Structural History
Precambrian
Little is known of the Precambrian geology of Illinois. Precambrian rocks do not crop out anywhere in the state and have been reached by only 34 drill holes to date (fig. 4). Of these holes, only ten include cores of Precambrian rocks, and only two cored holes have penetrated more than a few tens of feet into basement. The deep basement penetrations (holes 29 and 30, Table 1) are both in Stephenson County near the Wisconsin border. These borings penetrated 3,264 feet (996 m) and 3,095 feet (944 m) of Precambrian rock, mainly biotite granite and granitic gneiss.
Granite and similar igneous rocks were found in 28 of the Precambrian borings that have been studied; rhyolite was encountered in four holes. Radiometric ages have been reported on samples of granite from four holes: two in Stephenson County, one in Henry County, and one in Madison County. All four dates range from 1,463 to 1,486 million years ago; similar Middle Proterozoic ages are reported on granites from nearby parts of Iowa, Wisconsin, and northern Indiana (Hoppe et al. 1983).
The oldest rock in the region is Archean migmatitic gneiss (3,000 to 3,600 million years old), which is exposed in southern Minnesota. Its southern extent is unknown. The gneiss is interpreted as a remnant of a sialic protocontinent (Sims et al. 1987). North of the gneiss is a greenstone-granite terrane, part of the Superior Province of the Canadian Shield (2,600 to 2,700 million years old). The greenstone/ granite represents subaqueous volcanic and volcaniclastic rocks and associated granites. These were part of an island arc system sutured to the gneissic terrane during the Late Archean Era (Sims 1985, Sims et al. 1987).
Subsequent crustal extension produced a sedimentary basin in the Lake Superior region during the Early Proterozoic Era about 1,900 to 2,100 million years ago. Among the sediments deposited in this basin were the banded iron formations of the Mesabi, Cayuna, and Gunflint Ranges in Minnesota. An island arc system developed to the south in what is now Wisconsin. Approximately 1,850 million years ago, collision between the island arc and the northern era too resulted in the Penokean Orogeny (Ojakangas and Matsch 1982, Sum et al. 1987).
After a lapse of roughly 100 million years, crustal extension south of the Penokean collision zone in present Wisconsin led to intrusion of anorogenic granite and extrusion of rhyolite. Next, sedimentary rocks including the Sioux and Baraboo Quartzites were deposited. Sedimentary rocks about 1,600 to 1,700 million years old crop out in the Baraboo Range about 60 miles (100 km) north of the Illinois border. The rocks of the Baraboo Range have been metamorphosed to greenschist facies and deformed into east-trending folds (Sims et al. 1987).
Another major collisional event, the Central Plains Orogeny, took place about 1,630 to 1,800 million years ago (Bickford et al. 1986, Sims and Peterman 1986). Granitic and metamorphic rocks of the Central Plains Orogeny underlie large areas of Nebraska, Kansas, and northern Missouri and may extend into Illinois just north of St. Louis (fig. 5). Widespread acidic igneous activity succeeded the Central Plains Orogeny. Anorogenic granite and rhyolite were emplaced in present northern Illinois and adjacent areas 1,450 to 1,500 million years ago (Hoppe et al. 1983). The name Transcontinental Anorogenic Province has been applied to these rocks (Sims et al. 1987). At nearly the same time (1,350 to 1,480 million years ago), the St. Francois granite-rhyolite terrane, part of the Eastern Granite-Rhyolite Province, developed in what became southern Illinois, southeastern Missouri, and adjacent areas.
Nearly all of the Precambrian rocks that have been encountered in Illinois wells can be ascribed to either the Transcontinental or St. Francois Terrane (fig. 5), although the boundary between these terranes is ill-defined. The St. Francois Terrane, which crops out and has been drilled extensively in southeastern Missouri, is much better known than the Transcontinental Terrane. Kisvarsanyi (1981) characterized the St. Francois Terrane as a series of granite ring complexes, similar to those of the African and Brazilian Shields. The St. Francois Terrane may have been a rift zone. Volcanic rocks seem to be confined between two major northwest-trending shear zones, the Grand River and Northeast Missouri Tectonic Zones (Kisvarsanyi 1984, Sims et al. 1987). The Ste. Genevieve Fault Zone (fig. 2) partly coincides with the Northeast Missouri Tectonic Zone. Other northwest-trending shear zones in Missouri, eastern Kansas, and southern Iowa are similarly defined by Proterozoic igneous intrusions, geophysical anomalies, cataclasis of basement rocks, and folding and faulting of the Paleozoic rocks (Guinness et al. 1982, Sims et al. 1987).
Middle Proterozoic granite and rhyolite in Illinois may be largely a veneer on top of older layered rocks. Both proprietary and published seismic reflection profiles (Braile et al. 1984, Pratt et al. 1989) reveal strong, moderately to highly continuous, subhorizontal reflectors at depths far below the top of granite-rhyolite. Xenoliths of schist and gneiss found in diatremes in southeastern Missouri (Tarr and Keller 1933) provide further evidence that the St. Francois Terrane rests on an older basement.
