EVIDENCE FOR SUBGLACIAL RECHARGE TO THE CONFINED MADISON AQUIFER AS A DRIVER FOR PLEISTOCENE WATER-TABLE FLUCTUATIONS AT WIND CAVE NATIONAL PARK, SOUTH DAKOTA, USA

Phreatic speleothems in the lower levels of Wind Cave preserve a 300-ka paleohydrologic record of water-table fluctuations along the east flank of the Black Hills in South Dakota. U-series dating of wall coatings and foundered cave rafts were used to construct a paleohydrograph that considers the modern potentiometric-surface gradient beneath the cave. Maximum water-table high-stands were less than 45 m; more typically water levels were less than 25 m. Periods of non-deposition represent subaerial conditions that, in some cases, include evidence for vadose flow. Basal layers of coatings have ages indicating subaerial conditions between ~1,000 and 300 ka. After that, water-level high stands correlate to interglacial/interstadial periods and low stands correlate to full glacial or stadial periods. Isotopic compositions of δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and ²³⁴U/²³⁸U in speleothems younger than 12 ka are consistent with compositions in modern groundwater within the cave; however, older calcite has lighter δ¹⁸O, higher ²³⁴U/²³⁸U, and ⁸⁷Sr/⁸⁶Sr values indicating water interacted with Paleozoic carbonate aquifer rocks rather than Precambrian basement. Comparisons to data for modern groundwater sources in the region indicate that water present during high-stands was more like regional artesian sources having warmer, deeper flow paths rather than shallower, cooler sources associated with local recharge.

Data do not support previous interpretations of a monotonic water-table decline caused by local hydraulic changes. Instead, low stands likely reflect cold, dry conditions that provided little local recharge to shallow aquifers. High stands likely reflect reorganization of regional flow systems caused by subglacial recharge into the Madison aquifer beneath the Laurentide ice sheet on the northeast side of the Williston Basin (Grasby et al., 2000, Geology 28). Increased hydraulic pressure in a confined aquifer is capable of causing water-level increases on the south flanks of the basin as long as hydraulic connectivity is maintained. Subglacial recharge resulting in overpressuring within the confined Madison aquifer is considered the most likely driving force for increased heads within the flanks of the Black Hills, even though the timing of ice sheet advance and Wind Cave high stands were not synchronous.