A SUMMARY GEOLOGIC HISTORY OF THE ILLINOIS BASIN
David H. Swann
Nearly three miles of Paleozoic sediment, largely shallow water marine, fill the Illinois Basin. Dolomite is the dominant lithology, followed by limestone, shale, sandstone, chert, anhydrite, and coal, in that order. The entire basin was emergent at least 50 times and marginal areas were dry land about 100 times during the Paleozoic. Arches defined the western, northern, and eastern flanks during deposition, but the basin was not closed on the southwest until the end of the Paleozoic. Erosion has dominated post-Paleozoic history.
The Illinois Basin is a broad, rather gentle, structural depression. Layers of sedimentary rock a few thousand feet below the surface near the center of the basin rise gradually toward surrounding arches 150 to 400 miles distant (fig. 1.) Rock layers generally dip from the top of the arches to the bottom of the basin in an unbroken slope so gentle that it is measured in feet per mile rather than degrees (fig. 2 and Regional Geologic Cross Sections 1, 2, 3). Structurally the basin extends to the axes or crests of the surrounding arches, but the term is commonly applied to a smaller area within some boundary chosen part way up the slope. The choice among several such boundaries (e.g., edge of Pennsylvanian, edge of Chester, or sea level contour on Devonian, fig. 1) is arbitrary, and basin dimensions will vary with the choice. The Illinois Basin is oval, elongate north to south. We can compare the layers of sedimentary rock to a nest of graduated measuring spoons. The spoons themselves are Paleozoic sedimentary rock 530 to 280 million years old. The set of spoons is underlain by a basement of igneous and metamorphic rock about 1.3 billion years old. The Southwest edge of the basin is overlapped by Mesozoic and Cenozoic sediments 90 to 50 million years old. Except for receiving a thin veneer of Pleistocene glacial drift deposited during the last million years, the rest of the basin has undergone erosion above sea level since the Paleozoic. The ancient rocks of the basement lie nearly 3 miles beneath the surface near the mouth of the Wabash River where Illinois, Indiana, and Kentucky join. They outcrop on the Wisconsin Arch and a few places in the Ozarks and rise to within a mile of the surface in the other arches. The spoons in our model are not smooth and unblemished, but are dented and even crumpled. The three major lines of uplift within the basin are the LaSalle Anticlinal Belt, the DuQuoin-Louden Anticlinal Belt, and the Rough Creek-Shawneetown Fault Zone (See Index Map and folded map in pocket by Howard Schwalb). Each rises abruptly from the smooth floor of an inner central deep basin, the Fairfield Basin. Least prominent of the three is the DuQuoin-Louden structure, comprising several individual anticlines and eastward-facing monoclines. The belt trends slightly east of north and separates the Fairfield Basin from a shallow western shelf. The LaSalle structure is an enlarged mirror image of the DuQuoin structure, containing more and larger anticlinces and monoclines that rise 500 to 2,500 feet above the basin floor.
The Rough Creek-Shawneetown Fault Zone is markedly different from the LaSalle and DuQuoin-Louden structures. Whereas they rise from the Fairfield Basin in slopes of a few degrees, the Shawneetown is a reverse fault, a break rather than a bend. Beds from the south have been pushed up over their equivalents in the Fairfield Basin. The uplift along the Shawneetown structure is narrow, extending only a mile or so south of the reverse fault. South of it is the Moorman Syneline, a narrow east-west trough, even deeper than the Fairfield Basin. North of it, most structures run north-south, whereas south of it, structures are sharper, many run east-west and faults are numerous. The times of major movement along the Shawneetown axis do not correspond with those of the northern structures. Although the southern area has yielded much less oil per square mile than the northern region, there have been significant discoveries in it in recent years.
