DISCRIMINATION OF LOCAL AND GLOBAL EFFECTS ON UPPER MISSISSIPPIAN STRATIGRAPHY illinois basin

8/7/2019 DISCRIMINATION OF LOCAL AND GLOBAL EFFECTS ON UPPER MISSISSIPPIAN STRATIGRAPHY illinois basin

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JOURNAL OF SEDIMENTARY RESEARCH, V OL. 71, NO. 6, NOVEMBER, 2001, P. 9851002Copyright 2001, SEPM (Society for Sedimentary Geology) 1527-1404/01/071-985/$03.00

DISCRIMINATION OF LOCAL AND GLOBAL EFFECTS ON UPPER MISSISSIPPIAN STRATIGRAPHY,ILLINOIS BASIN, U.S.A.

LANGHORNE B. SMITH, JR.,* AND J. FRED READDepartment of Geological Sciences, Virginia Tech, Blacksburg, Virginia 24061, U.S.A.

* Present Address: Center for Stratigraphy and Paleontology, Room 3140, New York State Museum, New York 12230, U.S.A.e-mail: [email protected]

ABSTRACT: Interpretive cross sections based on detailed descriptionsof 33 outcrops and cores are used to better understand the relativeeffects of tectonics, eustasy, tides, and climate on Upper Mississippian(middle Chesterian) stratigraphy in the tectonically active, tide-domi-nated Illinois basin. The cross sections show that five mixed carbonatesiliciclastic, high-frequency sequences in the Bethel through Glen Deanformations can be correlated around the outcrop belt through areaswith very different subsidence histories.

The sequence boundaries are marked by paleosols and incised val-leys and can be correlated basin-wide within a framework of distinctivemarker beds. Because of its updip position, lowstand systems tracts arenot preserved in the Illinois basin. The transgressive systems tractsgenerally consist of one or two parasequences that are typically com-

posed of tidally influenced quartz sandstone filling incised valleys atthe base overlain by open-marine skeletal limestone, shallow-marineshale, and heterolithic siliciclastic tidal-flat deposits. The maximumflooding surface (MFS) for the sequences is picked at the base of thedeepest water limestone facies. Highstand systems tracts are composedof 1 to 6 regressive parasequences that consist of basal offshore skeletallimestone capped by laterally extensive shale and heterolithic silici-clastic tidal-flat facies.

The basin-wide extent of the sequence boundaries and maximumflooding surfaces across tectonic highs and lows suggests that the se-quences were produced by eustatic sea-level changes rather than localtectonics or autogenic processes. The sequences were likely producedby moderate- to high-amplitude (30100 m) fourth-order ( 400 ky)glacio-eustatic sea-level changes driven by the transition from thegreenhouse conditions of the Early Mississippian to the icehouse

conditions of the late Paleozoic. The lateral extent and frequency ofcomponent parasequences suggests that they were likely produced byfifth-order sea-level changes (10100 ky). The sequences may be bun-dled into third-order composite sequences, but the third-order signalis obscured by the magnitude of the fourth-order sea-level changes afeature typical of ice-house stratigraphies.

The sequences can be used as time slices to identify spatial and tem-poral variations in differential subsidence between the Cincinnati Archand the more rapidly subsiding Basin Interior. Episodes of high andlow differential subsidence occurred every two to three sequences.These subsidence variations had a major impact on lithofacies distri-bution and onlap and offlap geometries in sequences and parase-quences. The occurrence of some widespread seismically disturbedbeds suggests that active faulting occurred during deposition. Normalfaulting appears to have occurred during periods of high differentialsubsidence and reverse faulting during periods of low differential sub-sidence. Differential subsidence and related normal and reverse fault-ing may have occurred in response to phases of thrust loading andquiescence in the Appalachian orogenic belt to the east. Even in thistectonically active setting, however, it is the eustatic signal that gen-erates basin-wide, mappable stratigraphic sequences.

INTRODUCTION

The discovery of giant oil fields in Mississippian carbonates of the Cas-pian region has led to a renewed interest in the depositional environments,

eustatic sea-level changes, and sequence stratigraphy of same-aged rocksaround the world. In order to apply knowledge gained from study of UpperMississippian strata in other parts of the world, it is imperative that theglobal eustatic signal be discriminated from the effects of local tectonicsand local climate change. The purpose of this paper is to show how therelative effects of eustasy, tectonics, and climate can be discriminated usinghigh-resolution sequence stratigraphy in Upper Mississippian (middleChesterian) mixed carbonatesiliciclastic strata of the Illinois basin. Thisis an excellent succession for a study of the interplay between eustasy andtectonics because it is well exposed around the basin in areas with verydifferent Late Mississippian subsidence histories.

The succession also spans the critical time of transition from greenhouseconditions of the Early Mississippian to a period of major continental gla-

ciation in the Pennsylvanian and Permian (Frakes et al. 1992; Smith andRead 2000). Waxing and waning of these continental ice sheets caused sea-level changes that had a major impact on Upper Mississippian stratigraphyin the Illinois basin and same-aged strata around the world.

The Late Mississippian also marks a period of transition from a time oftectonic quiescence in the Late Devonian and Early Mississippian to a timeof major tectonic activity in the Pennsylvanian caused by continental col-lision in the Appalachians to the east (Fig. 1A). This collision caused majorspatial and temporal variations in subsidence in the Illinois basin duringthe Late Mississippian. The succession also spans a time of climate changefrom semiarid to humid, synchronous with buildup of global ice and thecollision of Gondwana with North America (Witzke 1990).

The complex interplay between eustasy, tectonics, and climate can beunraveled using high-resolution sequence stratigraphy. We constructed re-gional cross sections using detailed lithologic descriptions of 33 cores and

outcrop sections that span more than 600 kilometers from central Indianato southwestern Illinois. Despite the lateral heterogeneity of individualunits, sequence-bounding paleosols, which commonly pass laterally intoincised valleys, can be correlated over much of the basin within a frame-work of biostratigraphic and lithologic markers (Smith and Read 1995;Smith et al. 1995; Smith 1996; Smith and Nelson 1996). These unconfor-mities can be correlated through areas of both high and low subsidenceand are here interpreted to be eustatic in origin. On the basis of this inter-pretation, the unconformity-bounded sequences can be used as time slicesto better understand the timing of tectonic activity and climate change andtheir impact on the stratigraphy.

GEOLOGIC SETTING

Tectonic and Paleogeographic Setting

The Illinois basin was located between 5 and 15 degrees south of theequator during the Late Mississippian (Craig and Connor 1979; Scoteseand McKerrow 1990) (Fig. 1A). Throughout the Paleozoic, the Illinoisbasin was an embayment open to the present-day south. The basin coverspresent-day southern Illinois, southwestern Indiana, and western Kentuckyand was part of an extensive carbonate ramp that at times was continuousfrom Virginia to New Mexico (Craig and Connor 1979). The Illinois basinlay at least 300 kilometers away from the nearest ramp margin, which mayhave bordered the Ouachita Foredeep to the south.

The Illinois basin can be subdivided into three main provinces along the


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986 L.B. SMITH JR., AND J.F.READ

FIG. 1.A) Paleogeographic map showing regional setting of Illinois basin during Late Mississippian (from Smith and Read 1999; modified from Craig and Connor1979). Deeper water to south is future site of Ouachita foredeep that may also have been a deeper water environment in the Chesterian. B) Tectonic map of Illinois basin.Isopachs in feet for Bethel to top of Mississippian (from Swann 1963). WV Wabash Valley Fault System; RCS Rough CreekShawneetown Fault System; FD Fluorspar District Fault Complex.

outcrop belt: the Eastern Shelf, the Basin Interior, and the WesternShelf (Fig. 1B). The shelves on either side of the basin subsided at aslower rate than the Basin Interior. The basin was centered over northeastsouthwest trending Wabash Valley and Fluorspar Area Fault Systems,which were active during the Late Mississippian (Cole and Nelson 1995;Nelson 1996). The Eastern Shelf is subdivided into the Northeastern Shelf,which subsided at a rate close to that of the Basin Interior, and the South-eastern Shelf, which is most proximal to the Cincinnati Arch and commonlysubsided at a slower rate. Despite the varying subsidence rates, most of theIllinois basin remained a shallow epicontinental sea throughout most of theEarly and Middle Chesterian.

