Caseyville Formation

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Lithostratigraphy: Raccoon Creek Group >>Caseyville Formation
Chronostratigraphy: Paleozoic Erathem >>Pennsylvanian Subsystem >>Morrowan Series
Allostratigraphy: Absaroka Sequence

Primary source

Nelson, W.J., 2018, Pennsylvanian Subsystem in Illinois. Edited and figures drafted by Jennifer M. Obrad. Illinois State Geological Survey Bulletin (in press).

Contributing author(s)

W.J. Nelson


Original description

Referring to outcrops of quartz-pebble conglomerate and pebbly sandstone near Caseyville, Owen (1856, p. 48, 49, 56) introduced the name in the form “Caseyville conglomerate.”


Caseyville is an unincorporated community on the Ohio River in Union County, Kentucky. According to The Political Graveyard website (, Caseyville was founded prior to 1821 and later named for Samuel L. Casey, U.S. Representative from the 1st District of Kentucky. Casey was born here in 1821; he died in 1902 and is buried in the Caseyville cemetery.

Other names

Some early authors used “Millstone Grit,” a name borrowed from early usage for rocks of similar age and lithology in northern England. Others used generic terms such as “conglomerate.” Early authors, such as Shaw and Savage (1912) and Lamar (1925), applied the Pennsylvania name “Pottsville Formation” to the Caseyville and much of the Tradewater Formation in southern Illinois. The name “Mansfield Sandstone” (Hopkins 1896) became established in Indiana before widespread acceptance of Caseyville elsewhere and evolved into the current Mansfield Formation, which includes strata younger than the Caseyville.


Glenn (1912) was the first to use the name “Caseyville” in a formational sense. Lee (1916) described the type section, along with that of the overlying Tradewater Formation. The first geologist to map the Caseyville in Illinois was Butts (1925); however, Illinois State Geological Survey (ISGS) publications continued to use “Pottsville formation” for the lower part of the Pennsylvanian into the late 1930s. As cyclothemic classification took hold, the ISGS adopted the Caseyville as a group (e.g., Weller 1940). With their thorough overhaul of Pennsylvanian classification, Kosanke et al. (1960) essentially established the present usage. The name Caseyville Formation is currently used in Illinois and Kentucky, but not in Indiana (Greb et al. 1992, 2002; Tri-State Committee 2001).

Type section

Type location

The type section was “measured from outcrops on the Illinois shore of the Ohio River between the mouth of the Saline River and Gentry’s Landing below Battery Rock” in Hardin County, Illinois (Lee 1916, p. 15; Figure 2-1).

Type author(s)

Lee (1916, p. 15–16) created the original description (Figure 2-2). Kosanke et al. (1960) and Nelson (1989) reproduced the section. Geologic maps by Baxter et al. (1963), Kehn (1974), Denny et al. (2012), and Seid et al. (2013) cover the area.

Type status

Like other natural exposures of the Caseyville, the type section comprises prominent cliffs and ledges of sandstone, separated by poorly exposed or covered intervals of shaly strata.

Reference section (1)

Reference location

Railroad cuts on the Canadian National (formerly Illinois Central Gulf) Railroad from the NW¼ SW¼ SE¼, Sec. 31, T11S, R5E to NE¼ SE¼ NE¼, Sec. 18, T12S, R5E, Pope County, Illinois.

Reference author(s)

Kosanke et al. (1960, p. 28, 61–62) created the original description (Figure 2-2), although Potter (1957) described some features of the exposures. Geologic maps by Nelson and Lumm (1990) and Devera (1991) cover the site.

Reference status

The cuts and adjacent natural outcrops remain in good condition. The surrounding land is Shawnee National Forest. Train traffic along the line at the type section is heavy.

Reference section (2)

Reference location

Roadcuts on Interstate 24 just south of Exit 7, in Secs. 8, 9, 16, and 17, T12S, R3E, Johnson County, Illinois.

Reference author(s)

Published descriptions appear in Palmer and Dutcher (1979, p. 89–91), Cecil and Eble (1989, p. 33–37), and a thesis by Koeninger (1978). Devera (1989) described trace fossils from the upper part of the roadcut. The site is on the geologic map by Nelson et al. (2004). A graphic column is presented in Figure 2-2.

Reference status

Exposures in separate box cuts along northbound and southbound lanes remain mostly in good condition, although the shaly interval between the Pounds and Battery Rock Sandstones is largely grassed over. The Pounds Sandstone at this locality contains anomalous amounts of clay and dark mineral grains.

