The Puzzling Geology of Northern Slim Buttes

Slip slidin' away?

The Slim Buttes of northwest South Dakota are not your typical textbook buttes—not isolated hills with steep sides and flat tops. To me they look like a single ridge, or a "long and narrow tableland" (Gries 1993). Perhaps Slim Buttes are what North Dakota geologist John Bluemle (2016) calls "buttes in name only"—landforms that are buttes because that's what they were christened.

No matter; they're noteworthy whatever we call them. The tableland extends about 30 miles north to south, is generally less than a mile wide, and rises as much as 600 feet above the prairie. With ponderosa pine on the upper slopes and crest, the Buttes stand in clear contrast to the grassland below, a rolling green sea that reaches to the horizon in every direction.

In the northern Buttes, in the area of Reva Gap, the land has slid, fractured and tilted, producing the Castles—a mix of white badlands and green grass (in spring). The site was designated a Registered Natural Landmark in 1979 because it "illustrates the Nation's natural heritage" and "contributes to a better understanding of man's environment". Or rather it will once someone figures out what happened!

We do know that the Slim Buttes tableland is a product of erosion—a remnant of the Tertiary sedimentary rocks that used to cover the region. It's capped with hard sandstone of the Arikaree formation, which has protected older rocks below from erosion. Those softer underlying rocks are exposed at Reva Gap.

Castles stratigraphy. Not visible but important to the story is the Ludlow member of the Fort Union formation.

This stratigraphic column spans c. 50 million years, from late Cretaceous (c. 70 my ago) through lower Miocene (c. 20 my ago). The Brule and Chadron belong to the White River Group. The Chadron includes three informal units—typical Chadron, dazzling white beds, and golden beds (top to bottom).

The Castles have features that geologists love—angular unconformities. They are easy to spot, and nicely illustrate important concepts. A geologist can stand next to one and expound on some of the most basic principles of geology: original horizontality, superposition, and cross-cutting relationships. And they provide clues as to how the landscape formed.

Behold an angular unconformity!

This schematic shows the birth of a textbook angular unconformity:

It seems this story would apply to the Castles, where tilted beds of the Brule formation are capped with horizontal beds of the Arikaree formation. The rocks of the Brule, being sedimentary, must have begun as horizontal beds of sediment (principle of original horizontality), but now they're tilted. Therefore they were deformed somehow, maybe during mountain-building. The Arikaree rocks atop the Brule are younger (principle of superposition), and being horizontal, were deposited after deformation of the Brule ceased (principle of cross-cutting relationships).

However, for the Castles this story is incomplete, and could lead us to false conclusions. The final diagram in the schematic above should also include strata below the tilted beds, for herein lies a problem:

Oil and gas exploration has revealed that older sedimentary rocks out-of-sight below the tilted Brule and Chadron are horizontal! This is very hard to explain. How were beds tilted independent of beds below? Furthermore, this deformation is limited to northern Slim Buttes; no tilted Brule and Chadron have been found in the southern part. What a puzzle this jumble of rocks presents! Geologists still struggle to solve it.

John Paul Gries (1911-2003) was on the faculty of the SD School of Mines and Technology for fifty years. He's best known for Roadside Geology of South Dakota; more than 500,000 copies have been sold.

Roadside Geology of South Dakota (Gries 1993) is a wonderful guidebook, because it covers much more than what's visible from roads. For example, three pages are devoted to the Reva Gap area, including a history of geologists trying to explain its geologic history.

They came up with diverse theories that mostly failed: sharp folding and erosion; large-scale cross-bedding; major folding at a regional scale; and tilted blocks slumping off steep cliffs undercut by streams. The list ends with a "recent suggestion"—failure in very weak clay layers causing dropping and tilting of overlying beds. Gries didn't dispute this hypothesis, but concluded that "The field of speculation and interpretation is still wide open."

That recent suggestion appeared in "Stratigraphy, structure, and vertebrate fossils of the Oligocene Brule formation, Slim Buttes" (Lillegraven 1970; "recent" as of 1993). The author noted that faulting and tilting affected all strata from the Brule and Chadron formations down to at least the upper part of the Ludlow member. The Ludlow is key: "There are many shaley layers within the Ludlow or upper Hell Creek beds that could have acted as glide planes." Lillegraven suggested the underlying cause was "minor northeast movement of Ludlow through Brule sediments along a gravity-controlled detachment fault, or series of faults, somewhere deep in the Ludlow."

Detachment faults are low-angle faults along bedding planes that allow overlying strata to slide. This can cause faulting and tilting of the moving strata. Lillegraven theorized that "as the overlying sediments moved along the low-angle glide plane, there was a tendency to break into individual blocks, and superficial normal faulting [and tilting] occurred due to lateral spreading of soft clay from beneath firmer material."

A textbook detachment fault slip slidin' away. Is this what happened at Reva Gap?

Forty-five years after Lillegraven proprosed his detachment hypothesis, Ferguson et al. (2015) announced the presence at Reva Gap of a low-angle detachment in the basal orange and white sandstones of the White River Group [dazzling white beds and golden beds of the Chadron]. High-angle normal faults bounding tilted blocks of Chadron and Brule rocks connected to the detachment. But this can't be the full story. As they noted, underlying Fort Union rocks (Ludlow member) also are tilted. Maybe there's another detachment fault, deeper and out-of-sight. "This idea requires further field research," they concluded.

So the geologic story of the Castles remains unfinished, and the recommendation that Dr. Gries made to Castle visitors back in 1993 still stands: "Poke around and form your own theory." And don't worry if you're not up to the task. Even if no theory presents itself, I bet you will enjoy the experience.

Photogenic Chadron formation; Brule behind.

Yet another mystery to be grateful for.

Sources

Bluemle, JP. 2016. North Dakota's Geological Legacy. North Dakota State University Press.

Diffendal, RF. 2017. Great Plains Geology. University of Nebraska Press.

Ferguson, S, et al. 2015. Synsedimentary low angle normal detachment in White River Group strata of NW South Dakota. Abstracts with Programs, GSA 45 (7).

Gries, JP. 1996. Roadside Geology of South Dakota. Mountain Press Publishing Co.

Lillegraven, JL. 1970. Stratigraphy, structure, and vertebrate fossils of the Oligocene Brule formation, Slim Buttes, northwestern South Dakota. GSA Bull. 81:831-850.