Abstract This field trip examines the geology and geohydrology of a dissected part of the Salem Plateau in the Ozark Plateaus province of south-central Missouri. Rocks exposed in this area include karstified, flat-lying, lower Paleozoic carbonate platform rocks deposited on Mesoproterozoic basement. The latter is exposed as an uplift located about 40 mi southwest of the St. Francois Mountains and form the core of the Ozark dome. On day 1, participants will examine and explore major karst features developed in Paleozoic carbonate strata on the Current River; this will include Devil’s Well and Round Spring Cavern as well as Montauk, Round, Alley, and Big Springs. The average discharge of the latter is 276 × 10 6 gpd and is rated in the top 20 springs in the world. Another, Alley Spring, is equally spectacular with an average discharge of 81 × 10 6 gpd. Both are major contributors to the Current and Eleven Point River drainage system which includes about 50 Mesoproterozoic volcanic knobs and two granite outcrops. These knobs are mainly caldera-erupted ignimbrites with a total thickness of 7–8 km. They are overlain by post-collapse lavas and intruded by domes dated at 1470 Ma. Volcaniclastic sediment and air-fall lapilli tuff are widely distributed along this synvolcanic unconformity. On day 2, the group will examine the most important volcanic features and the southernmost granite exposure in Missouri. The trip concludes with a discussion of the Missouri Gravity Low, the Eminence caldera, and the volcanic history of southern Missouri as well as a discussion of geologic controls on regional groundwater flow through this part of the Ozark aquifer.

ABSTRACT The four largest spring systems in the mid-continent receive recharge through large interconnected voids in fractured and solution-weathered dolostones of the Ordovician and Cambrian systems. Cumulative thickness of the carbonate bedrock aquifer ranges up to 700 m in the Ozark region. Recharge from the surface occurs through weathered overburden, sinkholes, and losing streams and has been traced up to 60 km (straight-line horizontal distance) using fluorescent dyes. Mean discharge of the combined flow of these four spring systems is ~1400 cubic feet/second (ft 3 /s) or 40 m 3 /second (m 3 /s). All four spring systems will be visited while discussing the karst terrane that recharges them. Environmental and engineering challenges in the region will be discussed, such as wastewater treatment systems, solid waste disposal, and failed reservoirs. Hodgson Mill Spring represents a branch of the Rainbow/North Fork/Hodgson Mill System. While it receives base flow from the main system, it also receives local recharge that Rainbow and North Fork springs do not. A portion of the Mammoth Spring recharge system will be viewed at Grand Gulf State Park in Missouri, where a cave collapse has created cliffs and a natural bridge and exposed a small losing tributary that flows into a cave that has been traced to the spring. Mammoth Spring State Park in Arkansas offers a historical perspective of the development and use of large springs. Greer Spring in Missouri was used as a power source for grist, flour, and lumber mills, but has now largely returned to its predevelopment state and is managed by the U.S. Forest Service. Big Spring, featured in a former state park in Missouri, is now part of the Ozark National Scenic Riverways.

Abstract Cathodoluminescent microstratigraphy in epigenetic dolomite cements correlates over a north-south distance of >275 km across the Ozark Mountains. Trace elements in dolomite show coherent regional variations. Ubiquitous coeval Pb- Zn mineralization contains fluid inclusions with homogenization temperatures >100°C. All suggest the flow of brines (mainly) north from the Arkoma Basin through Cambrian sandstone and carbonate aquifers. The observation that brines still fill Ordovician and Cambrian strata ringing the Ozark Plateau constrains the cumulative flow that has occurred. If the Mid-Continent sediment cover was thick and insulating, brine flow driven by topographic differences in hydrologic head could have been slow enough to avoid salt flushing and still accommodate the fluid inclusion homogenization data. If the cover was thermally conductive and thin, as seems most geologically reasonable, flow at the rates required to explain the homogenization temperatures would have quickly flushed salt from the aquifers in contradiction to present observations. Topographically driven hydrologic flow across the Arkoma basin could not have continued uninterrupted for protracted periods. Given the present high permeability of the Pb-Zn deposits, there is no obvious way to limit or pulse cross-basin hydrologic flow. The simplest explanation is that brines were expelled by compaction or gas displacement. High temperature, low salinity fluid inclusions in the Ozark Cambrian aquifers probably represent the incursion of meteoric water into outcrop areas warmed by pulses of brine outflow. Channeling of fluid flow and the relation of alteration and fluid inclusions to flow channels need to be further investigated theoretically and in the field.