PRECAMBRIAN AND EARLY CAMBRIAN
Few wells have reached the ancient rocks beneath the sediments of the Illinois Basin only 4 in the central hundred counties, but several at the north, west, and east edges. Particularly along the west and north flanks, these have found pink granite (rated radiometrically at 1.2 to 1.4 billion years. Its coarse grain indicates the granite cooled slowly under (-over of miles of older rock. The metamorphosed covering and enclosing rock squeezed, heated, with radiometric clocks reset during the granite intrusion has been penetrated in a few wells, particularly along the east flank of the basin. Granite emplacement was but one phase of an early orogeny or mountain-building cycle whose details are hidden by later events. It was followed by three quarters of a billion years of erosion which leveled the heights, stripped away the covering rocks, and exposed the very roots of the mountain systems -all this before sedimentation began in the northern half or two thirds of the Illinois Basin during Upper Cambrian time, a little more than a half-billion years ago. By this time, the surface on which the basin was to develop seems fairly level, although hills of resistant basement rock several hundred feet high can be demonstrated in Wisconsin, Missouri, and western most Illinois. However, basement outcrops and wells reaching basement are concentrated in structurally high areas and may present a distorted picture. We may be sampling hilltop rock. Scarps one to three thousand feet high and many miles long are suggested by geophysical studies in northeastern Illinois and southwestern Indiana. Gravity and magnetic highs indicate denser and darker basement rock in structurally low areas than that found in the wells. Neither character nor age of basement or of fill in the scarp-lined troughs suggested by the geophysical data is yet known. The early history of the southern part of the Illinois Basin is fragmentary and confusing. Until recently, the history in the south has been considered parallel to that in the rest of the basin. Now, however, the situation appears comparable to that in Oklahoma. Recent Geological Survey research has shown an early history of the basins of Oklahoma differing profoundly from that of the rest of that state. Events in northern and central Oklahoma parallel those in the northern and central parts of the Illinois Basin. But in southern Oklahoma a tremendous wedge of layered volcanic and sedimentary rock, 2 to 5 miles thick, 500 to 600 million years old (Middle and Lower Cambrian) underlies the Upper Cambrian. Around the south end of the Illinois Basin there are but a few deep tests, in south-central Kentucky, western Tennessee, and southeastern most Missouri. Considered collectively, these fit the pattern of southern Oklahoma better than the northern patterns. A thick sequence of layered rock south of the Shawneetown Fault Zone would explain differences in structural pattern between north and south. If this interpretation should prove correct, the southern part of the Illinois Basin has a longer history than the northern part. In the Lower and Middle Cambrian the southern part may have been an extension or enlargement of the Rome Trough which occupied southeastern Kentucky and reached the Appalachian Basin in southwestern Virginia and Northeastern Tennessee.
In addition to being a structural basin today, the Illinois Basin was a basin of deposition in the Paleozoic. For a quarter of a billion years it sank and trapped sediment. The sinking was slow, averaging only two-thirds of an inch per thousand years. Except for minor variations due to the amount of sediment supply and the direction of its source, all major stratigraphic units thin east and west from axes of greatest thickness near the Indiana-llliiiois line (fig. 2). Nearly all thin northward toward the Wisconsin and Kankakee Arches. The Paleozoic basin was structurally open only to the southwest, where the Pascola Arch is situated today. This is shown by thicker, finer deep-water sediments in this direction. Much of the time the basin was covered by shallow warm seas, but it was drained more than 50 times and its margins were dry land about 100 times. In addition to the profound uncomformities at the top and bottom of the Paleozoic rocks, there are major regional unconformities beneath the Middle Ordovician, Middle Devonian, and Pennsylvanian sediments. Each unconformity marks a change in the pattern of basin architecture as well as an episode of uplift and erosion. The unconformities serve to define major rock units which have been named the Sauk, Tippecanoe, Kaskaskia, and Absaroka Sequences. Marine far outweighs non-marine rock in the fill of the basin. Dolomite is most abundant, limestone next, then shale (largely marine but partly continental), sandstone (also partly continental), chert, anhydrite, and coal. Although the sedimentary cover on the neighboring arches is generally much thinner than in the basin, these arches also sank and received sediment. They did not supply material to the basin. The chemical sediments limestone, dolomite, chert, and anhydrite came as ions in solution in sea water. The clastic sediments shale and sandstone reached the basin as mature sand and mud grains from distant sources to the north and east. During the Upper Ordovician and Upper Devonian the Appalachian Basin was oversupplied with detritus from erosion of the Taconic and Acadian mountain systems of Appalachia, so that sediment spilled over the Cincinnati Arch into the Illinois Basin from the east. At most other times the greater part of the detrital sediments came from lands to the north, in and beyond the Canadian Shield. In the Silurian and again in the Mississippian, sinking outstripped accumulation, so that the central part of the basin dropped below effective wave base and received little sediment while thicker deposits accumulated in the shallow water on the arches and basin flanks. Both times the deep-water basin was eventually filled with sediment that included thick cherty formations as well as more normal marine rocks.
