MORE THAN A DECADE OF U.S. GEOLOGICAL SURVEY GEOLOGIC MAPPING IN THE OZARK NATIONAL SCENIC RIVERWAYS AREA, SOUTHEASTERN MISSOURI

During the years 1996 to 2009, the USGS performed detailed geologic mapping and associated geologic studies in the Ozarks of southeastern Missouri under the FEDMAP component of the National Cooperative Geologic Mapping Program (NCGMP). These studies include: 1) delineation of regional structures and stratigraphic framework; 2) mapping of geologic structures in caves; 3) conodont-based biostratigraphic studies of lower Paleozoic carbonate rocks; 4) age and geochemical sampling and analyses of Mesoproterozoic basement rocks; 5) core-logging studies to establish the distribution of lithofacies and their diagenetic and hydrothermal overprinting in the subsurface; and, 6) audio-magnetotelluric resistivity soundings to image the subsurface geologic framework. This area of Missouri has been the focus of government-sponsored studies because of concerns for potential groundwater contamination within a world-class karst aquifer and spring system by proposed base metal exploration and production in the Mark Twain National Forest. In addition, there was a need for a complete detailed geologic inventory of the Ozark National Scenic Riverways (ONSR) for use in multidisciplinary studies of the park and surrounding lands drained by the upper Current and Eleven Point River systems. This project is culminating with the publication of nineteen 1:24,000 scale geologic quadrangle maps (USGS, I and SIM series) and a 1:100,000 scale geologic map and comprehensive geologic report, which encompasses the ONSR as well as the upper Current and Eleven Point River basins in southern Missouri. Significant findings include: 1) identification and delineation of a Mesoproterozic caldera; 2) identification of numerous strike-slip faults, which appear to exert strong controls on ground-water flow in the region; 3) documentation of stratigraphic control on cave and conduit formation; 4) refinement of the conodont biostratigaphy of the lower Paleozoic rocks exposed in southern Missouri and identification of multiple, coalescing unconformities at the Gasconade Dolomite-Eminence Dolomite contact; and 5) identification of structural and stratigraphic influences on groundwater flow and on the courses of this region’s streams and rivers.