Late Pleistocene sedimentation in glacial Lake Missoula and revised glacial history of the Flathead lobe of the Cordilleran ice sheet, Mission Valley, Montana

During the late Pleistocene the Flathead lobe of the Cordilleran ice sheet and 18 of 25 Mission Range alpine glaciers terminated in glacial Lake Missoula, in the Mission Valley, Montana. A 150-m-thick sequence of diamict intercalated with laminated silt and clay, previously interpreted as a series of till sheets, is present in the Mission Valley. The vertical lithofacies sequences and lateral facies variations of the postulated till beds reveal that they do not possess attributes of ice-contact deposits. In contrast, the stratigraphy and sedimentology of the diamict and related fines document proglacial lacustrine deposition. Therefore, the Mission Valley contains the thickest exposed section of sediment continuously recording the history of glacial Lake Missoula, in the largest and lowest lake basin. Because the Mission Valley does not contain ice-contact deposits, there is no record of a grounded Flathead lobe extending into the valley south of the Polson moraine.; The stratigraphy shows that sedimentation in glacial Lake Missoula was variable and episodic, and is best explained as a record of instability of the Flathead lobe and the alpine glaciers. Roughly every 30 to 60 years surging or calving of the glaciers resulted in increased sedimentation. The average depositional rate ranges from about 1.2 to 3.9 cm/yr with a maximum of 320 cm/yr. Diamict deposition took place at about twice this rate. Counting exposed varves reveals that glacial Lake Missoula existed for about 3240 to about 3610 years, and OSL geochronology indicates this was from at least 19.2 to 16 ka. The stratigraphic record does not lend support to hypotheses that the lake drained catastrophically and completely many times.; More than 2100 closed depressions on the floor of the Mission Valley have previously been interpreted as kettles from a late glacial advance of the Flathead lobe. However, these depressions are encircled by low ridges or ramparts, a distinguishing characteristic of collapsed pingos and pingo scars. The morphology of these ramparts, their internal structure, and the complex crosscutting patterns of depressions and ramparts, demonstrate that these closed depressions are pingo scars. The pingo scars formed in glacial Lake Missoula sediment limit the southern extent of the late glacial advance of the Flathead lobe and document the existence of permafrost after the lake drained.