Larry N. Smith

Columbia Mountain landslide: late-glacial emplacement and indications of future failure, Northwestern Montana, USA

Abstract The well preserved and undissected Columbia Mountain landslide, which is undergoing suburban development, was studied to estimate the timing and processes of emplacement. The landslide moved westward from a bedrock interfluve of the northern Swan Range in Montana, USA onto the deglaciated floor of the Flathead Valley. The landslide covers an area of about 2 km2, has a toe-to-crown height of 1100 m, a total length of 3430 m, a thickness of between 3 and 75 m, and an approximate volume of 40 million m3. Deposits and landforms define three portions of the landslide; from the toe to the head they are: (i) clast-rich diamictons made up of gravel-sized angular rock fragments with arcuate transverse ridges at the surface; (ii) silty and sandy deposits resting on diamictons in an internally drained depression behind the ridges; and (iii) diamictons containing angular and subangular pebble-to block-sized clasts (some of which are glacially striated) in an area of lumpy topography between the depression and the head of the landslide. Drilling data suggest the diamictons cover block-to-slab-sized bedrock clasts that resulted from an initial stage of the failure. The landslide moved along a surface that developed at a high angle to the NE-dipping, thinly bedded metasediments of the Proterozoic Belt Supergroup. The exposed slope of the main scarp dips 30–37°W. A hypothetical initial rotational failure of the lower part of a bedrock interfluve may have transported bedrock clasts into the valley. The morphology and deposits at the surface of the landslide indicate deposition by a rock avalanche (sturzstrom) derived from a second stage of failure along the upper part of the scarp. The toe of the Columbia Mountain landslide is convex-west in planview, except where it was deflected around areas now occupied by glacial kettles on the north and south margins. Landsliding, therefore, occurred during deglaciation of the valley while ice still filled the present-day kettles. Available chronostratigraphy suggests that the ~1-km thick glacier in the region melted before 12,000 14C years BP—within 3000 years of the last glacial maximum. Deglaciation and hillslope failure are likely causally linked. Failure of the faceted interfluve was likely due tensile fracturing of bedrock along a bedding-normal joint set shortly after glacial retreat from the hillslope. Open surficial tension fractures and grabens in the Swan Range are limited to an area above the crown of the landslide. Movement across these features suggests that extensional flow of bedrock (sackung) is occurring in what remains of the ridge that failed in the Columbia Mountain landslide. The fractures and grabens likely were initiated during failure, but their morphologies suggest active extension across some grabens. Continued movement of bedrock above the crown may result in future mass movements from above the previous landslide scarp. Landslides sourced from bedrock above the scarp of the late-glacial Columbia Mountain landslide, which could potentially be triggered by earthquakes, are geologic hazards in the region.

Pleistocene mountain glaciation in Montana, USA

Publisher Summary Montana is unique in hosting multiple Pleistocene examples of all glacier types including continental ice sheets, large mountain ice caps, small mountain ice caps and transection glaciers. The chapter reviews that uncounted valley, cirque, and niche glaciers also existed in more than 60 distinct mountain ranges. The distribution of these glaciers, with equilibrium-line altitudes rising from north-west to south-east, is consistent with control by moist air masses entering the north-western comer of the state. Prevailing winter westerly winds immediately south of the Cordilleran Ice Sheet at the glacial maximum are indicated. Mapped extents of Pleistocene glaciers in the mountains of western Montana were achieved from topographic map and aerial photographic interpretation, with limited field verification. Nowhere in Montana are the ages of glacial episodes well-constrained. Most Last-glacial moraines are correlated with the Pinedale glaciation elsewhere in the Rocky Mountains, rather than dated directly. Radiocarbon dates in the Yellowstone region and south of Glacier National Park and occurrences of Glacier Peak volcanic ash in the Flathead region show massive retreat or disintegration of the major ice caps prior to the Younger Dryas Chronozone. It is assumed that similar ages apply to the many mountain glaciers as well.