Stratigraphic Setting

The Bethel through Glen Dean formations described in this paper weredeposited over a thick interval of shallow marine carbonate ramp deposits(Muldraugh through Paoli formations and their equivalents) (Figs. 2, 3).The underlying Ste. Genevieve and Paoli formations are composed of ooidgrainstone, skeletal grainstone, muddy carbonates, and multiple disconfor-mities marked by calcrete, breccia, and eolianites (Smith and Read 1999;Smith 1996). There is a major unconformity between the Bethel Formationand the underlying Paoli Formation marked by well-developed paleosolsand incised valleys up to 75 meters deep (Friberg et al. 1969). The Bethelto Glen Dean interval is composed of interbedded carbonates (Beaver Bend,Reelsville, Beech Creek, Haney and Glen Dean formations) and siliciclas-

tics (Bethel, Sample, Cypress, Big Clifty and Hardinsburg formations). Inpart of the Basin Interior, the Bethel, Sample and Cypress formations areamalgamated into the West Baden Clastic Belt (Sullivan 1972) which is anortheastsouthwest trending, 25 km wide, 75 m thick accumulation ofclean quartz sandstone in Indiana and southeastern Illinois. The overlyingUpper Chesterian units are similar to the Bethel to Glen Dean interval butcontain more fluvial facies and fewer carbonates (Swann 1963).

CONSTRUCTION OF LITHOFACIES CROSS SECTIONS

Cross sections through the study interval are based on detailed measuredsections of outcrops and cores (Fig. 4). Each core and outcrop was logged

bed by bed, noting the thickness, color, grain type, grain size, depositionaltexture, biota, sedimentary structures, and evidence for subaerial exposure.Units in outcrop were traced laterally to determine facies relationshipswhere possible.

Initial correlations between sections were made using distinctive markerbeds such as paleosols and laterally extensive limestone units. Some pa-leosols can be traced laterally into incised valleys in outcrop sections. Inthese cases, limestone units overlying the paleosols can be traced to aposition immediately overlying the siliciclastic incised-valley fill with no

intervening paleosol. On the basis of that relationship, siliciclastic unitsthat grade up into limestone in cores where paleosols normally occur areinterpreted to be incised-valley fills. Due to a lack of data, correlations onthe Western Shelf are speculative and will not be discussed extensively inthe text.

LITHOFACIES AND DEPOSITIONAL ENVIRONMENTS

The cross sections and other published work suggest that the Illinoisbasin was a gently sloping ramp throughout the Bethel to Glen Dean in-terval (Treworgy 1985, 1988). The slope on the ramp was probably nevermore than 7 cm/km (0.004) at any time during deposition of the studyinterval (Smith 1996).

Early and Middle Chesterian siliciclastics were previously interpreted tobe fluvial, deltaic, and shallow marine facies that were deposited in a flu-

vially dominated birds-foot delta setting analogous to the modern Missis-sippi Delta (Swann 1964; Potter 1963; Seyler 1982). Recent studies havereinterpreted most of the siliciclastics as shallow marine and tide-dominateddeltaic facies (Visher 1980; Treworgy 1985; Specht 1985; Cole and Nelson1995).

Minor Middle Chesterian ooid grainstone and bidirectional cross-beddingin some skeletal grainstone units updip on the ramp suggest that somecarbonates were also tidally influenced. However, fossiliferous shale, skel-etal wackestone, and some of the skeletal packstonegrainstone units in theBasin Interior have graded beds caused by storms rather than tides (Tre-worgy 1985; Harris 1992). This suggests that some of the carbonate units


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987UPPER MISSISSIPPIAN SEQUENCE STRATIGRAPHY, ILLINOIS BASIN

FIG. 2.Generalized stratigraphic column(left) for Mississippian of Illinois basin andmore detailed stratigraphy of the lower andmiddle Chesterian Units (right) (modified fromSmith and Read 1999). This paper examines themiddle Chesterian Bethel to Glen Dean interval.Numbered sequences correspond to numberedsequences in Fig. 4.

FIG. 3.Biostratigraphy and correlation chartfor St. Louis through Tar Springs formations.This paper focuses on the Bethel through GlenDean formations (from Smith and Read 1999;modified from Sable and Dever 1990, andMaples and Waters 1987).


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FIG. 4.Interpretive cross sections of sequences 6 through 10 in the Bethel to Glen Dean interval. Cross section orientations shown in inset maps and locations ofmeasured sections are summarized in Appendix 1. Cross section EE (sequences 6 and 7) is hung on a basin-wide disconformity that caps the basal parasequence insequence 8. The major incised valley at the base of sequence 8 made it impossible to find a reliable datum below that horizon. Incised valley at base of Bethel Formationin section EE is not drawn to scale, its true depth is up to 75 m deep. Cross section FF (sequence 8) is hung on the basin-wide paleosol at the top of sequence 8.Cross section GG (sequence 9 and the base of sequence 10) is hung on the base of the Glen Dean Limestone. Locations 33 and 31 are closest to the Cincinnati Archand thinning in these locations is due to proximity to the arch. The cross sections do not go over the arch. Locations are equally spaced except where there are major gapsbetween sections. See location maps for actual distance between sections. Unconformities are lettered from J to Q and parasequences are numbered from 26 to 45.


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FIG. 5.Schematic representation of depositional environments for Bethel to Glen Dean interval (not drawn to scale). Key to symbols shown in Fig. 4.

TABLE 1.Mixed carbonate-siliciclastic ramp facies.

Lithology(DepositionalEnvironment)

Blocky mudrock andcaliche/breccia

(paleosol)

Non-fossiliferousshale/coal (marginal

marine/swamp)

Heterolithic sandstoneand shale (tidal flat;

transition zone)Quartz sandstone (tidalsand ridge, channel fill)

Oc cu rr en ce C om mo n t hr ou gh ou t b as in , b ett er d ev el -oped on shelves. Commonly underliesand overlies marginal marine shale

with plant fragments.

Underlies and overlies mudrock paleo-sols; thin coal beds in Bethel, Cypressand Hardinsburg.

Common throughout basin; transitionalbetween quartz sandstone and fossilif-erous and nonfossiliferous shale.

Color Most commonly red/maroon, but alsogreen and black; dolomitic nodulesare tan; Caliche is light brown.

Dark gray to olive green; common thin,black coal beds.

Sand is white and green, shale is darkgray and green.

White to light gray and green.

Depositional texture andgrain type

Mudrock is blocky, brecciated, slicken-sided clay w/dolomitic nodules. Cali-che is stringy to laminated micrite.

Clay with low-quality, thin coal beds. Ranges from 10% to 85% very fine- tofine-grained quartz sand or silt inter-laminated with shale.

Very-fine to medium-grained, subangu-lar, well-sorted; may have up to 15%shale.

Bedding and sedimentarystructures

Mudrock occurs in massive beds up toone meter thick; rooting and reductionhalos common.

Poorly fissile shale; coal beds most com-monly 130 cm thick

Flaser-, wavy- and lenticular-beddingand tidal rhythmites (Huff 1993).

Cross-bedded, flat-bedded and massivebedded; flaser-bedding common.

Biota Plant fragments common. Rare echinoderm fragments in transitionfacies.

Rare echinoderm fragments, could betransported or eroded from underlyinglimestone.

were deposited in deeper-water settings where tidal currents were dimin-ished (Fig. 5).

Lithofacies are summarized in detail in Table 1. Most siliciclastics weredeposited in nearshore tide-dominated settings, and carbonates were de-posited in both tide-dominated and storm-dominated settings (Fig. 5). Fromoffshore to onshore, the lithofacies in the Bethel to Glen Dean interval

include:Skeletal Wackestone (Foreshoal).Skeletal wackestone (Table 1) is

the most distal facies in the study interval and formed in laterally extensivesheets that can be correlated across the Eastern Shelf and into the BasinInterior in the Beaver Bend and Beech Creek formations (Harris 1992).The dark color and relatively high fossil diversity suggest that it formed

offshore in an unrestricted, low-energy, sub-wave base environment sea-ward of skeletal grainstone banks (Harris 1992).

