THE GEOLOGY OF JEWEL CAVE: NEW PIECES TO AN UNEXPECTED PUZZLE

Purpose

Over the last 20 years, exploration and mapping of Jewel Cave, as well as geological mapping of the Jewel Cave Quadrangle, have documented several unexpected relationships between cave features and surface geology; and also within and between the Pahasapa Limestone and Minnelusa Formations themselves. This paper presents several of these observations in order to encourage further study.

Results

Geological mapping has identified six subunits within the Minnelusa: I) 40 feet of basal sandstone; II) 50 feet of thin-bedded limestone; III) 120 feet of sandstones; IV) 120 feet of dolomite with interbedded sandstones; V) 120 feet of medium-to-coarse grained sandstones, and VI) over 100 feet of brecciated sandstone layers.

Unit thicknesses were constant throughout the quadrangle, allowing the mapping of subtle faults and folds. The folds express themselves in extensive dip-slope topography and exhibit a strong correlation between synclines and surface valleys.

The most striking observation is the fact that Jewel Cave exists almost exclusively in limestone that is capped with the Minnelusa Formation. This relationship holds throughout the southern Black Hills to the point that, without exception, no cave over 200 feet in length is known to exist within uncapped limestone.

Furthermore, there is no mappable paleotopographical relief within the quadrangle. Rather, evidence within Jewel Cave strongly suggested that “paleofill” developed contemporaneously with the development of the cave – after lithification of the basal Minnelusa sandstone.

Finally, the only known natural entrance to Jewel Cave is located precisely at the crest of a broad anticline.

General Conclusions

There is a strong correlation between the passages of Jewel Cave and modern geological features. The evidence suggests that Jewel Cave formed as a result of the most recent processes that shaped the present-day stratigraphy, structure, and topography. There is virtually no evidence of a Mississippian paleokarst development. This gives pause for reevaluation of the origin of caves in the southern Black Hills.