GLACIAL LAKE MISSOULA DEPOSITS ALONG THE CLARK FORK RIVER DOWNSTREAM FROM MISSOULA, MONTANA: STRATIGRAPHIC CONTEXT OF THE NINEMILE SECTION

Ice dams created by the Purcell lobe of the Cordilleran ice sheet near the current Idaho/Montana border impounded Glacial Lake Missoula, which inundated valleys of NW Montana to altitudes of 1265 m. Varved, cyclic, and silt-dominant glacial-lake deposits along the Clark Fork River valley, such as the Ninemile section, have been argued to represent one or more lake stands. Recent mapping of unconsolidated deposits from Missoula, downstream to the confluence of the Clark Fork and Flathead Rivers, revealed a variety of deposits that are interpreted to be products of glacial-lake impoundment and catastrophic drainage. The stratigraphic relations between the glacial-lake silt and older and younger gravelly deposits, and erosional and depositional landforms in the valleys suggests that some lake stands reached vastly different altitudes.

Where exposed, the alluvium is stratified, with a few <50 cm-thick interbeds of laminated silty clay. However one exposed paleovalley contains a basal diamicton of cobble- to very coarse boulder-sized clasts, possibly a debris flow deposit. Imbricated boulder-sized clasts and planar cross-stratified gravel, with set heights of 2 to >35 m, display down-river, and up-tributary paleocurrents, indicating a high-energy, high-volume alluvial environment. In limited exposures, the contact between underlying alluvium and overlying glacial-lake silt shows soft-sediment deformation, suggesting a rapid transition in depositional conditions. The glacial-lake silt unit is overlain by thin (mostly < 2 m) gravel deposits on strath terraces and fans, and alluvium in channels and floodplains.

Landforms within the 0.6 to 3.5 km-wide alluvial valley are 0.1–0.5 km-wide bedrock channels or alluvial floodplains; strath terraces; upper flats covered by glacial-lake silts, 50–110 m above local base level (albl); streamlined alluvial bars and large-scale dunes, with crests 95–170 m albl; and scabland erosional topography and gulch fills that are 40–270 m albl. The large-scale landforms represent erosion and deposition by catastrophic draining of an early, high stand (~1200 m) Glacial Lake Missoula. Most glacial-lake silt, such as at the Ninemile section, was deposited on the earlier high-energy deposits, and must represent one or more later and much lower lake stand(s) (<1000 m?).