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.