The Bug Creek problem and the Cretaceous-Tertiary transition at McGuire Creek, Montana

Bug Creek assemblages from Montana, transitional in composition between typical Cretaceous and Paleocene vertebrate faunas, are critical to K-T extinction debates because they have been used to support both gradual and catastrophic K-T extinction scenarios. Geological and palynological data from McGuire Creek indicate that Bug Creek assemblages are Paleocene and restricted to channel fills entrenched into older sediments, suggesting that the Cretaceous component of the assemblage was reworked. Thus, the author concludes, "Paleocene dinosaurs" are an illusion and the K-T survival rate of mammals is low because the presence of Cretaceous mammals in Bug Creek assemblages is also the result of reworking

Priabonian, late Eocene chronostratigraphy, depositional environment, and paleosol-trace fossil associations, Pipestone Springs, southwest Montana, USA

Sanidine 40Ar/ 39Ar ages of lapilli tuffs and the mammalian fauna of Pipestone Springs strata provide a high-resolution chronostratigraphy for upper Eocene (Priabonian) rock units in southwestern Montana. Two felsic lapilli tuffs with weighted-mean 40Ar/ 39Ar single crystal sanidine ages of 37.50±0.02 Ma and 36.00±0.20 Ma both fall within the Priabonian, late Eocene. These tuffs occur within the basal to upper part of the 55 m of exposed Pipestone Springs strata. The uppermost 15 m yield a diverse and abundant assemblage of mostly small-bodied middle Chadronian (Priabonian, late Eocene) mammals. The older lapilli tuff is an ashfall tuff, whereas the younger lapilli tuff exhibits minor aeolian reworking. The new 40Ar/ 39Ar age constraints significantly increase the age range of Pipestone Springs strata to include uppermost Duchesnean–lowermost Chadronian (Priabonian, upper Eocene) deposits in addition to its well-known mid- dle Chadronian vertebrate assemblage. These new 40 Ar/39Ar ages combined with its mammalian fauna further support Pipestone Springs strata as age-correlative to the Flagstaff Rim section in central Wyoming, and provide a basis for better determining late Eocene mammalian paleogeography and regional paleolandscapes in the United States Rocky Mountain to Great Plains areas. Loessites intercalated with paleosols dominate Pipestone Springs deposits. The recognition of loessites comprising these strata is a new depositional interpretation of Pipestone Springs strata, making these loessites some of the oldest known aeolian Eocene strata in the Great Plains–Rocky Mountains region. Pipestone Springs paleosols developed on lapilli tuffs are vertisols. Alfisols and inceptisols, developed from a parent material of volcanic glass mixed with non-volcanic grains, are the remaining paleosols within the loessite strata. Additionally, a new and important discovery in this project is the recognition that all paleosols are extensively bioturbated, containing trace fossils similar to Rebuffoichnus and newly identified trace fossils resembling Feoichnus, Eatonichnus, Fictovichnus, and Coprinisphaera

New records of eutherian mammals from the Goler Formation (Tiffanian, Paleocene) of California and their biostratigraphic and paleobiogeographic implications /

no. 379

Cretaceous extinctions: multiple causes

[Letter