THE ORIGIN AND SIGNIFICANCE OF A POSSIBLE "FIRECLAY" IN THE CRETACEOUS KOOTENAI FORMATION, SW MONTANA

The Kootenai Formation provides an important record of terrestrial sedimentation during the Early Cretaceous in the northern Rocky Mountains. In southwestern Montana a distinctive package of highly-indurated mudrocks, colloquially referred to as a “fireclay”, is present in some exposures of the upper Kootenai Formation. This interval is important because the presence of a true high-alumina fireclay would require a period of prolonged tropical weathering that is incompatible with current paleoclimate interpretations. In this study we use field observations, thin section petrography, and major element geochemistry to provide insight into the origin of the Kootenai “fireclays”.

We examine two outcrops of the upper Kootenai Formation approximately 5 km northeast of Mt. Doherty near Cardwell, MT. In both sections, the “fireclay” package is composed of a purple mudrock sharply overlain by a fine-grained olive-colored bed. The purple mudrock is internally massive aside from three distinct calcareous horizons. These horizons are composed of 20-70% calcareous nodules that are up to 7 cm in diameter. Although field characteristics suggest that this interval is a composite paleosol complex, neither the matrix nor the nodules display primary textures in thin section. The fine-grained purple matrix does not exceed 16% alumina which does not suggest pedogenic enrichment. The olive-colored bed has a sharp basal contact with the underlying purple mudrock. Although no large-scale stratification is present, locally it contains rip-up clasts, laminated zones, and soft-sediment deformation. The olive-colored bed largely lacks primary textures in thin section and is geochemically difficult to distinguish from the underlying purple mudrocks.

Our results indicate that these beds are not fireclays. The highly-indurated nature of these beds and the destruction of almost all of the original fabrics and textures suggests that they were profoundly overprinted by diagenetic processes. These rocks were overlain by the Elkhorn Mountain Volcanics (Cretaceous) which may have provided a source of chemically active groundwater that altered or replaced material within the parent beds. However, historical reports of high-alumina clays north of our study area merit further investigation.