UPPER CAMBRIAN AND LOWER ORDOVICIAN (SAUK SEQUENCE)
After a few miles of Precambrian rock had been worn from the Illinois region, the beginning of subsidence allowed sand-sized detritus to accumulate. The resulting thick, very pure, rather coarse-grained, white and pink, unfossiliferous, noncalcareous sandstone nearly blankets the basement rocks. It is the Mt. Simon Sandstone of Wisconsin, Illinois, and Indiana, and the Lamotte Sandstone of Missouri. Included in the Upper Cambrian because it is conformably overlain by fossiliferous rocks of that age, the Mt. Simon may be in part somewhat older. Nearly everywhere it lies on very much older Precambrian rocks. However, on the crest of the Cincinnati Arch at the southeast corner of the basin it is underlain by limestone and shale of possible Middle Cambrian age. The most direct evidences for marine origin of the Mt. Simon are a small amount of dolomite in a well in eastern Iowa, a trace of dolomite in a well in northern Illinois, and a few feet of black shale in another Illinois well. However, the uniformity, bedding, and thickness also suggest marine origin. The formation is water-laid, better sorted than non-marine units, and more uniform than any continental unit its size. It is more than 2,500 feet thick in north-central Illinois and exceeds 1,000 feet over most of Illinois and western Indiana. An amazing concentration of the most stable products of rock weathering, it is free of everything that can be dissolved or decomposed into mud.. Even feldspar is a very minor constitutent except in the basal part. The sea is the agent best fitted to carry away the finer and the soluble components while retaining the sand-sized residue in the region. Cross-bedding studies in Wisconsin show that the Mt. Simon was derived from the northeast. The Mt. Simon is overlapped by younger rocks at the top of some Precambrian hills in Missouri and western Illinois, in one test very deep in the Illinois Basin, and in another southeast of the basin in central Tennessee. The succeeding unit is the Eau Claire Formation of Wisconsin, Indiana, and Illinois or the Bonneterre Dolomite of Missouri. The most varied of the Cambrian and Ordovician units, it is sandstone north of the basin, shale to the east, and dolomite to the west. The lithologic facies intermingle within the basin. In most localities a vertical zonation into a lower sandy unit, a middle calcareous one, and an upper shaly one is evident. The entire sequence represents a fluctuating marine transgression that eventually pushed the land-bordering sandstone belt northward beyond the Illinois Basin. The Eau Claire is marine and fossiliferous. The association of sandstone, oolitic limestone, and fossiliferous marine shale'makes the Eau Claire the most attractive target in the Cambrian and Lower Ordovician for oil exploration. The thickest stratigraphic unit in the Illinois Basin, the Knox Dolomite Megagroup, thickens from a few hundred feet on the flanks to 4,000 and possibly 6,000 feet in the deep part of the basin near the Pascola' Arch. Algal reefs, oolites, and ripple marks show that much of it was deposited in shallow water. There are minor unconformities in the outcropping areas, but it is not clear whether any of these extend into the deep part of the basin. The rock is dominantly light gray to buff, fine-grained dolomite, partly sandy and partly cherty. Major sandstone wedges on the north and west flanks allow recognition of formations that can only be tentatively identified in the thicker lowsand sections deep in the basin. Shaly sandstone, shaly dolomite, and evaporate are virtually lacking. Original limestone has been altered to dolomite except in a few parts of the section in the deepest part of the basin. Although precise correlation is difficult, the Lower Ordovician greatly exceeds the Cambrian in thickness in the deep part of the basin, but the Cambrian exceeds the Ordovician on the basin flanks. This relation is partly explained by the post-Knox pre-St. Peter unconformity that truncates the Knox, cutting deeply into or through the Ordovician on the arches and flanks. A major puzzle of Upper Cambrian and Lower Ordovician sedimentation is the lack of significant amounts of shale. Clay is the most abundant ultimate product of rock weathering and the argillaceous sedi ments as a group are therefore much more abundant than other sedimentary rocks. Yet they form less than 10 per-cent of the Cambrian and Lower Ordovician column in the basin and they appear deficient on the entire North American Continent.