Abstract The Central Nonglaciated Plains of North America (Fig. 3; Table 2, Heath, this volume) extend from Montana to the Bal- cones Escarpment of central Texas (Fig. 1). Not all the regional aquifers that are found within the province are described in this chapter; specifically, the High Plains aquifer and the aquifers of alluvium along the major streams are described in Weeks and Gutentag (this volume) and Rosenshein (this volume). Climate, especially precipitation and evapotranspiration, is widely recognized as the dominant factor controlling streamflow, which plays a major role in modifying landforms in the region. However, the role of climate in controlling the occurrence and movement of ground water is not as widely recognized. Nevertheless, climate is the primary control on the amount of recharge to aquifers in the region. Recharge to the water table from precipitation ranges from less than 25 mm/year near the Front Range of the Rocky Mountains to more than 300 mm/year in the Ozark Plateaus of Arkansas. The dominant geologic characteristic of the Central Nonglaciated Plains is the extensive, nearly horizontal post-Precambrian sedimentary rocks that occur at nearly all locations (Fig. 2), although, the complete post-Precambrian Stratigraphie sequence does not occur at any one location. Major geologic structural features are shown in Figure 3. Geohydrologie units, as designated herein, are rock units that have similar characteristics. They are defined on permeability and their relationship to a reasonably distinct regional hydrologic system. The flow system of most regional aquifers is controlled by altitude of recharge and discharge areas. Major recharge and discharge areas for regional aquifers typically correspond to large physiographic features. The features were used to delineate the geohydrologic units discussed in this chapter. The Central Non-glaciated Plains have been subdivided into hydrophysiographic regions because several regional ground water-water flow systems occur within them. The major hydrophysiographic regions include the Northern Great Plains, the Central Great Plains, the Ozark Plateaus, the Southern Great Plains, and the Edwards Plateau-Llano Hills region (Fig. 1).

ABSTRACT After the first saltwater disposal (SWD) wells associated with the development of the Fayetteville Shale became operational in April 2009, central Arkansas experienced an increase in the rate of earthquakes with M ≤4.7. This seismic activity in the Fayetteville Shale development area (FSDA) included both natural and induced earthquakes that often occurred in spatial and temporal clusters. The Center for Earthquake Research and Information (CERI) and the Arkansas Geological Survey (AGS) closely monitored the area using a permanent network of seismograph stations, augmented by portable real-time broadband stations, to focus on specific sites of suspected induced seismicity. Any concerns were communicated directly to the Arkansas Oil and Gas Commission (AOGC), the agency charged with regulating SWD wells in the state. The largest and most significant cluster of induced earthquakes in the FSDA occurred between August 2010 and July 2011, when thousands of earthquakes (M ≤4.7) occurred on the Guy-Greenbrier fault, which had not been mapped prior to the earthquakes. Since a 13-km-long fault can potentially produce a M 6 earthquake if it slips as a single event, CERI and AGS staff testified at an AOGC hearing in July 2011 that continued fluid injection into SWD wells near the fault could potentially trigger a damaging earthquake. Operators voluntarily shut down and plugged three nearby SWD wells. Another SWD well operator was ordered (by the AOGC) to shut down and plug their well. Earthquake activity dramatically decreased on the Guy-Greenbrier fault and ceased soon after shut-in, thus providing strong supporting evidence that the earthquakes were induced. The AOGC also established a moratorium that prohibited SWD well operations in the area surrounding the fault and rules governing proximity of new SWD wells to mapped faults and proximity of SWD wells to each other. When the price of natural gas rises to make development cost effective, we anticipate the new rules will help to minimize the occurrence of felt and potentially damaging earthquakes. Although the Guy-Greenbrier earthquake sequence ended in 2011, earthquakes still occurred within the FSDA. While some of these earthquakes were within 10 km of active SWD wells, many other earthquakes were not close to any SWD well. We identified 156 earthquakes M ≤3.4 from the regional catalog that occurred between 2012 and 2016 that were within 5 km of a natural-gas production well during the hydraulic fracturing window posted by the AOGC. This suggests that these earthquakes may have been induced by hydraulic fracturing.