No record is known in Illinois of geologic events between about 600 and 1,350 million years ago. The Midcontinent Rift System, which curves from southern Michigan through Lake Superior and then southwestward to Kansas, formed approximately 1,000 to 1,200 million years ago. At roughly the same time, continental collision along the eastern and southern margin of North America resulted in the Grenville Orogeny (King 1977). After the Grenville Orogeny, what is now Illinois was situated in the interior of a Late Proterozoic supercontinent.
The Precambrian supercontinent broke apart near the end of the Precambrian Era about 600 million years ago. The North American craton, as part of a continent called Laurentia, separated from the southern landmass of Gondwanaland, and ocean basins developed between the two continents. The line of separation roughly coincided with the present trends of the Ouachita and Appalachian Mountains (fig. 5). Several aulacogens extended into Laurentia, roughly perpendicular to the coastline (Burke and Dewey 1973). Among these intracratonic aulacogens was the Reelfoot Rift and its eastward extension, the Rough Creek Graben. The Reelfoot Rift (Ervin and McGinnis 1975) underlies the present Mississippi Embayment from northeastern Arkansas and western Tennessee to southern Illinois. The Rough Creek Graben (Soderberg and Keller 1981, Schwalb 1982) trends eastward into Kentucky. The Reelfoot Rift and Rough Creek Graben are bounded by large listric normal faults that penetrate crystalline basement (fig. 6).
Cambrian Period
The Reelfoot Rift and Rough Creek Graben actively subsided during the Cambrian Period and received thick successions of sediments, while areas outside the grabens remained as eroding uplands (Houseknecht and Weaverling 1983, Howe and Thompson 1984, Nelson and Kolata 1988, Consortium for Continental Reflection Profiling [COCORP] Atlas 1988, Profiles AR-6 and TN-3, Bertagne and Leising 1991). Rapid sedimentation took place in these trenches concurrent with movement along their bounding faults. Faulting continued through the early St. Croixan Epoch when the Mt. Simon/Lamotte Sandstone was being deposited (Howe and Thompson 1984, Bertagne and Leising 1991).
A hilly topography with as much as 800 feet (240 m) of documented local relief was carved from Precambrian granite and rhyolite in the rest of Illinois. In the St. Croixan Epoch (Late Cambrian), the sea invaded the continental interior, depositing the Mt. Simon/ Lamotte Sandstone (fig. 7). The lower part of the Mt. Simon/Lamotte, derived from weathered granite, is generally coarse and arkosic; upward it becomes finer grained and more quartzitic. Two basins of deposition developed (fig. 8). One was located over the rapidly subsiding Reelfoot Rift and Rough Creek Graben in southernmost Illinois; the other was in northeastern Illinois and parts of adjacent states. This northern basin disappeared after Mt. Simon/Lamotte deposition. The Sparta Shelf was a positive area; the Mt. Simon/Lamotte is thin or absent in the area. An ancestral Du Quoin Monocline may have formed a structural hinge line on the east margin of the Sparta Shelf. The St. Francois Mountains and other large Precambrian hills remained above sea level, so the Mt. Simon/Lamotte does not overlap them.
By the end of Mt. Simon/Lamotte deposition, active movement ceased on the faults that bounded the Reelfoot Rift and Rough Creek Graben, but the rifted area continued to subside rapidly. The resulting structural depression, of which the deepest point lay south of Illinois, is called the Reelfoot Basin (Schwalb 1969). The configuration of the Reelfoot Basin is poorly known; it may have been a trough that connected with the open ocean south of Laurentia. The Reelfoot Basin was a Cambrian-Ordovician predecessor to the Illinois Basin.
After deposition of the Mt. Simon/ Lamotte, a carbonate bank (Bonneterre Dolomite) developed on the Ozark Dome and extended into southwestern Illinois. The Bonneterre intertongues northeastward with siliciclastics of the Eau Claire Formation (Workman and Bell 1948, Howe et al. 1972, Schwalb 1982, Sargent 1991). Within the Reelfoot Basin, this interval becomes predominantly shale and more than doubles in thickness (Schwalb 1982). Only a few of the highest knobs in the St. Francois Mountains and on the Sparta Shelf remained above water during late St. Croixan time (Dake and Bridge 1932).
The patterns established during Eau Claire sedimentation continued for the rest of the Cambrian Period. Subsidence was most rapid in southern Illinois but the sea was shallow; carbonate sedimentation (Knox Group) was predominant. Tongues of supermature quartz sandstone, derived and extended from the north, interfingered with carbonates in northern Illinois (Buschbach 1975, Ostrom 1970, 1978).
Ordovician Period
Sedimentation continued with little or no break from the Cambrian into the Ordovician Period in Illinois. Rapid deposition of shallow water carbonate took place in southern Illinois during Canadian (Lower Ordovician) time. Knox strata may exceed 7,000 feet (2,100 m) in the Reelfoot Basin, and they thin rapidly northward (Schwalb 1982). Equivalent rocks of northern Illinois are less than 500 feet (150 m) thick and consist mainly of cherty dolomite and dolomitic sandstone.
