Stratigraphy at Toadstool

Harmon D. Maher Jr., Dept. of Geography and Geology, University of Nebraska at Omaha, 2004

The audience for this document is students who are engaging in the EUR STEP funded Toadstool project this summer. This is a distillation of what turns out to be a complex topic. You will learn to distinguish between the various units when mapping. There is a relationship between the type of minerals seen in the veins and in the fault rocks and the enclosing sediments. It will help for you to commit to memory the names of the geologic units described below.

Introduction: When you move to a new place, you learn the place-name geography, the names of the streets, etc.. This is a link to a map showing where Toadstool Park is geographically. In a fashion, you do the same when you start working in a new area geologically. You learn the names of the geologic units, major structures, geomorphic features, etc.. In this way you can mentally navigate the geologic landscape and more easily communicate with others doing the same. For sedimentary rocks, the types of units have been formalized and the science of defining, correlating and understanding these sedimentary units is known as stratigraphy. The largest units are Groups. Within them you have Formations, and within formations you can have Members, and very distinctive layers can even be named as Beds. Ideally, the unit is named after a geographic locality (called the type locality) where it is well exposed, and has been studied and defined as a unit. We will be dealing with lithostratigraphic units, based on the sediment type and characteristics. As our understanding of the geology evolves, so do the stratigraphic designations.

Broad geologic setting for Toadstool strata: Rocky Mountain uplifts, including the Laramies, the Big Horns, and the Black Hills were produced by tectonic forces some 60 million years ago, during the Laramide orogeny. One result was an apron of sediments shed from the uplifts that extended hundreds of kilometers east into the more stable continental interior. The building of this apron was episodic, both spatially and temporally. It is these sediments that dominate the landscape of western Nebraska and the Toadstool and Pine Ridge area. This occurred during the Tertiary (65-1.8 million years ago), and so we will need to learn the Tertiary stratigraphy. In NW Nebraska these strata lie disconformably (i.e. the boundary is a significant time gap in the local geologic record) on top of dark marine shales of the Pierre Formation.

Tertiary stratigraphy of the Toadstool area: The apron of Tertiary strata is broken up into three major groups in western Nebraska (although some have called for 4 to 5). From the lowest and oldest, to the youngest and highest, these are the White River Group, the Arikaree Group, and the Ogallala Group. The White River Group is the one we need to focus in on here. The White River Group is traditionally broken up into two groups, the lower Chadron Formation, and the overlying Brule Formation. It has been suggested that another thin Formation exists beneath the Chadron Formation, and while this is interesting when considering the entire geologic history of the area, for our purposes we can wait to learn about it.

Brule Formation: In the Toadstool area, the Brule Formation can be divided into the lower Orella Member, and the overlying Whitney Member. These are relatively easy to tell apart. The Orella Member has a much greater of sandstones within multicolored siltstones. These sandstones are part of river channel complexes. It is not difficult with practice to pick out the channel walls. One channel complex is filled with particularly coarse sandstone. Others can be filled with fine sandstone or even siltstone. The channels are considered nested because they are so close to each other that one cuts into the other. Banding (bedding) in the siltsones is much better developed than in the rest of the White River Group, and distinctive red-brown bands may represent old soil horizons where oxidation was prevalent in the sediment. It is the Orella Member that creates some of the most intricate badlands topography. The overlying Whitney Member is in contrast light tan colored and relatively massive siltsones with very poor definition of bedding. the contact between the two is gradational.

Image of nodular sandstone layers within siltstones of the Orella Member of the Brule formation at Toadstool Geologic Park.

Typical intricate badlands topography associated with the Orella Member in the Brule Formation of Toadstool Geologic Park.

Chadron Formation: The Chadron formation can also be subdivided, but at present, for our purposes it is not necessary to do so. It consists primarily of siltstones. The banding is thicker ( > several feet) and generally more diffuse than in the overlying Brule Formation. It is multicolored, but whereas in the Brule red-browns dominate, in the Chadron there is proportionally more green. The badlands form are more rounded and have been called haystacks. There are sandstone channel complexes in the Chadron, but these are isolated, distinctly smaller, and steeper sided than those in the Brule. There are also some distinctive white ash layers in the Chadron. The position of the contact of the Chadron with the Brule Formation is a matter of debate, but we will take it as the lowest good sandstone associated with the nested channel complex. It will not be difficult in the field to identify this boundary.

Diagram of stratigraphy at Toadstool Park from Chadron State College.