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.

The Weaubleau structure consists of a 19-km circular feature that contains deformed Mississippian limestones. The age of the structure is stratigraphically constrained between deposition of the deformed Osagean limestones and the overlying undeformed Pennsylvanian (Desmoinesian) units. Paleomagnetic samples were collected from tilted Burlington-Keokuk Limestone (undivided), a polymict breccia inside the structure, and undeformed Burlington-Keokuk Limestone outside of the structure. Stepwise thermal and alternating-field demagnetization of tilted limestone samples reveals a characteristic remanent magnetization (ChRM) with southeasterly declinations and shallow positive inclinations with maximum unblocking temperatures of 475 °C. The ChRM is post-tilting and resides in magnetite. The pole is 30.2°N, 135.4°E (d p = 4.1°, d m = 7.9°), which lies on the Late Mississippian portion of the apparent polar wander path. The breccia samples only contain a present-day field component. Many of the samples from outside the structure contain a present-day field component residing in magnetite, although some contain a poorly defined component with southeasterly declinations and moderate positive inclinations. The ChRM is apparently localized within the deformation feature. Since the ChRM is post-tilting, the age of the deformation has been constrained better than the stratigraphic age. The post-deformational ChRM is not a shock magnetization and is interpreted as a chemical remanent magnetization (CRM). One hypothesis for the origin of the CRM is hydrothermal fluids that were activated as a result of the impact. This hypothesis is consistent with 87 Sr/ 86 Sr values in the deformed limestones, which suggest alteration by radiogenic fluids.

The Mississippian formations and their varied faunas in the type area in the upper Mississippi Valley suggest a wide variety of paleoecological environments. The rock types include black paper-thin shale, greenish clay shale, massive mudstone and siltstone, sandstone, conglomerate, breccia, lithographic limestone, oölitic limestone, fine-grained earthy and dolomitic limestone, dolomite, coarse-grained crinoidal and other types of limestone. The sediments were laid down in relatively shallow seas. The area is structurally a part of the Central Interior Lowlands. The structure is essentially that of a platform of crystalline igneous rocks overlain by a relatively thin cover of sediments. Larger positive areas that surrounded the area include the Wisconsin lobe of the Canadian Shield, the Cincinnati anticlinal area, the Nashville dome, the Ozark dome area, and a landmass along an anticlinal fold that extended northeasterly across Nebraska and adjacent States. Local folds may also have been landmasses at times. Important among these are the Lincoln and Pittsfield-Hadley anticlines and an unnamed feature that extended northeasterly as a slightly submerged area across Fayette, Shelby, Douglas, and Champaign counties, Illinois. Sediments were probably received from all these postulated landmasses. Some of these landmasses served as barriers at times and thus made local seas. At other times the entire area was submerged. The oldest fauna considered is that of the Grassy Creek shale of Devonian or Mississippian age. This unit consists largely of paper-thin black shale beds. It was probably formed in stagnant water in an area partly enclosed by land barriers. The fauna consists of conodonts, other fish remains, spores, and linguloid brachiopods. The muds that formed the Grassy Creek shales were derived from low-lying lands. The Saverton fauna lived in an environment in which greenish-gray mudstones were being deposited. It had a large benthonic invertebrate fauna but included conodonts, other fish remains, and spores. The Louisiana limestone is typically a dense lithographic rock with dolomitic clay partings. The fauna includes a large and varied benthonic assemblage. A relatively thin series of shales, oölites, and limestones lying on the Louisiana limestone has been referred to the Glen Park formation by some authors but is called the “Hamburg” oölite by others. This series is overlain by the greenish-gray siltstone, dark clay shales, and fine-grained olive sandstones of the Hannibal shale. The “Hamburg” strata contain an assemblage of small brachiopods, pelecypods, pieces of Bryozoa, and some small gastropods. The assemblage has been called a dwarf fauna, but the smallness of the individuals that make up most of the assemblage may be a result of sedimentary sorting. The “Hamburg” oölite is thought to have been deposited in very shallow water. The Hannibal fauna consists mainly of brachiopods and pelecypods and suggests a shallow marine benthonic environment with several burrowing types of life being prominent. The Chouteau limestone is principally an argillaceous fine-grained limestone, but it also contains beds of medium crystalline limestone and some dolomitic limestone. It contains a large and varied fauna mainly of benthonic invertebrates. The Sedalia-Burlington-Keokuk limestone series is predominantly a coarsely crystalline cherty crinoidal limestone, with minor amounts of shale and dolomite. The shallow benthonic fauna is dominated by crinoids, although many other forms are quite numerous. Crinoid columnals probably were moved about before consolidation. The Warsaw and Spergen rocks contain more shale and argillaceous limestone than the immediately underlying rocks. Sedimentary structures suggest shallow water as do the faunules. Oölites may be rare or absent; fenestellate bryozoans are important components of the diverse shallow benthonic faunas. The St. Louis and Ste. Genevieve formations are typically fine-grained to dense limestone, and considerable thicknesses of oolitic limestone occur in each formation. Gypsum and anhydrite are known from subsurface sections of the St. Louis. Breccias and conglomerates are conspicuous locally, and dolomites and fine sandstones may be present. Sedimentary structures suggest a shallow-water origin. Shallow benthonic faunas are more abundant in some beds than in others but are less varied as a rule than in the Burlington limestone and adjacent formations. Nektonic life is represented by fish remains. Conspicuous forms in the benthonic fauna are the coral Lithostrotion proliferum and the echinoid Melonechinus. The area was probably land during most of Chester time.