MIDDLE ORDOVICIAN THROUGH LOWER
DEVONIAN (TIPPECANOE SEQUENCE)
Withdrawal of the seas from the Illinois Basin region between the Lower and Middle Ordovician was complete. A valley system with canyons 200 or more feet deep and with extensive karst topography and solution features developed upon the readily dissolved Lower Ordovician terrane. Some minor structures in the positive areas bordering the basin are beveled by the unconformity. A clay and chert residuum from the solution of Knox units occurs at the bottom of sink holes and in pockets on the upland surface of the unconformity. However, at most places the initial deposit of the Middle Ordovician is an exceptionally pure well-rounded and well-sorted sandstone, the St. Peter. No new sand entered the region, and the St. Peter represents further reworking of the already mature sands of the Cambrian and Lower Ordovician. St. Peter deposition accompanied the northward advance of the returning Middle Ordovician sea. A good supply of sand in the west resulted in a formation 100 to 300 feet thick over the uplands and twice as thick in the valleys. The St. Peter breaks into discontinuous sandstone lenses on the east flank of the basin. Sand was so scarce on the Cincinnati Arch in Ohio that the basal Middle Ordovician is argillaceous carbonate and is so tight that it seals oil in hills of cavernous Knox Dolomite beneath the unconformity. A complex of bars, lagoons, and embayments in which clay and carbonate were deposited followed behind the St. Peter shoreline in its slow northward advance. While St. Peter was still being deposited in central Illinois, the black fetid Dutchtown Limestone overlapped the St. Peter south of the Shawneetown Zone. The youngest part of the St. Peter is a bar of coarse sand lying across north-central Illinois separating contemporaneous Glenwood Shale on the north from Joachim Dolomite on the south. Oceanic circulation to the Joachim Basin of southeastern Illinois was restricted and 100 feet of anhydrite was deposited by partial evaporation of seawater. By the middle part of the Middle Ordovician a succession of southward-thickening carbonate units, alternately pure and slightly shaly, were laid down in wide spread open seas. Several layers of ash a fraction of Aii inch to 3 or 4 inches thick were blown from volcanoes east of the Appalachian Basin. At the end of the Middle Ordovician, two positive areas appeared part way down the flanks of the Illinois Basin. They were centered near St. Louis on the west slope and in southeastern Indiana on the east where the youngest. Middle Ordovician carbonates are entirely lacking. Middle Ordovician oil in western Illinois appears associated witl@ the erosion of the western positive area, but none has been found in conjunction with the eastern one. The Upper Ordovician Maquoketa Shale Group is the thin western part of a wedge of debris eroded from the rising Taconic mountain system of Appalachia. Clastic sediment overflowed the Appalachian Basin, spilled over the Cincinnati Arch, filled the Illinois Basin, and reached the Mid-Continent region. Eastward tilting of the eastern United States under the unequal sediment load temporarily obscured basin architecture. The Cincinnati Arch became a hinge line between an eastern segment of the sediment wedge that thickened into the Appalachian Basin at 10 to 15 feet per mile and a western segment thinning into the Illinois Basin at 2 feet per mile. The LaSalle Antielinal Belt provided a second hinge line between the 2 foot per mile segment and a region covering m. ost of Illinois in which the western thinning is only 3 inches a mile. The end of the Ordovician saw a withdrawal of the sea and development of erosional grooves or valleys as much as 5 feet deep, a mile or so wide, and several miles long. Several channel segments are known, but they have not been fitted into an integrated pattern, nor is any regional truncation evident. The returning Silurian seas reoccupied the Ordovician areas. The earliest sediments filling the valleys incorporated much reworked Maquoketa