Geology of Wind Cave National Park, South Dakota
Caverns Form in the Limestone Bedrock of the Black Hills
Oct 13, 2009
With 132.05 miles of mapped passages, Wind Cave in the southern Black Hills is the world’s fourth longest cave. Its incredibly complex and numerous caverns are still being explored. At approximately 300 million years old, it is one of the oldest caves on Earth. Wind Cave’s unique features and exposed rock help scientists construct the region’s paleoenvironments.
Geologic History of Wind Cave
According to the National Park Service, a warm, shallow sea existed in what is now southwestern South Dakota about 350 million years ago. The calcite shells of organisms living in this sea eventually formed the Madison Limestone deposit. Locally, this formation is called the Pahasapa Limestone. In the vicinity of the park, the limestone is 275 to 325 feet thick, and it is in this bedrock that Wind Cave began to form some 320 million years ago.
For 240 million years, the shallow sea advanced and retreated. During arid periods, gypsum, a mineral containing calcium and sulfur, crystallized as the seawater evaporated, leading to irregular shaped gypsum beds in the limestone. Gypsum is chemically unstable, and over time it slowly converted to calcite, the mineral that comprises limestone. The result of this chemical reaction was that the sulfur in the gypsum’s crystal structure was freed to mix with groundwater. This resulted in a diluted form of sulfuric acid, which dissolved the surrounding limestone bedrock, forming Wind Cave.
When the shallow sea advanced, sediments filled the previously formed caverns. These deposits are called paleofills, and in Wind Cave they are formally named Minnelusa sediments. Minnelusa sediments are typified by red sand, silt, clay, and fragments of limestone, sandstone, and chert. When the sea retreated again, cave formation resumed in the limestone bedrock. This process occurred several times in the history of the cave.
The Laramide Uplift began about 40-60 million years ago. This uplift formed the Black Hills and sped up the process of cave formation. During this time, groundwater also began draining from parts of the cave. Currently, groundwater in Wind Cave is found in an area named the Lakes, which is located approximately 500 feet below the ground’s surface.
Speleothem Formation in Wind Cave
Once the groundwater drains from a cave, speleothems can develop. Speleothems are cave decorations that form when small amounts of water travel through the cave. Calcite precipitates on the cave wall as the water evaporates, creating stunning formations.
Wind Cave boasts many kinds of common speleothems, such as dogtooth spar, frostwork, cave popcorn, and helictite bushes. Gypsum speleothems are found in the drier parts of the cave. These decorations appear in the form of needle-like crystals, puffs of cotton, and curved or coiled crystals called gypsum flowers.
There are very few flowstone and dripstone speleothems, such as stalactites or cave bacon, in Wind Cave. These types of speleothems require larger amounts of flowing water. This amount of water is not available in Wind Cave due to the region’s semi-arid climate. Additionally, the limestone is overlain by semipermeable clay beds that prevent surface water from reaching the cave.
Wind Cave is truly renowned, however, for its abundant and well-formed boxwork speleothems. Boxwork are thin calcite formations resembling honeycombs. While the development of boxwork is somewhat of a mystery, geologists believe that its origin lies in the gypsum beds that formed when the region’s shallow seas retreated. As the seas retreated and advanced, the gypsum beds expanded and contracted, creating cracks in the surrounding limestone. These cracks filled with newly crystallized calcite, which was more resistant to weathering than the adjacent limestone bedrock. The gypsum eventually converted to limestone and was dissolved away by acidic groundwater, leaving behind the more resistant calcite deposits.
Rocks Exposed at the Surface
Wind Cave National Park’s history is not only told by underground rock exposures. The oldest rocks in the park outcrop in its northwest section. Metamorphic schists formed during mountain-building events believed to have occurred about two billion years ago. Slightly younger pegmatites about 1.7 billion years in age point to later mountain building. Pegmatite is an intrusive igneous rock similar to granite.
To the southeast of these Pre-Cambrian age schists and pegmatites, progressively younger sedimentary rocks are found, ranging in age from 600 million years to 60 million years. These outcrops show the advance and retreat of the shallow seas, which occurred until the Black Hills began to form during the Laramide Uplift. Since that time, sediments from the erosion of the Black Hills have filled some of the park’s valleys.
Ongoing Exploration in Wind Cave
No end to Wind Cave has yet been found, and geologists and spelunkers continue to explore its depths. New passages and rooms are constantly being mapped, which may lead to a further understanding of the region’s paleoenvironment and what processes can be expected to occur in the future.
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