RECORD OF CATASTROPHIC CALVING OF THE FLATHEAD LOBE OF THE CORDILLERAN ICE SHEET INTO GLACIAL LAKE MISSOULA

During the late Pleistocene the Flathead lobe of the Cordilleran ice sheet terminated in glacial Lake Missoula, in the Mission Valley, Montana. A 150-m-thick sequence of diamict intercalated with laminated silt and clay document proglacial lacustrine deposition in up to 350 to 400 m of water. Due to the geometry of the valley floor, both diamict and varved sediment were deposited on the floor of the Mission Valley. The stratigraphy of the Mission Valley shows that sedimentation in glacial Lake Missoula was highly variable and episodic. It is best explained as a record of instability of the Flathead lobe as it retreated from the valley as lake level steadily increased. Roughly every 30 to 60 years surging or calving of the ice resulted in a dramatic increase in sedimentation. The average depositional rate ranges from about 1.2 to 3.9 cm/yr with a maximum of 320 cm/yr. Correlation to sections containing diamict indicates that diamict deposition took place about twice this fast.

In stratigraphic sections consisting only of silt/clay rhythmites, interpreted as varves, the temporal pattern of deposition resulted in distinctive “packets”. These packets are characterized by thick silt beds at the base, a few centimeters to more than three meters thick, that thin towards the top of the packet. The packets are topped by an average of 20 to 45 microvarves; varves that are about 1 to 5 mm thick. In general, rapid deposition ranging from an average of 11 to 81 cm/yr follows immediately after 20 to 45 years of sedimentation on the order of millimeters. After this rapid deposition, the rate drops off for 10 to 20 years until it reaches slow but constant levels. There are 66 of these packets exposed that generally become thinner towards the top of the sections. The packets record the general retreat of the Flathead lobe, as a result of rising lake level, punctuated by sudden, episodic instability of the ice sheet over about 3240 to about 3610 years. Once Lake Missoula drained, the Flathead lobe advanced to it maximum grounded position at the Polson moraine. Thin deposits of subaerial outwash suggest it remained at his position for a relatively short time before again retreating.