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.