Three well-exposed channels of the lower Kootenai formation have several unusual features in common. The channels are 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 (985 ft) wide and 35 m (115 ft) 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 moved from one lobe to another. The lithology of the channel-fill deposits appears to be a function of the abandonment mechanism. A mud-filled channel forms where abandonment is rapid, as is the case with upstream diversion of a trunk river system. Sand and mixed sand-mudfills predominate where a distributary is progressively abandoned, for example where the discharge is diverted into an alternate favored distributary.

Superimposed channels are difficult to map in the subsurface by geologic means alone. They cut across the trend of adjacent facies so their presence cannot be predicted from analysis of the containing strata.

End_of_Article - Last_Page 487------------