and are quite silty and difficult to distinguish from the underlying units. However, the Cincinnati Arch now acted as an efficient sediment trap, confining mud and sand to the Appalachian Basin so that pure carbonates dominate the Lower and early Middle Silurian of the Illinois Basin. At first these were thickest in the deep part of the basin, but toward the end of Middle Silurian time the basin dropped more rapidly than sedimentation could fill it, creating a sediment-starved condition. Coral reefs grew to sea level on the northeast and north flanks, the I)asinward ones towering 600 to 800 feet above the adjacent deep-water bottom. Clusters of reefs on the crest of the Kankakee Arch prevented two-way water communication between the Michigan and Illinois Basins. Normal marine water from the Illinois Basin filtered through the reefs into the Michigan Basin where it was concentrated by evaporation and filled that basin with saline evaporates. Sedimentation in the deep part of the Illinois Basin was continuous from Silurian through Lower Devonian time, but the flank and arch areas were exposed after Middle or early Upper Silurian. The Lower Devonian deposits in the deepest part of the basin are (-bert and cherty limestone more than a thousand feet thick. On the flanks of the basin the cherty beds are intercalated with wedges of pure crinoidal limestone that formed in the shallow near-shore water. The seas withdrew at the close of the Lower Devonian, so that the pre-Middle Devonian surface truncates units ranging from latest Lower Devonian near the basin center to Lower Ordovician on the arches. Despite the amount of truncation, there is little direct evidence of erosional valleys cut into the unconformity surface. Abundant solution-widened joints, cavities, and fissures 50 feet and more beneath the unconformity suggest master drainage channels this deep, even though they have not yet been found.
MIDDLE DEVONIAN THROUGH
MISSISSIPPIAN (KASKASKIA SEQUENCE)
The Mt. Simon Sandstone at the base of the Sauk Sequence is 1,000 feet thick, the St. Peter at the base of the Tippecanoe Sequence 100 feet thick, but the Dutch Creek Sandstone above the third major unconformity is only a few feet or a few inches thick. It is exceptionally mature, formed by yet another reworking of the sand grains already sorted and rounded in the two earlier cycles. In some parts of the basin the sand occurs only in pockets or as filling of the solutionenlarged joints or "fissures" in the older carbonates. The Middle Devonian seas extended up the arches, beyond the Lower Devonian and in many places, beyond Silurian paleo-outerops. Sediment from distant sources did not reach the basin during the earlier part of the Middle Devonian, so that locally derived sand is the only significant acid-insoluble component of the pure limestones and dolomites. In addition to sand grains, light brown organic tinting deep brown in the Geneva Dolomite -helps distinguish the Middle Devonian from underlying rocks. In the last half of the Middle Devonian, small amounts of clay from the rising Acadian mountains of Appalachia again reached the basin, producing darker, finer limestone and shaly limestone less readily dolomitized than the earlier carbonates. By Upper Devonian time the mud from the east reached across the Illinois Basin producing the black shale of the New Albany. A floating mass of seaweed whose spores are abundant in the shale prevented oxygen from reaching the ocean bottom, so that benthonic life and the production of limestone were inhibited. Accumulation again began to lag behind sinking. Clastic sediment from a far distant northern region, probably the Franklin Mountains of the Canadian Arctic, reached the northern edge of the basin in the waning stages of Devonian and became of great iniportance during the Mississippian. The latest Upper Devonian and the Lower Mississippian Kinderhookian) rocks in the deep part of the basin are only a few feet thick, but they reach 100 to 200 feet on the north flank of the basin and are many hundreds of feet thick