Reference section (3)

Reference location

Roadcuts on Interstate 57 beginning just south of Exit 36, in Secs. 24 and 25, T11S, R1E, Johnson County, Illinois.

Reference author(s)

Published descriptions appear in Ethridge et al. (1973, Stops 4 and 5, p. 63–78), Koeninger (1978), Palmer and Dutcher (1979, Stop 8, p. 84–88), Cecil and Eble (1989, p. 30–32), and Nelson and Weibel (1996, p. 18, 24). Devera (1989) described trace fossils from the upper part of the roadcut. This site is on the geologic map of Weibel and Nelson (1993). A graphic column is presented in Figure 2-2.

Reference status

Exposures on both sides of the highway remain in good condition, although the shaly interval between the Pounds and Battery Rock Sandstones was never visible.

Reference section (4)

Reference location

A core that represents the entire Caseyville Formation is ISGS #1 Berry, drilled in Sec. 8, T9S, R3W, Jackson County, Illinois (ISGS county #26246). The Caseyville is 285.2 ft (86.9 m) thick in the Berry core, extending through a depth range from 105.1 to 390.3 ft (32 to 119 m; Figure 2-3). The upper contact is an abrupt change from clay- and iron-rich sublitharenite above to white, pure quartz arenite below. The lower contact is unconformable between conglomerate of the Caseyville and the Mississippian Kinkaid Limestone below.

Reference author(s)

This report.

Reference status

Records of the Berry test hole, including the core description and electric and gamma-ray logs, are on file at the Geologic Records Unit of the ISGS and are accessible via the ISGS website. Core is archived at the ISGS Samples Library in Champaign under call number C-15356.

Stratigraphic relationships

Resting with major unconformity on older rocks, the Caseyville is the oldest Pennsylvanian formation in the Illinois Basin. It is also the only Pennsylvanian formation that sometimes can be recognized on purely lithologic grounds, without identifying specific named members. Nearly pure quartz sandstone (quartz arenite or orthoquartzite) characterizes the Caseyville, whereas the sandstone in younger Pennsylvanian formations is subarkose and sublitharenite.

A number of beds and members in the Caseyville Formation have been named. Most of the names were originally used informally and then formalized by Kosanke et al. (1960). Formalizing these units may have impeded more than helped understanding of the Caseyville. Formalizing gave the impression that the Caseyville comprises two major sandstone members, Pounds and Battery Rock, bounded by shaly units. In reality, many sections of the Caseyville contain five or more sandstone units (e.g., Figure 2-3). In my own mapping experience, the existence of named sandstone members created a bias toward connecting isolated sandstone outcrops in the field. None of the named members of the Caseyville can be traced more than a few miles into the subsurface.

Extent and thickness

Owing to the difficulty of identifying the top of the Caseyville Formation in the subsurface, the thickness and lithofacies of the formation have not been mapped in detail from well data. Small-scale maps that depict the thickness of roughly the Caseyville interval appear in McKee and Crosby (1975, Plate 3A), Greb et al. (1992, Figure 15), Droste and Furer (1995, Figure 4), and Droste and Horowitz (1998, Figure 4). Compiled from these sources, the map shown here (Figure 2-4) is essentially an isopach map of the Caseyville.

These maps indicate that the Caseyville Formation and equivalent strata lie largely southeast of a line that runs from Vigo County in west-central Indiana to Randolph County in southwestern Illinois. Northwest of this line, Caseyville rocks are largely confined to paleovalleys on the sub-Pennsylvanian surface. A sizeable outlier of the Caseyville occurs in the Quad Cities area in northwestern Illinois and adjacent Iowa.

Along its outcrop, the exposed Caseyville shows in map view as multiple semicircular arcs extending southeast and east from Randolph County, Illinois, into western Kentucky. Resistant Caseyville sandstone forms scenic bluffs at many localities, including Pounds Hollow, Garden of the Gods, Dixon Springs, and Ferne Clyffe State Parks, Lusk Creek Canyon, Bell Smith Springs, and Burden Falls. Caseyville outcrops continue southeastward to Muhlenberg County, Kentucky. Farther southeast in Kentucky, geologic mappers did not differentiate Caseyville and Tradewater Formations in areas where quartzose sandstones are thin or absent above paleo-uplands on the sub-Pennsylvanian surface. In Indiana, the lower part of the Mansfield contains thick units of massive and cross-bedded quartz arenite that contains abundant rounded granules and small pebbles of quartz. These outcrops are identical in lithology and inferred age to typical Caseyville Formation in southern Illinois.

