Cave speleogenesis constrains landscape evolution and drainage integration in southern Nevada since 1.6 Ma

We describe geochemical and observational evidence for the mode and timing of speleogenesis in King Solomon’s Cave and Morning Star Cave in southern Nevada (USA), which feature subaqueous calcite speleothems, including mammillary speleothems and a new speleothem we name scabbards, selenite swords, and evidence of vapor condensation-dissolution of massive gypsum crystals (i.e., selenite, CaSO4·2H2O). A U/Pb isochron age on a calcite scabbard of ca. 1.61 ± 0.29 Ma is identical to an age of 1.63 ± 0.26 Ma on a mammillary calcite at Morning Star Cave (10 km to south), placing both caves at or below the water table at that time. Stable oxygen isotopic data on a calcite mammillary and selenite sword suggest these subaqueous speleothems formed in warm geothermal waters with temperatures of ∼40−57 °C, assuming near-modern source water δ18O values. Following cave dewatering, venting of ∼27 °C warm vapors and subsequent condensation released latent heat to maintain a modern temperature of ∼23−24 °C at King Solomon’s Cave, higher than the outside mean annual temperature of 20.4 °C. Because the selenite and calcite speleothems formed subaqueously, yet the caves are now on ridges >190 m above the nearby Muddy River, our data require some combination of mountain uplift and water table lowering since 1.6 Ma, as basin-fill deposits were eroded from the landscape to expose the modern bedrock and topography at Valley of Fire, with implications for river basin integration into the Colorado River drainage.