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.

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).

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!

Irish Geo Travels – Northern Ireland

Traveling to Ireland has been something I’ve wanted to do. So, when the opportunity came up to go to Scotland, I couldn’t leave the general area without seeing at least some of both Northern Ireland and the Republic of Ireland. I only made it as far south as Dublin, but I guess on a positive side, that leaves many places that I need to visit on a future trip. I really wanted to go on a Cliffs of Moher Tour for example, as I’ve heard so many good things about them, but that’s one of the many things that will have to wait until next time unfortunately.

I flew from Glasgow into Dublin, rented a car, and first headed for Northern Ireland which is the subject of this blog. The causeway coastal route in Northern Ireland (from the North Channel coast eastward to the Irish Sea coastline) is a drive that I wanted to try. I ended up driving only about half of it – from Ballycastle east to Port Stewart because I spent so much time stopping to look at rocks and scenery.

The area that I drove through is a part of the Causeway coastline that cuts into the Antrim lava plateau. Beginning about 62 million years ago and continuing for several million years, extensive volcanic activity associated with the opening of the north Atlantic Ocean occurred here. In fact, igneous activity was so extensive in the nascent north Atlantic area, that the Antrim plateau basalts are only a small part of the North Atlantic Igneous Province, which is centered on Iceland. But – coming back more locally to the Antrim area, basaltic lava here intruded into Cretaceous marine strata, mainly chalk beds (which makes a striking visual contrast along the coastline). As noted on a Queen’s University Belfast website for the Giant’s Causeway:

The total area of these flows is now much reduced compared to their original extent, but they still constitute, at 3,800km2, Europe’s most extensive lava field. Traditionally the lavas of the Antrim Lava Group have been divided into three main phases of activity, separated by two extended periods of quiescence or limited, local activity.

The two areas that I spent most time at during my coastal causeway drive are the Carrick-a-rede Bridge and the Giant’s Causeway. These areas are developed within the Lower and Middle Basalts of the Antrim Lava Group and contain an Inter-basaltic Bed of reddish-weathered regolith and paleosols. A photo tour of the two areas are shown below –

Carrick-a-Rede Rope Bridge

A rope bridge connects the mainland with Carrick-a- Rede island. The first rope bridge was built in 1755 to facilitate fishing of Atlantic salmon. The salmon fishery has since died out, but the bridge is maintained as part of National Trust lands.

The Coastal Highway is cut into the Antrim Plateau where Paleocene basalt overlies Cretaceous chalk strata. The Lower and Middle Basalts of the Antrim Lava Group are in this area separated by a reddish-colored paleosol zone.

The hike to the Carrick-a-Rede Bridge goes over Paleocene basalt of the Antrim Lava Group.

A closer view of bridge – not too much wind when I visited, so it was a pleasant walk across the bridge.

Giant’s Causeway:

The Giant’s Causeway is a UNESCO World Heritage Site. As noted on its UNESCO website:

The Giant’s Causeway lies at the foot of the basalt cliffs along the sea coast on the edge of the Antrim plateau in Northern Ireland. It is made up of some 40,000 massive black basalt columns sticking out of the sea. The dramatic sight has inspired legends of giants striding over the sea to Scotland.

UNESCO World Heritage Site signage at the entrance to the Giant’s Causeway.

A Giant’s Causeway marker – This area was inscribed as a World Heritage site in 1986.

The paleosol zone of the lower Inter-Basaltic Bed exposed on the road to the Giant’s Causeway.

The onion skin basalt rocks at Windy Gap, on the road to the Giant’s Causeway basalt columns. These rocks have undergone much spheroidal weathering.

Causeway basalt columns…

More columns…

…and more columns. Halfway up the far slope is the reddish-colored lower inter-Basaltic bed that separates the Lower Basalt from the Middle Basalt of the Antrim Lava Group.

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:

The Field Season Is Going Strong in Southwestern Montana

My field season is in full swing. I recently spent time with students from the Webb Schools in Claremont, CA, during their annual sojourn to southwestern Montana. We prospected a few Tertiary localities, with the students making some good fossil mammal and fossil invertebrate finds. We were also extremely lucky to have a southwest Montana landowner give us a tour of a buffalo jump that is on his land. The following photos are from our various fossil site and buffalo jump field adventures.

woodin-snails

Tertiary fossil snails (about 25 My in age) at one locality captured the interest of students. Once one snail was found, everyone was intent on finding more.

Bob Haseman talks about a buffalo jump in the Toston Valley. He is standing by one of the many tepee rings associated with the jump site.

Bob Haseman talks about a buffalo jump in the Toston Valley of southwestern Montana. He is standing by one of the many tepee rings associated with the jump site. The small boulders on the surface between Bob and the students are part of a tepee ring.

Webb School students hiking up to the "Looking-Out" site associated with the buffalo jump. A eagle catchment area is immediately below the highest point of the "Looking-Out" site.

Webb School students hiked up to the “Looking-Out” site associated with the buffalo jump. A eagle catchment area is immediately below the highest point of the “Looking-Out” site.

eagle-catchment

The eagle catchment area is a shallow depression where a person would hide beneath brush awaiting the approach of an eagle. A nearby animal carcass would aid the quest to capture a eagle which was then used for its feathers.

Chadronian (about 36 Ma) age rocks yielded a few brontothere teeth and bone fragments.

Chadronian (about 36 My in age) rocks near Three Forks, Montana yielded a few brontothere teeth and bone fragments for the curious students.

Chadronian strata in this area contain brown to reddish, popcorn textured floodplain deposits and whitish-colored fine-sand channel deposits.

Chadronian strata in this area consist of brown to reddish popcorn-textured floodplain deposits that contain paleosols and whitish-colored fine-sand channel deposits.