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 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.

Abstract Riverbluff Cave developed near the southern margin of the Springfield Plateau as a single passage between James River and its tributary, Ward Branch. Portions of the cave preserve a general fining-upward sediment sequence, but with highly fossiliferous gravel beds near the middle. These gravel beds include fragments of various vertebrates, including mammoth and horse. Trackways and claw marks are also preserved atop the sediment in numerous locations. Cosmogenic isotope data provide burial dates for some of the sediment layers and fossil remains. The earliest sediment (reverse magnetic polarity) entered the cave at ~1.1 Ma, while the fossiliferous gravel bed is dated at ~0.74 Ma. The overlying laminated silts and clays have normal polarity with a burial date of ~0.65 Ma at the base. Thus, the sediment sequence spans the Matuyama/Brunhes paleomagnetic datum, and records at least 450 ka of sedimentation within the cave.