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 A succession of Ordovician and Mississippian carbonates, separated unconformably, is exposed across the southern flank of the Ozark Dome in southwest Missouri. Deposits of both periods exemplify typical facies of the Midwestern United States: carbonate tidal-flat assemblages for the Early Ordovician and carbonate shelf environments for the Early–Middle Mississippian. The basic stratigraphic sequence of these deposits has been known for over a century, but interesting features remain to be addressed. Thin discontinuous sandstones are present within the Early Ordovician Cotter Dolomite, but the informal Swan Creek sandstone member seems anomalous. This sandstone can exceed 5 m in thickness and is fairly continuous across southwest Missouri. Most Ordovician sandstones in Missouri mark major transgressions above regional unconformities, but not the Swan Creek, and there is no obvious source of the sand. Therefore, we hypothesize that the Swan Creek represents reworked eolian dunes blown across the broad peritidal environment. Clastic sandstone dikes, apparently sourced from the Swan Creek, cut across beds of Cotter Dolomite near faults. We propose that these dikes are evidence of local faulting and seismicity during the Early Ordovician. Early and Middle Mississippian limestones comprise a sequence of shelf deposits, although mud mounds and other facies changes near the Missouri-Arkansas line mark the edge of the Mississippian shelf and the transition to a ramp setting. Early Mississippian carbonate deposition was interrupted by a short and localized influx of siliciclastic sediment comprising the Northview Formation. The Northview has additional characteristics consistent with a river-dominated deltaic deposit, which we suggest as its origin. If correct, this hypothesis implies that the history of tectonic features in the Midwest is more complicated than yet known. Finally, facies changes within and between the local Mississippian formations may record an early crustal response to the impending Ouachita orogeny farther to the south.

ABSTRACT The Mississippian system in the midcontinent of the United States is a complex carbonate- and chert-dominated system with a large degree of reservoir variability and heterogeneity. An outcrop study was done in the state of Arkansas on the Middle Mississippian (Visean) Burlington-Keokuk Formation to analyze the depositional setting and high-resolution sequence stratigraphic architecture to better understand the reservoir distribution of similar units in the subsurface. The outcrop location, in the northwestern portion of the state of Arkansas, was studied using an integrated sequence stratigraphic approach, combining high-resolution photography for tracing bed boundaries and lithologic contacts along with facies determination from outcrop and thin section analysis. A range of skeletal packstones to grainstones dominated by crinoidal fragments and an abundance of void-filling syntaxial calcite cements comprised the majority of the outcrop facies. Nodular to bedded siliceous limestone to carbonate-rich chert facies were observed containing up to approximately 50% microporosity. Based upon facies assemblages and the presence of meter-scale sand waves with faint cross bedding on outcrop, these units were likely deposited in a high-energy sand shoal or sand bar in a proximal position on a distally steepened ramp. Within the outcrop, multiple shoaling upward packages were observed, consisting of siliceous limestones and cherts at the bases overlain by coarsening and thickening upward grainstone bodies. This stacking pattern was observed at two different scales. Larger-scale packages 15 to 35 feet (5–10 m) thick were mappable and continuous across the entire outcrop (1320 ft [400 m]), and are inferred to be controlled by eustatic sea-level change. A smaller-scale stacking pattern was observed on the meter (several feet) scale and were mappable for 165–500 ft (50–150 m) laterally. The lack of limited lateral correlation is inferred to be due to autocyclic controls within the active sand body. The observed shoaling upward patterns create a hierarchy of stacked reservoir and seal units with superimposed variability. These findings illustrate the potential for high-frequency sea-level change and autocyclic control on facies and reservoir distribution that may be seen in the subsurface. Two-dimensional geostatistical modeling further illustrates the need for this level of characterization, as variogram inputs are biased significantly by the segregation of high-frequency sequences and dominant eustatic or autocyclic controls on deposition.

ABSTRACT Facies analysis utilizing a conodont biostratigraphic framework is a powerful tool for evaluating genetic relationships of Osagean–basal Meramecian strata within the Ozark region (Arkansas–Missouri–Oklahoma) of the southern midcontinent. This investigation builds upon previous work cited herein, and suggests that some lithostratigraphic divisions, although useful in differentiating strata in a localized setting, may not be suitable for regional correlations within the Boone Group. High-resolution conodont biostratigraphy demonstrates the diachronous nature of lithostratigraphic divisions within the Boone Group, with both the Reeds Spring Formation and Bentonville Formation (Burlington–Keokuk) clearly becoming younger as they are traced from southwestern Missouri into northern Arkansas and northeastern Oklahoma. Subsequent facies analysis shows that the Reeds Spring Formation represents deposition within outer ramp through proximal middle ramp settings (low to moderate energy), whereas the Bentonville Formation (Burlington–Keokuk) records deposition within proximal middle ramp to inner ramp settings (moderate to high energy). Integration of facies analysis and conodont biostratigraphy-based relative chronostratigraphy provides the basis for construction of four time-slice maps illustrating the distribution of time-correlative facies belts. Together, these time-slice maps deliver a clearer representation of the evolution of Boone Group carbonate ramp deposition during the Osagean, which was characterized by overall shallowing-upward and progradation to south and southwest.