DYNAMIC TOPOGRAPHY, PRESSURIZED SANDSTONES AND HYPOGENE SPELEOGENESIS IN THE OZARKS

Large-scale crustal subsidence proposed for the New Madrid region (Forte et al., 2007) can explain the upwelling of saline groundwater in the Northern Mississippi Embayment and parts of the adjacent Ozarks. As rock moves downward, deep groundwater is displaced upward. Raising the Ozarks relative to the Mississippi Embayment increases hydraulic pressure in sandstone aquifers and stresses the interlayered, low-conductivity, telogenetic, carbonates. In response to the increased pressure, groundwater finds new pathways to exit freshwater confined aquifers. Upward cross-formational discharge of deep groundwater can produce cave passages that do not correlate with joints as well as huge spring flows that do not correlate with catchment areas. Recognition of the widespread regional discharge regime in the Ozark region has been hampered by the unfounded assumption of unconfined conditions based on water levels from wells open to large intervals. Recent studies, using techniques such as flow meters, reveal relatively thin, pressurized sandstones at surprisingly high elevations in areas that were once thought to be recharge zones. The pressurized aquifers limit downward infiltration of meteoric recharge and create a complex flow field: a topography-driven meteoric regime perched above an overpressured regime driven by neotectonics. The boundary between the two competing regimes shifts continuously as valleys entrench and karstification changes the hydraulic function of carbonates. As denudation progresses, different parts of the Ozark stratigraphic column are in different karstification zones at the same time. Beneath uplands, groundwater is confined by relatively shallow unfractured carbonates. Ubiquitously ascending waters move upward into sandstone layers and then laterally toward the valleys. Mixing creates large conduits beneath sandstones and eventually unroofing opens fractures in the carbonates, allowing epigenic processes to become dominant. Wells that seal off or miss the upper pressurized sandstone can produce low water levels, a condition the USGS refers to as ‘mature karst’. In epigenic zones near deep valleys, sandstones that extend back into the uplands can continue to supply deep groundwater to perennial cave streams.