The Quad Cities outlier is called Caseyville Formation because its lithology and age are similar to the type Caseyville (Ravn et al. 1984; Ravn 1986; Isbell 1985). Outcrops occur within and west of the Quad Cities in Mercer and Rock Island Counties, Illinois, and Muscatine and Scott Counties, Iowa. As in the southern part of the basin, Caseyville sandstones of the northwestern outlier are quartz arenites; however, quartz granules are rare. Shale, siltstone, and at least three coal layers make up the rest of the formation. These rocks have a maximum observed thickness of 100 ft (30 m) and are overlain unconformably by lower Desmoinesian rocks of the upper Tradewater Formation. The Caseyville of the Quad Cities area has been established as Morrowan age via palynology of its coal beds (Ravn and Fitzgerald 1982; Ravn et al. 1984).

A few authors have described Caseyville strata in the subsurface of Illinois. Lowenstam (1951, p. 45–46) reported Caseyville sandstone and quartz-pebble conglomerate occupying what is now known to be a paleochannel cut into the Mississippian surface in the Bible Grove area of Clay County (Figure 2-5). Winslow (1959) identified Caseyville-age fossil spores in drill cores from the Allendale oil field in northern Wabash County (Figure 2-5). Although not identified as “Caseyville,” the quartz-rich sandstone serving as an oil reservoir in the Hardinville field of Crawford County (Figure 2-5) probably represents this formation. Sealed by overlying shale, the productive sandstone lies at the bottom of a sub-Pennsylvanian paleovalley (Howard and Whitaker 1988). Using limited palynological control, Droste et al. (2000) published a series of generalized maps and cross sections showing Morrowan (Caseyville age) depositional patterns in part of the Illinois Basin.

During the 1990s, Russell Jacobson and I constructed a network of subsurface cross sections of Caseyville and Tradewater strata in southern Illinois. Starting from the outcrop and the few available cores, we tried to extend correlations into the deep Fairfield basin by using geophysical logs supplemented by sample studies in which special attention was paid to the character of the sandstone. T. Michael Whitsett carried out a similar investigation in western Kentucky with the cooperation of the Kentucky Geological Survey. Through our collective efforts, we determined that with sufficient attention to samples, the Caseyville–Tradewater contact could be approximated in the subsurface and that depositional packages or sequences could be correlated across distances of 5 to 10 mi (~8 to 16 km) and occasionally farther. We were unable to develop any means for distinguishing the Caseyville from the Tradewater solely from geophysical logs, and we failed to trace any named members of the Caseyville or lower Tradewater more than a few miles from their type localities. The cross sections we developed remain unpublished in ISGS archives.

The thickness of the Caseyville varies greatly because the unit was deposited on a deeply incised surface. The maximum thickness is more than 600 ft (180 m) in the Evansville paleovalley of western Kentucky (Greb et al. 1992). Local thickness variations as great as 200 ft (60 m) are not unusual. The oldest strata are confined to valley bottoms, whereas the younger units are continuous across the interfluves.


The Caseyville Formation is primarily sandstone, siltstone, and shale, with lenticular coal beds and rare limestone. The sandstones are dominantly composed of quartz and contain very few lithic fragments and very little feldspar, clay, or mica. Caseyville sandstone is consistently classified as orthoquartzite or quartz arenite (Potter and Siever 1956b; Potter and Glass 1958; Potter and Pryor 1961; Morrow 1979; Bohm 1981). Quartz granules and pebbles, mostly less than half an inch (12 mm) in diameter, commonly occur either scattered throughout the sandstone or concentrated in beds of conglomerate. The coarser sandstones reach about 100 ft (30.5 m) thick and show relatively uniform cross-bedding, with dip directions to the west, south, or southwest (Potter and Olsen 1954; Potter 1962, 1963; Greb 1989a) parallel to the direction of elongation of the sandstone bodies. In the finer grained sandstone units, which are usually less than 25 ft (7.6 m) thick, the most prevalent sedimentary structure is ripple bedding. Shale or siltstone interbeds are common.

