The unit is mostly a black shale, although there are plenty of fossils and gypsum layers, as well as layers of a clay known as “bentonite” formed from volcanic ash settling into water. The unit varies in thickness but typically is on the order of 600 meters (~2000 feet) thick.
This shale is a marine shale, formed in fairly quiet seawater. It formed during the Cretaceous period, over stages known as the Campanian and Maastrichtian, from about 85 million to about 65 million years ago.
This shale is one of many shales sitting in the interior of North America formed at this time. One interesting question should arise from these details…why exactly was there seawater in South Dakota 80 million years ago? South Dakota today is pretty far away from the ocean, how does the ocean get there?
The answer to that question is a major part of the geologic history of inland North America, and perhaps of the globe itself. During the latter part of the Cretaceous, the ocean actually covered a portion of interior North America, creating the Western-Interior Seaway; a seaway of ocean-water running all the way from Northern Canada through the modern-day Gulf of Mexico. This seaway was enormous and deep enough that ships could have navigated through it with ease; hundreds of meters deep. The seaway persisted for millions of years, depositing marine rocks like this shale.
So what made the oceans come onto North America like that? Well, there are several reasons, a couple of which I’ll highlight here. First, at the time, the current Rocky Mountains were at their peak of being built. When a mountain range is built, it builds up enormous amounts of weight that actually can drag the Earth’s crust downwards around it. The weight of the Rocky Mountains drove subsidence to the East, dropping the land surface down in the area the seaway would form.
Second…the seas themselves were high at this time period because of the formation of new ocean floor. The ocean floor actually has a different height depending on how old it is. When new ocean crust is made, that crust is fairly warm and buoyant, it literally floats upwards on the mantle. But, as that crust ages, it cools off and becomes denser, starting to sink. When lots of new ocean crust is being made, the ocean floor rises up and can push the oceans onto the continents.
The Cretaceous Seaway was created during one of these ocean highstands. Why the oceans rose up is debated; it may have been related to a volcanic hotspot or series of them, it may have been related to the opening of the Atlantic Ocean, etc. But units like this black shale, the Pierre shale, deposited in quiet water in a giant, continent-sized seaway, are the end result.
Image Credit: National Park Service:
Research article with ages:
Pierre Shale (Britanica):