in the Michigan Basin. Disappearance or reduction of the floating algal mat allowed the precipitation of a thin persistent deep-water limestone, the Chouteau or Rockford, over most of the basin floor near the end of the Kinderhookian. By the beginning of Middle Mississippian (Valmeyeran) time, the bottom of the basin had dropped to a thousand feet below sea level. A major river system, the Michigan River, brought silt and fine sand across the Canadian Shield from the rising Franklin Mountains of the Arctic. After filling the Michigan Basin, the river fashioned a deltaic complex, the Borden Siltstone, on the north and east borders of the Illinois Basin. On the west margin of the basin the low-lying Ozarks were fronted by clear water with abundant crinoids and other lime-secreting organisms whose skeletal remains formed a carbonate platform comprising the Fern Glen, Burlington, and Keokuk Limestones. The platform ended eastward in a steep submarine escarpment three hundred feet high facing the deep water of the central basin area. Eventually a tongue of the Borden Delta reached entire IN, across the north end of the basin. It then turned southward down the west edge of the deep section along the Burlington-Keokuk scarp. Mud from this tongue covered the western platform, overwhelmed the Keokuk lime-secreting organisms, and formed the Warsaw Shale. While hundreds of feet of limestone and siltstone were thus being deposited in the basin margins, the deep central part of the basin was receiving only a few feet or inches of clay. Toward the end of the major Borden Delta-building episode, dark chert and siliceous limestone of the Fort Payne Formation began to partly fill the deep area. During the middle 1)art of the Valmeyeran the Michigan River was kept from emptying into the Illinois Basin by some combination of a rise in sea level, a decrease in sediment load of the river, or an increase in the capacity of the Michigan Basin farther upstream. Crinoids, bryozoans, and other limesecreting organisms repopulated the shallow shelves and delta platforms. Their remains form the Ullin (Harrodsburg) Limestone. The finer fragments were swept over the edges of the platforms to fill the remaining deep holes, restoring shallow-water conditions to the entire basin. The formations in the upper part of the Valmeyeran are largely shallow-water limestone which varies widely in its properties. The Salem is fossiliferous limestone composed of forms less robust than those of the Harrodsburg, and includes a belt of oolitic limestone across central Illinois. The St. Louis is generally finer-grained than the Salem, represents a low-energy environment and contains few fossils, and contains some evaporate. The Ste. Genevieve includes much oolite deposited in water only a few feet deep. Michigan River sediment again reached the Illinois Basin during Ste. Genevieve time in the earlier cycles of a long series of advances and retreats of the shore line that characterize the remainder of deposition in the basin. The Upper Mississippian rocks are half shale and a quarter each sandstone and limestone, arranged in alternating limestone-and-shale and sandstone-and shale formations. Limestone sheets a few feet or tens of feet thick cover large sections of the basin and are separated by elastic units that are mostly shale but contains systems of elongate sandstone bodies like the veins of a leaf. The typical major body in the sandstone network is 30 to 200 feet thick, I to 6 miles wide, and 10 to 40 or more miles long. Each elastic unit marks an advance of the Michigan River delta for hundreds of miles. The pro-grading delta encroached upon and filled the shallow sea that had occupied the basin. The overlying limestone records a return of the sea. The return was caused by a decrease in the river's sediment load and by continued sinking of the basin. Sediment supply was linked to climatic cycles lasting a few hundred thousand years. Toward the end of the Mississippian, the region was uplifted, tilted up to the north, and beveled by normal stream erosion.
