GEOLOGIC HISTORY OF THE BLACK HILLS CAVES, SOUTH DAKOTA. USA

Large caves of the Black Hills (e.g., Wind and Jewel Caves) are located in the semi-confined Madison carbonate aquifer and bear a strong paleokarst influence. Petrographic and geochemical analyses show a complex history. Soon after Mississippian deposition, carbonate breccias and solution voids developed from deformation and redox reactions involving interbedded sulfates. Broad uplift accelerated the inflow of meteoric water, which replaced sulfates with calcite and formed late Mississippian karst and caves. These features were buried by 1.5–2 km of Pennsylvanian–Cretaceous strata.

Laramide uplift and erosion allowed much groundwater flow through the Madison. The present caves developed partly by enlargement of paleokarst voids. Many cave walls are lined by early diagenetic features with little post-Paleozoic modification. Past speleogenetic interpretations favored artesian flow or rising thermal water, but are suspect. Few passages extend down-dip below the present water table or up-dip to Madison outcrops. None extend to the base of the Madison and few reach the top. All major caves underlie a thin cover of Minnelusa Formation (clastics with carbonate and sulfate interbeds). Infiltration through alternating non-carbonate / carbonate strata provides closed-system dissolution and very low PCO₂, while other sources (e.g. through Madison outcrops) have much higher PCO₂. During cave enlargement, all sources were calcite-saturated; but mixing of waters of contrasting CO₂ produces undersaturation. Water flow and dissolution were focused along the high-permeability paleokarst zone.

Caves experienced at least three episodes of calcite deposition: (1) coating paleo-voids during deep burial (Mesozoic); (2) by blockage of springs by Oligocene-Miocene continental sediments (U/Pb dates 26–14.7 Ma); (3) in lower levels during late Pleistocene water-table fluctuations, with minor vadose inflow (Th/U dates to ~276 ka). All were preceded by subaerial weathering, shown by altered substrates, aragonite growth, and evaporative chemical signatures. Late Tertiary faulting locally disrupted crust 2. Exhumation of springs and caves after (2) left caves and related topography as genetic relics. The caves reveal >300 My of hydrologic and geochemical history with a close relation to regional geologic events.