The sea withdrew from most of the North American craton at the end of the Canadian Epoch, and the region was subjected to subaerial erosion. The resulting unconformity separates the Sauk Sequence below from the Tippecanoe Sequence above (Sloss et al. 1949, Sloss 1963). The Kankakee Arch emerged, a karst topography developed in exposed Canadian carbonates, and channels were locally cut as deep as the Franconia Formation (Cambrian), (Buschbach 1964, 1975). Southward, this sub-Tippecanoe unconformity becomes less pronounced. Sedimentation possibly was uninterrupted in the deep part of the Reelfoot Basin. Sandstone and sandy dolomite of the Everton Formation overlie the sub-Tippecanoe unconformity in southern Illinois (Schwalb 1982 ). The St. Peter Sandstone unconformably overlies the Everton, and oversteps it northward. The almost pure, well rounded, frosted quartz sand that constitutes the St. Peter probably was recycled from older sandstones to the north of Illinois. The thickness of the St. Peter varies in irregular fashion, reflecting both its deposition upon an erosion surface and its intertonguing facies relationship with overlying carbonates (Templeton and Willman 1963).
After St. Peter sedimentation, carbonate deposition resumed early in the Champlainian Epoch. The Dutchtown and Joachim Formations and Platteville Group steadily increase in thickness from northwestern to southeastern Illinois toward the Reelfoot Basin (Willman and Buschbach 1975). The Decorah Subgroup, a wedge of fine grained, siliciclastic sediment derived from the Transcontinental Arch far to the northwest, was deposited above the Platteville in northwestern Illinois and around the east margin of the Ozark Dome. The succeeding upper Champlainian-Cincinnatian Galena (Trenton) Group, unlike most older units, is more than twice as thick in northwestern Illinois as it is in the south (Willman and Buschbach 1975). This trend suggests that by early in the Cincinnatian Epoch the Reelfoot Basin was no longer subsiding more rapidly than the rest of Illinois. Such interpretations must be made cautiously, however, because thickness trends of carbonate rocks are not necessarily reliable indicators of subsidence rates. In some instances, carbonates accumulate more rapidly on stable shelves than in basins.
The Taconian Orogeny took place east of Illinois late in the Champlainian and during the Cincinnatian (Late Ordovician) Epochs as a consequence of collision between North America and an eastern landmass. The chief effect in Illinois was the introduction of fine siliciclastic sediment that was eroded from distant highlands raised during the orogeny. These siliciclastics constitute the Cincinnatian Maquoketa Group, which is predominantly shale. The Maquoketa thickens eastward from about 300 feet (90 m) in eastern Illinois to more than 1,000 feet (300 m) in western Ohio (Kolata and Graese 1983, Whitaker 1988). This eastward thickening may reflect downwarping of the crust in a foreland basin adjacent to the orogenic belt, and it also reflects proximity to the source area. Local thickening of the Maquoketa in extreme western Kentucky indicates that part of the Reelfoot Basin continued to subside more rapidly than most of Illinois. Other local structural movements in and near Illinois, possibly triggered by the Taconian Orogeny, are indicated by Cincinnatian sedimentation patterns. The Thebes Sandstone of southwesternmost Illinois and adjacent Missouri may have been derived from uplift of part of the Ozark Dome. In northern Illinois, thickness and lithofacies distribution of Maquoketa rocks indicate slight concurrent uplift of the Wisconsin Arch, La Salle Anticlinorium, and related structures (Kolata and Graese 1983, Graese 1988).
Silurian Period
The Silurian Period was a quiet time tectonically in Illinois and surrounding areas. Marine sediments, now largely dolomite in the north and limestone, shale, and siltstone in the south, were laid down in Illinois during the Silurian Period.
Alexandrian (Lower Silurian) carbonates were deposited on the Maquoketa Group after a brief episode of erosion. The Alexandrian Series is less than 50 feet (15 m) thick in most of central Illinois. Its maximum thickness of 125 to 150 feet (38-46 m) is attained in a few areas of northeastern Illinois and Franklin County in southern Illinois (Willman and Atherton 1975). It is unknown to what degree, if any, the thickness trends reflect structural movement.
Pinnacle reefs first appeared early in the Niagaran (Silurian) Epoch and grew throughout the Niagaran and Cayugan (Late Silurian) Epochs. Known reefs in Illinois lie mainly in the Chicago area and along a broad zone that trends northeastward from St. Louis toward Terre Haute, Indiana. Reefs in Illinois appear to have grown preferentially along the margins between areas of shallow and deep water. Reefs in the Chicago area are part of an archipelago that surrounds the Michigan Basin. Reefs in southern Illinois flanked a depression known as the Vincennes Basin (Droste et al. 1975), which was similar in shape and size to the present Fairfield Basin; but the axis of the Vincennes Basin lay east of that of the Fairfield Basin. The Vincennes Basin extended south of the Rough Creek Fault System into Kentucky and thus was a proto-Illinois Basin, successor to the Reelfoot Basin of Cambrian and Ordovician time. The thickest Silurian strata are not found in the Vincennes Basin, but in the reef areas--a significant fact indicating that carbonate thickness is not a reliable guide to subsidence rates. Interpretation of Silurian paleogeography is based more on lithofacies than on isopach mapping (Droste and Shaver 1980, 1987, Whitaker 1988).
