(Submitted Abstract to the 2003 Geological Society of America Meeting, Seattle, Washington)

Geochemical and Isotopic Constraints on the Source of Groundwater to Lower Kane Cave, Wyoming
* Edwards, Melissa C., Bennett, Philip C., and Engel, Annette S.
Department of Geological Science, The University of Texas at Austin 1 University Station C1100, Austin, TX 78712

Most karst features occur due to the dissolution of limestone by carbonic acid charged phreatic or meteoric water. However, an important subset of caves forms when anaerobic groundwater transports hydrogen sulfide into an oxidizing environment, resulting in speleogenesis via sulfuric rather than carbonic acid. The actively forming Lower Kane Cave in the Mississippian Madison Limestone of the Bighorn Basin near Lovell, Wyoming, is an accessible example of this alternative method of cave development. Located along the fold axis of the Little Sheep Mountain anticline of the Bighorn Basin, this system hosts a diverse range of microbial organisms, including acid-producing and sulfide and sulfate utilizing species, whose role in speleogenesis is currently under investigation. Water samples were collected from cave springs, nearby springs, freshwater wells and produced water from oil wells in the local area. Samples were analyzed for major and trace elements, stable isotopes and Sr isotopes by multi-collector ICP-MS, as well as dissolved gas and organic acid analyses. These data were used to examine the regional flow of groundwater to the cave and potential oil-field sources of hydrogen sulfide. The Madison Aquifer in this area is characterized by relatively fresh water, and in the cave vicinity is the source of municipal water supplies for the towns of Cowley and Greybull. The Madison water samples collected in the area are Ca-HCO₃ to Mg-SO₄ type, with relatively little Na and Cl. Overall the cave water chemistries are characterized as Ca-Mg-HCO₃-SO₄ waters; Ca = 70 ppm, Mg = 25 ppm, HCO₃ = 205 ppm and SO₄ = 110 ppm. However, when compared to other Madison water samples, the waters of Lower Kane Cave are slightly higher in TDS (around 400 ppm), significantly warmer (22^o C versus between 6-12^o C), and contain much higher dissolved sulfide (up to 2ppm). Additionally, Sr isotope signatures for the cave waters are significantly more radiogenic than that of other Madison water samples collected in the region. The ⁸⁷Sr/⁸⁶Sr of cave water samples ranged from 0.710009 to 0.710124. Madison water samples from other sampling sites ranged from 0.708907 to 0.709253, while water samples from the overlying Amsden and Phosphoria were consistently less radiongenic than the Madison waters, with ⁸⁷Sr/⁸⁶Sr values ranging from 0.707889 to 0.708561. Previous investigators suggested that the sulfate and high TDS of the cave waters derived from mixing between the low TDS Madison formation waters and those of the more saline overlying Phosphoria-Tensleep aquifer. Preliminary results of this investigation suggest that a simple Phosphoria-Madison mixing model, while able to account for the major element chemistry and raised TDS, is insufficient to explain the higher temperatures and radiogenic Sr characteristics observed. Alternative sources of the observed water chemistries include upward leakage of water from underlying units, including the leaky aquitards of the Devonian Jefferson and Cambrian Gros Ventre Formations, or the Cambrian Flathead sandstone aquifer. The cave waters may also demonstrate heterogeneity within the Madison aquifer and tap into a particular siliclastic and sulfidic zone, the presence of which is masked at other sampling sites that derive a greater percentage of water from the main body of the aquifer. While the exact origin of the water to Lower Kane Cave remains unclear, it is evident that the flow regime at this location is more complicated than originally believed.

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