If you’ve been following my first three installments on the Black Hills (Mesozoic, Paleozoic, and Proterozoic rocks) it should be clear by now that the Black Hills rocks are layer out in a series of concentric rings that get progressively older toward the middle, with the older rocks exposed by the erosion of younger sediments. Yet, even though the exposed rock sequences are determined by erosion, the Black Hills are still mountains that tower over the surrounding landscape; they are clearly visible from the Badlands, more than 100 km away (above).
Based on what we’ve seen in the earlier posts, it’s possible to make an extremely generalized cross section through the Black Hills:
In the diagram, gray represents Paleoproterozoic metasediments, orange is the Harney Peak granite, yellow are Paleozoic sediments, red is the Triassic Spearfish Formation, and greens are Jurassic and Cretaceous sediments. Not to scale.
From this, we can match the sediments on each side of the mountains to see the original, pre-erosion structure:
The Black Hills are an anticlinal dome that has been bent upward by tectonic forces. Of course, there was no point at which the Black Hills looked like the dome in my diagram, because erosion would have started as soon as the uplift began. The relatively soft Mesozoic and Paleozoic sediments would have eroded quite quickly, with the rate erosion slowing down once the more resistant Proterozoic rocks were exposed.
This raises several intertwined questions. What caused the uplift, when did it happen, and where did all the eroded sediment go?
Let’s start with the cause. The Black Hills don’t exist in isolation. Rather, they are the easternmost example of a whole series of mountain ranges, which include the Big Horn Mountains in Wyoming, 260 km further west:
The event responsible for these mountains is called the Laramide Orogeny, named for the Laramie Mountains in Wyoming. Nearly all the surface exposures of Archean Wyoming Craton rocks are the result of uplift during this orogeny. The Laramide Orogeny is generally though to be associated with the subduction of the Pacific Ocean seafloor under the North American plate. The subduction zone was located far to the west, but apparently it went through a period in which the subduction angle of the ocean crust was shallow. This moved the effects of subduction (like folding) further east than they would normally occur.
When did this event occur? We can get a clue by returning to the Badlands. While the Black Hills are the easternmost Laramide mountain range, there are smaller-scale Laramide structures further east, including the Sage Creek Anticline. As we’ve already seen, the Sage Creek deformation tilted Cretaceous sediments, but not the overlying Eocene sediments:
This brackets the age of the Laramide Orogeny from roughly the Late Cretaceous to the Eocene, perhaps 80 to 50 million years ago. We can see additional evidence for this in other areas around the Black Hills. For example, there are nearly flat-lying Eocene to Oligocene lake sediments in a few places within the Black Hills:
There is also an impressive series of igneous bodies scattered across the northern part of the Black Hills and into Wyoming, including Crow Peak and Citadel Rock, near Spearfish…
…Bear Butte, near Sturgis…
…Green Mountain, near Sundance…
…and, of course, Devils Tower and Missouri Buttes:
There is quite a bit of variation in the composition of these bodies, although they tend to be diorites or rhyolites. There are also many smaller scale igneous features throughout the northern Black Hills, such as these rocks cutting through the Deadwood Formation in Spearfish Canyon…
…or these light-colored igneous rocks cutting across Proterozoic rocks in the Homestake Gold Mine in Lead:
In spite of their varied composition, these igneous bodies all have some things in common. Nearly all of them are intrusive, meaning that they didn’t erupt at the surface (although one or two of them may have erupted). All are located in a band that runs approximately east-west, with Missouri Buttes at the west end and Bear Butte at the east end, which clips the northern end of the Black Hills. They cut across whatever rocks happen to be in the area, suggesting that they are among the youngest features present. Indeed, radioisotope dates back this up; all of these bodies were emplaced during the Eocene, and most likely are formed from magmas formed during the Laramide Orogeny. The fact that none of these bodies is younger than Eocene corresponds nicely with the mostly undeformed White River Group sediments to the east, confirming that by the end of the Eocene the Laramide Orogeny was essentially complete.
That leaves our third question: if the Black Hills dome has been so deeply eroded, what happened to the all the eroded rock? Roughly 5,000 cubic kilometers of Paleozoic sediment alone has been removed; this doesn’t include all the Mesozoic and Proterozoic rocks that are also missing. Again, the Badlands provide an answer:
The massive amount of Eocene and Oligocene sediment in the Badlands had to come from somewhere. Some of it is volcanic ash, blown in from as far away as Nevada. But a lot of this sediment is derived from the eroded Black Hills. The Powder River Basin in Wyoming on the other side of the Black Hills is similarly filled with early Cenozoic sediments.
With that I’m going to wrap up talking about South Dakota for the time being. We’ve been very busy in the lab over the last few weeks, and I have lots of new fossils to talk about!