Abstract

Three large channels of the lower Kootenai Formation are exposed in the walls of the Missouri River valley east of Great Falls, Montana. Although the channels occur in two different stratigraphic units, they have several features in common. Each channel is contained within crevasse and bay-fill sequences, but the contacts between channel-fill deposits and laterally adjacent strata are erosional. The channels have a broad U-shape, range up to 300 m wide and 35 m deep, and exhibit a distinctive style of fill. Channel-filling occurred in increments by accretion from the bottom up and sides in, to form a concave layering which is more or less symmetrical about the axis of each channel. Lithology of the fill of each channel is quite different, however, and ranges from mudstone, to interbedded sandstone and mudstone, to sandstone. The channels are interpreted as superimposed distributaries formed by avulsion when the locus of sedimentation shifted from one lobe to another. The lithology of the channel-fill deposits appears to be a function of the abandonment rate. A mudstone-filled channel results where abandonment is rapid, as is the case with upstream diversion of a trunk river system. Sandstone and mixed sandstone-mudstone fills predominate where a distributary is progressively abandoned, for example, where the discharge is diverted into an alternately favored distributary. Superimposed channels are difficult to map in the subsurface by geologic means. They cut across the trend of adjacent facies, and so their presence cannot be predicted from analysis of the containing strata.

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