Reactivated Basement Faulting as a Hydrogeologic Control of Hypogene Speleogenesis In the Southern Ozarks of Arkansas

Integration of recent data from diverse types of research have been used to develop a conceptual model of hypogene speleogenesis for Chilly Bowl Cave and the nearby karst features of the Ozark Plateaus that overlie deep basement faults in northern Arkansas. The model is based on and is constrained by geologic mapping, cave mapping, structural reconstruction, gravity mapping, hydrogeology, ground-water tracing, endangered species distribution mapping, cave mineralogy, stable-isotope geochemistry, and fluid inclusion studies. This conceptual model draws on preexisting basement faults that were reactivated during tectonic pulses of the Ouachita orogeny, which was centered about 150 kilometers to the south. Confined, geothermally heated water was expelled from the foreland basin, flowed in aquifers toward the Ozark Dome, and recharged from below along integrated faults and systematic joints in the overlying Paleozoic rocks, creating caves and depositing minerals. Cave erosional features and deposits have been found that indicate past episodes of thermal ground-water circulation. These include upper-level paleo-cave passages and chambers, now reactivated by present-day stream drainage, scallops in conduits and flow tubes showing upward flow, dolomite breccia linings, and remarkable dog-tooth spar (calcite) crystals, some of which are as long as 1.9 meters. The stable isotopic signature of these crystals is anomalous, and inconsistent with a meteoric fluid origin of meteoric water percolating downward from the surface.

Gravity data and deep drilling records reflect the displacement of basement rocks across major faults, and distribution of several endangered species suggest that brittle overlying carbonate rocks were fractured and that the faults served not only as flow conduits, but as pathways for dispersal of these species.