FINDING HOLES IN A THEORY OR FINDING A THEORY IN HOLES – THE POTENTIAL ROLE OF HYPOGENE SPELEOGENESIS FOR CAVES OF THE CUMBERLAND PLATEAU IN SOUTHEASTERN KENTUCKY

Caves along the Cumberland Plateau in southeastern Kentucky serve as classic examples of epigenic caves where water stairsteps through the lithology toward base-level drainages. These drainages convey the water to springs along the plateau margin. Traditional interpretation of gypsum deposits in these caves is alternatively attributed to the seepage of sulfate-enriched waters from the oxidation and leaching of pyrite in overlying siliciclastic caprock or the pyrite within the host limestone. Closer inspection of these caves reveal features noted by Ralph Ewers in the 1970s where he popularized the term “paragenesis,” in which the bottom of the cave passage was the inception horizon and the dissolution progressed upward. Recent research on hypogene speleogenesis, in which rising fluids dissolve limestone, raises a question of whether the observed paragenetic features are in fact hypogene features.

In this study, we document instances of upward flow in a sub-aqueous environment, such as rising half-tubes leading toward cupolas, blind passages, and possible bubble tracks. We also note that these caves are located near or within shallow oil reservoirs and that natural seeps of petroleum persist along surface and cave streams. Our hypotheses for this upward flow ranges from sediment aggregation linked to glacial cycles, localized confinement by the Lost River Chert, or rising sulfide-rich brines from underlying petroleum reservoirs. We are using δ³⁴S within petroleum seeps and gypsum samples from caves to test the last of these three hypotheses. The range of δ³⁴S in gypsum is 0.0 and +11‰. The range in dissolved SO₄ and H₂S at seeps is +17 to +24‰ and -13 to +6‰, respectively. The depleted δ³⁴S of H₂S in the brine indicate microbial-mediated fractionation. While these results may support the possibility that microbial-derived H₂S plays a role forming the in-cave gypsum, it is very important to note that other possibilities, such as pyrite oxidation, have not been tested.