CONSTRAINING CHRONOSTRATIGRAPHIC BOUNDARIES WITHIN THE CRETACEOUS AND PALEOCENE FORMATIONS OF THE EASTERN CRAZY MOUNTAINS BASIN, MONTANA

The basis of temporal benchmarks in Cretaceous Kootenai to Paleocene Melville (Tongue River) Formations in the eastern Crazy Mountains Basin (CMB) and Shawmut Anticline of south-central Montana is considered in the correlation of chronostratigraphic boundaries within these strata. Although significant work on mammals is published, much more evidence is identified as available to document the mammalian biostratigraphic record and independently provide temporal control based on initial paleomagnetic and molluscan studies.

Lithostratigraphic units in the considered area, comprising nearly 4400 m of strata, include the Lower Cretaceous Kootenai and Colorado Formations; Upper Cretaceous Eagle, Claggett, Judith River, Bearpaw, Lennep, and Hell Creek Formations; Upper Cretaceous–Lower Paleocene Bear (Tullock) Formation; and Paleocene Lebo and Melville Formations. Biostratigraphic correlation of continental horizons within the Cretaceous strata are based on the occurrence of dinosaurs and mollusks; Cretaceous marine units are correlated based on numerous marine/brackish invertebrates; and Paleocene continental strata are correlated using vertebrates (predominantly mammals), mollusks, and plant fossils. Radiometric dates are known from some of these and correlated units elsewhere in the Western Interior, but none, as yet, from the CMB.

Paleomagnetic studies within the CMB section to date span the uppermost part of a thick (~490-m) Hell Creek Formation through the lower part of the Melville Formation. The approach taken better constrains the Lancian- through Tiffanian-age strata (lower and upper part of the CMB section). Extensive search has yet to produce useful fossil mammals in the uppermost ~500 m of exposed Melville Formation, indicating that paleomagnetic and other data will be necessary to precisely constrain the upper age of the entire section.