Abstract

Arikareean strata exposed in several intermontane basins of western Montana are composed mostly of aeolian derived tuffaceous material deposited in fluviatile, lacustrine, and paludal environments. Streams were principally aggrading; lakes were shallow, ephemeral and confined mainly to two adjoining basins; and swamps were very local with abundant plant material and occasional lignites. Bedded gypsum and gypsum cement in lacustrine strata indicate periods of dryness. Locally abundant invertebrate and vertebrate fossils are found mainly in fine-grained fluviatile overbank, lacustrine, and lacustrine delta-fill deposits; logs, branches, and other large pieces of wood are found in fluviatile channel deposits; leaves, wood, fusain, and other plant fragments are common in paludal deposits; and root traces and burrows are very common in all fluviatile and paludal deposits and some lacustrine deposits.

The Arikareean strata are bounded by unconformities and have been subdivided into three biostratigraphic intervals using diagnostic mammalian and molluscan fossil assemblages. Mammalian assemblages and magnetostratigraphy data allow important correlations with Arikareean strata in the northern Great Plains and in the Columbia Plateau. The lithostratigraphy and fossil assemblages indicate climatic conditions with mild winters and alternating humid and semiarid seasons.