Devonian Period
Sedimentation apparently continued without a break from the Silurian into the Devonian Period in southern Illinois, where the Silurian-Devonian systemic boundary has not been accurately located. Because pre-Middle Devonian erosion moved Lower Devonian rocks elsewhere, they are extant only in southern Illinois and thicken southward. A basin probably existed with its trough or axis in southernmost Illinois and adjacent part of Kentucky. Rogers (1972) called this feature the Metropolis Depression, but the term Vincennes Basin (Droste et al. 1975) is more widely used. Lower Devonian strata within this basin are dominantly fine grained, highly siliceous limestone and dolomite interbedded with chert. A coarse bioclastic limestone, the Backbone Limestone, represents basin-fringing shoals (Rogers 1972, Droste and Shaver 1987).
At the end of the Early Devonian Epoch, the sea withdrew from much of the North American craton, including all except southernmost Illinois. Subaerial erosion exposed strata as old as the Galena Group (Upper Ordovician) in western Illinois (Willman et al. 1967, Collinson and Atherton 1975). The resultant unconformity separates the Tippecanoe Sequence from the overlying Kaskaskia Sequence (Sloss et al. 1949, Sloss 1963).
As Middle Devonian sedimentation began, two areas in Illinois remained above sea level. One was the Sparta Shelf in the southwestern part of the state (fig. 9). The other was the Sangamon Arch, a broad northeast-trending arch that divided the Devonian seaway into two basins, one in southern Illinois and the other in northwestern Illinois and Iowa. Both basins primarily received carbonate sediments, but the detailed successions differ.
Another episode of continental collision along the eastern coast of North America brought on the Middle to Late Devonian Acadian Orogeny. Active faulting in and near Illinois may have been triggered by Acadian stresses. The buried Rough Creek Graben and Reelfoot Rift subsided, creating a Middle Devonian depocenter in southernmost Illinois and western Kentucky. The north side of the Rough Creek Fault System was raised in Kentucky (Freeman 1951). To the northwest, the Sparta Shelf was uplifted relative to the Ozark Dome along the Ste. Genevieve Fault Zone shortly after deposition of the Grand Tower Limestone (lower Middle Devonian). The fault zone, known mostly from subsurface data, trends eastward from Missouri into Jackson County, Illinois (fig. 10). Devonian, Silurian, and Ordovician strata were eroded from the northern block; elastic detritus was incorporated into upper Middle Devonian units south of the fault scarp (S. Weller and St. Clair 1928, Nelson and Lumm 1985). In Iowa and northern Illinois, the east-trending Plum River Fault Zone was also active during Middle Devonian sedimentation (Bunker et al. 1985).
Meanwhile in the eastern United States, the Catskill Delta Complex prograded westward from sources in new mountains uplifted during the Acadian Orogeny. Distal, dark organic clay and silt (New Albany Group) reached Illinois late in the Middle Devonian Epoch and continued to accumulate through the Late Devonian into the Kinderhookian (lower Mississippian) Epoch. By the Late Devonian, the Sangamon Arch and Sparta Shelf were again largely submerged and received marine sediment. Regionally the New Albany is thickest near the western end of the Rough Creek Graben. Isopach mapping (Schwalb and Potter 1978) revealed that both the northern and southern boundary faults of the graben were active during New Albany sedimentation. Also active was the Media Anticline in Henderson County, Illinois, and several northwest-trending anticlines nearby in Iowa. Folding evidently was caused by movement on basement faults reactivated under stresses associated with the Acadian Orogeny.
Mississippian Period
The basin or embayment that was established in southern Illinois during the Devonian Period continued into the Mississippian Period, while the basin that had existed northwest of the Sangamon Arch became a shelf. Nevertheless, Kinderhookian strata are considerably thicker in west-central than in southern Illinois. Lineback (1969) referred to the southern basin early in the Mississippian as a starved basin. Only a little mud and fine silt (Springville Shale) accumulated there. The shallow, well oxygenated water on the Western Shelf was conducive to carbonate formation. By early in the Valmeyeran (middle Mississippian) Epoch, a carbonate bank (Burlington and Keokuk Limestones) flourished on the Western Shelf.
An Acadian elastic wedge, the Borden Delta, prograded into Illinois from the east or northeast early in the Valmeyeran (Swann et al. 1965). The delta eventually reached southwestern Illinois and overlapped the Burlington-Keokuk carbonate bank. A relatively deep basin remained on the south, flanked by the Borden foreset beds. Siliceous lime mud (Fort Payne Formation) began to fill this basin and overlapped the flanks of the Borden Delta (Lineback 1966).
Tectonic stability prevailed in Illinois for the rest of Valmeyeran time. Carbonates gradually filled the southern basin and overlapped the Borden Delta and Western Shelf. The Ullin, Salem, St. Louis, and Ste. Genevieve Limestones record gradual shoaling and infilling of the basin. By the end of the Valrneyeran, the sea was very shallow across most of Illinois. The epicontinental shelf and adjacent coastal plain sloped very slightly southwest toward the rapidly deepening Ouachita geosyncline.
