First today, I think I have been remiss in not pointing you to a great paleontology podcast, Palaeocast. All things fossils. The most recent episode is about ceratopsians, the group of dinosaurs that included Triceratops. Check it out. And speaking of ceratopsians, the oldest one known was just announced last week. It was a raven-sized little thing that lived in Montana about 105 million years ago. LINK • Link2
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Yesterday I promised that today we’d talk about some of the Oligocene debris that helped bury the Rocky Mountains. We’ll go to South Dakota to do that.
You can find the sediments that were shed off the mountains up in the mountains, but because of the Miocene and later erosion, a lot of that stuff is gone, eroded and washed onto the plains or down the Missouri River. In South Dakota’s Badlands, a nice pile of the Oligocene sediment has been preserved.
It’s eroded, to be sure, into fanciful angular shapes – that’s why the place is called Badlands, the word for a region of exposed rock, often fairly soft rock, that has eroded into steep slopes separated by an intricate meshwork of canyons and ravines. They tend to exist in arid country today, where erosion rates are relatively low and the erosion that does happen is usually catastrophic, as in flash floods. The rocks eroded tend to be relatively young, because younger rocks are often more poorly consolidated than older rocks, but there’s nothing sacred about that.
|Exposure of Oligocene Brule Formation in South Dakota Badlands (National Park Service photo)|
The White River Badlands of South Dakota preserve a thick wedge of the Oligocene sediments that eroded off the nearby Black Hills, a Laramide uplift. The sediment was carried to the east by rivers of various sizes, some that flowed strongly enough to carry gravels, and some that were parts of shallow braided streams carrying sand and silt. Flood plains saw extensive mud deposits. Over time, from the late Eocene into the early Oligocene, the deposits built up a pile that totaled more than 100 meters thick – 300 feet – in the Badlands National Park area. Across the eastern Rockies region, equivalents of these Oligocene deposits reach thicknesses of 700 feet or more.
The first package of rocks that makes up the South Dakota Badlands is called the Chadron Formation, gravels, sands, silts, muds. As the sediment was being laid down, this area was certainly not badland, but harbored abundant life. The Chadron is famous for its titanotheres or brontotheres. The nature of the cement and porosity in the Chadron formation is such that the rocks weather into smooth, rounded hillocks.
River and flood deposition wasn’t continuous over the 11 million years of the Oligocene, and in the Badlands of South Dakota, a lot of the reddish layers interbedded with the white rocks are paleosols – ancient soil horizons, red because iron was concentrated in them. There were also occasional lakes in which limestones formed, and one of them marks the break between the Chadron Formation and the overlying, younger Brule Formation. It’s really quite similar to the Chadron, but the cement in it includes considerable volcanic ash and calcite, so it holds together well and tends to form steep slopes and knife-edge ridges. The Brule is famous for turtle fossils and oreodonts. Oreodonts had teeth that were suited for eating oreo cookies – well, no, not really – oreodonts were fox-sized mammals that are not very closely related to any modern animals, but camels and pigs are not too far from oreodonts on the evolutionary tree. Technically they are called artiodactyls, even-toed herbivores. Lots of their fossils have been found in the Badlands rocks.
There’s a distinctive volcanic ash bed at the top of the Brule Formation, erupted about 30 million years ago, and it was followed by more deposition in the mid- to late Oligocene of the Arikaree formation, generally the highest unit in the Badlands National Park area.
Try to envision a vast sheet of these Oligocene strata laid out across the intermountain basins of the Rockies as well as much of the western Great Plains. There were rivers, certainly, and both erosion and deposition were both still going on, but at a rate that was nothing like it had been in Eocene and Oliogene time. The region was relatively level for millions of years – close to 25 million years. The modern badlands topography is a very recent development, within the past million years and more like just 500,000 years ago. Significant rainfall and snowmelt associated with the glacial period provided the water that cut aggressively into the Oligocene sediments, creating the modern badlands. They continue to erode and develop today.
—Richard I. Gibson
Links & References
Chadron, Brule, Arikaree
Reference: Geology of National Parks, by Ann Harris and Esther Tuttle, Kendall/Hunt Publishing.