Skeletal Grainstone and Packstone (Shoal, Bank).Skeletal grain-stone and packstone (Table 1; Fig. 6A) are common throughout the studyinterval, and their stratigraphic distribution, lack of mud, grain size, dom-inance of suspension feeding fauna, and cross-bedding suggest that they

formed in shallow, high-energy storm- and tide-dominated settings.Ooid Grainstone (High-Energy Shoal).Ooid grainstone (Table 1)

was deposited in clear, warm, shallow, high-energy settings above and ad-jacent to skeletal banks that served as preexisting topographic highs (Harrisand Fraunfelter 1993).

Muddy Carbonates (LagoonIntershoal).Muddy carbonates (Table


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TABLE 1.Extended.

Fossilferous Shale(carbonateclastic

transition)Muddy Carbonates(Lagoon/intershoal)

Ooid Grainstone(high-energy shoal)

Skeletal grainstone andpackstone (bank, shoal)

Skeletal wackestone(foreshoal)

Common throughout basin. Transition be-tween siliciclastic and carbonate rocktypes.

Occurs on Southeastern Shelf in Bea-ver Bend and Beech Creek forma-tions.

Most common on Eastern Shelf inBeaver Bend, Reelsville and Haneyformations.

Common throughout basin in all lime-stone units. Forms sheet-like unitsat base of most regressive parase-

quences.

Occurs in sheet-like units in BeaverBend and Beech Creek formations.Patchy downdip in Haney and Glen

Dean formations.

Olive green and dark gray. Tan dolomite; l ight gray-brown l ime-stone.

White to light gray. Light to medium to dark gray. Dark gray.

Wackestone/fissile shale; whole brachiopodsand bryozoans.

Microcrystalline dolomite, lime mud-stone/wackestone, pelletal limestone.

Grainstone with fine to mediumgrained ooids, echinoderms and bra-chiopods.

Grainstone and packstone with up to15% shale. Medium to very coarsegrained.

Wackestone with pelletal, argillaceousmatrix. Whole brachiopods andlarge echinoderm fragments com-mon.

F issi le, fla t- be dd ed . M assi ve, r ar e la min it es ; d ol om ite v ug -gy.

Cross-bedded, thick-bedded, and mas-sive.

Thick-bedded, massive, and cross-bed-ded.

Massive bedded and thick bedded.

Bryozoans, brachiopods and echinoderms. Gastropods, common echinoderm frag-ments.

Echinoderms and brachiopods. Abundant echinoderms, brachiopods,bryozoans, common mollusks andforaminifera.

Abundant echinoderms, brachiopods,bryozoans, common corals, mol-lusks, foraminifera, gastropods.

1) formed in low-energy, restricted environments in areas of little silici-clastic influx.

Fossiliferous Shale (CarbonateSiliciclastic Transition).Fossilifer-ous shale (Table 1; Fig. 6B) forms sheet-like caps to many parasequencesand fills lows between sand ridges. This facies commonly interfingers withheterolithic tidal-flat facies, which suggests a backshoal setting betweenoffshore carbonates and nearshore siliciclastics (Treworgy 1985). Stormbeds formed when skeletal debris was transported landward from the more

distal skeletal banks.Quartz Sandstone (Tidal Sand Ridge and Channel and Incised-Val-

ley Fill).Quartz sandstone (Table 1; Fig. 6C, D) occurs in ridges, channelfills, and incised-valley fills and less commonly in sheets (Seyler and Cluff1990; Zuppann and Keith 1988; Grube 1992). Tidal sand ridges are elon-gate convex-up bars up to 18 meters thick, 2 kilometers wide and severalkilometers long, which commonly trend northeastsouthwest parallel to thepaleoslope and inferred tidal-current direction (Figs. 6D and 7; Off 1963;Specht 1985; many others). Almost all channel and incised-valley-fill sand-stones are tidally influenced and commonly contain basal conglomerates,marine skeletal debris and marine trace fossils (e.g. Friberg et al. 1969;Ambers and Robinson 1992). Basal parts of some incised-valley fills maybe fluvial deposits. Modern analogs and the bi-directional cross-bedding,dip-parallel orientation of sand ridges, marine trace fossils, and abundant

tidal sedimentary structures all suggest that the sand ridges, tidal-channelfills and incised-valley fills were deposited in tide-dominated shallow-ma-rine environments (Specht 1985; Grube 1992).

Heterolithic Facies (Tidal Flat and Transition-Zone).The heterol-ithic facies consists of flaser-bedded, wavy-bedded and lenticular-beddedsandstone and shale and tidal rhythmites (Fig 6E; Table 1). These faciesformed in transition zones between areas of sand and mud deposition suchas tidal flats (cf., Van Straaten 1954) and the subtidal zone between tidalsand ridges and more distal fossiliferous shale. Modern siliciclastic tidalflats commonly occur near tide-dominated deltas and estuaries where thereis mixing of sandy offshore facies and muddy salt marsh or delta-plainfacies. Modern tidal rhythmites are only found in mesotidal to macrotidal,intertidal and subtidal, settings (Kvale and Mastalerz 1998). The presenceof tidal rhythmites, siliciclastic tidal-flat facies, and tidal sand ridges all

suggest a strong tidal influence and that some parts of the basin may haveexperienced mesotidal to macrotidal conditions.Nonfossiliferous Shale and Coal (Marginal Marine or Swamp).

Nonfossiliferous shale with common plant fragments and thin coal bedsoccur above and below paleosols. The presence of plant fragments, the lackof marine fossils, and the stratigraphic association with paleosols suggest

that these facies were deposited in a swamp or marginal marine environ-ment.

Blocky Mudrock, Calcrete, and Breccia (Paleosol).Blocky, slick-ensided mudrock paleosols are typically red, but some are green or darkgray. These paleosols are commonly brecciated and root-disrupted and maycontain brecciated carbonate nodules (Fig. 6F). These paleosols formed onshale, and less commonly sandstone, during periods of prolonged exposurein an arid or seasonal semiarid climate (Ambers and Petzold 1992). Some

mudrock paleosols can be correlated laterally into caliche paleosols thatformed on exposed carbonates (Smith 1996).

SEQUENCE STRATIGRAPHY

A sequence is defined as a genetically related succession of strata lackingapparent internal unconformities, composed of parasequences and parase-quence sets arranged in systems tracts and bounded by unconformities ortheir correlative conformities (Mitchum and Van Wagoner 1991). This def-inition was created for seismic-scale data sets that extend from shorelineto basinal settings. The Illinois basin was situated hundreds of kilometersupdip from the platform margin, and, as a result, only the updip parts ofsequences are preserved. Lowstand systems tracts are absent, and lowstandsare represented only by unconformities. It is difficult to recognize parase-quence sets, and systems tracts may be composed of as little as a single

parasequence. Stratigraphic units will be called sequences in this study ifthey are bounded by basin-wide unconformities and have maximum flood-ing intervals marked by any extensive carbonate deposition on the easternshelf. There are some unconformity-bounded stratal units in the study in-terval that do not have maximum flooding intervals marked by carbonatedeposition on the eastern shelf, and they are here treated as disconformity-bounded parasequences.

Most parasequences are regionally traceable across the Eastern Shelf andinto the Basin Interior and are either transgressiveregressive or simpleregressive parasequences. Transgressiveregressive parasequences are typ-ically composed of transgressive sandy siliciclastics overlain by carbonateunits that mark the maximum transgression, and regressive shale-dominatedsiliciclastics. Transgressiveregressive parasequences either lie entirelywithin transgressive systems tracts or contain the maximum flooding sur-

faces for the sequences within their carbonate units. In these cases, thetransgressive part of the parasequence is within the transgressive systemstract (TST) and the regressive part is within the highstand systems tract(HST). Regressive parasequences occur only in HSTs and have skeletallimestone bases and shallow upward into shale-dominated siliciclastic caps.