The shales and siltstones are not as widely exposed. The thicker shale units are sandy and contain several beds of sandstone, some relatively coarse grained. A few shales associated with coals are dark, relatively fine, and uniform, and their clay mineral content is high. Most of the Caseyville in northwestern Illinois is composed of medium gray to dark gray brittle shales interbedded with silty shales and a few clean quartz sandstones.

Several coal seams, most of them somewhat lenticular, are found in the Caseyville Formation, although only one—the Gentry Coal Bed in southeastern Illinois—is named. In Rock Island and Mercer Counties, as many as seven impure coals up to 2 ft (0.6 m) thick occur in the Caseyville (Searight and Smith 1969). Limestone is rare, and marine body fossils have been reported from only a few localities.




The Caseyville rests unconformably on Mississippian and older rocks throughout the Illinois Basin. In outcrops and in wells where samples are available, the upper contact can be placed at the highest occurrence of quartz arenite, in contrast to overlying sandstones that are less mature. This contact is locally sharp, commonly gradational, and partly intertonguing. In parts of western Kentucky, the contact has been mapped at the base of the Bell or Hawesville coal beds. When subsurface geophysical logs are used, the top of the Caseyville is generally at the top of a thick, upward-fining sequence, but correlating such sequences among wells can be problematic. In many outcrops and well records, the top of the Caseyville cannot be identified with confidence.

The sub-Pennsylvanian surface coincides with the unconformity that separates the Kaskaskia and Absaroka Sequences throughout the North American craton (Sloss 1963; Sloss et al. 1949). This unconformity has been mapped throughout the Illinois Basin by using records from tens of thousands of boreholes (Bristol and Howard 1971, 1974; Davis et al. 1974; Droste and Keller 1989; Greb 1989b). Detailed studies of individual paleovalleys (Potter and Desborough 1965; Shawe and Gildersleeve 1969; Pryor and Potter 1979) shed further light on this subject. Maps from these studies depict a dendritic system of entrenched paleovalleys that trend dominantly southwest across the basin. Paleovalleys have been deflected by tectonic activity locally in the southern part of the basin (Desborough 1961; Nelson and Lumm 1985; Greb 1989a, 1989b). Regionally, the youngest pre-Pennsylvanian rocks belong to the late Chesterian Kinkaid Limestone in far southern Illinois, and the oldest is the Middle Ordovician St. Peter Sandstone at the crest of the La Salle Anticlinorium near Ottawa in northern Illinois. The oldest Pennsylvanian rocks are poorly documented but probably occur in the bottoms of the deepest paleovalleys in the southern part of the basin. Caseyville Formation is absent in much of northern and western Illinois, where Tradewater and, locally, Carbondale strata rest directly on the sub-Pennsylvanian unconformity.

Rexroad and Merrill (1985) hypothesized that continuous sedimentation took place across the Mississippian–Pennsylvanian boundary in far southern Illinois. This conclusion was based on similar-appearing conodont faunas from the Grove Church Member of the Kinkaid Formation and overlying Wayside Member of the Caseyville in Johnson County. However, follow-up studies by Jennings and Fraunfelter (1986) and Weibel and Norby (1992), who considered conodonts, other fossils, and physical evidence, supported a substantial hiatus between Grove Church and Wayside strata at the same locality. Moreover, mapping completed a short distance west revealed a Mississippian unit younger than the Grove Church, the Dutchman Limestone Member, in clearly unconformable contact to overlying Caseyville strata (Nelson et al. 2004). Clearly, a contact that is unconformable within paleovalleys cannot be conformable on interfluves. Conformity would prevail where paleovalleys entered the ocean below the range of eustatic fluctuations. This situation prevailed in the Arkoma Basin, southwest of Illinois.

Well log characteristics

Atherton et al. (1960) outlined ways to differentiate Pennsylvanian from Mississippian rocks on resistivity logs as well as in well cuttings. The typical electric-log pattern in southern Illinois reflects the composition of shale, siltstone, sandstone, and a few thin resistive beds that may be coal, limestone, or tightly cemented sandstone and conglomerate. The base of the Caseyville Formation is often picked at the base of the lowest blocky sandstone signature or the top of the uppermost recognizable rock-unit signature in underlying strata. For example, in the southern part of the basin, where the Caseyville is incised into Upper Mississippian (Chesterian) units, the log pick is generally straightforward because the underlying Chesterian carbonates have distinctive log signatures. Accurately picking the Pennsylvanian base where it is cut into Mississippian shale or sandstone is much more difficult. Likewise, the top of the Caseyville may be picked on top of the uppermost thick sandstone with a blocky gamma-ray, spontaneous potential, and resistivity log signature or at the base of the lowest coal just above a blocky sandstone signature. However, where thick sandstones are absent (such as on the southeastern margin of the basin), this convention does not work.