During the Chesterian Epoch (late Mississippian), terrigenous elastics once again were introduced into Illinois and neighboring areas. According to Swann (1963), a series of streams called the Michigan River System delivered clay, silt, and fine quartz sand from source areas far to the northeast. Deltas prograded into the epicontinental ocean; the shoreline periodically advanced and retreated, most likely because of eustatic changes in the sea level. As a consequence, the Chesterian Series consists of numerous alternating units of limestone and terrigenous elastics. Chesterian structural movements in Illinois were subtle. Lithofacies mapping suggests that the Du Quoin Monocline, La Salle Anticlinorium, and Rough Creek Graben were elevated slightly during deposition of the Golconda Group (middle Chesterian) (Treworgy 1988).
Widespread structural deformation took place in Illinois and through much of North America near the end of the Mississippian Period. Major episodes of mountain building took place both east and west of Illinois at this time. On the east, an early episode of the collisional Alleghenian Orogeny occurred in latest Mississippian through the early Pennsylvanian time. Mountains rose from Alabama to Nova Scotia, while adjacent foreland basins such as the Black Warrior (Alabama) and Pocahontas (Virginia and West Virginia) sank. West of Illinois, many basement-cored· fault blocks were uplifted, notably the Ancestral Rockies (Colorado and Wyoming), Wichita-Amarillo Mountains (Texas and Oklahoma), and the Nemaha Anticline and Central Kansas Uplift (Kansas). Uplifts in Illinois were similar in style, although smaller in scale, than the western examples. Most late Mississippian uplifts in Illinois involved movement on north- to northwest-striking, high-angle reverse faults in basement, and the movements forced folding of sedimentary cover. Active structures included the La Salle Anticlinorium; Clay City, Salem, Louden, and Waterloo-Dupo Anticlines; Du Quoin Monocline; Ste. Genevieve Fault Zone; and the Lincoln Anticline/Cap Au Grès Faulted Flexure (fig. 10). Most of these structures continued to undergo movement both during and after Pennsylvanian sedimentation.
Late Mississippian deformation in Illinois was a compressional event analogous to the Laramide foreland deformation in the Wyoming Province (Stearns 1978). As a consequence of either the widespread tectonic activity or a eustatic drop in sea level, all of Illinois was exposed to subaerial erosion at the end of the Mississippian Period. The resultant unconformity separates the Kaskaskia and older sequences from the overlying Absaroka Sequence. Erosion was deepest on active uplifts; St. Peter Sandstone (Middle Ordovician) underlies the unconformity near the northern end of the La Salle Anticlinorium. Progressively younger rocks are preserved beneath the unconformity southward. A system of anastomosing rivers cut southwest-trending valleys into Mississippian strata of central and southern Illinois (Bristol and Howard 1974, Howard 1979).
Pennsylvanian Period
After erosion of the sub-Absaroka surface, sedimentation resumed in the Morrowan (early Pennsylvanian) Epoch. First the valleys aggraded, then the intervening divides were covered. Rivers flowing from the northeast were the primary source of elastic sediment, as in Chesterian time. Basal Pennsylvanian deposits (Caseyville Formation) of southern Illinois represent a variety of fluvial, deltaic, coastal swamp, and marginal marine environments. A small area of the Caseyville, probably from a northwestern source, also accumulated in Rock Island County and adjacent parts of Iowa. The rest of Illinois was still undergoing subaerial erosion, but the sea gradually encroached northeastward. Almost all of Illinois received sediments during Atokan time, although a few structural uplifts were not covered until early in the Desmoinesian Epoch. Uplift along the Du Quoin Monocline continued; the Fairfield Basin sank more rapidly than did the Sparta Shelf. The Fairfield Basin probably was connected to the Arkoma Basin, so that sediment was carried around the southeast side of the Ozark Dome (Houseknecht 1983). The La Salle Anticlinorium and other anticlines and monoclines in Illinois rose intermittently throughout the Pennsylvanian Period (fig. 11).
By middle Desmoinesian time, Illinois had become a level plain or shelf. Vast coal swamps flourished. Highly continuous but thin marine limestones were interlayered with fluvial and deltaic elastic units and coal. Local, minor movements on the Du Quoin Monocline, La Salle Anticlinorium, and other structures have left an imprint on Desmoinesian sedimentary patterns.
These conditions persisted, with minor and subtle variations, through the Missourian and Virgilian Epochs and probably into the Permian Period. Very early Permian rocks identified in a drill core in western Kentucky (Kehn et al. 1982) imply that marine strata of this age formerly covered at least the southern part of the Illinois Basin.
Late Paleozoic (?) Compressional Events
Reverse and strike-slip faulting and folding occurred in southern Illinois late in the Paleozoic Era. The resulting structural pattern indicates compression from the south or southeast. Associated with this deformation is alkalic ultrabasic igneous activity; these rocks have been dated as Early Permian (Zartman et al. 1967). The style and timing of deformation indicate a relation to the Alleghenian and Ouachita Orogenies, which were caused by continental collision. Many compressional faults in southern Illinois are reactivated faults that originated during Cambrian rifting. Major events of the late Paleozoic compressional phase were as follows (fig. 11):
- The south side of the Rough Creek-Shawneetown Fault System rose along a reverse fault that dips steeply to the south. Maximum uplift was at least 3,500 feet (1,050 m) at the "Horseshoe Upheaval" in eastern Saline County (Nelson and Lumm 1987, Bertagne and Leising 1991).
- In similar fashion, the southeast side of the Lusk Creek Fault Zone was raised (Nelson 1986).