Before Pennsylvanian sedimentation began, renewed uplift had entrenched a southwest-flowing system of youthful valleys 200 feet below the smooth upland surface. The deep part of the basin began to fill with non-marine floodplain sandstone, shale, and conglornerate. Cyclicity is masked by lack of marine beds and by the poor lateral continuity of non-marine units. Because the sediments were in large part derived from ggnew" mountain systems, they are much less mature than the sandstones at the base of the three earlier Paleozoic sequences. Concurrent with early Pennsylvanian sedimentation, uplift of the LaSalle and ,DuQuoin-Louden Antielinal Belts sharply differentiated the Fairfield Basin for the first time. At first, marine conditions seldom reached up the regional slope into the Illinois region, but toward the middle of the Pennsylvanian the seas extended much farther and the sedimentary cycles became striking. Clastic rocks, however, continued much more abundant than carbonates throughout the system. The clasties are derived largely from the north but some Appalachian sediment reached the Illinois Basin. Several times a delta platform, whose top had been barely above sea level during the deltaic advance, was converted to a fresh-water coal swamp by the gradual relative rise of sea level. In most cycles the sea eventually overrode the swamp and covered the coal with a thin layer of black pyrite-bearing shale and with beds of fossiliferous limestone. Later in the Pennsylvanian the marine phase was more persistent giving thinner coals and thicker limestone sheets. The compactness of sediments and rank of coal in the youngest preserved Pennsylvanian strata show that they were once buried under several thousand feet of younger rock. Analogy with basins to the east' and southwest suggests that sedimentation continued through the Pennsylvanian and into the Permian, but the record has been destroyed during the long post Paleozoic interval.
EROSION, AND GLACIATION
By late in the Pennsylvania, 75 to 90 percent of all movement within the central part of the basin and along its northern, eastern, and western edges had already occurred. The basin was still structurally open to the southwest it had no southwest flank. During the 190 million years that elapsed between deposition of the youngest Pennsylvanian rocks in the basin and the oldest Upper Cretaceous rocks at the north edge of the Gulf Coastal Plain, the southwest flank came into being. The basin became closed structurally when the Pascola Arch was uplifted 10,000 feet or more and eroded to base level during and after uplift. Subsidiary movements included nearly all of the normal faulting within and along the southern margin of the basin, as well as the uplift and thrusting of the ShawneetownRough Creek structure against the south edge of the Fairfield Basin. The relative age of a few faults can be determined, but systematic ordering and dating of the sequence of major structural events is impossible. Some faults have been active since the Cretaceous, but most of the structural deformation occurred near the end of the Paleozoic. Since then, the entire region has generally been above sea level undergoing erosion. The northern edge of the wedge of Cretaceous and Cenozoic coastal plain sediments overlaps the southern edge of the basin. However, the history of these sediments is linked to that of the Gulf rather than the Illinois Basin region. During the last million years, four major Pleistocene ice sheets covered the northern part of the basin, depositing 50 to 200 feet of glacial drift over large areas and as much as 400 feet where end moraines cross bedrock valleys. The drainage pattern of the region was disrupted so that landscape changes and erosion are more rapid today than during most of the half billion years in the history of the Illinois Basin.
Recent references for Precambrian history include Ham et al. (1964), Rudman et al. (1965), Bradbury and Atherton (1965), Lidiak et al. (1966), and McGinnis (1966); for the Cambrian and Ordovician, Templeton and Willman (1963) and Bell et al. (1964); for the Silurian and Devonian, Lowenstam (1949,1950) and Collinson et el. (1967); for the Mississippian, Workman and Gillette (1956), Swann (1964), Swann et al. (1965) and Lineback (1966); for the Pennsylvanian, Wanless (1962), Potter (1963), and Simon and Hopkins (1966); and for structure and general history, Swann and Bell (1958), Swann and Willman (1961), Cohee et al. (1961), and Sloss (1964).
BELL, A. H., ATHERTON, ELWOOD, BUSCHBACH, T. C., and SWANN, D. H., 1964 Deep oil possibilities of the Illinois Basin: Illinois Geol. Survey Circ. 368, 38 p.