Construction and analysis of a regional network of cross sections failed to uncover reliable criteria for differentiating Caseyville from Tradewater strata solely on the basis of the well log response. The larger sandstone bodies tend to be highly permeable and, depending on the setting, may be saturated with freshwater or saltwater, oil, or natural gas. In some areas, Caseyville sandstone tends to be more permeable and “cleaner” than overlying Tradewater sandstone, which contains more interstitial clay and iron oxide cement. However, the only reliable way to identify the Caseyville–Tradewater contact in the subsurface is to examine samples (cores and cuttings).


Invertebrate fossils are rare. Rexroad and Merrill (1985) and Weibel and Norby (1992) described collections from the basal Caseyville sandstone of Johnson County, Illinois. Wanless (1939, p. 36) stated that M.W. Fuller identified nearly 100 species of marine invertebrates from the Sellers Limestone Bed; unfortunately, this list was never published. Devera et al. (1987) collected a molluscan fauna from black shale in Pope County, Illinois. Reports of Caseyville fossil plants are scattered in the literature. Winslow (1959) investigated spores from Upper Mississippian and Caseyville strata of Illinois. Peppers (1996) correlated the Caseyville regionally on the basis of palynomorphs. Devera (1989) described Caseyville trace fossils from three sites in southern Illinois. Whaley et al. (1979, p. 9) reported marine fossils from the Caseyville in western Kentucky. Reworked silicified fossils from Mississippian and older rocks occasionally are found in the basal Caseyville (Poor 1925).

Age and correlation

Although Caseyville fossil localities are few, available data indicate that the formation spans nearly all of the Morrowan Stage. David White (in Kindle 1895, p. 354–355) described the fossil plants of the Hindostan whetstone in southern Indiana as being comparable to those that accompany the Sharon coal in Ohio and the Sewell coal in West Virginia. Read and Mamay (1964, p. K7) stated that the whetstone “appears to represent the transition between” their floral zones 4 and 5, which occurs near the contact of the Pocahontas and New River Formations (below Sewell coal) in West Virginia. On the basis of palynology, Peppers (1996, p. 11–12) placed the French Lick, Sharon, and Sewell coals together in Namurian C or early Morrowan stages and as correlative with the lower part of the Wayside Member in southern Illinois.

Conodonts have been recovered from basal Caseyville sandstone at the type locality of the latest Mississippian Grove Church Member in Johnson County, southern Illinois. Without specifying age in terms of stages, Rexroad and Merrill (1985) interpreted the faunas as showing no break in deposition across the systemic boundary. At the same locality, Weibel and Norby (1992, p. 39) reported “Lochriea commutata and Rhachistognathus cf. R. websteri, which suggest an early Morrowan age.” These same workers affirmed the presence of an unconformity at the base of the Caseyville, as elsewhere throughout the Illinois Basin. Rexroad and Merrill (1985, p. 47) identified conodonts from the Sellers Limestone and stated that they “represent a slight evolutionary advance and an environmental setting intermediate between that of the type section of the Wayside and the Wayside of the Sifford School locality.” Where this zone or subzone falls on the Midcontinent or global time scale was not mentioned. Clearly, further investigation of the Sellers is in order.

Marine shale between the Battery Rock and Pounds Sandstones along Bear Branch in Pope County, Illinois, yielded a molluscan fauna, including the goniatites Axinolobus and several species of Gastriosceras, together with brachiopods, undescribed conodonts, and a wide variety of palynomorphs. Together, these fossils indicate a late Morrowan or approximately middle Westphalian A age (Devera et al. 1987).

Jacobson et al. (1983) described a marine fauna from limestone overlying the Bell coal bed near the base of the Tradewater Formation in Johnson County, southern Illinois. The conodonts, particularly abundant specimens of Idiognathoides ouachitensis, indicate an age close to the Morrowan–Atokan boundary.

