Map showing geologic structure and altitude of the top of the Minnelusa Formation and orientation of mapped cave passages in the Madison Limestone, southern Black Hills, South Dakota
K.D. Peter, K.R. Mills, and C.L. Loskot
The primary purpose of this report is to show the geologic structure and the altitude of the top of the Minnelusa Formation in the southern Black Hills. It can be used in conjunction with topographic maps to estimate the depth to the Minnelusa Formation at a specific site. The Minnelusa Formation was mapped because it is the deepest aquifer for which there is extensive data. Similar maps for the northern Black Hills and Bear Lodge Mountains and the northeastern Black Hills also have been prepared by Peter and others (1985a, b).
The secondary purpose of this report is to describe the possible effects of geologic structures and hydrologic conditions on cave formation in the Madison Limestone, which underlies the Minnelusa Formation. Two hypotheses of cave formation were tested by investigating five caves. Caves are large-scale examples of secondary permeability in the Madison Limestone. The relationship of secondary permeability to geologic structures has been determined to have significant relation to water development (Lattman and Parizek, 1964, and Siddiqui and Parizek, 1971). It has been suggested that caves in the Bighorn Basin in Wyoming were formed as the result of a steep hydraulic gradient imposed on a uniformly jointed limestone (Huntoon, 1985, p. 443). The orientation and length of the passages in the five caves were measured and qualitatively compared to mapped structural features in the vicinity to identify any relation.