Flagstaff Rim, Wyoming – A Classic Area of Continental Eocene Tuffs and Fossil Vertebrates

Flagstaff Rim strata, in central Wyoming, contain numerous Eocene tuffs and fossil vertebrates.

The Flagstaff Rim area in central Wyoming contains a classic geological section of Tertiary continental rocks that, for the most part, range in age from approximately 37 million years to about 35 million years. These strata are then capped by gravels that may be late Tertiary in age (probably younger than 20 million years in age, although there are no age constraints on them). I became interested in this section because the 37-35 million year part of it has strong similarities in terms of age and fossil vertebrate assemblages with Eocene continental rocks at Pipestone Springs, southwestern Montana where I’ve been working.

Eocene rock section locations for Pipestone Springs, southwest Montana and for Flagstaff Rim, central Wyoming.

Much work has already been done at Flagstaff Rim for both fossil vertebrates and Tertiary tuff ages (see Emry 1973; Emry 1992; Emry and Korth 2012; Sahy et al. 2015 for some background). But – a group of us working on continental Tertiary strata in the US Great Plains-Rocky Mountains decided it was time to resample all the tuffs in the Flagstaff Rim section and do 40Ar/39Ar single crystal sanidine age analyses and high-precision U–Pb dating of zircon on these tuffs and several of the section’s detrital beds. Emmett Evanoff, now at the University of Northern Colorado, graciously arranged our field work/camping venue. Bill McIntosh, at the New Mexico Geochronology Lab, and Steve Hasiotis, at the University of Kansas Geology Department, were also a part of our field crew. Bob Emry, Smithsonian Institution emeritus, joined us for a day, and told us about his decades-long work with fossil vertebrates at Flagstaff Rim. We had a very productive field time – and all section tuffs as well as some detrital beds were sampled. A back-breaking, sample-hauling hike at times, but always an amazing place as shown by the numerous photos below.

A white-colored tuff from the lower Flagstaff Rim section crops out in the central part of the photo.
Sampling the lowermost tuff from the Flagstaff Rim section.
The basal part of the Flagstaff Rim section is a paleochannel complex, so needless to say, it contains coarse-grained deposits. Hard to find a prospective bed for sampling detrital sanidine, but we may have found one. We’ll see!
The upper part of the Flagstaff Rim Section containing tuffs G through J. The dark-colored beds at the section’s top are the overlying, later Tertiary gravels.
An Isolated channel tuff occurs in the upper part of the Flagstaff Rim section. No radioisotopic or zircon age exists for this tuff, so it will be good to add these to the tuff age database.
Tuff J-1 near the top of the Flagstaff Rim section must give off a lot of energy as our hardy field crew levitates above it at the end of the field day.

Background Reading:

Emry, R.J. 1973. Stratigraphy and preliminary biostratigraphy of the Flagstaff Rim area,

Natrona County, Wyoming. Smithsonian Contributions to Paleobiology 18: 48 pp.

Emry, R.J. 1992. Mammalian range zones in the Chadronian White River formation at

Flagstaff Rim, Wyoming. In: D.R. Prothero and W.A. Berggren (eds.), Eocene–

Oligocene Climatic and Biotic Evolution, 106–115, Princeton University Press. Princeton, New Jersey.

Emry, R.J. and Korth, W.W. 2012. Early Chadronian (late Eocene) rodents from the

Flagstaff Rim area, central Wyoming. Journal of Vertebrate Paleontology 32:

419–432.

Sahy, D., Condon, D.J., Terry, D.O., Fischer, A.U., and Kui­per, K.F. 2015. Synchronizing

terrestrial and marine records of environmental change across the Eocene–

Oligocene transition. Earth and Planetary Science Letters 427: 171–182.

Welcome To My 2021 Field Office

This is my summer receptionist… a Yellowstone 2020 wolf pup of the Junction Butte pack that roams mainly in the Lamar Valley of northern Yellowstone National Park.

My geological field work lately has taken me to several areas of western Montana, so I thought I’d do a visual collage of a few of the landscapes where I’ve been working. To start with, I’ve been spending time flying drones over Tertiary exposures in southwestern Montana, Great fun and good insight into Tertiary geology. Many of my flights are focused on Eocene strata at Pipestone Springs. Along with 3 co-authors (Don Lofgren, Stephen Hasiotis, and Bill McIntosh), we have a paper on Pipestone chronostratigraphy, trace fossils, and depositional environments that is now in review. Below are a couple of drone photos from Pipestone Springs.

Pipestone Springs Eocene strata with the basal lapilli tuff (37.5 million years in age) and the upper trace fossil bed (36 million years in age) indicated. Note the person standing on the surface of the upper trace fossil bed.
A somewhat closer drone view of the upper trace fossil bed with me as pilot standing off to the right side of the photo. My drone landing pad is my friend’s political campaign sign!

My travels through a part of northwestern Montana last week put me in a very different geologic setting from southwestern Montana. Proterozoic rocks are the mainstay in this area, and they make for some spectacular landscapes. So spectacular in fact, that I’ll just do a barrage of photos from the east side of Glacier National Park…

Chief Mountain, located a few miles north of Babb, Montana, is the iconic geologic view of major thrust fault where Proterozoic rocks (Chief Mountain rocks) are pushed over Cretaceous strata (green area of this photo).
No matter how much has changed in this past year because of the pandemic, it is just astounding to me to see a sign that says the Canadian border crossing is closed. It was a heavy mix of emotions being out in the early morning to view Chief Mountain, but then to see this sign and know that we’re still all in this even a year out. Whew…
Of course, no visit to Many Glacier is complete without marveling at Grinnell Point.
One of the positive aspects of getting up very early on a spring day is the sun rise near the Many Glacier Lodge. It didn’t last long as rain clouds pushed in but what a view for a time!
And another plus to being up early in the Many Glacier area is to watch a black bear amble down the road.
Coming back south, down the Front Range of the Rockies, we got out from under the rain clouds. Once again, geologically, the rocks that comprise Dancing Lady Mountain have been thrust over younger, Cretaceous strata (basically the area in green on the photo).

In summary, this is just a quick view of a couple western Montana areas where I’ve been recently. I have to say that I’m really looking forward to more amazing places to work this field season. I may swap out my office receptionist, though.

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).