Abstract The Chouteau Limestone is late Kinderhookian in age in Illinois and late Kinderhookian to Valmeyeran in Indiana (Rexroad and Scott, 1964). The Chouteau conformably overlies the Kinderhookian Hannibal Shale of the New Albany Group (Figures 7-1 and 2-12), except in southwestern Illinois where the Hannibal is absent and it overlies Devonian and older strata (Willman et al., 1975). In places, the lower part of the Chouteau grades laterally into upper beds of the Hannibal. The Chouteau is widespread in the basin and is generally less than 20 ft (6 m) thick. It is a carbonate mudstone and wackestone that is dolomitic and cherty in places and commonly wavy-bedded. The depositional extent of the Chouteau (Figure 8-9) coincides with the shelf-margin, slope and basin areas (Figure 8-5) of the New Albany deposition (Cluff et al., 1981). It coincides with the area of the overlying Borden delta and with the fringes of the Burlington-Keokuk carbonate bank. The Chouteau was probably deposited at a time when siliciclastic influx was minimal. With a minor unconformity, the Chouteau is overlain by a variety of lithofacies in the lower Valmeyeran Series, including Borden Siltstone in central and eastern Illinois and adjacent parts of Indiana, Burlington Limestone in western Illinois, Meppen or Fern Glen formations in southwestern Illinois, Springville Shale in southeastern Illinois, and New Providence Shale in Kentucky (Figure 9-1).

Abstract This field trip provides an overview of geological features in southwestern Missouri that are related to the American Civil War and to human culture. This includes the geology and history of the Wilson’s Creek National Battlefield (where the second important battle of the American Civil War was fought on 10 August 1861), Zágonyi’s Charge (25 October 1861), the Battle of Springfield (8 January 1863), and the gravestones and monuments of the National Cemetery in Springfield in which many of those who fought at Wilson’s Creek and other Civil War conflicts are buried. Other stops include the Springfield Underground and the quarries and facilities at what was once the town of Phenix (which, along with Carthage, Missouri, was the home of some of the largest dimension-stone quarries west of the Mississippi River); and a reconstructed mill site in Point Lookout, just south of Branson. Most of the field trip involves outcrops, quarries, and bedrock composed of the Mississippian Burlington-Keokuk limestones (undivided), providing numerous chances to examine outcrops and products made of limestone and chert.