supporting their interpretation as ice-rafted debris deposited as icebergs grounded near glacial lake shorelines. Five “bath-tub rings:” 809-812 m (correlative with the Deep Coulee deposit?); 867-876 m; 911-924 m; 928-933 m (correlative with the Cat Creek delta); and 956-966 m indicate that Lake Musselshell was filled and drained a maximum of five times. Cosmogenic isotope dating of each altitude cluster is in progress.