The following is a description of each of the sequences. Sequences 1


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FIG. 6.Photographs of important rock types. A) Cross-bedded skeletal grainstone (15 cm pen for scale); B) fossiliferous shale (lens cap is 5 cm in diameter); C) cross-bedded quartz sandstone (hammer for scale); D) Tidal Sand Ridge composed of quartz sandstone from base of outcrop to dashed white line. The sand ridge is 16 m highat the highest and thins to zero within 200 meters. E) Heterolithic facies (lens cap is 5 cm in diameter); F) Mudrock paleosol overlies brecciated dolomite (lens cap is 5cm in diameter).

through 5 from the underlying Ste. Genevieve and Paoli formations aredescribed in Smith and Read (1999).

Sequence 6

Sequence 6 includes all of the Bethel and Beaver Bend formations andthe basal part of Sample Formation (Fig. 4). The top of the sequence cannotbe recognized in the Basin Interior, where the Bethel, Sample, and Cypressformations merge to form the West Baden Clastic Belt. On the Southeastern

Shelf, sequence 6 consists of the uppermost disconformity-bounded car-bonate unit with the crinoid Talarocrinus within the Girkin Formation.

The basal sequence boundary (unconformity J) is marked by paleosolson most of the Eastern and Western Shelves and incised valleys that cutdown as much as 75 meters into the underlying carbonates (Reynolds andVincent 1967; Friberg et al. 1969). The TST is composed of transgressiveregressive parasequence 26 and the basal transgressive part of transgres-siveregressive parasequence 27. Most of parasequence 26 is containedwithin the incised-valley fill at the base of the sequence. The basal part ofthe incised-valley fill is composed of quartz sandstone with unidirectional


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FIG. 7.Isopach map (thickness in feet) of total sand thickness in Big CliftyFormation of southwestern Indiana (modified from Specht 1988). Northeastsouth-west trending sand-ridges are more than 50 feet (15 m) thick and thin to zero overapproximately 0.4 km.

planar and trough cross bedding that dips in what would have been a down-stream direction (Friberg et al. 1969). The unidirectional orientation of thecross bedding suggests that the basal part of the fill may be fluvial. Duringthe maximum transgression, quartzose skeletal limestone and heterolithicfacies were deposited near the top of the incised-valley fill (Friberg et al.1969). Minor siliciclastics overlie the limestone interval and mark the poor-ly developed regressive part of the parasequence.

On the Northeastern Shelf and in the eastern Basin Interior, parasequence27 is a transgressiveregressive parasequence that consists of a thin (30 cmor less), laterally extensive coal bed overlain by sandstone and fossiliferousshale, skeletal grainstone, dark gray skeletal wackestone, skeletal grain-stone, fossiliferous shale, and heterolithic facies. At the Battletown Quarry(location 45) in north-central Kentucky, a tidal sand ridge more than 18meters thick occurs in the transgressive part of the parasequence in theBethel Formation. The MFS for sequence 6 is picked at the base of theskeletal wackestone unit in the Beaver Bend Limestone because it is thedeepest-water facies deposited on the Eastern Shelf. In the central BasinInterior, the Beaver Bend either interfingers with quartz sandstone of theWest Baden Clastic Belt or was removed by subsequent incision.

The HST of sequence 6 consists of the upper part (regressive interval)of parasequence 27. On the Eastern Shelf and in the eastern Basin Interior,it consists of the dark gray skeletal wackestone and overlying skeletalgrainstonepackstone of the Beaver Bend, and fossiliferous shale and tid-ally influenced siliciclastics of the Sample Formation.

Sequence 7

Sequence 7 is composed of the upper Sample, Reelsville, Ridenhower,and lower Cypress formations (Fig. 4). The basal sequence boundary (un-conformity K) is a paleosol that can be traced laterally into incised valleys(Fig. 4) where at least 15 meters of incision occurred on the Northeastern

Shelf (Fig. 8; Kissling 1967; Ambers and Robinson 1992). Paleosols occurat the base of incised-valley fills at locations 45 and 67 on the NortheasternShelf, suggesting that the channels were eroded and subsequently subaer-ially exposed. A well-developed calcrete and exposure breccia with teepeestructures developed on top of sequence 6 on the Southeastern Shelf, whichremained emergent throughout deposition of sequence 7.

The TST includes transgressiveregressive parasequences 28 and 29. Onthe Northeastern Shelf, the base of parasequence 28 consists of tidally

influenced quartz sandstone units up to 12 meters thick that partially fillincised valleys (Fig. 8). In outcrop, these quartz sandstone channel fills arecommonly flaser-bedded and contain abundant marine trace fossils (Ambersand Robinson 1992). Some channels were not completely filled with quartzsandstone, and skeletal grainstonepackstone of the lower Reelsville For-mation was deposited in remaining topographic lows. In the eastern BasinInterior (location 20), parasequence 28 is composed of a basal fossiliferousheterolithic bed overlain by quartz sandstone and fossiliferous shale. Trans-gressiveregressive parasequence 29 has a thin quartz sandstone at the baseoverlain by argillaceous skeletal limestone, fossiliferous shale, and a welldeveloped paleosol across the eastern side of the basin (unconformity L).Parasequences 28 and 29 are unrecognizable in the West Baden ClasticBelt and cannot be distinguished from one another in the western BasinInterior because of lack of data.

Unconformity L is overlain by a discontinuous carbonate unit, composedof skeletal grainstonepackstone with minor lagoonal mudstone on theEastern Shelf and skeletal packstone in the central Basin Interior. The MFSfor sequence 7 is picked at the base of the limestone in parasequence 30because it is the first carbonate unit in sequence 7 to be deposited on theWestern Shelf and in most of the West Baden Clastic Belt. The HST ofsequence 7 consists of regressive parasequence 30, which is composed oflimestone overlain by heterolithic tidal-flat facies and patchy quartz sand-stone channel fills.

Sequence 8

Sequence 8 includes the upper Cypress Formation, the Beech CreekLimestone, and most of the Big Clifty Sandstone in Indiana and the Beech

Creek and Fraileys members of the Golconda Formation in Illinois andwestern Kentucky (Fig. 4).Sequence 8 is bounded at the base by unconformity M, which is marked

by a regional paleosol on the Eastern and Western Shelves and up to 45meters of incision in the Basin Interior. This boundary may coincide witha conodont zone boundary between the Reelsville and the Beech Creek(Collinson et al. 1962). The TST includes a disconformity-bounded trans-gressiveregressive parasequence (31) and the transgressive part of trans-gressiveregressive parasequence 32. Parasequence 31 is up to 35 metersthick in the Basin Interior but thins to less than five meters on the westernand northeastern shelves and pinches out completely on the SoutheasternShelf. In the Basin Interior, it is composed of a thick quartz sandstonemarine channel fill that passes laterally into storm-bedded fossiliferousshale (locations 1, 2, 19, and 20) and is overlain by heterolithic facies. Onthe Northeastern Shelf, the parasequence consists of shale and tidal rhyth-

mites capped by a paleosol that can be correlated basin-wide and tracedinto a 6 meter incised valley on the Northeastern Shelf (unconformity N).In the Basin Interior, the base of overlying parasequence 32 consists ofbasal siliciclastic tidal-flat facies and shale with poorly developed coal andpaleosol horizons and NESW trending lenticular quartz sandstone bodiesin the Basin Interior that are important hydrocarbon reservoirs (Grube1992). These strata are overlain by the Beech Creek Limestone, which iscomposed of a patchy basal skeletal grainstone unit overlain by a laterallyextensive deeper-water skeletal wackestone and a second skeletal grain-stonepackstone unit on the Eastern and Western Shelves (Harris 1992).The grainstone units pinch out in the Basin Interior where the Beech Creek


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FIG. 8.Incised valley in Sample Formation, Cape Sandy Quarry, southern Indiana. This photograph shows an incised valley that cuts from the middle of the SampleFormation through 6 meters of the lower Sample Formation and up to 5 meters into the underlying Beaver Bend Formation. The surface at the base of the incised valleycan be traced laterally to a paleosol (marked with vertical lines extending down from surface).

thins and is composed of the open skeletal wackestone (Harris 1992). TheMFS of sequence 8 is picked at the base of the skeletal wackestone, whichis the deepest-water facies deposited on the Eastern Shelf (Harris 1992).The Beech Creek Limestone was the first unit to be deposited on the South-eastern Shelf after sequence 6.

