Geochemical and isotopic tracing of Paleozoic groundwater flow in breached anticlines : a case study at Lower Kane Cave, Bighorn Basin, Wyoming

Abstract

Lower Kane Cave is forming in the upper Mississippian Madison Limestone by sulfuric acid speleogenesis. The cave is located along the axialtrace of the Little Sheep Mountain anticline where the Paleozoic units have been exposed in a canyon cut by the Bighorn River. The Madison Limestone comprises the upper section of the Madison aquifer, which serves as an important regional aquifer for water supply and petroleum production in much of Wyoming, Montana and the Dakotas. Compared to other Madison springs and wells in the region, the cave springs are characterized by a higher concentration of TDS, SO₄ and H₂S, differences which likely contribute to the localization of cave formation. This study used geochemical and strontium isotope data to determine signatures for the Madison aquifer and other Paleozoic aquifers of the Bighorn Basin to constrain the origin of groundwater to Lower Kane Cave. Mississippian Madison aquifer waters are characterized by lower [Sr] and higher ⁸⁷Sr/⁸⁶Sr (between 0.70891 - 0.70925), than groundwater in the overlying Pennsylvanian Amsden and Tensleep and Permian Phosphoria aquifers, which have ⁸⁷Sr/⁸⁶Sr values between 0.70789 - 0.70856. These values are slightly greater than established marine values of ⁸⁷Sr/⁸⁶Sr for the respective depositional periods. Coupled with the increased concentrations of TDS, SO₄ and H₂S, the distinctly radiogenic ⁸⁷Sr/⁸⁶Sr ratios of 0.71001 to 0.71012 measured at the cave springs suggest that the springs of Lower Kane Cave are the result of mixing between Madison waters and a thermal, saline, radiogenic endmember. Data from the Thermopolis Hot Springs in the southern Bighorn Basin support the existence of such a water within the lower Paleozoic section in the Bighorn Basin of Wyoming, suggesting that similar flow systems operate at the Thermopolis and Little Sheep Mountain anticlines, and potentially at Sheep Mountain anticline as well. These results further demonstrate the importance of structural controls on groundwater flow in the Bighorn Basin, and have implications for our understanding of cave localization and fracture controlled flow at anticlines within the Bighorn Basin, as well as at similar zones of foreland compression in other areas.