1959 NSS Expedition to Wind Cave Stratigraphic Relationships
The Pahasapa limestone is fine crystalline, white to light gray limestone. Near Wind Cave it is apparently about 300 feet thick. The uncomformably overlying Minnelusa formation has been eroded away to expose the limestone north and west of the cave. The natural entrance to the cave is near the top of the Pahasapa limestone, and most of the cave still lies beneath Minnelusa rocks south and east of the entrance. From the entrance the cave immediately drops 100 feet into the limestone, and the lower parts of the cave are consistently about 120 to 150 feet below the Minnelusa contact, the entire cave being developed in the upper 150 feet of the Pahasapa limestone. The cave, since it remains in the upper part of the limestone, has an over-all slope southeast along the 4°regional dip.
The Pahasapa limestone was exposed to sub-aerial erosion prior to the deposition of the Minnelusa formation. The complex breccia and filled sinkhole and crevice structures in the bedrock in Wind Cave suggest that the limestone was heavily karsted during this period of erosion.
A section of the exposed bedrock in the cave was measured (Figure 5), starting at the base of the Three-way Stairs and passing upward through the Bachelor's Quarters into the Fairgrounds, where its highest part is in the large crevice in the ceiling in the north end of the room.
The lettered units were used for convenience. Many of the units probably cannot be traced throughout the cave cave. The chert bed between L and M persists in the ceiling in much of the Fairgrounds Loop.
While it was not possible due to time limitations, to correlate these units throughout the cave, it is interesting to look at the gross aspect of this section.
The lower 55 feet of limestone generally contains boxwork, often profusely, but sometimes absent. The limestone varies in massiveness of bedding. In general, most of the cave may be in this part of the limestone, and contains boxwork.
The middle part of the section is most unusual, consisting of 17 feet of a breccia of white limestone and some chert, in pebbles 0.25 - 5 inches in size, in a yellowish-brown silty matrix. Boxwork occurs locally in the brecciated zone. Cavities are often filed with calcite crystals. This is the cemented fill breccia (see the section on sedimentation). The breccia may be seen in the Bachelor's Quarters, and the Attic passage is developed in this horizon.
The upper part of the measured section includes about 50 feet of limestone, contrasting sharply with the lower and middle parts of the section by the virtual absence of boxwork, except just above the breccia. The limestone is massive, and develops spongework and other smooth solution forms. In this part of the section occur filled crevices, and zones where the "bedrock" appears to be made up of all sizes of solution-rounded limestone blocks with red silt and clay matrix. This upper part of the section is seem in the Fairgrounds Loop and the Garden of Eden. The Loose Fill Breccias occur in this horizon (see section on sedimentation).
Interestingly, Plummers Pit and one other shaft lead upward to loose fill breccia for distances up to 150 feet above the breccia zone. By current reasoning these shafts should extend 50 feet or more into the Minnelusa formation. If they do not, it may mean that the upper eroded surface of the Pahasapa limestone is very irregular, and the shafts may extend into ancient hills on this surface.
Although the measured section is in one part of the cave, a similar sequence appears to exist elsewhere. Careful stratigraphic work is called for. Much information about the nature of the erosion surface on the Pahasapa limestone could be extracted from a careful study of the stratigraphy. For instance, it is not clear what kind of sinkholes and perhaps caves existed before deposition of the Minnelusa. Whether these followed the same structural trends as Wind Cave does is not known. Wind Cave cuts through some of the ancient features in a seemingly random fashion. The breccia zone is perhaps 100 feet below the top of the limestone. Its function in the formation of the old karst erosion surface may be very interesting.
One working hypothesis for the study of the ancient karst surface might be that the Pahasapa limestone is 300 feet thick near Wind Cave, compared to 650 feet reported to the north, because it has been reduced 300 feet by pre-Minnelusa erosion.
It appears that the boxwork and associated fractures die out at the breccia. This is an important relationship to investigate in connection with the origin of the boxwork fractures and cores.
Did You Know?
Fire is an important factor in protecting the prairie. Historically, fires burned across the prairie every 4 to 7 years. Fires burn the small trees that would otherwise march across the prairie and turn the grasslands to forest.