Glacial Lake Missoula Bottom Sediments
Richard L. Chambers, PhD
Geologist, Educator, Author, Photographer
Ninemile Creek Roadcut
Although no deep soil profiles were found, thin zones, 2-20 cm thick, of desiccated and weathered varves were found at the top of 22 rhythmites. Two of the 22 varve sequences were so deeply weathered that no varve count was possible. These zones provide reasonable evidence for subaerial exposure of a drained lake floor subject to frost action. What appears to be a frost crack filled with weathered and desiccated varves is evidence of periglacial conditions between lake fillings (Chambers, 1971, 1984; Hanson, et al. 2012). Smith and Hanson (2014) also found strong evidence for periglacial features, contorted bedding, desiccation, and paleosols in outcrop that provide clear evidence of multiple exposure surfaces: each representing a lake-lowering event.
Chambers (1971, 1984) counted 729 varves at the Ninemile Creek roadcut; averaging 24 per cycle, however this number does not take into account the 22 zones of weathered varves, or the varves in cycles 33 to 40 which are too highly weathered to count. Hanson, et al. (2012) visited the Ninemile Creek exposure and counted only 583 varves. The discrepancy between 729 and 583 could be explained by 1) weathering of the upper part of the exposure over the past several decades preventing Hanson, et al. to count varves in the upper section, 2) Chambers (1971) counted composite varves as simple varves, thereby increasing the number, or 3) that Hanson et al. (2012) grouped thinner varves into composite varves resulting in a fewer number of varves.
Evidence of subaerial exposure and frost action
Small streams flowed across an exposed lake floor
Depending upon where I cleaned, measured and described sections of the Ninemile Creek roadcut, I came across the Pebble-Gavel Facies. This facies is interpreted as non-channelized fluvial sedimentation on the exposed lake floor deposited following a lake-level lowering event. The local, small streams eroded into the underlying silt-clay facies depositing a pebble-gravel lag which shows crude, imbricate cross-bedding. This is a localized facies and was observed in only 4 of the rhythmites in the Ninemile Creek roadcut. Hanson, et al (2012) made similar observations and conclusions at the Rail Line section.
Details of the Silt-Clay "varve" Facies
The Silt-Clay facies typically grades upward into darker-toned rhythmically laminated silt and clay couplets that are interpreted to be varves. The silt-clay couplets thin upward and are unconformable with the overlying sand-silt facies of the next rhythmite. Individual varves tend to thin upward within each rhythmite, evidence of a continually deepening lake and increasing distance from the sediment source. Simple varves have a light-colored, more silty lower layer (summer deposition), that grades into a darker-colored, more clay rich upper layer (winter deposition). The varve couplet highlighted by the yellow bracket is a composite varve; which in this case has a light-colored, silty layer, within the winter layer, possibly an indication of a short, winter thaw. I've also observed just the opposite, where dark layers occur in the "summer" layer, indicating a short cold period. Evidence of "graded" bedding is seen in the thin section through several valve couplets.
