Glacial Lake Missoula: The Montana Sedimentary Record of Multiple Drainings

Glacial Lake Missoula inundated valleys of northwest Montana to altitudes of ~1270 m. Channeled Scablands downstream of the lake show that numerous floods occurred during the latest Pleistocene. The record from Glacial Lake Missoula has been elusive because of the lack of deposit dates. However, previous workers regarded glacial-lake silts at the Ninemile section to record 8–40 lake fillings and drainings, even though the cycles have not been correlated within the lake basin.

Recently recognized catastrophic flood deposits along the Clark Fork River lie beneath the silts. This unconsolidated alluvium contains imbricated boulder-sized clasts, 2– >35-m-thick cross-stratified gravel, and 70–100-m-high bedforms which all indicate a high-energy, high-volume alluvial environment. Gravel bars and scablands were formed by catastrophic draining of one or more early, high lake stands (1200–1270 m). Glacial-lake silt, the youngest glacial deposit in the basin, include the 25-m-thick Ninemile section and the 40– >100-m-thick sections downslope of the Flathead glacier terminus. The silt exposures are notable for the lack of channels that would indicate lake-drainage events. The erosional top of the glacial silts are incised relict valley networks formed during the last glacial-lake drainage. Preservation of these networks shows that significant erosion associated with the last lake draining was confined to the small, inner Clark Fork River canyon. The lake silts represent either a lower lake stand (1050–1150 m?) or a less catastrophic failure than events responsible for the older gravelly flood alluvium. The lake that deposited the silts was not responsible for the highest discharge events. Preliminary dating of catastrophic flood sediments in the lake basin suggests that they are older than the latest Pleistocene flood deposits in the Channeled Scablands. This interpretation suggests that multiple latest Pleistocene floods in Washington may have been sourced from reservoirs other than Glacial Lake Missoula.