SEDIMENTOLOGY OF A LOWER CRETACEOUS FLUVIAL SYSTEM, KOOTENAI FORMATION, SOUTHWESTERN MONTANA

Sheet sandstone units exposed in a strike belt three kilometers long within the Kootenai Formation of southwestern Montana are composed of multilateral and to some extent multistory channel deposits. Sandstone units range from two to seven meters in thickness and are encased in calcareous mudstone deposits of overbank and flood plain origin. Channel deposits are composed of medium- grained chert arenite and conglomerate. The framework grains of the conglomerate beds are calcareous mudstone clasts derived from adjacent floodplain deposits. Beds of conglomerate compose approximately 25% of the channel sequences. Fluvial facies within the channel complex include midchannel sandstone, channel margin conglomerate, and levee sandstone. Midchannel sandstone sequences are characterized by large-scale trough cross stratification and by the absence of conglomerate. Conglomerate beds with granule and pebble sized mudstone clasts occur at channel margins and interfinger with the lateral edges of the midchannel sandstone beds. Levee deposits are composed entirely of sandstone. They are poorly developed and occur as thin beds in interchannel areas. Floodplain deposits overlying and underlying the sandstone intervals contain nodules of calcareous mudstone and less abundant nodules of calcite. The nodules are identical in size and composition to the mudstone clasts of the channel margin conglomerate. The abundance of locally derived clasts from adjacent floodplain deposits, the absence of clearly developed channel margins and the sheet geometry of the channel sequences suggests that Kootenai streams had laterally unstable margins and migrated by erosion of floodplain deposits. This pattern is consistent with braided stream systems with multilateral channels. Abundant calcareous nodules in the flood plain deposits may have been formed by pedogenic processes and suggest arid climatic setting.