Ravn and Fitzgerald (1982) and Ravn et al. (1984) established the Quad Cities outlier of the Caseyville as Morrowan age through palynology of its coal layers. On the basis of extensive analysis of palynomorphs from the Illinois Basin and other North American coal basins, Peppers (1996) assigned the entire Caseyville Formation to the Morrowan Stage and placed the Morrowan–Atokan boundary near the base of the overlying Tradewater Formation, between the Reynoldsburg and Bell coal beds. Peppers’ findings agree with those cited above, based on marine invertebrates.

Morrowan-age strata throughout the eastern United States strongly resemble the Caseyville Formation, comprising clean quartz sandstone with common quartz pebbles. Caseyville-correlative units include the Olean Conglomerate of New York, lower part of the Pottsville Formation of Pennsylvania, Sharon Conglomerate of northeastern Ohio, most of the New River Formation in West Virginia, pebbly quartz arenites formerly called “Lee Formation” in eastern Kentucky (Chesnut 1991, 1992), Gizzard and Crab Orchard Mountain Groups of eastern Tennessee, Parma Sandstone of Michigan, and Prairie Grove Formation of northern Arkansas (Nelson 1989).

Environments of deposition

The primary factors controlling deposition of the Caseyville Formation were the (1) configuration of the sub-Pennsylvanian surface, (2) glacially induced eustasy, and (3) periodic changes in climate. Tectonic activity and autogenic processes, such as channel avulsion, played locally important roles. For example, tectonic movements controlled the development of Caseyville paleovalleys in parts of western Kentucky (Greb 1989a, 1989b).

Maps of the sub-Pennsylvanian surface depict a system of southwest-oriented linear and anastomosing valleys, with tributaries making V shapes downstream and preserving examples of stream piracy. This is not a simple dendritic system resulting from headward erosion on an emerging coastal plain; rather, the sub-Pennsylvanian surface likely records superimposed valley incisions. The largest valleys are as wide as 20 mi (32 km) and nearly 200 ft (61 m) deep. Valleys have relatively flat bottoms and steep walls, commonly marked by landslide blocks of Chesterian strata. Some of the valleys are terraced, with the Chesterian limestone and sandstone units holding up benches (Bristol and Howard 1974; Howard 1979). The Rio Caroni system of tropical Venezuela has been offered as an analogue (Shawe and Gildersleeve 1969; Howard 1979). As H.F. Garner (1966a, 1966b, 1974) related, the linear, anastomosing pattern of Rio Caroni developed under alternating wetter and drier climatic phases of the late Quaternary. The system downcut during humid stages and aggraded during drier stages. Howard (1979) proposed a similar mechanism in the Illinois Basin. Howard further invoked multiple episodes of eustasy-driven downcutting and aggradation. At the same time, Pryor and Potter (1979, p. 53) recognized that limestone beds holding up benches or terraces “does not easily harmonize with a presumed warm, humid, subtropical climate.” Here was the first recognition that wetter and drier, more seasonal climatic episodes alternated during the Pennsylvanian as they are doing during the Quaternary.

Paleocurrent studies, based chiefly on cross-bedding, indicate prevailing southwesterly transport of sediment (Potter and Siever 1956a; Potter 1963; Potter and Desborough 1965). This southwesterly flow, toward the Ouachita trough and Arkoma Basin, was inherited from Late Mississippian time (Swann 1964).

Much of the Caseyville is composed of incised valley deposits. Multiple episodes of valley incision are evident, not only at the base but throughout the formation. As Kvale and Archer (2007) illustrated with examples from Indiana, two markedly different styles of valley fill are present. Major trunk valleys typically are filled with thick fluvial, cross-bedded pebbly sandstone and quartz-pebble conglomerate. In contrast, tributaries have a thin basal sandstone or conglomerate overlain by thick shale that shows “marine influences manifested by tidal rhythmites, certain trace fossils, and macro- and microfauna” (p. 809)

A prime example of a trunk valley is the Brownsville paleovalley near the southeastern margin of the basin in Kentucky (Shawe and Gildersleeve 1969; Pryor and Potter 1979). Terraced or stepped valley walls demarcate the trunk with a deep, narrow inner valley filled with coarse, pebbly sandstone. Unidirectional cross-bedding indicates paleocurrents paralleling the valley walls as flow was confined to the inner channel. This was evidently a low-sinuosity, braided channel. Younger sandstone occupying the upper valley is finer grained, less pebbly, and multistory, having three or more upward-fining cycles. Wide dispersion of paleocurrent orientations, along with lateral accretion structures, indicates a high-sinuosity, meandering fluvial system (Pryor and Potter 1979). Integrating evidence from Appalachian and Illinois Basins, Archer and Greb (1995) characterized the Morrowan drainage system as possibly being on the scale of Earth’s largest extant drainage basin, that of the Amazon River.

