Badlands N.P. | GeologyVirtualTrips

BADLANDS N.P.

FIELD TRIP STOP – BADLANDS TOPOGRAPHY, HIGH RATES OF EROSION, AND PRESERVATION OF THE RISE OF MAMMALS IN THE CENOZOIC ERA

LOCATION: The Badland National Park is located in southwestern South Dakota, about 50 miles east of the Back Hills.

GEOLOGIC FEATURES: Semi-Consolidated Cenozoic sediments experiencing extremely high rates of erosion.

DESCRIPTION: “Badlands Topography” refers to the significant erosion of soft (poorly cemented) sediments, commonly by cloudbursts in arid to semi-arid environments. to produce what have been variously described as Buttes, Pinnacles, Gullies, Steep ridges, Spires, Ravines, Canyons, and Flat-topped tables. The rate of erosion is significant, currently estimated at 1 inch per year. Photos taken in the mid-1970s stand in contrast to current views of the park.

Sediments of The Badlands are mainly composed of Sand, Silt, Clay, and Volcanic Ash deposited between 75 and 26 million years ago.

Between 75 and 69 mya the black Pierre Shale was deposited in and inland shallow sea having marine fossils. Subsequent uplift in the area caused the sea to withdraw, creating the Yellow Mound Paleosol seen in the Park.

As uplift continued to the west due to the formation of the Rocky Mountains in the area of the Black Hills, sediments were carried to the east to the present area of the Badlands and were deposited as the grayish Chadron Fm. (180 ft thick) between 37 and 34 mya (Upper Eocene) in warm and humid climates. These sediments represent rivers and floodplains. Evidence of early mammals are plentiful.

Above the Chadron Fm is found the tan-brown Brule Fm (400-465 ft thick) deposited between 34-30 mya (Lower Oligocene). These sediments represent alternating river channel sandstones and paleosols developed in alluvium (producing the reddish bands of strata in the striking alternating bands of red and white) in cooler and drier conditions. Mammal fossils are plentiful and easily erode from the sediment, even after a single cloudburst. (It is illegal to touch any fossil in the Park – as is the case for any National Park!)

Above the Brule Fm. is the lighter-colored Sharps Fm. laid down between 30 and 28 mya (Upper Oligocene). The bottommost layers of the Sharps represent the whitish Rockford Ash, volcanic deposits originating near the present Yellowstone N.P.

The most rugged (resistant to erosion) beds in the Park are the sandstones of the upper Brule and Sharps Formations.

Significant erosion began 500,000 years ago and continues to this day. In 500,000 years, the current Badlands N.P. will have disappeared.

STUDENT QUESTIONS:

(1) What dictates whether or not a sediment will be cemented into a solid rock?

(2) What is a Paleosol?

(3) CHALLENGE: Draw a vertical column of the Tertiary Geologic Time Scale indicating the age of the boundaries between the Quaternary Period, the Tertiary Pliocene, Miocene, Oligocene, Eocene, and Paleocene Epochs, and the Cretaceous Period. Indicate the age (in mya) of each boundary. Then, show the position of the Formations (and unconformities) present within the National Park. Indicate the gaps of time indicated by unconformities.

SELECTED REFERENCES:

-Harris, A. and E. Tuttle. 1983 (3rd ed).Geology of the National Parks. Kendall/Hunt Publishing Co., Dubuque, IO. 554 pp.

-Harris, D.V. and E.P. Kiver. 1985 (4th ed.). The Geologic Story of the National Parks and Monuments, John Wiley and Sons, New York, 464 pp

-NPS. Badlands N.P. - Geologic Formations. Accessed on 8/2/20:

https://www.nps.gov/badl/learn/nature/geologicformations.htm

PHOTOS:

Figure 1 - Alternating bands of reddish paleosols within the Brule Formation (Lower Oligocene).

Figure 2 - Similar view as for Figure 1.

Figure 3 - Steep-walled faces of readily eroded semi-consolidated sediment. Present rate of erosion is a large 1 inch per year.

Figure 4 - A clastic dike (arrows) cuts through horizontal layers of sediment. These types of structures are commonly thought to have been formed in two possible ways - a large desiccation crack that is filled with sediment from above or injection of fluidized sediment from below. Being more resistant to erosion than surrounding rock, the dike stands out in relief.

Figure 5 - Another view of Figure 4.

Figure 6 - Story of the Badlands.

Figure 7 - The Badlands topography.

Figure 8 - The Badlands topography.