The HST is composed of the regressive interval of transgressiveregres-sive parasequence 32 and regressive parasequences 33 to 35 except onmuch of the Eastern Shelf, where it consists of the Big Clifty Sandstone.In the Basin Interior, the HST consists of limestoneshale parasequences32 to 35, which can be correlated regionally in the Beech Creek and Frail-eys Shale members of the Golconda Formation. The fossiliferous shaleparasequence caps are progressively more quartzose toward the top of theFraileys. On the Eastern Shelf, the regressive part of parasequence 32 con-sists of Beech Creek open skeletal wackestone and grainstone overlain inplaces by dark gray shale. In most locations on the Eastern Shelf, the BigClifty Sandstone lies directly on top of the Beech Creek Limestone. Onlyin a few locations is the base of parasequence 33 is preserved (locations54 and 55). Tidal-channel fills occur at the base of the Big Clifty Sandstone(Visher 1980), which suggests that erosion of tidal channels occurred priorto and possibly during progradation. The Big Clifty Sandstone may nothave started prograding from the east until after deposition of parasequence33, and may have eroded underlying shaly units. The Big Clifty Sandstoneappears to be a single regressive unit consisting of tidally influenced quartzsandstone (Visher 1980; Treworgy 1985) with tidal sand ridges up to 16meters thick (Fig. 7; Specht 1985).

Sequence 9

Sequence 9 is composed of the uppermost Fraileys/Big Clifty, Haney,and lower Hardinsburg formations (Fig. 4). The basal sequence boundaryis marked by a basin-wide paleosol (Treworgy 1985) (unconformity O).The TST of sequence 9 is composed of transgressiveregressive parase-quence 36. This parasequence consists of a thin shale at the top of theFraileys/Big Clifty, which is overlain by a skeletal packstone in the BasinInterior and is capped by a basin-wide fossiliferous shale in the HaneyLimestone. The MFS is picked at the base of parasequence 37, the firstlaterally extensive limestone unit to be deposited across the Eastern Shelf.

The HST of sequence 9 is composed of up to 6 regressive parasequences(37 to 42). Although these units have not been previously recognized asparasequences, subsurface mapping has illustrated that limestoneshaleunits in the Haney Limestone can be correlated for tens of kilometers (Tre-worgy 1985, 1988). Regressive limestoneshale parasequences 40 and 41are present only in the western part of the basin (locations 4 and 9), wherethe Hardinsburg thins and pinches out. This suggests that these parase-quences are laterally equivalent to the siliciclastics of the Hardinsburg orwere eroded in the rest of the basin during progradation of the tidal delta.In the Basin Interior, HST parasequences 37 and 38 are discrete parase-quences, but on the Eastern Shelf they are amalgamated into an ooid grain-stone at the base overlain by a skeletal grainstonepackstone unit. Locally,ooid grainstone units occur in bars with muddy carbonates filling swales(location 53; Fig. 4). The Hardinsburg Formation is composed of heterol-


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ithic sandstoneshale tidal-flat facies and quartz sandstone lenses on theNortheastern Shelf, and a basal quartz sandstone unit overlain by tidal-flatfacies in the Basin Interior. The sandstone in the Basin Interior fills tidalchannels locally scoured into the underlying limestone and shale by strongtidal currents prior to and possibly during progradation of the tidal flat andtidal-delta facies. There is a plant fossil-coal horizon near the base of theHardinsburg that can be traced in western Kentucky (locations 19 and 20),and several other coal and plant fossil horizons have been reported from

the Hardinsburg (Swann 1963). A disconformity-bounded parasequence(42) occurs at the top of the sequence between unconformities P and Qand consists of siliciclastic tidal-flat facies. The sequence is capped byunconformity Q.

Sequence 10

Sequence 10 consists of the upper Hardinsburg, Glen Dean Limestone,and the lower part of the Tar Springs Formation (Fig. 4), but only the basalpart of the sequence is included in this study. The Glen Dean is roughlyequal in thickness across the basin except in places on the NortheasternShelf where erosion occurred at the base of the Tar Springs.

The basal sequence boundary (unconformity Q) is marked by a well-developed paleosol, which is interpreted to correlate laterally with incised

valleys that cut through more than 50 meters of the lower Hardinsburg,and the Haney and Fraileys Members of the Golconda Formation (Potter1963; Treworgy 1988; Droste and Horowitz 1988). Conodont, coral, andforamaniferal biostratigraphic zone boundaries between the Haney andGlen Dean limestones may coincide with this unconformity (Maples andWaters 1987; Collinson et al. 1962).

The TST is composed of transgressiveregressive parasequence 43,which includes siliciclastics filling incised valleys of the Hardinsburg, apatchy skeletal grainstone unit confined to the Basin Interior, and a fossil-iferous shale that can be correlated basin-wide. The MFS for sequence 11occurs at the base of parasequence 44, which contains the first grainstonepackstone unit to be deposited basin-wide in the sequence. The HST con-sists of parasequences 44 and 45 and siliciclastics of the overlying TarSprings Formation. Channels filled with heterolithic facies cut through theparasequences and fossiliferous shale units within the Glen Dean Lime-

stone. The top of the sequence was not studied.

Composite-Sequences

A composite sequence is a succession of genetically related sequencesin which the individual sequences stack into lowstand, transgressive, andhighstand sequence sets (Mitchum and Van Wagoner 1991). It is not pos-sible to trace the downdip extent of sequence boundaries because the Illi-nois basin was situated far updip from the regional ramp margin. It istherefore not possible to unequivocally pick composite sequences in theMiddle Chesterian section of the Illinois basin. The boundaries at the baseof sequences 6, 8, and 10 appear, however, to have deeper incised valleysthan those at the bases of the other sequences in the study area. The deeperincised valleys likely represent greater relative sea-level falls, suggestingthat the sequences may be bundled in groups of two into lower-frequencycomposite sequences. The basal composite sequence in the Bethel to GlenDean interval includes sequences 6 and 7 and is bounded at the base byunconformity J and at the top by unconformity M. The upper compositesequence includes sequences 8 and 9 and is bounded at the base by un-conformity M and at the top by unconformity Q. The basal sequences inthe sequence pairs have: (1) deeper incised valleys than the boundariesinternal to the sequence-pairs; (2) incised-valley fills overlain by the best-developed coal horizons in the study interval; (3) tidal sand ridges over-lying the coal horizons, which are important reservoirs; and (4) open skel-etal wackestone in their maximum flooding intervals, which is the mostdistal limestone facies and is not common in internal sequences. The upper

sequences in the composite sequences have a greater number of regressiveparasequences in their HSTs and are bounded at the base by non-erosionalsequence boundaries or relatively shallow incised valleys.

Duration of Sequences and Parasequences

Based on foramaniferal and conodont zones, the Ste. Genevieve to GlenDean interval is uppermost Visean (middle of V3b to V3c) (Baxter and

Brenckle 1982). This interval has a duration of roughly 3 My, judging fromSHRIMP dating of volcanics interbedded with marine strata in Australia(Roberts et al. 1995) or between 4.8 to 5 My judging from Ross and Ross(1988) and Harland et al. (1990). There are five unconformity-boundedsequences in the Bethel to Glen Dean interval (Smith 1996) and five in theunderlying Ste. Genevieve to Paoli interval. Ten sequences divided into 3to 5 million years gives an average period of 300 to 500 ky for the se-quences.

The calculated duration for the sequences is suggestive of the long-term(414 ky) Milankovitch eccentricity signal (Berger 1988). If the sequencesare 300 to 500 ky duration then the composite sequences formed in 600to 1000 ky, and would be in the range of third-order units (0.55 My;Weber et al. 1995). There are 2 to 7 parasequences in each sequence, whichgives average durations between 40 to 250 ky.

Age-Equivalent Stratigraphy from Other Basins

There is a similar change from Chesterian (Upper Visean) carbonate-dominated sequences with non-erosional disconformities to mixed carbon-atesiliciclastic sequences with erosional unconformities in age-equivalentstrata in Great Britain (Walkden 1987), Kansas (Montgomery and Morrison1999), Poland (Skompski 1996), and elsewhere. Additionally, all of theboundaries of these mixed carbonatesiliciclastic sequences can be corre-lated from the Illinois basin across the Cincinnati Arch into the Appala-chian foreland basin where the interval is almost entirely composed ofcarbonates (Smith et al. in press; Al-Tawil 1998). The similarity of UpperMississippian sequence architecture in different basins on multiple conti-nents suggests that these sequences were produced by a global forcingmechanism.

