USGS geologic Mapping and karst research in the Ozark National Scenic Riverways, Missouri, USA

Introduction Ozark National Scenic Riverways (ONSR), a unit of the national park system, was created in 1964 to protect 134 miles of the Current River and its major tributary, the Jacks Fork, that are located in south-central Missouri (Figure 1). The park includes numerous large karst springs, including Big Spring, which is the largest spring in the national park system by flow volume. The National Park Service (NPS) administers a narrow, nearly continuous corridor of land adjacent to the two rivers. Base flow for the rivers is chiefly supplied by groundwater that has traveled through the karst landscape from as far as 38 miles away (Imes and Frederick 2002). The watershed is vulnerable to pollution, but the area remains largely rural with few industries. The springs and rivers provide habitat for numerous aquatic species as well as recreational resources for floaters, fishermen, and campers. ONSR is a major cave park with hundreds of known caves and diverse in-cave resources. The geology of ONSR comprises a Mesoproterozoic (~1.4 giga-annum) basement of intrusive and extrusive igneous rocks discomformably overlain by relatively flat-lying Cambrian and Lower Ordovician (~500– to 470–mega-annum) sedimentary rocks, on which various surficial and residual sedimentary units are superposed. The basement rocks, chiefly granite and rhyolite, are exposed as erosional remnant hills in an uplifted area near the central part of the park area (Figure 2). The Paleozoic sedimentary rocks are chiefly dolomite that contain interlayerings of quartz sandstone and chert. The bedrock is cut by chiefly vertical strike-slip faults of the Ouachita Orogeny to the south. The entire exposed Paleozoic section is pervasively karstified.