The New England Appalachians contain two north-south–trending sets of gneiss domes. The western belt, which includes the Chester dome, contains 13 domes that expose either 1 Ga Laurentian basement rocks or ca. 475 Ma rocks of the Shelburne Falls arc. The eastern belt contains 21 gneiss domes cored by either 600 Ma crust of possible Gondwanan affinity or ca. 450 Ma rocks of the Bronson Hill arc. Domes in both belts are surrounded by Silurian and Early Devonian metasedimentary rocks, which were deposited in two north-south–trending basins before the Acadian orogeny. The Chester dome in southeastern Vermont, the main focus of this study, is an intensively studied, classic example of a mantled gneiss dome. Lower Paleozoic units around the Chester dome are dramatically thinner than they are elsewhere in southern Vermont, and are locally absent. A strong spatial correlation between the highly attenuated mantling units and highly strained, mylonitic rocks suggests the presence of a ductile, normal-sense shear zone. Garnet-bearing rocks in the core of the dome record metamorphism during decompression of 2–3 kbar, whereas rocks above the high-strain zone were metamorphosed during nearly isobaric conditions. Strain markers and kinematic indicators suggest that extension occurred during northward extrusion of lower- to middle-crustal wedges of Proterozoic and Ordovician quartz-feldspar–rich gneisses below and up into a thick tectonic cover of Silurian mica-rich metasediments that had been transported westward in large-scale nappes. If the ductile, normal-sense shear zone was responsible for synmetamorphic decompression, as we propose, extrusion occurred at ca. 380 Ma.

Abstract The Omaha pool was discovered in November, 1940, by the Carter Oil Company’s York No. 1, SE. ¼, SE. ¼, SW. ¼ of Sec. 33, T. 7 S., R. 8 E., Gallatin County, Illinois. The producing area is now defined and extends over 450 acres located generally southwest of the discovery well. Production is from the Palestine and Tar Springs formations of the Chester (Upper Mississippian) series. The pool is on the crest of a large dome and is exceptional in that igneous rock is found in intrusive contact with the producing sands. Sills and dikes ranging from less than one foot to 50 feet in thickness, composed of mica-peridotite, occur at many levels in the Pennsylvanian and Chester series. Contact effects indicate that some oil was in the sands before intrusion of the igneous material, suggesting a certain amount of prior uplift. The pronounced doming of the structure and the intrusion of dikes and sills may have accompanied intrusion of a hypothetical subjacent laccolithic or stock-like igneous body, probably in post-Pennsylvanian-pre-Cretaceous time. Minor folding occurred earlier at the close of the Mississippian period.

Abstract The Black Warrior basin of northeast Mississippi and northwest Alabama is a triangular area of 35,000 sq mi of Paleozoic strata bounded on the north by the Nashville and Ozark domes, on the southeast by the folded Appalachians, and the southwest by the buried Ouachita mountains. The western two-thirds of the basin is buried beneath Mesozoic and Tertiary strata of the Gulf Coastal Plain and Mississippi embayment. The first gas production in the basin was established in 1909. Since that time exploration has been sporadic, but in the early 1950's seven Upper Mississippian gas fields and a one-well Ordovician oil field were discovered in the exploration program which followed the development of the Muldon gas field. During most of Paleozoic time, the basin was part of the stable continental interior and received more than 17,000 ft of sediment typical of that deposited in the shallow inland seas of the era. The center of the basin shifted in each Paleozoic period. In Early Pennsylvanian time, there was a great downwarping of the basin accompanied by an uplift of orogenic belts on the southeast and probably on the southwest; more than 10,000 ft of Pottsville sediment was deposited in the Black Warrior trough. After Pennsylvanian time, the basin was uplifted and received no Permian sediment. In early Mesozoic time, the province was downwarped on the southwest and was covered with the Mesozoic and Tertiary sediment of the Mississippi embayment. The Paleozoic strata of the basin are complexly faulted, and most of the gas fields are on faulted structural features which were located by geophysical methods. All of the gas production is from Upper Mississippian Chester sandstone beds which have a marked lack of lateral persistence and extreme lateral and vertical variations in porosity and permeability. The eight-well Muldon field is the largest gas field in the basin, It is on a horst-like fault block, and production is from the Sanders sandstone, of middle Chester age, at a depth of 5,500 ft. Total recoverable reserve of the Muldon field is estimated to be 70 billion cu ft of gas. Most of the Paleozoic rocks of the basin have abundant oil and gas shows where there is porosity. There are several unconformities in the basin which offer the possibility of large-scale stratigraphic traps, and there are many undrilled structural anomalies. It seems probable that much more oil and gas will be found in the basin in the future.

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.