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

Ischyromys articulated skeleton from late Eocene strata, Pipestone locality, southwest Montana.

Finally – the work done by myself and my co-authors, Don Lofgren, Steve Hasiotis, and Bill McIntosh, is published in the new issue of Acta Palaeontologica Polonica 67 (1): 5–20. Our work combines chronostratigraphy with depositional environment interpretations and paleosol-trace fossil associations for a new view of a well-known Eocene vertebrate locality in southwest Montana. We had fun and learned much from integrating various aspects of Pipestone’s Eocene geology, vertebrate paleontology, ichnofossils, and radioisotopic age constraints to better understand this amazing locality.

Here’s the abstract:

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 middle
Chadronian vertebrate assemblage. These new 40Ar/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.

Link to publication pdf: Pipestone_hanneman and others

LATE EOCENE CHRONOSTRATIGRAPHY, DEPOSITIONAL ENVIRONMENT, AND PALEOSOL-TRACE FOSSIL ASSOCIATIONS, PIPESTONE SPRINGS, SOUTHWEST MONTANA

I just received notice from the Geological Society of America (GSA) that our abstract is now accepted for the GSA 2020 annual meeting. I was very much looking forward to going to Montreal for the meeting, but like much else, it will now be virtual. Our presentation is scheduled for the session titled “D23. Recent Advances in Understanding Environmental Changes and Their Effects on Sedimentation”, which will be on Monday, 26, October 2020, beginning at 1:30 PM. And I say our abstract, because my co-authors are: Steve Hasiotis (Department of Geology, University of Kansas, Lawrence, Kansas), Don Lofgren (Raymond M. Alf Museum of Paleontology, Claremont, California,) and Bill McIntosh (New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico). We’re excited to get this abstract out in the public domain as it details the first single-crystal sanidine 40Ar/39Ar ages for the well-known vertebrate locality of Pipestone Springs in southwestern Montana. We also have other significant findings, such as newly-identified trace fossils and the presence of loessites in the Pipestone Springs section. Our paper on these findings is nearing completion, soon to be submitted to a peer-reviewed journal. Anyways, here’s our Pipestone Springs abstract:

Sanidine 40Ar/39Ar ages of lapilli tuffs and the mammalian fauna of Pipestone Springs Main Pocket provide a high-resolution chronostratigraphy of late Eocene strata in the Pipestone Springs area of 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, occur within the basal to mid-section of the 55 m of exposed Pipestone Springs strata, whereas the upper 15 m yields a diverse and abundant assemblage of mostly small-bodied middle Chadronian mammals. The older lapilli tuff is an airfall tuff whereas the younger lapilli tuff exhibits some aeolian reworking. Loessites intercalated with paleosols dominate Pipestone Springs deposits. Andic paleosols are developed on the lapilli tuffs. Buried B cambic to weakly developed argillic horizons characterize the remaining paleosols that are also classified as andic because there is a significant component of volcanic grains mixed with identifiable non-volcanic grains in their parent material. All paleosols are extensively bioturbated, containing newly identified trace fossils likely constructed by dung beetles (Coleoptera) based on comparisons to modern and ancient traces attributed to this group. Close examination shows that the tracemakers built these structures in a helical pattern from the inside and outside by adding pelletized sediment from the base upward, such that the architectural elements resemble features of Rebuffoichnus, FeoichnusEatonichnus, and Coprinisphaera. The preserved forms likely reflect a continuum of state of completion by adults and usage by larvae and pupae, and final preservation in the paleosols. The new isotopic age constraints significantly increase the age range of the Pipestone Springs strata to include early Chadronian deposits in addition to its well-known middle Chadronian vertebrate assemblage. Recognition of loessites comprising these strata is also a new interpretation, making these deposits some of the oldest known aeolian Eocene strata in the Great Plains–Rocky Mountains region.

Pipestone Springs Main Pocket vertebrate locality (middle Chadronian).