Glacial Lake Missoula Bottom Sediments
Richard L. Chambers, PhD
Geologist, Educator, Author, Photographer
Jocko Valley Roadcut
This aerial photo was taken by Amy Howie on September 29, 2016 as part of a new documentation of Glacial Lake Missoula for the Ice Age Floods Institute, Missoula Chapter. This roadcut does not look like the roadcut I studied in 1969-1971, as it has been trimmed back due to highway construction. I tried to visit the roadcut during August 2006, but couldn't due to widening this section of Highway 93. The numbered boxes are locations of photos showing details of the road cut.
Photo 1, taken in 1970, shows the basal rhythmite at this location. There are 308 varves in the 17 rhythmites I measured and described in this roadcut. The roadcut ranges in thickness from about 1 to 10-m.
In this photo, we see a rather sharp contact between the Sand-Silt facies and the Silt-Clay (varve) facies. Notice that the upper portion of the varves are highly disturbed. I not sure if this is soft-sediment deformation due to compaction, or perhaps a seismite attributed to an earthquake. These zones be contorted beds created under periglacial conditions. I observed such zones in other outcrops in the Missoula Valley and at the Ninemile Creek roadcut.
The probable sediment sources for these lake bottom sediments were the local alpine glaciers at the eastern end of the Jocko Valley.
In Photo 2 observe that the Lake Missoula Rhythmites overlie valley fill of probable Tertiary age and channel fill of possibly late Pleistocene age. Photo taken September 22, 2016.
Photo 3 shows the very thick Sand-Silt facies of rhythmite 1 above the channel fill (contact is where the machete is stuck in the ground). Sedimentary structures include planar and ripple cross-bedding, flame structures, and soft sediment deformation. Photo taken July 1984.
Details of sedimentary structures in Photo 3 above.
