FAR-FIELD TECTONIC FACTORS OF THE OUACHITA OROGENY AND THEIR INFLUENCE ON KARST HYDROGEOLOGY OF THE SOUTHERN OZARKS

The karst hydrogeology of the southern Ozarks in Arkansas and Missouri is unique because it contains hugely varying groundwater attributes, which can make resource utilization complicated and contaminant clean-up difficult. Herein, we describe three major factors of a conceptual model that we feel are significant controls of groundwater flow in the region, and relate these to tectonic events associated with the Ouachita orogeny. The area of interest lies about 240 km north in the distal end of the stress field (far-field) created by oblique closure of the Ouachita System with the Ozarks. Tectonic closure was from east to west. Preexisting basement faults, reactivated during tectonic pulses of the Ouachita orogeny, are recognized as a critical framework element in control of regional karst development. In northwest Arkansas, these have a dominant northeast-southwest trend, whereas in southern Missouri, these features are predominantly northwest-southeast. Differential movement on these basement faults allowed vertical displacement of as much as several hundred meters, which fractured the overlying sedimentary cover into a series of slightly tilted blocks (< 3 degree dip). The blocks are bound by fractures which define spring basins; these typically do not coincide with surface drainage basins. Pure carbonate lithologies have been orthogonally jointed as a result of uplift. These pure lithologies are the cave- and karst formers, and typically develop phreatic conduits that occur near the upper boundary of the limestone and the overlying confining unit. Surface downcutting of base-level streams allowed later vadose entrenchment of the caves. The final major factor controlling karst development is the occurrence of secondary chert, which decreases with distance from the Ouachitas. The insoluble nature of the chert occludes developing zones of permeability, it masks the developing karst, and where continuous, serves as a regional confining layer.