Glacial Lake Missoula Bottom Sediments
Richard L. Chambers, PhD
Geologist, Educator, Author, Photographer
Glacial Lake Missoula Rhythmites
A record of lake fillings and drainages
Rather than finding single thick sequences of glacial lake varves, Alt and Chambers (1970) and Chambers (1971, 1984, 1989) discovered a much more complex and far more interesting situation. What they found was small-scale cycles, up to a meter or more thick. The rhythmically bedded deposits show a very distinctive pattern of alternating light and dark-colored layering at the outcrop scale. A typical Lake Missoula rhythmite has a light-colored base of fine-grained sand and silt grading 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 layer of the next rhythmite. Beds of pebble and gravel, weathering zones, and evidence of frost action were found at the tops of many rhythmites. The repeated lake floor exposures do not necessarily indicate catastrophic drainings or even that the lake drained completely. The left photograph shows a portion of the Ninemile Creek roadcut, while the detailed photo on the right shows a rhythmite from the Jocko Valley roadcut. Each rhythmite is bounded by an unconformity along its base and top.
Some Definitions
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Rhythmites are individual units (i.e., couplets) of rhythmic bedding with no time or seasonal connotations (A.G.I. Glossary, p.246). About 16 Lake Missoula rhythmites are shown in the left photograph.
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Varves are couplets, regardless of origin, that represent an annual period (De Geer, 1912). In the right photograph, we observe a very sharp contact between the Sand-Silt facies and the varves that comprise the Silt-Clay facies.
Lithofacies:
The Lake Missoula bottom sediments are classified into three lithofacies: 1) pebble-gravel facies, 2) sand-silt facies, and 3) silt-clay facies (Chambers, 1971, 1984; Hanson, et al., 2012). Details and evidence fro subaerial exposure are illustrated on the Ninemile Creek and Jock Valley roadcut pages.
Pebble-Gravel Facies
The lower contact between each rhythmite is unconformable and often erosive. The pebble-gravel facies, shown in this image from the Ninemile Creek roadcut, has cut into rhythmite 2 and is overlain by the Sand-Silt facies of rhythmite 3. This facies is interpreted as non-channelized fluvial sedimentation on the exposed lake floor deposited following a lake-level lowering event which eroded the underlying silt-clay facies deposited in the previous lake. 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.
Sand-Silt Facies
The sand-silt facies is present in every rhythmite, with individual occurrences typically ranging from about 5-160 cm or even up to several meters thick. Lower contacts are often sharp and characterized by planar or ripple cross bedding; most of the lower contacts are clearly erosional. This facies was probably deposited by hyperpycnal flow entering a rising lake-level, with possible reworking of the sediment by wave action (Chambers, 1971, 1984; Hanson, et al., 2012). This photograph shows a very thick Sand-Silt facies unit at the Jocko Valley roadcut site. In this image we see current ripples, laminar (planar) and massive bedding, flame structures, and soft sediment deformation.
Details of the Sand-Silt Facies
Type-A and type-b ripples within the Sand-Silt facie at Ninemile Creek.
Silt-Clay Facies
The sand-silt facies always grades upward into the silt-clay facies and was deposited in deeper water. This facies is composed of rhythmically laminated silt-clay couplets that resemble varves and has a sharp, unconformable contact with the overlying sand-silt facies. The varves tend to thin upward within each rhythmite, evidence of a continued deepening lake and increasing distance from the sediment source (Chambers, 1971, 1984; Hanson, et al., 2012).