Comparison of Chesterian High-Frequency Sequences withPennsylvanian Cyclothems

Sequences 6 and 8 resemble typical Pennsylvanian cyclothems from theIllinois basin (Fig. 9). Both these sequences and typical cyclothems havedeep incised valleys at the base filled with tidally influenced siliciclastics.Both have coal beds overlying paleosols and incised-valley fills. In boththe sequences and the typical cyclothem, coal beds are overlain by marinesiliciclastics and carbonate units. Pennsylvanian cyclothems commonlyhave deeper-water black shale bounded by two carbonate units, whereasthe Chesterian examples have offshore skeletal wackestone bounded byshallow-water skeletal grainstone. Both are capped by marine siliciclasticsand disconformities. The black shales from the cyclothems likely formedin deeper water than the wackestones in the Late Mississippian, but bothare the deepest-water facies present in their respective sequences. Both havesiliciclastic deltaic facies overlying the carbonate-dominated intervals.

DISCUSSION

Origin of Sequences and Parasequences

Moderate- to high-amplitude (25100 m) glacio-eustatic sea-level chang-es are the best explanation for the basin-wide unconformities, the verticaljuxtaposition of deeper-water limestone facies over paleosols, the deep in-cised valleys, and similar sequences in other parts of the world. Althoughno glacial deposits are currently dated as late Visean, the similarity betweensome of these sequences and the overlying Pennsylvanian cyclothems (Fig.


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FIG. 9.Comparison of sequences 6 and 8from this study with Pennsylvanian completeIllinois cyclothem (modified from Heckel1980). Patterns are the same as in Fig. 4.

9) suggests that they were produced by similar processes, and there is ampleevidence for major continental glaciation in the Early Pennsylvanian (Frak-es et al. 1992). It is likely that late Visean glacial deposits would havebeen reworked by subsequent, larger glaciations.

It has been suggested that unconformities in the Illinois and Appalachianbasins could have been caused by flexural uplift of the Cincinnati Arch, amigrating peripheral bulge (Quinlan and Beaumont 1984). Local tectonicuplift on the Cincinnati Arch, however, would have created local uncon-formities on or near the Arch that could not be correlated away from thetectonic high. The sequence-bounding unconformities and deepening eventsin the Chesterian can be correlated basin-wide and into the Appalachianbasin. Unconformities generated by a migrating peripheral bulge would be

diachronous, and should cut across biostratigraphic zone boundaries, andunconformities in the proximal fold-thrust belt should be out of phase withthe unconformities on the arch. Yet all of the biostratigraphic zone bound-aries studied appear to coincide with the unconformities.

In their modeling study of the link between thrust loading in the Ap-palachian basin and the tectonic response in the Appalachian and Illinoisbasins, Quinlan and Beaumont (1984) modeled the crust as a uniform flex-ural beam with no preexisting faults or weaknesses. However, there arenumerous faults in both the Illinois and Appalachain basins that are rootedin the crust. Waschbush and Royden (1992) showed that preexisting faultsor weaknesses in the crust impede migration of a peripheral bulge. It is ouropinion that the stress caused by loading in the foreland basin was accom-modated by these preexisting faults rather than by a migrating peripheralbulge. Evidence for this is the relatively stationary position of the Cincin-nati Arch throughout the Paleozoic. Furthermore, Nelson (1995) showedthat some faults in the Illinois basin were reactivated as both normal andreverse faults a minimum of five times during the Paleozoic.

The only way that the sequence boundaries could have been producedby tectonics would be repeated, synchronous uplift and subsidence on theorder of 30 to 95 meters of the Illinois basin and all of the other placeswhere similar sequences have been recognized. Although continents do riseand fall over millions of years due to heating and cooling of the crust,there is no known mechanism that would periodically produce such aneffect on the order of hundreds of thousands of years.

Swann (1964) and Droste and Horowitz (1990) attributed the alternationof carbonate and siliciclastic units in the Illinois basin to cyclic climate

change under static sea level. Cyclic climate change alone, however, couldnot have produced the basin-wide paleosols that can be correlated fromclastics to carbonates, the deep incised valleys, or the extent of marineinundation required to deposit deeper water carbonates on the shelves. Sub-tle variations in the duration of the wet season may have caused moresiliciclastic influx in some of the siliciclastic units but could not have pro-duced the unconformity-bounded sequences.

Parasequences.The regional extent and high frequency of the parase-quences suggest that they were also produced by glacio-eustatic sea-levelchanges. Pulses of siliciclastics driven by cyclic climate change would haveproduced local parasequences near the siliciclastic source rather than basin-wide parasequences developed in a variety of rock types. The magnitude

of relative sea-level change forming the parasequences was variable de-pending on their position within the sequences and the magnitude of thehigh-frequency fluctuations.

Transgressiveregressive parasequences occur only within the TSTs ofhigh-frequency sequences because incised valleys provided accommodationspace, quartz sand was available for reworking, and the duration and mag-nitude of the fifth-order sea-level rises were both greater because they co-incided with fourth-order sea-level rises. Asymmetric regressive skeletallimestoneshale parasequences are best developed in the HSTs of sequenc-es. They formed when rapid, high-frequency sea-level rises halted silici-clastic influx, allowing carbonate-producing organisms to flourish and skel-etal limestone to be deposited (Smith 1996). As sea level slowly fell, fos-siliferous shale prograded over the basin and capped the parasequences.

Effects of Tectonics on the Stratigraphy

The sequences can be treated as time slices to determine the relativetiming of tectonic activity in the basin (Fig. 10). Significant variations inthickness trends within sequences between the Basin Interior and theshelves, apparent synsedimentary faulting in some units, and the presenceof seismically disturbed beds or seismites, suggest that the Illinois basinwas tectonically active and undergoing differential subsidence during theLate Mississippian. Thickness ratios from the Western Shelf:Basin Interior:Southeastern Shelf are 1.5:10:1 for the interval spanning sequences 6, 7,and the basal parasequence in sequence 8; 1:1:1 for the rest of sequence 8and the basal part of sequence 9; and 1.4:3:1 for the upper part of sequence


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FIG. 10.Variations in differential subsidenceand reactivation of pre-existing faults in theIllinois basin.

9. Uplift on the shelves and/or downwarping in the basin caused relativelyhigh differential subsidence ( 5:1) during deposition of sequences 6, 7,and the basal parasequence in sequence 8. This was followed by a periodof low ( 2:1) differential subsidence in the upper part of sequence 8 andbase of sequence 9, and a subsequent increase to moderate differential

subsidence in the upper part of sequence 9.High differential subsidence was likely associated with normal faulting

in the Basin Interior during deposition of sequences 6, 7, and the basalparasequence in sequence 8, which are amalgamated in the West BadenClastic Belt (Fig. 10). This clastic belt directly overlies the northeastsouth-west trending Dixon Springs Graben, which is part of the Fluorspar AreaFault Complex and extends over part of the Wabash Valley Fault Zone(Cole and Nelson 1995). There is evidence for thickening in the intervalabove unconformity N and below the Beech Creek Limestone at locations12 and 13 relative to other nearby sections that may have been caused bynormal faulting. As a result of relatively high differential subsidence, sil-iciclastics in these sequences were funneled into the tectonic low in theBasin Interior and pinched out on the more slowly subsiding SoutheasternShelf, which also remained too high for marine deposition throughout se-quence 7.

During the period of low differential subsidence during deposition of theupper part of sequence 8 and the lower part of sequence 9, reverse blockfaulting occurred in the Basin Interior (note absence of parasequences 34and 35 at location 20 and absence of parasequences 36 and 37 at location11 in Figure 4). As a result of the low differential subsidence, siliciclasticswere no longer funneled into the Basin Interior, and thick quartz sandstoneswere deposited over a wide area in sequence 8.

