Badlands National Park in South Dakota is one of the world’s classic geological and paleontological localities. Brett, Tim, and I spent a couple of days in the park to collect material for Brett’s virtual field trip project. The Badlands Loop Road is a beautiful drive and most of the park’s major geological features can be seen from the road’s overlooks and adjacent trails.
The oldest rocks exposed in the park are marine units, the Cretaceous Fox Hills and Pierre Formations. In the park the Cretaceous beds generally dip to the south, away from the Sage Creek Anticline, a subsurface fold associated with the Laramide Orogeny. The tilted Cretaceous units are eroded on the top surface and form an angular unconformity with the overlying Paleogene units, constraining the age of deformation to between the Cretaceous and the Eocene:
Two prominent paleosols are the next highest units in the section. The Yellow Mounds Paleosol is mostly developed in the Cretaceous beds, while the Interior Paleosol is largely developed in Eocene sediments, but the source rock for the soils can change depending on the location. These are thick, well-developed soils that probably formed over a long period of time under humid conditions (Retallack, 1985).
The bulk of the Badlands section is made up of the White River Group; in fact, the Badlands include the type section for this group. The whole group is made up mostly of alluvial and fluvial (river-related) sediments. Most of the sediment itself is debris eroded from the Black Hills and reworked volcanic ash blown in from the west.
The oldest of these units is the Chadron Formation, which weathers to gray, rounded mounds. The Chadron was originally thought to be Early Oligocene, but is now considered to be Late Eocene (Terry, 1998).
The Brule Formation is perhaps the unit for which the Badlands are best known. It forms steep-sided cliffs when it weathers, and includes numerous brightly-colored paleosols; over 80 of them! The paleosols tend to be redder and perhaps thicker in the lower part of the Brule (the Scenic Member) than in the younger Poleslide Member. The Brule is Oligocene in age.
The youngest unit on the Badlands Loop is the Sharps Formation, which is part of the Arikaree Group and is Late Oligocene in age. Like the Brule, the Sharps contains a lot of ash, including the Rockyford Ash that marks the base of the unit and is several meters thick. Also like the Brule, there are a number of paleosols in the Sharps, but they tend to be white or gray rather than red.
Moving vertically through these units, the paleosols generally become less brightly colored, due to an increase in the amount of carbonates. This can also be seen in the nodular texture present in both the Poleslide Member and the Sharps:
An increased carbonate content in soils is generally associated with more arid conditions, indicating that this area was becoming drier through the Oligocene (Retallack, 1985).
Another interesting feature in the Poleslide and the Sharps is the abundant sedimentary dikes – linear vertical fractures filled with sediment, mostly reworked volcanic ash in the case of the Badlands dikes:
At least some workers (for example, Retallack, 1985) believe that the sedimentary dikes are also associated with more arid conditions, and that they’re essentially giant desiccation cracks. However, the dikes have two main trends, suggesting that something else may control their distribution.
Even though the bulk of the Laramide uplift in this area was completed by the Eocene, there is lots of evidence that some relatively minor structural deformation continued into at least the Oligocene. There are numerous faults throughout the Badlands that offset the younger sedimentary units. One of the larger normal faults is easily visible where it offsets the gray Chadron Formation next to the brightly colored Interior and Yellow Mounds Paleosols:
And here’s another offsetting paleosols in the Scenic Member of the Brule:
Given the presence of all this faulting it wouldn’t surprise me if the same forces influenced the formation, or at least the position, of the dikes.
In the next post I’ll talk a bit about the fossils found in these units.
Retallack, G. J. 1985. An excursion guide to fossil soils of the mid-Tertiary sequence in Badlands National Park, South Dakota, p. 277-302 in J. E. Martin (ed.), Fossiliferous Cenozoic Deposits of western South Dakota and northwestern Nebraska. Dakoterra V. 2, Pt. 2.
Stoffer, P. W. 2003. Geology of Badlands National Park: A Preliminary Report. USGS Open File Report 03-35, 63 p. http://geopubs.wr.usgs.gov/open-file/of03-35/of03-35.pdf
Terry, D.O. Jr., (ed.) 1998. 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, 216 p.
I was finally able to upload a Gigapan that shows the Badlands stratigraphic section, from the Fox Hills all the way to the Sharps.
* Edited to correct typographical errors.