Abstract 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.

With the retreat of the Chesterian sea to the south, the area now occupied by the Illinois basin was exposed to subaerial erosion. In most of this area a linear drainage system of southwestward-flowing streams developed, which eroded valleys as much as 450 ft. deep and 20 mi. wide. The topography of the eroded Chesterian surface strongly influenced initial Pennsylvanian sedimentation when the eroded valleys were filled with fine-to-coarse quartz sand, siltstone, and dark gray shale. A detailed study of the Chesterian-Pennsylvanian unconformity in the subsurface of northeastern Wayne County, Illinois, revealed extensive rotational slumping of Chesterian strata along some steep (at least 34° slope) valley walls. Displaced blocks of Chesterian strata penetrated by drill holes range in thickness from 10 ft. to 125 ft., and some may exceed 660 ft. in maximum dimension. Maximum observed displacement is 170 ft. Failure of Clore and Menard shales, which are rich in expandable clay minerals, allowed downward displacement of overlying strata capped by the Negli Creek Limestone Member of the Kinkaid Formation. Slumping of strata along valley walls where the valleys cut through the northeast regional escarpment of the Negli Creek Limestone produced the greatest known concentration of Chesterian slump blocks within the Illinois basin.

Abstract Numerous biohermal buildups occur in Mississippian (Lower Carboniferous) strata in the Illinois Basin and adjacent regions. They developed as mud mounds, biodetrital calcisiltite mounds, and bryozoan frame thickets (fenestrate-frame coquina or rudstone) during the Kinderhookian and early Meramecian (Tournaisian and early Viséan), and as microbial mud mounds, microbial-serpulid-bryozoan boundstones, and solenoporoid (red algal) boundstones during the Chesterian (late Viséan and Serpukhovian). True Waulsortian mounds did not develop in the Illinois Basin, but echinoderm (primarily crinoids)-bryozoan carbonate banks and bryozoan frame thickets generally occupied the same niche during the Kinderhookian-early Meramecian. Nutrient availability and the resulting increase in the productivity of echinoderms and bryozoans were apparently detrimental to Waulsortian mound development. Deposition of crinoidal-bryozoan carbonates during the Kinderhookian-Osagean initially occurred on a ramp setting that later evolved into a platform with a relatively steep margin through sediment aggradation and progradation. By mid-Osagean-early Meramecian, two such platforms, namely the Burlington Shelf and the Ullin Platform, developed adjacent to a deep, initially starved basin. Sedimentologic and petrographic characteristics of the Kinderhookian-earliest Meramecian carbonates resemble the modern cool-water Heterozoan Association. This is in contrast with post-earliest Meramecian carbonates, which are typically oolitic and peloidal with common peritidal facies. The post-earliest Meramecian carbonates, therefore, resemble those of the warm-water Photozoan Association. The prevalence of Heterozoan carbonates in the Illinois Basin correlates with a rapid increase in the rate of subsidence and a major second-order eustatic sea-level rise that resulted in deep-water starved basins at this time. In the starved Illinois Basin, deposition was initially limited to a thin phosphatic shale that was followed later by deposition of up to 200 m of siliceous, spiculitic, and radiolarian-bearing limestone. The starved basin was connected to the deep open ocean through a bathymetric depression, which was centered over the failed late Precambrian-Early Cambrian Reelfoot Rift, which extended from the deep-water Ouachita Trough in central Arkansas to southern Illinois, approximately parallel to the trend of the modern Mississippi River. We believe that upwelling of cool, nutrient-and silica-rich deep oceanic water, which entered the basin through this bathymetric depression, resulted in proliferation of pelmatozoans and bryozoans. The subsequent change from cool-water-like carbonates to warm-water-like carbonates appears to be related to decreased subsidence and gradual shallowing of the basin.