Renewed moderate differential subsidence in the upper half of sequence9 appears to have been accompanied by normal faulting in the Basin In-terior. The Hardinsburg Formation thickens by more than 25 meters over2 km into a graben in the Fluorspar District without complementary thin-

ning of the underlying Haney Limestone (Nelson 1996). Moderate differ-ential subsidence in the upper half of sequence 9 apparently led to funnelingof siliciclastics into the tectonic low in the Basin Interior.

Further evidence of tectonic activity is the presence of seismites,which consist of contorted bedding and ball-and-pillow structures, primar-

ily in the heterolithic facies. Synsedimentary faulting commonly generatesseismites when earthquakes cause sediment liquefaction or dewatering,which distorts original bedding and lamination in strata within 15 metersof the sedimentwater interface (Seilacher 1984; Scott and Price 1988). Aseismite horizon occurs in the uppermost Cypress Formation at locations11 and 12 in the Fluorspar District and could be related to normal faultingand thickening of the section at locations 12 and 13 (Fig. 4). A secondseismite horizon at the top of the Big Clifty Sandstone can be correlatedfor more than 75 kilometers between locations 48 and 67 (Fig. 4). A thirdseismite horizon occurs at the base of the Hardinsburg Formation at loca-tions 19 and 20. Earthquakes with magnitudes between 6 and 8 probablywould have been required to produce these liquefaction features, judgingfrom calculations made for modern seismites in Japan (cf. Kuribayashi andTatsuoka 1975).

Tectonic Linkage between the Illinois and Appalachian Basins.Variations in the rates of subsidence in the Illinois basin were probablyrelated to phases of thrust loading in the Appalachian foreland basin to theeast (Fig. 1A). Study of time-equivalent units in southwest Virginia revealsthat relatively high subsidence rates associated with thrust loading com-menced in the Early Chesterian (Al-Tawil 1998). Appalachian thrust load-ing likely caused the Cincinnati Arch to resist subsidence (Quinlan andBeaumont 1984) while the horizontal stress associated with continentalcollision may have caused greater subsidence rates in the Illinois basininterior (DeRito et al. 1983). Uplift on the northeastsouthwest trendingCincinnati Arch and greater subsidence in the Basin Interior likely initiatedreactivation of normal faults in the northeastsouthwest trending Wabash


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FIG. 11.Estimation of the magnitude of sea-level changes that produced fourth-order sequences 6 through 10 in the Bethel to Glen Dean interval.

FIG. 12.Comparison of Mississippian onlap curve of Ross and Ross (1988)(left), transgressionregression curve of Swann (1963) (center), and relative sea-level curve prepared for this paper (right).

Valley Fault System and Fluorspar Area Fault Complex (Fig. 1B). Phasesof high differential subsidence would have occurred as a response to Ap-palachian thrusting and lower differential subsidence would have occurredduring periods of relative quiescence.

Effects of Climate on the Stratigraphy

Paleoclimatic indicators suggest that the climate in the Illinois basinchanged from arid in the late Meramecian to dominantly dry wetdry sea-

sonality in the early Chesterian to more humid wetdry seasonality in themiddle Chesterian. The upper Meramecian St. Louis Formation has wide-spread, bedded evaporite deposits (Shaver et al. 1986), which suggest anarid climate. The overlying lower Chesterian Ste. Genevieve and Paoliformations have widespread caliche and breccia (Smith and Read 1999),which typically form in a wetdry seasonal climate (Esteban and Klappa1983). The abundant eolianites and dolomite in the formations suggest thatthe climate was dominantly dry during their deposition. In the middle Ches-terian Bethel to Glen Dean interval, which is the subject of this paper, redmudrock paleosols with slickensides, brecciated dolomite nodules, caliche,and breccia again suggest a wetdry seasonal climate (Ambers and Petzold1992). However, the increased siliciclastic influx and the presence of thincoals along with decreased ooid production and a lack of eolianites suggestthat the climate was wetter than during deposition of the underlying Ste.Genevieve. This trend of progressively more humid climate continued intothe Pennsylvanian, which has more siliciclastics, less limestone, and thick-er, better-developed coal beds (Witzke 1990).

The upward shift to more humid wetdry seasonality may have beendriven by the buildup of ice-sheets on Gondwana. Larger ice sheets at theSouth Pole may have caused the wet intertropical convergence zone to shiftto the north, especially during sea-level lowstand, when ice sheets were attheir largest. The increase in siliciclastic content from the Early Chesteriansequences 1 through 5 into the Middle Chesterian Bethel to Glen Deaninterval is likely related to onset of a more humid climate in the region incombination with increasing-amplitude sea-level changes. A similar in-crease in siliciclastic sedimentation occurred in time-equivalent strata in

Great Britain, which was attributed to increasing-amplitude sea-levelchange coupled with tectonics (Walkden 1987). Higher-amplitude sea-levelchanges caused greater base-level falls, which in turn caused greater erosion

in the source areas and increased supply of siliciclastic sediment. At thesame time, synchronous climate change and increasing humidity wouldhave increased influx of clastics during times of lowered sea level.

Sea-Level Curve for the Middle Chesterian

The depth of incised valleys, paleosols, the interpreted water depth offacies deposited during maximum flooding, and the occurrence of parase-quences were used to construct a detailed relative sea-level curve for theBethel to Glen Dean interval (Figs. 11, 12). The magnitude of sea-levelchanges that made each sequence is estimated by adding the interpretedmagnitude of sea-level fall below the platform and rise above the platformfor each sequence. The depth of incision associated with each sequenceboundary is the only evidence available to estimate the magnitude of thesea level falls for each sequence (Fig. 11).

Ross and Ross (1988) constructed an onlap curve for late Paleozoiclarge-scale third-order sequences. Their sequence boundaries occur at thebase of sequence 8 and at the base of the Tar Springs Formation, whichoverlies the Glen Dean Formation. Swann (1963) constructed a transgres-sionregression curve based on the seaward extent of siliciclastics withinthe Illinois basin. Swanns curve roughly parallels the high-frequency se-quence curve presented here because siliciclastic units commonly prograd-ed toward the tops of the sequences (Fig. 12). Swann (1963) did not picka regression at the top of the Fraileys/Big Clifty (unconformity O) becausethe Big Clifty Sandstone did not prograde across the entire basin and basin-wide paleosols were not used in construction of the curve. The refined sea-


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FIG. 13.Frequency of late Paleozoic ice-rafted deposits worldwide (modified from and Frakes et al. 1992, and Smith and Read 2000; age from Roberts et al. 1995)and composite sea-level curve for early and middle Chesterian (Late Visean) from this study. The increase in the amplitude of sea-level changes between sequences 5 and6 suggests that major glaciation on Gondwana began in the Late Visean rather than in the Namurian as was previously thought.

level curve prepared for this paper could only be developed by recognitionof parasequences, paleosols, and incised valleys.

The linkage between the depth of the incised valleys and the magnitude

of sea-level fall is complicated by the possibility that subaqueous fluvialor tidal erosion may have occurred below sea level or that sea level mayhave fallen farther than the bases of the incised valleys. Paleosols formedat the bases of incised valleys in the Sample Formation, which indicatesthat sea level fell below the base of those incised valleys. Narrow, deepincised valleys are typical of areas far from the mouth of the river, andshallow, wide incised valleys are typical of areas closer to the mouth ofthe river (Schumm and Ethridge 1994). The deep incised valleys at thebases of sequences 6 and 10 are less than 2 km wide (Friberg et al. 1969;Treworgy 1985) which suggests that they developed updip under subaerialrather than submarine conditions. If subsea erosion was a major factor insome of the channel formation, then the estimates for sea-level fall maybe too high. However, if the bases of the incised valleys were above sealevel, the estimates for sea-level fall here are too low. Furthermore, com-paction has not been taken into account in the construction of these cross

sections, and the actual depth of the incised valleys was greater than re-ported here.

Subsequent sea-level rises can be estimated by the height of tidal sandridges and the water depth in which limestone facies were deposited. Tidalsand ridges up to 16 meters thick occur in the early TST of sequence 6and the HST of sequence 8, suggesting that sea level rose at least 16 metersabove the exposed platform. The deeper-water skeletal wackestone faciesformed above storm wave base but below the effects of strong tides, sug-gesting a water depth of at least twenty meters. Confirmation of this esti-mated water depth comes from sequence 8, where skeletal wackestone andgrainstonepackstone are overlain by 16-meter-thick tidal sand ridges of

the Big Clifty Formation (Specht 1985), which formed during progradationand sea-level fall.

