ABSTRACT

Paleoclimatic interpretations of sedimentation patterns, paleosol mineralogy, and vertebrate faunal assemblages indicate that the extensive Tertiary continental clastics which fill the basins of southwestern Montana were transported and deposited in a predominantly arid to semi-arid climatic regime. The arid conditions were preceded by a major middle Eocene wet interval and interrupted by a major early Miocene wet interval. That pattern of Tertiary climatic changes coincides with worldwide climatic changes, indicated by deep ocean sedimentation rates and oceanic oxygen isotope paleo-temperature data. Transport and deposition of continental clastics under predominantly and to semi-arid conditions have profound effects upon sediment types and distribution patterns which in turn lead to models of uranium migration and distribution in the Tertiary basins of southwestern Montana.