Episodic Sediment-Discharge Events in Cascade Springs, Southern Black Hills, South Dakota

By Timothy S. Hayes


Water-Resources Investigations report 99-4168


Prepared in cooperation with the
South Dakota Department of Environment and Natural Resources and the West Dakota Water Development District


The report is available in PDF format.


Cascade Springs is a group of artesian springs in the southern Black Hills, South Dakota, with collective flow of about 19.6 cubic feet per second. Beginning on February 28, 1992, a large discharge of red suspended sediment was observed from two of the six known discharge points. Similar events during 1906-07 and 1969 were documented by local residents and newspaper accounts. Mineralogic and grain-size analyses were performed to identify probable subsurface sources of the sediment. Geochemical modeling was performed to evaluate the geochemical evolution of water discharged from Cascade Springs. Interpretations of results provide a perspective on the role of artesian springs in the regional geohydrologic framework.

X-ray diffraction mineralogic analyses of the clay fraction of the suspended sediment were compared to analyses of clay-fraction samples taken from nine geologic units at and stratigraphically below the spring-discharge points. Ongoing development of a subsurface breccia pipe(s) in the upper Minnelusa Formation and/or Opeche Shale was identified as a likely source of the suspended sediment; thus, exposed breccia pipes in lower Hell Canyon were examined. Upper Minnelusa Formation breccia pipes in lower Hell Canyon occur in clusters similar to the discrete discharge points of Cascade Springs. Grain-size analyses showed that breccia masses lack clay fractions and have coarser distributions than the wall rocks, which indicates that the red, fine-grained fractions have been carried out as suspended sediment. These findings support the hypothesis that many breccia pipes were formed as throats of abandoned artesian springs.

Geochemical modeling was used to test whether geochemical evolution of ground water is consistent with this hypothesis. The evolution of water at Cascade Springs could not be suitably simulated using only upgradient water from the Minnelusa aquifer. A suitable model involved dissolution of anhydrite accompanied by dedolomitization in the upper Minnelusa Formation, which is caused by upward leakage of relatively fresh water from the Madison aquifer. The anhydrite dissolution and dedolomitization account for the net removal of minerals that would lead to breccia pipe formation by gravitational collapse. Breccia pipes in the lower Minnelusa Formation are uncommon; however, networks of interconnected breccia layers and breccia dikes are common. These networks, along with vertical fractures and faults, are likely pathways for transmitting upward leakage from the Madison aquifer.

It is concluded that suspended sediment discharged at Cascade Springs probably results from episodic collapse brecciation that is caused by subsurface dissolution of anhydrite beds and cements of the upper Minnelusa Formation, accompanied by replacement of dolomite by calcite. It is further concluded that many breccia pipes probably are the throats of artesian springs that have been abandoned and exposed by erosion. The locations of artesian spring-discharge points probably have been shifting outwards from the center of the Black Hills uplift, essentially keeping pace with regional erosion over geologic time. Thus, artesian springflow probably is a factor in controlling water levels in the Madison and Minnelusa aquifers, with hydraulic head declining over geologic time, in response to development of new discharge points.

Development of breccia pipes as throats of artesian springs would greatly enhance vertical hydraulic conductivity in the immediate vicinity of spring-discharge points. Horizontal hydraulic conductivity in the Minnelusa Formation also may be enhanced by dissolution processes related to upward leakage from the Madison aquifer. Potential processes could include dissolution resulting from leakage in the vicinity of breccia pipes that are abandoned spring throats, active spring discharge, development of subsurface breccias with no visible surface expression or spring discharge, as well as general areal leakage from the Madison aquifer into the Minnelusa Formation.




General Geohydrology and Possible Sediment Sources


Methods of Data Collection and Analyses

Evaluation of Possible Sediment Sources

Mineralogy of Suspended Sediment and Possible Sediment Sources

Examination of Lower Hell Canyon and Breccias

Geologic Examination

Mineralogic Analysis of Minnekahta Limestone Breccia

Grain-Size Analyses of Upper Minnelusa Formation Breccia Pipes and Wall Rocks

Geochemical Evolution of Cascade Springs Water

Previous Modeling Efforts

New Modeling Efforts

Input Data Sets

Chemical Composition of Ground Water

Stoichiometries of Aquifer Minerals

Isotopic Values for Aquifer Minerals and Ground Water

Model Results

Implications for the Role of Artesian Springs in Regional Geohydrology

Summary and Conclusions

References Cited


For additional information write to:


District Chief

U.S. Geological survey

1608 Mt. View Road

Rapid City, SD 57702


Copies of this report can be purchased from:


U.S. Geological Survey

Branch of Information Services

Box 25286

Denver, CO 80225-0286

Download the PDF version of the report for high-resolution, printable pages (1MB).

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