- Right-lateral movement and intrusion of ultramafic igneous rocks took place along the Cottage Grove Fault System and at the Omaha Dome.
- Hicks Dome and its radial and concentric faults were produced by cryptovolcanic explosions at depth. Diatremes and ultramafic dikes, dated as early Permian (Zartman et al. 1967), appeared in the vicinity. The Tolu Arch probably also formed at this time.
- The McCormick and New Burnside Anticlines developed, probably in response to horizontal thrusting along one or more décollements northwest of the Lusk Creek Fault Zone (Nelson 1987a).
In addition, post-Pennsylvanian uplift affected many structures in the Fairfield Basin, including the Du Quoin Monocline, the La Salle Anticlinorium, and the Salem, Louden, and Clay City Anticlines. Whether these movements were related to compressional deformation farther south is not known. The Pascola Arch is also a post-Pennsylvanian feature, but its structural style is not understood and timing of uplift is poorly constrained. The Pascola Arch was formed sometime between the Late Pennsylvanian and Late Cretaceous period.
Mesozoic (?) Extensional Events
Reversal of the stress field from the compressional phase described above resulted in a period of extensional stress typified by normal faulting. In many cases, reverse faults formed in the compressional phase were reactivated as normal faults. Major structural events include the following (fig. 12):
- Normal faulting occurred along the Rough Creek-Shawneetown Fault System. The southern block sank back to approximately its pre-Permian position; drag along the fault zone created the northern limb of the Eagle Valley-Moorman Syncline (Nelson and Lumm 1987).
- Similarly, the Lusk Creek Fault Zone underwent normal faulting. Normal faults along the McCormick and New Burnside Anticlines probably formed at the same time (Nelson 1986b, 1987).
- The northeast-trending normal faults of the Fluorspar Area Fault Complex developed; mineralization took place.
- The Wabash Valley Fault System developed.
The cause and timing of extensional faulting are poorly understood. Several observations suggest, however, that the most likely time of normal faulting was Triassic or Jurassic. Normal faults in the fluorspar district displace the Permian dikes and are therefore younger than the dikes. Drag and the position of fault slices in the Lusk Creek Fault Zone and Rough Creek-Shawneetown Fault System indicate that the last movements on these faults were normal and down to the south or southeast. Displacement of Cretaceous and Tertiary rocks of the Mississippi Embayment by faults of the Fluorspar Area Fault Complex is small compared with offsets of Paleozoic bedrock (Rhoades and Mistler 1941, Kolata et al. 1981 ). The stress regime implied by northeast-trending normal faults is inconsistent with inferred stresses that produced the Permian Alleghenian Orogeny; also, it does not match the modern, measured stress regime. The Triassic was a period of widespread graben formation in the eastern United States. Most of the Triassic grabens trend north-south to northeast-southwest. The Atlantic Ocean, with its north-south-trending central rift, opened during the Jurassic Period. These observations indicate that normal faulting in southern Illinois probably took place in Triassic or Jurassic time.
Cretaceous to Recent Events
Documenting post-Cretaceous structural movement in Illinois is difficult because Cretaceous and Tertiary strata are restricted to small areas and, where present, are poorly exposed. Cretaceous rocks occur only in the Mississippi Embayment of southernmost Illinois and in several small outliers in Adams, Brown, and Pike Counties in western Illinois. Clay, silt, and sand of Paleocene and Eocene age overlie Cretaceous strata in the Embayment. Scattered deposits of Pliocene or early Pleistocene gravel variously overlie Tertiary, Cretaceous, or Paleozoic bedrock in both southern and western Illinois, beyond the limits of glacial drift. Exposures of these materials are Limited to occasional river bluffs, stream cuts, and small quarries for clay, sand, and gravel.
Faulting of Cretaceous and Tertiary sediments has been documented adjacent to Illinois in both Kentucky and Missouri. Rhoades and Mistler (1941) reported Cretaceous and possibly Tertiary deposits offset along northeast-trending faults of the Fluorspar Area Fault Complex near Paducah, Kentucky. Geologic maps of the same area show faults displacing the Cretaceous Tuscaloosa Gravel and McNairy Sand (Amos 1967, 1974, Amos and Wolfe 1966, Amos and Finch 1968). Offsets of Cretaceous units are small, 100 feet (30 m) or less, in contrast to displacements as great as 2,000 feet (600 m) along the same faults in Paleozoic bedrock. In Missouri, Tertiary strata are folded and faulted along the southeast face of Crowleys Ridge, a linear northeast-trending scarp within the Mississippi Embayment. Mapping in this area demonstrates displacements of units as young as the Mounds Gravel (Pliocene to early Pleistocene?) and possible offsets of Quaternary loess and alluvium (Grohskopf 1955, McCraken 1971, W. Johnson 1985, Harrison and Schultz 1992, Nelson and Harrison 1993). In the Thebes Gap area of Scott County, Missouri, and Alexander County, Illinois, late Tertiary faults mostly strike northeast and exhibit right-lateral slip.
