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

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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).

EOCENE AND OLIGOCENE MAMMALS FROM THE GRAVELLY RANGE OF SOUTHWEST MONTANA

Our first paper on work that several of us are doing in the Gravelly Range, southwestern Montana, was just published in a special issue of Paludicola, Scientific Contributions of the Rochester Institute of Vertebrate Paleontology. This issue contains papers in honor of James Gilbert Honey, a paleontologist and stratigrapher who focused on the Cenozoic, particularly the paleontology/evolution of camels and the Paleocene’s Fort Union Formation geology and paleontology. We’re pleased to have our work included in this volume! You can find our entire paper at:

Rochester Institute of Vertebrate Paleontology – Paludicola:

Donald Lofgren, Debra Hanneman, Jackson Bibbens, Liam Gerken, Frank Hu, Anthony Runkel, Isabella Kong, Andrew Tarakji, Aspen Helgeson, Isabel Gerard, Ruoqi Li, Sihan Li, Zhihan Ji. 2020. Eocene and Oligocene mammals from the Gravelly Range of southwestern Montana. Paludicola 12: 263-297.

Our paper’s abstract is: High elevation outcrops of Tertiary strata in the Gravelly Range of southwest Montana yield late Uintan to Whitneyan vertebrates that comprise five mammalian assemblages; Rapamys Site, Black Butte Low, Teepee Mountain, Black Butte High, and Lion Mountain High. The Rapamys Site and Black Butte Low are late Uintan or early Duchesnean. Two new species are present at the Rapamys Site (the carnivore Lycophocyon tabrumi and the rodent Pareumys muffleri). Small mammalian assemblages from Teepee Mountain and Black Butte High are late Duchesnean-early Chadronian and Chadronian, respectively. The most diverse assemblage is from Lion Mountain High, which is correlative with Whitneyan faunas from Wyoming, Nebraska, and South Dakota. The Whitneyan age of the Lion Mountain High assemblage is further age constrained by an underlying tuff with a weighted mean 40Ar/39Ar age of 31.7 +- 0.02 Ma and an overlying basalt flow with a K/Ar age of 30.8 +- 0.7 Ma. Paleogeographic range extensions into Montana for Lion Mountain High taxa include Diceratherium tridactylum and Oxetocyon cuspidatus. The taxonomic composition of the combined Rapamys Site/Black Butte Low mammalian assemblage is most similar to those from southern California, rather than geographically closer assemblages found in Wyoming and Utah. Comparison of undescribed middle Eocene mammalian assemblages from southwest Montana to those from southern California will further elucidate the middle Eocene Montana-California paleobiogeographic affinity.

Our geology paper on this area is soon to follow….

Tertiary geology and paleontology of the central Gravelly Range – a project update

The 2017 field crew working at Lazyman Hill. The strata are late Eocene (probably 34-36 million years in age) tufa deposits.

It’s time for our yearly update talk on field work and data compilation for the Tertiary geology and paleontology of the central Gravelly Range project in southwestern Montana. The Madison Ranger District in Ennis, Montana (5 Forest Service Road) will be hosting my talk on Monday, April 2nd at 10am in the Madison Ranger District conference room. We have a project permit from the US Forest Service because our project area lies within the Madison Ranger District – and the USFS District people have been really helpful with our project logistics. Thus, this is the perfect way to let them know what we did this past field season and how the whole project is coming together. The Madison District just sent their public announcement for the talk:

Dr. Hanneman and Dr. Don Lofgren, PhD (Director, Raymond M. Alf Museum of Paleontology, Claremont, CA 91711) and their team have been executing a multiyear study in the Gravelly Range near Black Butte resulting in many interesting paleontological findings right here in our own back yard.  Please join Dr. Hanneman and the Madison Ranger District for an update on this project and what they hope to unearth this year!

It’s a very intriguing project on high-elevation, mainly Eocene-Oligocene Tertiary geology and paleontology (mostly vertebrate and floral). So – anyone with an interest in this and who is in the geographic area, is welcome at the talk!

