Abstract

An unusual limestone channel complex is present at the top of the Kootenai Formation in the southeastern foothills of the Elkhorn Mountains, Montana. Eight calcirudite-filled channels and one quartz arenite-filled channel are present within the channel complex which is laterally traceable for 200 meters before becoming covered or complexly faulted. In apparent cross-sectional view, individual channels are 0.9 m by 11.0 m to a maximum of 7.0 m by 59.4 m. The quartz arenite-filled channel is fine- to medium-grained and moderately to well-sorted; in contrast the limestone-clast-filled channels are very poorly-sorted with a grain size range from clay size to 37 cm (maximum diameter). Thin section modal analyses reveal the following percentage breakdown of major clast types: micrite-microspar 44 percent, ostracod biomicrite 24 percent, gastropod biomicrite 19 percent, biopelmicrite 5 percent, quartz + chert 6 percent and coated grains 2 percent. Clast composition reflects derivation primarily from the upper calcareous member of the Kootenai Formation. Individual clasts range from angular to well rounded. The channel complex is interpreted as having its origin along a storm-dominated coastline within overall transition from a nonmarine (Kootenai) to marine (Blackleaf) depositional setting. The following phases are postulated for the process of formation: 1) upper calcareous member deposition and lithification; 2) formation of a sedimentary sill; 3) fair-weather channel cutting; 4) storm cutting and filling; and 5) onlap.

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