As we saw in the last post, the Black Hills are surrounded by a ring of Mesozoic and Paleozoic sedimentary rocks that get progressively older toward the interior of the Black Hills. However, after just a few kilometers we’ve passed all the way through to the bottom of the Cambrian. What rocks are found if we move even deeper into the mountains?
Mostly, we get vast thicknesses of highly metamorphosed sediments like the ones shown above. They are pretty seriously deformed, with both large and small faults (like the small ones shown below):
There are also lots and lost of folds, again including small ones…
…and some that are a bit larger:
These rocks may look pretty uniform in appearance (even to me!), but the original sediments (protoliths) were quite diverse, and include conglomerates, sandstones, and mudstones. There are some igneous protoliths as well, and at least some of these rocks have been interpreted as tectonic rift sequences.
Much of the core of the Black Hills are made up of these metamorphic rocks, but if we move to the south-central part of the range, near the town of Keystone, we start seeing a different kind of rock, seen below sitting above the metasediments:
This is the Harney Peak Granite, a classic intrusive igneous rock (meaning that it cooled below ground rather than erupting at the surface, as indicated by the large crystals it contains). The Harney Peak is perhaps most famous as the source rock for the carvings of the Mt. Rushmore and Crazy Horse memorials:
In both these cases the metasediments can be seen surrounding the carved granites. (We’ve also posted a Gigapan of Mt. Rushmore, if you want to see the boundary between these rocks up close). The Harney Peak is widespread from Keystone south to near Custer. Where it is the dominant surface rock, weathering along intersecting joint surfaces has eroded it into impressive spires along the Needles Highway:
The presence of the Harney Peak gives us some tools to work with in reconstructing the history of these rocks. Because it is an igneous rock, it’s possible to use radioisotopes to determine when the Harney Peak cooled; it turns out to be around 1.715 billion years old, putting it in the late Paleoproterozoic Era. Moreover, the Harney Peak doesn’t show any evidence of the intense metamorphism that affected the metasediments that make up the bulk of the Black Hills. On the contrary, the metasediments show a zone of contact metamorphism where they’re close to the Harney Peak, that cuts across the other metamorphic features. This all shows that, not only are these rocks older than the Harney Peak, but they had most been deformed and metamorphosed prior to the Harney Peak’s emplacement as well. Most of this metamorphism is thought to have take place between roughly 1.8 and 1.7 billion years ago.
(If those numbers seem really close to the Harney Peak age – and they do overlap – keep in mind that we’re dealing with billions of years here. That 0.1-billion-year span is longer than the entire Cenozoic Era, which is only 0.065 billion years long.)
Like other deformed regions, these metamorphic rocks were folded and metamorphosed in an orogenic (mountain-building) event. These occur at tectonic boundaries where two plates collide, indicating that this was a collision zone during the Paleoproterozoic. But what was colliding?
If we leave the Black Hills and travel west into Wyoming, we can see one of the probable colliding bodies. In the core of many of Wyoming’s mountain ranges and uplifts, including along Wind River Gorge and in the Bighorn Mountains (below) are granites that are much older than the Harney Peak:
These are Archean Eon granites that are around 2.9 billion years old – more than a billion years older than the Harney Peak Granite! These rocks are part of the Wyoming Craton, an Archean microcontinent. During the Paleoproterozoic, the Wyoming Craton collided along its (current) eastern margin with another continental block. Interpretations vary as to what was on the other side of this collision; it may have been the Superior Craton, or the subsurface “Dakota Block”. There is also a lot of uncertainty about the complexity of the collision, but it probably included multiple island arc collisions as well as the opening and closing of back-arc basins. At any rate, the marine sediments surrounding the Wyoming Craton were folded and metamorphosed during the collision, with the Harney Peak Granite being emplaced near the end of this episode.
There is another piece of this story. Way out on the eastern side of the Black Hills, not far from the Paleozoic sediments, there is small patch of metamorphosed rock called the Little Elk Granite that’s not located particularly close to the Harney Peak Granite exposures:
Associated with these granites are additional heavily-metamorphosed sedimentary rocks:
When it was dated, the Little Elk Granite turned out to be about 2.5 billion years old; it’s an Archean rock that has nothing to do with the Harney Peak, which is almost a billion years younger. Moreover, the associated metasediments apparently have a metamorphic fabric that does not cut across the Little Elk Granite, indicating that the metasediments are even older than the granite. Along with a similar exposure on the western side of the Black Hills, these are the oldest rocks in South Dakota.
Much of the information in this post came from the following sources: