An Afternoon Spent Packing Heat, or, An Introduction to the Chadron

The focus on Badlands National Park persists, with a post inspired by a recent field outing. In this case, the objective was potentially radioactive sedimentary rocks, but a brief introduction to one of the formations of Badlands NP is warranted:

Act I: A troubled upbringing

The Chadron Formation (32 – 37 Ma) has a troubled history, not unlike many formations in the western United States. It was originally (and still commonly) distinguished by its “Titanotherium beds/graveyards” (e.g. Hayden, 1857; Clark, 1937), referring to the remains of the huge (Asian elephant-sized) animals now called brontotheres (reconstruction seen at right from here). This is a problematic term, as brontotheres are actually rare in the formation (Prothero and Whittlesey, 1998), and the famous “graveyards” may not actually be plural (see Harksen and Macdonald, 1969). Regardless, brontotheres are very large and distinct fossils, and are restricted to the Chadron in the area of Badlands NP, as far as I know. A type section for the Chadron was eventually assigned (Harksen and Macdonald, 1969), but is admittedly of poor quality, and the boundary with the overlying Brule is “basically continuous” (Stoffer, 2003), causing some individuals (including myself) to wonder why they are considered separate formations…

Anyways, the Chadron Formation is one of the lowermost beds in the stratigraphy of Badlands National Park, directly underlying the cliff-forming Brule Formation. It is predominantly green to grey massive mudrocks and (particularly towards the top) thin marls and limestones. It is also relatively shy in the north unit of the park, but is (allegedly) beautifully exposed in the south unit*. Assigning rocks to the Chadron is a rather casual affair in the park, as most poorly consolidated mudrocks of Late Eocene age are uncritically tossed into the formation (Stoffer, 2003). The high clay content in the Chadron causes them to weather into rolling “haystack” buttes, in contrast to the sharp cliff-like buttes of the Brule (image at left from Stoffer, 2003). The Chadron exhibits the last of the environment that existed in the Eocene in the Badlands NP area, documenting extensive streamside forests, low sedimentation rates, and strongly developed paleosols (Retallack, 1983). Also, in contrast to later beds, mammal fossils are most common in channels sands, not paleosols (Retallack, 1983).

* I have not yet explained – Badlands NP has a north unit and a south unit, with a thin strip connecting the two; the park is shaped like a big, sheared dumbbell. And there’s also a chunk of park to the east of the south unit. 

Act II: I defy the basic physics of radiation

Anyways, in the upper Chadron Formation just outside the park boundary in the south unit is an exposure of channel sandstones with uranium-rich minerals at its base. It is suspected that downward-moving meteoric water carried uranium from overlying ash beds until it reached the base of the sandstone, which is underlain by an impenetrable claystone (Moore and Levish, 1955). I made it my duty last Sunday to try and find this exact sandstone. Thankfully, the original publication on the sandstone (Moore and Levish, 1955), includes a marked topographic map and photographs, so I was in luck. After roughly two hours of hiking over distinctly different terrain depending on which side of the tables I was on, I came upon the following exposure (photo from Moore and Levish (1955) on left, with the same ridge marked "A" in both images for comparison):

As I approached the area marked by the arrow of Moore and Levish (1955) (it's in the middle, and very small), the infamous sandstone was exposed in all its glory (backpack at lower right for scale):

I collected a few samples from the base of the (very well cemented) sandstone, but I won’t know if I was truly successful until I find a Geiger Counter or a bored petrologist. Interestingly, some bone fragments were also found nearby, with distinct coloration. Here, for instance is a cross-section of a distal ungulate tibia (cyan book cover background for color enhancement):

The yellow "rim" is notable - bright oranges and yellows are not uncommon for fossils from the area of Badlands NP, and are suspected to indicate the fossils might be radioactive (M. Cherry, personal communication). Considering the nature of the nearby uranium-bearing sandstones, this is not particularly surprising. As I mentioned before, fossils are most common in channel sandstones in the Chadron (Retallack, 1983), and bones commonly show high concentrations of certain minerals relative to the surrounding rocks, due to their difference in porosity. However, while radioactive fossils are not uncommon (e.g. Farmer et al., 2008), I have not been able to find literature that details why these brightly-colored fossils in Badlands NP are specifically suspected to potentially be radioactive. For now, I am left unsure if this assumption is based on mineralogical studies (good) or just color comparison (bad).

 

REFS

Clark, J. 1937. The stratigraphy and paleontology of the Chadron Formation in the Big Badlands of South Dakota: Carnegie Museum Annals, v. 25, p. 261-350. 

Farmer C.N., Kathren, R.L., and Christensen, C. 2008. Radioactivity in fossils at the Hagerman Fossil Beds National Monument: Journal of Environmental Radioactivity, vol. 99, no. 8, p. 1355-1359. 

Harksen, J.C., and Macdonald, J.R. 1969. Type sections for the Chadron and Brule Formations of the White River Oligocene in the Big Badlands of South Dakota: South Dakota Geological Survey Report of Investigations 99, 23 p.

Hayden, F.V. 1857. Notes on the geology of the Mauvaises Terres of White River, Nebraska: Proceedings of the Academy of Natural Sciences of Philadelphia, v. 9, p. 151-158.

Moore, G.W., and Levish, M. 1955. Uranium-bearing sandstone in the White River Badlands, Pennington County, South Dakota: U.S. Geological Survey Circular 359, 7 p.

Prothero, D. R., and Whittlesey, K.E. 1998. Magnetic stratigraphy and biostratigraphy of the Orellan and Whitneyan land mammal "ages" in the White River Group, in Terry, D.O., Jr., LaGarry, H.E., and Hunt, R.M., Jr., eds, Depositional Environments, Lithostratigraphy, and Biostratigraphy of the White River and Arikaree Groups (Late Eocene to Early Miocene, North America): Geological Society of America, Special Paper 325, p. 39-61.

Retallack, G.J. 1983. Late Eocene and Oligocene paleosols from Badlands National Park, South Dakota: Geological Society of America Special Paper 193, 82 p.

Stoffer, P.W. 2003. Geology of Badlands National Park: A Preliminary Report: U.S. Geological Survey, Open-File Report 03-35, 62 p. (pdf available here)