Uranium-series dating of the draining of an aquifer: The example of Wind Cave, Black Hills, South Dakota

Wind Cave, South Dakota, is a three-dimensional, rectilinear maze cave developed in 90 m of Mississippian limestone and dolomite that is overlain by thick clastic cover strata. Beneath an area of 1.8 km 2 , 87 km of small, drained, phreatic passages are known. Exploration terminates at a water table 160 m below the highest parts of the cave. As the cave was draining, calcite precipitated from its waters. Uranium-series measurements (85) were taken on fragments of broken calcite crusts collected in the lower 93 m of the cave. The measurements reveal that it has been draining as a backwater for at least the past 300,000 yr. 234 U/ 238 U ratios in the water were remarkably constant, allowing estimation of ages beyond 400 ky B.P. After deducting glacial-interglacial cyclic oscillations (of 10- to 20-m amplitude), the water table appears to have been lowered at a steady rate of 0.4 m per 10 3 yr since ≥400 ky B.P. Most of this fall is attributed to the increase of hydraulic conductivity in the ~260 m of cover strata that separates the carbonate aquifer from its spring discharge points. A subaqueous calcite deposition zone extended to ~70 m below the contemporary water table at all times, where mean rates of deposition were 0.12 mm per 10 3 yr or less. Modeling of the precipitation kinetics suggests a well-mixed water body, with a Ca 2+ concentration close to the thermodynamic equilibrium value and variations of depositional rate being controlled by hydrostatic pressure.