Pennsylvanian rocks of the Eastern Interior basin underlie about 53,000 square miles in Illinois, Indiana, and western Kentucky, with small outliers in Missouri and Iowa, attaining maximum thickness of about 2,500 feet in southeastern Illinois. At the beginning of the Pennsylvanian there was a regional southwest slope furrowed by numerous subparallel valleys as deep as 200 feet. An eastward slope prevailed along the western border of the basin with smaller valleys. Subjacent strata range in age from the Middle Ordovician St. Peter sandstone to the upper Mississippian Kinkaid limestone.Because of topographic unevenness and early Pennsylvanian tectonism, the early Pennsylvanian Caseyville and Tradewater units range from zero to 1,200 feet. Movements along the LaSalle anticline and Duquoin monocline are reflected in an isopach map of this interval. In later Pennsylvanian Carbondale and McLeansboro time the intervals increase regularly toward the southeast, showing that the present southeast border of the basin is due to post-Pennsylvanian deformation. Structural relief on the No.2 coal ranges from +700 to —913 feet. Similar systems of nomenclature are used in Illinois and Kentucky but different names are employed in Indiana. The significant features of 46 key beds are described.
Clastic ratios average about 8 parts clastic to 1 non-clastic. Sand-shale ratios are extremely variable owing to lenticular sandstones at more than 20 positions. Many sandstones occupy channels excavated into underlying strata; othersseem to be offshore barrier beach deposits. There are marine limestones at about 25 different positions, generally thickening toward the western border of the basin, but each has its individual pattern of distribution. Lower Pennsylvaniansandstones are highly quartzose and derived from older sediments; later sandstones are micaceous and feldspathic with much interstitial clay, evidently a first-cycle deposit from a metamorphic terrane. Coals are generally banded, and range in rank from high-volatile to medium-volatile bituminous. Refractory underclays are found in the lower Pennsylvanian in northern Illinois and in the area near St. Louis, Missouri.
Cyclic sedimentation is well displayed with at least 25 alternations of coal, marine limestone, shale, sandstone, and underclay. Early Pennsylvanian sediments came from the east and northeast but the source of the later micaceous sandstones is still uncertain. Most marine invasions came from the west, north of the Ozark uplift, but some in the early Pennsylvanian probably entered the basin from the east and south.
The basin has produced about 5 billion tons of coal and 325 million barrels of Pennsylvanian oil. Other industries based on Pennsylvanian materials are refractories, common brick and tile, road metal, agricultural limestone, portland cement, rock asphalt, glass sand, foundry sand, and whetstone.
Figures & Tables
Pennsylvanian System in the United States
Rocks of the Pennsylvanian System are the bed rock of approximately 10 per cent of the land area of continental United States. These rocks yield 17 per cent of the petroleum, most of the coal, and most of the ceramic raw material of the United States.
Areas of deposition of Pennsylvanian rocks are naturally discriminated as the New England trough, the Appalachian trough, the Eastern Interior basin, the Michigan basin, the Western Interior basin, the Ardmore basin, the Ft. Worth syncline, the Permian basin, the Rocky Mountain geosyncline, and the Cordilleran trough. These genetic areas and the Ouachita fold belt are the regions described.
The Pennsylvanian System in Michigan has been described in detail by W. A. Kelly (Mich. Geol. Survey, Pub. 40, part 2, p. 155-226, 1936). The section is truncated and consists of the Parma sandstone (below), the Saginaw group of cyclical formations, and the Grand River group, which contains red sandstones and gypsum. The Parma and Saginaw are Pottsvillian in age; possibly, Atokan and Desmoinesian. The Grand River, placed by some geologists in the Permian, is probably early Missourian.
The project for a special volume was initiated in 1953, at which time it was decided that the Springer Series was tobeconsidered as Pennsylvanian, the Wolfcamp and Admire-Council Grove-Chase, as Permian. Evidence has since accumulated that the Springer is Mississippian, that the Ouachita Jackfork and Stanley are Mississippian, that the Dunkard is, atleast in part, Pennsylvanian, and that the “Lyon series” and Wolfcamp formation might better be classed