Wind Cave N.P., SD 2012
While exploring South Dakota in July of 2012, I made a stop at the Wind Cave National Park and Jewel Cave National Monument. Both Caves are located in the southern Black Hills of South Dakota and is most famous for their large concentration of a fairly rare cave formations.
This region of the country where Wind Cave National Park is located today was periodically under water during much of the past several hundred million years. About 350 million years ago, this region was covered by a sea with abundant shelled marine life, the remains of which are fossilized in the limestone that makes up Wind and Jewel Cave. Masses of gypsum (hydrated calcium sulfate, Ca2SO4-2 H2O), formed around the same time as the limestone, as the marine water evaporated and left gypsum salt deposits behind. As water came and went from the gypsum, it shrank and expanded, causing the gypsum and limestone to crack. The gypsum began to make its way into the cracks of the limestone and crystalized. Later, the gypsum transitioned to calcium carbonate (calcite) beginning the formation of the caves.
About 320 million years ago, the first cave passage was formed. The oceans retreated and allowed for acidic fresh water to take its place. This acidic water was the reason the gypsum was able to convert to calcite and create sulfuric acid and sulfurous acid. When the sulfuric acid and sulfurous acid found its way to the limestone, the acids dissolved the rock and created cave passages.
Over time, the seas came again and deposited red clay, sandstone, and more limestone. For about the next 240 million years, the seas came and went, depositing and eroding sediments as through transgressions and regressions. Between 40 and 60 million years ago, the Black Hills were tectonically uplifted, which further opened cavities within the limestone. The water in the cave is believed to have started draining about 40 to 50 million years ago. This process is extremely slow and there is still water in the cave today, continuing to shape the cave.
The Wind Cave National Park is recognized as the densest cave system in the world, with the greatest passage volume per cubic kilometer. Wind Cave is one of the longest caves in the world with 239.81 km of explored cave passageways. The three levels making up the Wind Cave system are located in the upper 76 m of the Mississippian Pahasapa (Madison) Limestone. Deposited in an inland sea, chert, gypsum, and anhydrite lenses within the limestone are evidence of high periods of evaporation.
Additionally, The cave passages in the Wind Cave NP and Jewel Cave NM are said to “breathe” as air continually moves into or out of them, equalizing the atmospheric pressure of the cave and the outside air. When the air pressure is higher outside the cave than inside it, air flows into the cave, raising the cave’s pressure to match the outside pressure. When the air pressure inside the cave is higher than outside it, air flows out of the cave, lowering the air pressure within the cave. A large cave such as Wind Cave with only a few small openings will “breathe” more obviously than a small cave with many large openings.
The cave is notable for its calcite formations known as boxwork, as well as its frostwork. Approximately 95 percent of the world’s discovered boxwork formations are found in Wind Cave.
While many speleothems have formed as water has dripped into the passages, the most conspicuous feature of Wind Cave, boxwork, has probably formed differently. Boxwork is made of thin blades of calcite that project from cave walls and ceilings, forming a honeycomb pattern. The fins intersect one another at various angles, forming “boxes” on all cave surfaces. Boxwork is largely confined to dolomite layers in the middle and lower levels of Wind Cave.
Many of the bedrock walls in Wind Cave have resistant fins of calcite from which the intervening limestone and dolomite bedrock has been removed by weathering. The veins in which the boxwork formed are along narrow fractures resulting from stresses produced when the mineral gypsum dried and rehydrated. The calcite formed in these fractures taking on the shape of the original gypsum crystals.
The bedrock is less resistant than the boxwork veins. This occurs not just because the bedrock is less crystalline, but also because it has been changed to a crumbly sand consisting of calcite crystals held together by a sparse cement of secondary quartz. The quartz is the remnant of an early matrix that formed around former dolomite crystals. Much of the original bedrock was apparently removed by hydrogen sulfide/sulfuric acid (H2S-H2SO4) solutional processes, which left many very small pores. These porous zones easily weathered away during cave development, as well as later when they were in contact with the moist cave atmosphere.
Jewel Cave National Monument is currently the third longest cave in the world, 322.4 km of mapped passageways. It is located approximately 21 km west of the town of Custer in Black Hills of South Dakota and quite close to Wind Cave National Monument.
Most of Jewel Cave formed within the Mississippian Pahasapa Limestone deposited 350 million years ago. It’s history of formation is similar to Wind Cave but a little older. However, Jewel Cave passages follow a pattern of joint development. The faults and joints are associated with the uplift of the Black Hills approximately 58 to 54 million years ago. After main cave dissolution, a thick layer of calcite lined the walls about 2.5 million years ago.
During cave development and afterwards, speleothems and speleogens formed, including the “jewels” or spar. Other examples include stalactites, stalagmites, flowstone, cave popcorn, etc.