TIMING AND SOURCES OF POSTGLACIAL GROUNDWATER DISCHARGE NEAR THE GLACIAL LIMIT OF THE JAMES LOBE OF THE LAURENTIDE ICE SHEET, SOUTH DAKOTA

Deposits of tufa, travertine, and carbonate cements are scattered north of the Missouri River within 20 km of the known glacial limit of the James Lobe of the Laurentide continental ice sheet. These deposits drape and cement late Wisconsin glacial sediments and are being studied to investigate potential constraints on conceptual models of groundwater flow associated with glacial advance and retreat. Five study sites are distributed on the south and west sides of the James Lobe near Yankton and Pierre, respectively. Modern springs with high dissolved-ion contents (conductivities >2000 µS/cm) discharge at or near all sites. Deposits near Yankton are poorly exposed in the eastern alluvial scarp of the James River valley and consist of calcite-cemented glacial sediment and porous tufa cut by banded travertine veins. Modern springs have δ²H and δ¹⁸O values (-63 and -8.8‰, respectively) that are consistent with recharge of modern precipitation in shallow aquifers, along with elevated U concentrations (19–31 ppb) indicative of oxidizing conditions. Deposits near Pierre are better exposed and include a variety of tufa with abundant iron and manganese oxides. Springs associated with these deposits have much lower δ²H values (-129 to -139‰) and indicators of reducing conditions including abundant Fe-Mn hydroxide precipitates and relatively low U concentrations (0.6-1.4 ppb). These springs have affinities with deep artesian groundwater with Pleistocene isotopic signatures and likely reflect upward leakage into shallow aquifers.

Yankton-area travertines have high U/Th and precise U-series ages ranging from 13.0 to 10.5 ka for main-stage vein material with late-stage vug-fillings as young as 5.8 ka. Both initial U isotopic compositions and δ¹³C and δ¹⁸O values vary slightly but systematically with age. Nevertheless, all calcite δ¹⁸O values are consistent with modern discharge rather than with values expected for glacial meltwater. Dating of Pierre-area tufas is complicated by lower U concentrations and U/Th due to high Fe-Mn oxide abundances, but also indicate late-Pleistocene to Holocene ages. Calcite δ¹⁸O values in these materials show a much larger range and are consistent with formation from mixtures of both deep and shallow groundwater sources.