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.

Tertiary Paleovalleys in the Laramie Mountains, Wyoming

The Laramie Mountains are part of the central Rocky Mountains in southeastern Wyoming. Archean and Proterozoic rocks form the bulk of the mountain range due to late Cretaceous–early Eocene (Laramide) basement-involved uplift. Hogbacks made of Paleozoic to Mesozoic age rocks flank much of the

The Laramie Mountains of southeastern Wyoming contain Proterozoic and Archean rocks that are now exposed by a late Cretaceous –early Eocene (Laramide) basement-involved uplift.

The Laramie Mountains of southeastern Wyoming contain Proterozoic and Archean rocks that are now exposed by a late Cretaceous–early Eocene (Laramide) basement-involved uplift. The Precambrian rocks are flanked by hogbacks of Paleozoic to Mesozoic age rocks as seen in the above photo.

Precambrian cored mountain areas. But what sets the Laramie Mountains apart from the adjoining Colorado Front Range and even the western Great Plains is that upper Eocene to Miocene strata are preserved within the Laramie Mountains and on its sides as paleovalley fill. The reasons for this unusual paleovalley fill preservation can probably be tied to the Laramie Mountains being much lower in elevation than the adjoining Colorado Front Range and that they were not glaciated during the Pleistocene.

I went on a field trip a few days ago specifically to look at the Laramie Mountains Tertiary paleovalleys. It was a really good trip. Emmett Evanoff led the trip and because he’s spent so much time working in the area, he had much info and insight on the paleovalleys. What follows are a few photos from the trip:

High Plains escarpment of Tertiary rocks on the eastern flank of the Laramie Mountains near Chugwater Creek. Eocene White River mudstone and siltstone, beds are capped by coarse sandstone beds. An overlying gravelly sandstone unit, probably of the upper Oligocene Arikaree Formation lies above the White River beds. The Miocene Ogallala Formation of stacked conglomerate sheets caps the entire section.

High Plains escarpment of Tertiary rocks on the eastern flank of the Laramie Mountains near Chugwater Creek. Eocene White River mudstone and siltstone beds are capped by coarse sandstone beds. An overlying gravelly sandstone unit, probably of the upper Oligocene Arikaree Formation lies above the White River beds. The Miocene Ogallala Formation containing stacked conglomerate sheets caps the entire section.

The walls to the Tertiary paleovalleys near Chugwater Creek are hogbacks of overturned rocks ranging from Pennsylvanian to Cretaceous in age.

The walls to the Tertiary paleovalleys near Chugwater Creek are hogbacks of overturned rocks ranging in age from Pennsylvanian to Cretaceous.

Daemonelix burrow in Arikareean strata. The burrow is a corkscrew shaped burrow made by the ground beaver Palaeocastor.

We found a Daemonelix burrow in Arikareean strata. The burrow is corkscrew shaped and was probably made by the ground beaver Palaeocastor.

foodtruck

Pat’s food truck was a welcome sight during the field trip. As she said – good food and good rocks – what’s better than that?

 

Large boulders occur at the base of White River Formation in the Toltec Tertiary paleovalley. The Toltec paleovalley is on the west side of the Laramie Mountains where basal Tertiary strata are exposed at and close to the range margins.

Large boulders occur at the base of the White River Formation in the Toltec Tertiary paleovalley. The Toltec paleovalley is on the west side of the Laramie Mountains where basal Tertiary strata are exposed at and close to the range margins.

Polished boulders of Precambrian granite are found in the Garrett paleovalley which now lies in the drainage area of the North Laramie River. Wyoming is known for wind and these boulders certainly attest to that.

Polished boulders of Precambrian granite are found in the Garrett paleovalley which now lies in the drainage area of the North Laramie River. Wyoming is well known for wind and these boulders certainly attest to that.

 

 

 

Glacial Geology Field Tripping in the Northern Yellowstone Area

Living near Yellowstone National Park has its advantages – and the best of these is being easily able to go on field trips to the Park area. A field trip opportunity came up last week when the Rocky Mountain section of the Geological Society of America came to Bozeman, Montana, for its annual meeting. One of the meeting field trips was the “Glacial and Quaternary geology of the northern Yellowstone area, Montana and Wyoming”. The trip was led by Ken Pierce, Joe Licciardi, Teresa Krause, and Cathy Whitlock. Having spent much time in the Yellowstone area, I was ecstatic about going along to find out about recent geological work. I won’t elaborate on the specifics of the trip, but for those interested in more than the photos posted below, the field trip guide is available in The Geological Society of America Field Guide 37, 2014, p. 189-203. It’s worth a read!

A few of the stops on the trip:

Paradise Valley – Chico Moraines and Chico Outwash (45.3402 N, 110.6967 W)

Chico moraine boulders have an average cosmogenic age of 16.1 +- 1.7 10BE ka.

 

A succession of outwash terraces border the melt-water channel which is now the Chico Hot Springs road.

North Gardiner Area – Giant Ripples (45.0551 N, 110.7659 W)

Giant ripples occur on a mid-channel bar a few miles north of Gardiner, Montana.

Cosmogenic ages on the flood deposit boulders of the giant ripples average 13.4 +- 1.2 10Be ka.

Northern Yellowstone NP – Blacktail Deer Plateau (44.9577 N, 110.5652 W)

The Blacktail Plateau is capped by moraines of Deckard Flats age - 14.2 +- 10Be ka.

The Blacktail Plateau is capped by moraines of Deckard Flats age – 14.2 +- 10Be ka.

Northern Yellowstone NP – Phantom Lake Ice-Marginal Channel (44.9554 N, 110.5289 W)

The ice-marginal channel that Phantom Lake lies in was cut into volcanic bedrock during the Pinedale glacial recession. The lake is dammed on its down-stream end by a post-glacial age alluvial fan.

The ice-marginal channel that Phantom Lake lies in was cut into volcanic bedrock during the Pinedale glacial recession. The lake is dammed on its down-stream end by a post-glacial age alluvial fan.

Northern Yellowstone NP – Junction Butte Moraines (44.9128 N, 110.3854 W)

The Junction Butte moraines have an age date of 15.2 +-1.3 10Be ka. Large  boulders of Precambrian crystalline rocks and several ponds typify the morainal surface.

The Junction Butte moraines have an age date of 15.2 +-1.3 10Be ka. Large boulders of Precambrian crystalline rocks and several ponds typify the morainal surface.