FLATHEAD LOBE RECESSION IN THE FLATHEAD VALLEY: IMPLICATIONS FOR ICE-MARGINAL LAKE DRAINING

Acquisition of a 60 m continuous core through about half of a buried glaciolacustrine deposits in in the middle of the Flathead Valley near Kalispell, Montana provide timing constraints on the retreat of the Flathead Lobe of the Cordilleran Ice Sheet and the northward expansion of glacial Lake Missoula (GLM). The varved glaciolacustrine deposits, from about 30 m to 90 m below land surface, contained a few horizons of plant material, dated at ~15.6 cal ka BP at 78 m and ~11.2 cal ka BP at 37 m (Beta 612642 and 608404). Well-log data suggests that glaciolacustrine deposits are interpreted to continue to ~170 m below land surface. These new data show that glacial Lake Missoula, previously known to exist from ~21-14 ka, occupied the Flathead Valley at least to 48.15° N at ~15.6 ka and possibly much earlier based on the depth of lacustrine deposits below the lowest dated horizon.

Along the western side of the Flathead Valley and in the nearby Little Bitterroot River (LBR) valley, topography shows that catastrophic flows cut bedrock canyons and scablands. Previous work in the southern Big Arm area and the northern Tally Lake area described ice-dammed lakes along the Flathead Lobe drained catastrophically. The new dates show that these ice dams occurred prior to ~15.6 ka.

In the LBR watershed, just west of the Flathead Valley, basal units of GLM glaciolacustrine sediments were optically dated to 21.8 ± 1.4 and 20.3 ± 1.2 ka, like those in other parts of the lake basin. The upper LBR watershed, about 40 km north of Camas Prairie, contains five enigmatic canyon reaches, three with significant waterfalls and one that is abandoned, and a catastrophic boulder fan with >5 m sized boulders. One canyon reach, a narrow 160 m-deep meandering gorge, contains the four “Hidden Lakes” where bedrock was excavated along the valley bottom, suggesting sculpting by high-velocity flood flows. The boulder fan was deposited where the valley widens below the gorge. The topography suggests that GLM, or a previously unrecognized ice marginal lake, drained rapidly through the area, causing canyon cutting and establishment of the upper LBR during the last glaciation. The LBR watershed provided a direct north-to-south route during drainage of GLM into the valley north of the classic Camas Prairie flood deposit.