Edwards, Jason, O., M.S., Autumn 2006 Geology
Evidence for Glacial Outburst Floods along the Lower Flathead River: Results from
Geologic Mapping, Geomorphologic Analysis, and a Gravity Survey near Polson,Montana
Committee Chair: Marc S. Hendrix
The Kerr Dam Canyon (KDC) and Buffalo Basin (BB) are located 10 km west of Polson, Montana along the Lower Flathead River. Quaternary sediments in this region were deposited as part of an ice marginal system that included the Flathead Lobe (FHL)of the Cordilleran Ice Sheet and associated outwash environments, Glacial Lake Missoula (GLM), and proglacial Lake Flathead.
In this study, I mapped approximately 63 square km in and around the former terminal position of the Flathead Lobe, in order to better understand the history of deglaciation in the region and better document evidence supporting the delivery of large discharge pulses of meltwater during terminal glacial retreat. Based on my field observations and facies descriptions, I defined and mapped eight geologic units; Ysr – St. Regis Fm., Yr - Revett Fm., Qal - Quaternary alluvium, Qalf - Quaternary alluvium related to flood events, Qgdm - massive diamict, Qgdl - layered diamict, Qo - glacial outwash, and Qgm - glacial
moraine.
Evidence for outburst flooding includes a eries of lobe-shaped Qalf deposits in the BB and several flights of strath terraces in the KDC. The Qalf deposits are composed of cobble- to boulder-bearing conglomerate (maximum clast size >4 m) that is organized by elevation into three depositional surfaces. I infer that these surfaces were formed as part of a proglacial fluvial system that included episodic jökulhlaups (outburst floods)associated with ablation of the Cordilleran Ice Sheet. Four terraces are carved into thick Qgdm deposits and can be traced the length of the KDC. The terraces correlate acrosscanyon at elevations 925 and 913 meters.
A depth-to-bedrock model inverted from gravity data in the BB provides the basis for
interpreting the shape of the valley fill/bedrock contact. I organized forty seven gravity
observations into four lines and inverted for depth to create cross sections. From these
data, I infer the presence of two normal faults on the east shoulder of the basin and
channel-shaped erosional features near the mouth of the canyon. The existence of the channel features strongly suggests that the ancient Lower Flathead River likely served as the principle drainage for the lower Flathead River drainage system prior to the most recent occurrence of glacial Lake Missoula and possibly deeper into the geologic past.