BRADBURY, J. C., and ATHERTON, ELWOOD, 1965, The Precambrian basement of Illinois: Illinois Geol. Survey Circ. 382,13 p.
COHEE, G. V., et al., 1961, Tectonic map of the United States U. S. Geol. Survey and Am. Assoc. Petroleum Geologists.
COLLINSON, CHARLES, et al., 1967, Devonian of the north central region, United States: Alberta Soc. Petr. Geologists, International Symposium on the Devonian System, v. 1, p. 933-971.
HAM, W. E., DENISON, R. E., and MERRITT, C. A., 1964 Basement rocks and structural evolution southern Oklahoma: Oklahoma Geol. Survey Bull. 95, 302 p.
LIDIAK, E. G., MARVIN, R. F., THOMAS, H. H., BASS, M. N., 1966, Geochronology of the Midcontinent Region, United States 4, eastern area: jour. Geophysical Research, v. 71, no. 22, p. 5427-5438.
LINEBACK, J. A., 1966, Deep water sediments adjacent to the Borden Siltstone (Mississippian delta in southern Illinois: Illinois Geol. Survey Circ. 401, 48 p.
LOWENSTAM, H. A., 1949, Niagaran reefs in Illinois and their relation to oil accumulation: Illinois Geol. Survey Rept. Inv. 145, 36 p.
_____, 1950, Niagaran reefs of the Great Lakes area: jour. Geol., v. 58, no. 4, 430-487.
McGINNIS, L. D., 1966, Crustal tectonics and Precambrian basement in northeastern Illinois: Illinois Geol. Survey Rept. Inv. 219, 29 p.
POTTER, P. E., 1963, Late Palcozoic sandstones of the Illinois Basin: Illinois Geol. Survey Rept. Inv. 217, 92 p.
RUDMAN, A. J., SUMMERSON, C. H., and IIINZE W-1@Geology of basement in Midwestern United States: Assoc. Petroleum Geologists Bull., v. 49, no. 7, p. 894-904.
SIMON, J. A., and HOPKINS, M. E., 1966 Sedimentary structures and morphology of late Paleozoic sand bodies in southern Illinois, Field Conference held in conjunction with 50th annual convention of the Am. Assoc. Petroleum Geologists, Illinois Geol. Survey Guidebook 7, 67 p.
SLOSS, L. L., 1964, Tectonic cycles of the North American Craton: Kansas Geol. Survey Bull. 169, p. 449-460.
SWANN, D. II., 1964, Late Mississippian rhythmic sediments of Mississippi Valley: Am. Assoc. Petroleum Geologists Bull., v. 48, no. 5, 637-658.
_______ and BELL, A. H., 1958, Habitat of oil in the Illinois Basin in Habitat of Oil: Am. Assoc. Petroleum Geologists, Tulsa, Oklahoma, p. 447-472.
_______, LINEBACK, J. A., and FRUND, EUGENE, 1965, The Borden Siltstone (Mississippian) delta in southu,cstern Illinois: Illinois Geol. Survey Cire. 386, 20 p.
_______, and WILLMAN, H. B., 1961, Megagroups in Illinois Am. Assoc. Petroleum Geologists Bull., v. 45, no. 4, p. 471-483.
TEMPLETON, J. S., and WILLMAN, 11. B., 1963, Champlainian Series (Middle Ordovician) in Illinois: Illinois Geol. Survey Bull. 89, 260 p.
WANLESS, H. R., 1962, Pennsylvanian rocks of Eastern Interior Basin in Pennsylvanian System in the United States (symposium): Am. Assoc. Petroleum Geologists, Tulsa, Oklahoma, p. 4 59.
WORKMAN, L. E., and GILLETTE, TRACEY, 1956, Subsurface stratigraphy of the Kinderhoook Series in Illinois: Illinois Geol. Survey Rept. Inv. 189,46 p.