Post-Cretaceous tectonic faulting in southernmost Illinois was postulated by Ross (1963, 1964) and on the basis of field observations and subsurface data. Kolata et al. (1981) disputed Ross's findings and attributed all observed deformation to nontectonic processes such as landsliding and solution-collapse; however, new mapping has uncovered tectonic deformation of Cretaceous and Tertiary sediments in several areas in southernmost Illinois. The Mounds Gravel and older units are offset by northeast-trending faults in the Illinois portion of the Thebes Quadrangles (Harrison and Schultz 1992). A zone of post-Eocene faults trends slightly east of south through southern Union and northern Alexander counties (Devera et al. 1994, Nelson and Devera 1994). These faults dip steeply and exhibit probable strike-slip with a component of extension. The McNairy Formation and Mounds Gravel are deformed with probable right-lateral slip in the Dixon Springs Graben near the edge of the Mississippi Embayment (W.J. Nelson, unpublished mapping).
The contemporary tectonic stress field on the central United States, including Illinois, has been measured by a variety of methods (Sbar and Sykes 1973, Zoback and Zoback 1980, Nelson and Bauer 1987). The principal compressive stress axis is oriented from east to west to east-northeast to west-southwest in southern Illinois (fig. 13, Table 5). Joint patterns in bedrock, directional ground failures in underground mines, and a small thrust fault in coal-bearing strata (fig. 14) apparently are products of present stress. Immediately south of Illinois, ancient faults of the Reelfoot Rift are being reactivated under contemporary stress, producing earthquakes in the New Madrid Seismic Zone (Braile et al. 1982, 1984, Hamilton and Zoback 1982, Russ 1982, Stearns et al. 1986). The most prominent faults in the New Madrid Seismic Zone (fig. 15) trend northeast and are undergoing right-lateral slip. Tertiary faults newly mapped in the Thebes area and Dixon Springs Graben have the same trend and sense of slip (Harrison and Schultz 1992, Nelson and Harrison 1993).
As this bulletin goes to press, research in southern Illinois continues in an attempt to determine whether any faults there are presently active.
Elsewhere in Illinois, Rubey (1952) reported that terrace deposits of the Grover Gravel were uplifted about 150 feet (45m) along the Cap Au Grès Faulted Flexure. The Grover Gravel is considered equivalent to the Mounds and Pliocene to early Pleistocene in age (Willman et al. 1975). Quaternary structural movements in southern Illinois and southeastern Missouri were inferred on the basis of anomalous drainage patterns and tilted terraces and peneplains (Shaw 1915). No later geologist has addressed Shaw's ideas.
Figure(s)
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Table(s)
| Table 1 Wells that reach Precambrian rocks in Illinois. See figure 4. | ||||||||
|---|---|---|---|---|---|---|---|---|
| Elevation (ft), datum msl | ||||||||
| Well | County | Sec.-T-R | Elev (ft) | TO (ft) | Top of Knox | Top of Mt. Simon | Top of basement | |
| 1 | Amboy Oil and Gas No. 1 McElroy | Lee | 30-20N-10E | 714 | 3,772 | -151 | -1,356 | -3,046 |
| 2 | Northern Ill. Oil and Gas No.1 Taylor | Boone | 28-43N-3E | 815 | 2,998 | absent | -510 | -2,104 |
| 3 | Schulte No. 1 Wyman | De Kalb | 35-41N-5E | 910 | 4,484 | -105 | -810 | -2,953 |
| 4 | Herndon No. 1 Campbell | Pike | 15-4S-5W | 716 | 3,207 | -374 | -2,044 | -2,488 |
| 5 | Panhandle Eastern No. 1 Mumford | Pike | 21-5S-4W | 812 | 2,226 | -58 | absent | -1,409 |
| 6 | Carr No. 1 Vedovell | Lee | 35-20N-10E | 812 | 3,653 | +288 | -888 | -2,653 |
| 7 | Miss. River Fuel No. A-15 Theobold | Monroe | 35-1S-10W | 666 | 2,768 | -293 | absent | -2,093 |
| 8 | Lawinger No. 1 Miller | La Salle | 1-36N-4E | 681 | 3,659 | +435 | -339 | -2,788 |
| 9 | Otto No. 1 Swenson | La Salle | 1-36N-5E | 659 | 3,725 | +539 | -473 | -3,041 |
| 10 | Vickery No. 1 Mathesius | La Salle | 32-35N-1E | 677 | 3,556 | +607 | -744 | -2,838 |
| 11 | H.L. Kelley No. 1 Fullerton | Mercer | 19-13N-4W | 584 | 3,716 | -356 | -1,916 | -2,671 |