In southern Illinois, the Battery Rock and Pounds Sandstone Members largely represent fluvial fill of trunk valleys. However, these valleys were confined by recently deposited sediment rather than bedrock, so they are wider and shallower than basal Caseyville paleovalleys (Archer and Greb 1995). Upper Caseyville valleys commonly were miles wide, resulting in sandstone bodies that can be mapped across large areas (potentially in the subsurface as well as on the outcrop). The upper and lateral portions of these sandstone units frequently contain tidal and marine indicators, including bidirectional paleocurrents (Koeninger and Mansfield 1979; Devera and Nelson 1995), tidal rhythmites (Kvale et al. 1989; Archer 2004), and trace fossils attributed to marine organisms (Devera 1989). Thus, trunk valleys can be viewed as rivers that became estuaries during times of rising sea level. Devera et al. (1987) described a probable tributary valley-fill in southern Illinois. Dark gray to black marine shale as thick as 82 ft (25 m) contains a fauna of nautiloids, gastropods, bivalves, brachiopods, and conodonts. Juveniles are abundant among the nautiloids, suggesting this protected estuary served as a breeding ground.

Without directly identifying source areas, several authors have inferred the origin of Morrowan quartz arenites based on regional relationships. Potter and Glass (1958), Sloss (1979), and Nelson (1989) proposed that Caseyville sand was recycled from older Paleozoic sedimentary rocks, including quartz-pebble conglomerates and quartz-rich sandstones, during early stages of tectonic uplift in the northeastern United States and in southeastern Canada. According to this model, by Atokan time the sedimentary cover had been removed from uplifts, exposing crystalline rocks that supplied more mica, feldspar, and lithic grains to the Illinois Basin. An alternative model invokes a climate change. Cecil (1990, p. 534) suggested that pure quartz sand “may have been derived from spodosols that developed under the effects of tropical rainy conditions.” This idea finds support from paleobotany, which indicates overall moderately wet conditions during the Early Pennsylvanian (Morrowan) and a drier, probably seasonal wet–dry regime during the early Middle Pennsylvanian (Atokan) time (Phillips and Peppers 1984).

In the Quad Cities outlier, trough cross-bedding and channel orientations indicate paleocurrents toward the north-northwest to east-northeast, which is 180 degrees opposite paleocurrents in the overlying Tradewater strata (Isbell 1985; Ludvigson and Swett 1987). Isbell (1985) hypothesized that the Mississippi River Arch was tectonically active, deflecting regional southwestward sediment transport toward the north into a local basin in the Quad Cities area.

Economic importance

Much of the early oil production in Illinois came from Pennsylvanian sandstone reservoirs along the La Salle Anticlinorium in the east-central part of the state. Geologic reports on these fields, such as those by Mylius (1921, 1927), are difficult to interpret because they were written before the advent of electric logging when understanding of subsurface stratigraphy was primitive. During the oil boom of the late 1930s, the emphasis of exploration shifted to Mississippian and deeper targets in the central and southern parts of the basin. New Pennsylvanian fields continued to be developed, yet few publications addressed their geology. Howard and Whitaker (1988) and Seyler and Cluff (1990) evaluated production from basal Pennsylvanian sandstone encased by shale within sub-Pennsylvanian paleovalleys. Schwalb et al. (1971) and Greb et al. (1992) covered some aspects of Pennsylvanian oil production in western Kentucky.

In the southeastern part of the basin, in western Kentucky, the lower Caseyville contains tar sands, which historically were quarried for asphalt (Orton 1891; Bowersox 2016b). Since the 1970s, several companies have investigated the Caseyville and older tar sands as oil sands when the price of conventional oil has been high (Noger 1987; Bowersox 2016a). Caseyville sandstones are also freshwater aquifers in parts of the basin (Maxwell and Devaul 1962; Davis et al. 1974). Thin, lenticular Caseyville coal seams never supported more than small, local mines. Sandstone has been quarried for use as building stone, flagstone, millstones, and grave markers.



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ISGS Codes

Stratigraphic Code Geo Unit Designation
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