Cenozoic Sequence Stratigraphy of Southwestern Montana

Much of my research has been focused on Cenozoic sequence stratigraphy of continental basin-fill in southwestern Montana. This approach to the stratigraphy of continental deposits has facilitated correlation of stratigraphic units both within and among the various basins of this area. I recently gave a talk about my work in this area at Montana Tech of the University of Montana. Here’s the You Tube version of my talk:

Earl Douglass and the Tertiary Geology of Southwest Montana’s Madison Bluffs

Most vertebrate paleontologists probably think of the spectacular dinosaur finds near Jensen, Utah, when the name Earl Douglass is mentioned. Douglass’s discovery of a partial Apatosaurus near Jensen in 1909  did spark the beginning of his long career with finding more dinosaur material in what we now know as Dinosaur National Monument. But Douglass began his quest for fossil vertebrates while he was in southwestern Montana – several years before he was summoned by the Carnegie Museum of Natural History’s director William Jacob Holland to find dinosaurs.

From the spring of 1894 to 1896, Douglass taught at a one-room school in the lower Madison Valley of southwestern Montana. The school house was located in the lower Madison Valley, directly west of the area known as the Madison Bluffs. These bluffs contain strata that range in age from probably as old as Eocene through the late Miocene. The strata are continental units that include alluvial fan to fluvial trunk stream deposits.

The school house near the Madison Bluffs, southwestern Montana, that Earl Douglass taught at from 1894-1896.

The school house near the Madison Bluffs, southwestern Montana, that Earl Douglass taught at from 1894-1896.

The Madison Bluffs consist of Tertiary fluvail/alluvial fan strata of probably Eocene to late Miocene age.

The Madison Bluffs consist of Tertiary fluvial/alluvial fan strata of probably Eocene to late Miocene age. The Madison Buffalo Jump State Park is located at the northwest edge of this photo.

During his tenure at the lower Madison Valley school, Douglass spent much of his spare time exploring the Madison Bluffs. At the beginning of his teaching contract in 1894, he had very little knowledge of vertebrate paleontology and of the area geology. He initially considered the Madison Bluff beds as Cretaceous in age. But when he found a “tooth very much like a Protohippus” (Earl Douglass journal entry on May 12, 1894), Douglass knew that the beds were younger in age. As time passed, he began to find a significant quantity of fossil vertebrate mammal material within the bluff’s deposits. Consequently, he immersed himself into reading about comparative anatomy so he could readily identify the fossil material. Douglass eventually used his collected fossil material for his 1899 Master’s thesis at the University of Montana – ostensibly the first Master’s degree awarded by the University.

horse jaw from douglass madbluff

Douglass kept journals of his time in the lower Madison Valley, and often detailed both the area geology as well as his fossil finds. Alan Tabrum and volunteers from the Carnegie Museum of Natural History have transcribed many of his journal entries from southwestern Montana. I’ve included two portions of journal entries to illustrate his finding of a horse jaw from the bluffs (above diagram) and one of Douglass’s drawings of “Big Round Top” (an area in the bluffs near the one-room school house) as compared to that same area today in a photo that I took about a week ago.

earldouglass_bigrt

It’s not difficult to understand how Earl Douglass became enthralled with the geology and paleontology of the Madison Bluffs. In addition to the fossil vertebrates, the bluffs contain many other fascinating geological features. Towards the central part of the bluffs (immediately south of the Madison Buffalo Jump State Park), calcic paleosol stacks mark the boundary between most likely Eocene and Miocene strata. The calcic paleosol stacks contain at least two generations of soil profiles (typically minus the A and upper part of the B horizons). Rootlets and burrows are commonly associated with these paleosols.

Volcanic tuffs also occur within the bluff’s strata, which is really handy for those of us who like isotopic age control for southwestern Montana Tertiary deposits. The tuffs could potentially help age constrain the paleosol stacks and sedimentation within the so far non-fossil bearing part of the bluffs. And with the help of the New Mexico Geochronology Lab, a group of us are working on just that aspect of Madison Bluff geology.

Calcic paleosol stacks in the central part of the Madison Bluffs, southwest Montana.

Calcic paleosol stacks in the central part of the Madison Bluffs, southwest Montana.

Roots within the calcic paleosols found at the Madison Bluffs.

Roots within the calcic paleosols found at the Madison Bluffs.

Burrows at the base of a calcic paleosol.

Burrows and roots at the base of a calcic paleosol.

Gray tuff found below calcic paleosol stacks.

Gray tuff found below the calcic paleosol stacks.