| 12 | Miss. River Transmission No. S-5 Baer | Madison | 27-3N-6W | 516 | 4,868 | -2,451 | absent | -4,341 |
| 13 | Humble Oil No. 1 Weaber-Horn | Fayette | 28-8N-3E | 538 | 8,616 | -4,097 | -6,352 | -7,676 |
| 14 | R.E. Davis No. 1 South | Henry | 30-16N-1E | 803 | 3,863 | -500 | -1 ,797 | -3,052 |
| 15 | I. Seele No. 1 Seele | Winnebago | 24-44N-2E | 870 | 3,385 | +255 | -381 | -1 ,786 |
| 16 | C. Reed No. 1 McCoy | Will | 20-35N-9E | 632 | 4,300 | -385 | -1,272 | - 3,593 |
| 17 | Texaco No. 1 Cuppy | Hamilton | 6-6S-7E | 393 | 13,051 | -7,365 | absent | -12,574 |
| 18 | Texaco No. 1 Johnson | Marion | 6-1 N-2E | 541 | 9,210 | 4,835 | -7,909 | -8,629 |
| 19 | Jones and Laughlin Steel Corp. No. 1 WD-1 | Putnam | 3-32N-2W | 527 | 4,877 | -1 ,063 | -2,608 | -4,315 |
| 20 | American Potash and Chemical No. WD-1 | Du Page | 9-39N-9E | 741 | 4,043 | -309 | -1 ,084 | -3,279 |
| 21 | Union Oil of California Cisne Comm. No. 1 | Wayne | 3-1 S-7E | 504 | 11,614 | -7,072 | -10,653 | -11 ,010 |
| 22 | Maryland Service No. S-1 Kircheis | Madison | 27-3N-6W | 504 | 5,018 | -2,446 | -4,462 | -4,506 |
| 23 | Miss. River Transmission Klein No. S-2 | Madison | 33-3N-6W | 513 | 5,213 | -2,655 | -4,631 | -4,688 |
| 24 | North. Ill. Gas No. 1 Lillard | Henderson | 14-9N-5W | 627 | 3,180 | -297 | -1 ,793 | -2,531 |
| 25 | Harza Engineering UPH-1 Commonwealth Edison | Stephenson | 18-29N-6E | 905 | 2,096 | +377 | -260 | -1,090 |
| 26 | R.W. Beeson No. 1 Poiter Unit | Perry | 28-5S-3W | 486 | 7,043 | -4,270 | absent | -6,464 |
| 27 | C.E. Brehm Drilling and Prod. No. 1 Hemminghaus | Clinton | 33-3N-1W | 486 | 7,040 | -4,384 | absent | -6,394 |
| 28 | C.E. Brehm Drilling and Prod. No. 1 Bochantin Comm. | Washington | 35-3S-2W | 458 | 7,332 | -4,588 | absent | -6,838 |
| 29 | Harza Engineering UPH-2 Commonwealth Edison | Stephenson | 12-28N-5E | 996 | 5,442 | +527 | -314 | -1 ,182 |
| 30 | Harza Engineering UPH-3 Commonwealth Edison | Stephenson | 7-28N-6E | 990 | 5,272 | +515 | -319 | -1 , 187 |
| 31 | Ross Production No. 1 Goodwin | La Salle | 30-29N-2E | 739 | 5,775 | -1,148 | -2,811 | -4,881 |
| 32 | U.S. Geological Survey No.1 IL. Beach State Park | Lake | 14-46N-12E | 585 | 3,500 | absent | -1,055 | -2,875 |
| 33 | Thor Resources No. 1 Sleight | Pike | 12-4S-3W | 624 | 3,602 | -682 | -2,362 | -2,946 |
| 32 | Wood Energy No. 1 Borowiak | Washington | 24-3S-1W | 522 | 9,222 | -5,469 | -8,176 | -8,186 |
| Information compiled by M.L. Sargent, ISGS, 1992 | ||||||||
| Table 5 In situ stress measurements in Illinois. See figure 13. | |||
|---|---|---|---|
| Principal stress orientation |
Method of measurement | Reference | |
| 1 | N 59 E | Earthquake focal solution | Herrmann 1979 |
| 2 | N 76 E | Strain gauge in coal mine | Y.P. Chugh 1984 (pers. comm.) |
| 3 | N 76 W | Strain gauge in coal mine | Y.P. Chugh 1984 (pers. comm.) |
| 4 | N 90 E | Earthquake focal solution | Y.P. Chugh 1984 (pers. comm.) |
| 5 | N 87 E | Strain gauge in coal mine | Blevins 1982 |
| 6 | N 83 E | Earthquake focal solution | Stauder and Nuttli 1970 |
| 7 | N 73 E | Borehole breakout measurement | Dart 1985 |
| 8 | N 29 E | Borehole breakout measurement | Dart 1985 |
| 9 | N 72 W | Borehole breakout measurement | Dart 1985 |
| 10 | N 67 W | Borehole breakout measurement | Dart 1985 |
| 11 | N 81 E | Borehole breakout measurement | Dart 1985 |
| 12 | N 50 E | Borehole breakout measurement | Dart 1985 |
| 13 | N 56 E | Borehole breakout measurement | Dart 1985 |
| 14 | N 88 E | Borehole breakout measurement | Dart 1985 |
| 15 | N 89 E | Borehole breakout measurement | Dart 1985 |
| 16 | N 72 E | Borehole breakout measurement | Dart 1985 |
| 17 | N 62 E | Hydrofracturing in boreholes | Haimson 1974 (exact location not specified) |
| 18 | N 41 E | Overcoring in coal mine | Hanna et al. 1985 |
| 19 | N 72 E | Overcoring in tunnel | Shuri and Kelsey 1984 |
| 20 | N 48 E | Hydrofracturing in boreholes | Haimson and Doe 1983 |
| 21 | N 90 E | Overcoring in coal mine | Ingram and Melinda 1988 |
| 22 | N 78 E | Overcoring in coal mine | Ingram and Melinda 1988 |