The magnitude of sea-level changes that produced the parasequences was

likely between 5 and 50 meters, depending on their position within thesequence. Lower TST and upper HST parasequences are commonly dis-conformity-bounded and were likely produced by higher-amplitude sea-level changes than regressive parasequences deposited in the lower part ofthe HSTs. Relatively minor sea-level changes would be required to makethe regressive limestoneshale parasequences and much greater sea-levelchanges would have been required to make some of the transgressiveregressive parasequences.

Onset of Late Paleozoic Glaciation on Gondwana

There is an abrupt change from carbonate-dominated sequences boundedby non-erosional disconformities in the underlying Ste. Genevieve to Paoliinterval (sequences 1 through 5; Smith and Read 1999) to the mixed car-bonatesiliciclastic sequences bounded by unconformities with deep incisedvalleys in the Bethel to Glen Dean interval (Sequences 6 through 10; thispaper) (Smith and Read 2000). The abundance of shallow marine faciesand lack of erosional unconformities in Sequences 1 through 5 suggest thatthey were produced by sea-level fluctuations of no more than 20 to 30meters (Fig. 13; Smith and Read 1999; Smith and Read 2000). The deeper-water carbonates and deep incised valleys in sequences 6 through 10 in theBethel to Glen Dean interval suggest that they were produced by sea-levelchanges of up to 95 meters. Similar transitions from carbonate-dominateddisconformity bounded fourth-order sequences to mixed carbonatesilici-clastic sequences with deep incised valleys occur in same-aged strata inKansas (Montgomery and Morrison 1999), Great Britain (Walkden 1987)and Poland (Skompski 1996).


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The approximate threefold increase in the amplitude of sea-level changesbetween sequences 5 and 6 suggests a similarly dramatic increase in thevolume of ice on Gondwana and probably marks the onset of the majorlate Paleozoic ice age (Fig. 13; Smith and Read 2000). The threefold in-crease in the amplitude of sea-level fluctuations should affect all same-agedmarine strata and should serve as an effective global stratigraphic markerin settings where incised valleys are preserved.

CONCLUSIONS

Five unconformity-bounded high-frequency ( 400 ky) sequences in theUpper Mississippian (middle Chesterian) Bethel to Glen Dean interval (upto 68 meters thick) can be traced across the Illinois basin in spite of verylarge spatial variation in subsidence rates. Four of the sequences haveboundaries marked by incised valleys up to 75 meters deep that can becorrelated laterally to paleosols formed on interfluves, and the other ismarked by a basin-wide paleosol. Lowstand systems tracts are not pre-served in the Illinois basin because of the updip position on the regionalramp. Transgressive systems tracts consist of one or more transgressiveregressive parasequences. The maximum flooding surfaces for the sequenc-es are picked at the base of the deepest-water carbonate units to be depos-ited on the shelves. The early highstand systems tracts are composed ofregressive limestoneshale parasequences, whereas late HST parasequences

consist of prograding siliciclastic tide-dominated deltaic facies. Correlationof the sequences across the Illinois basin and into the Appalachian basinindicates a eustatic origin with magnitudes of sea-level change of 25 to 95meters. Thus the study provides a local data set for refining the global sea-level curves for this part of the Chesterian.

The sequences can be used as high-resolution time slices to better un-derstand the Early and Middle Chesterian tectonic history of the basin.Periods of relatively high differential subsidence between the CincinnatiArch and the Basin Interior were accompanied by normal faulting, silici-clastic influx into the central Basin Interior, and pinchout of sequences andparasequences onto the Arch. Periods of lower differential subsidence wereaccompanied by reverse block faulting in the Basin Interior and quartzsandstone deposition on the Southeastern Shelf.

Basin-scale high-resolution sequence stratigraphy is a useful tool for dis-

criminating the relative effects of eustasy and tectonics. Only through de-tailed, basin-wide correlation using close spaced measured sections thatallow regional mapping of individual disconformities can local unconfor-mities produced by tectonic uplift be distinguished from regional uncon-formities produced by eustatic sea-level change.

ACKNOWLEDGMENTS

We would like to thank Erik Kvale, Brian Keith, Steve Graham, Jacqueline Hun-toon, John Southard, and Mary Kraus for their constructive reviews of this paper.We also thank the members of the Illinois, Indiana, and Kentucky Geological Sur-veys for providing cores and valuable discussion. Aus Al-Tawil, Mike Pope, TomWynn, John Nelson, and Garland Dever provided help and guidance throughout thecourse of this research. Financial Assistance was provided by the Petroleum Re-search Fund (ACS-PRF#30516-AC8), the Geological Society of America, Sigma Xi,and Virginia Tech.

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Received 6 December 1999; accepted 5 April 2001.

APPENDIX 1: OUTCROP AND CORE LOCATIONS FOR THISSTUDY AND SMITH AND READ (1999)

1. Core, Madison Co., C-141, Illinois State Geological Survey Core Facility,Champaign, IL

2. Core, Madison Co., C-140, Illinois State Geological Survey Core Facility,Champaign, IL

4. Core, Union Co., C-13,619, Illinois State Geological Survey Core Facility,Champaign, IL

5. Outcrop, Anna Quarry, East Side of Anna, IL, NW of Junction of S.R. 51and S.R. 146

6. Outcrop, I-57, milepost 27.4, south of Anna, IL Exit7. Outcrop, Cypress Quarry, Columbia Quarries Inc., S.R. 37, 3 miles south of

Cypress, IL8. Core, Pope County, C-46, Illinois State Geological Survey Core Facility,

Champaign, IL10. Core, Hardin Co., C-12781, Illinois State Geological Survey Core Facility,

Champaign, IL12. Core, Hardin Co., C-12782, Illinois State Geological Survey Core Facility,

Champaign, IL13. Core, Hardin Co., from Ozark-Mahoning Fluorspar mine near Denton, IL14. Outcrop, Hardin Co. Materials Co., Pit 2, junction S.Rts. 1 and 146, Cave-

in-Rock, IL15. Outcrop, Cave-in-Rock Quarry, Martin-Marietta, on county roads ENE of

Cave-in-Rock, IL16. Outcrop, Three Rivers Quarry, Martin-Marietta, U.S. 60 6.25 miles NE of

Smithland, KY17. Outcrop, Fredonia Quarry, Denny and Simpson, U.S. 641, 2.5 miles south of

Fredonia, KY18. Outcrop, Princeton Quarry, Kentucky Stone Co., S.R. 91, 2.75 mi. south of

Princeton, KY21. Core, Caldwell Co., C-220, Kentucky Geological Survey Core Facility, Lex-

ington, KY22. Outcrop, S. Hopkinsville Quarry, Rogers Group, U.S. Alt 41, 4.5 miles south

of Hopkinsville, KY23. Outcrop, Christian Quarries Quarry (abandoned), U.S. 68, 0.5 miles east of

Hopkinsville, KY24. Outcrop, Todd Quarry, Kentucky Stone Co. Quarry, U.S. 68, 7 miles west of

Elkton, KY25. Outcrop, new roadcut for U.S. 68 bypass, west of Russelville, KY26. Outcrop, Russelville Quarry and Core, Kentucky Stone Co., S.R. 79, 1 mile

east of Russelville, KY

29. Outcrop, roadcut at milepost 9.1 on Green River Parkway north of BowlingGreen, KY30. Outcrop, Rockfield Quarry, Kentucky Stone Co., U.S. 68, Rockfield, Ken-

tucky33. Outcrop, roadcut on I-65 N at milepost 48 north of Park City, KY Exit34. Outcrop, E. Park City Quarry (abandoned), Old Bardstown Rd., 2 miles east

of Park City, KY35. Outcrop, Cave City Quarry, Scottys Paving Co., Rt. 90, 5 miles east of

Cave City, KY36. Outcrop, roadcut on I-65 south, 1 mile south of Munfordville, KY Exit37. Outcrop, roadcut on I-65 North, 0.5 miles north of Munfordville, KY Exit38. Outcrop, Upton Quarry, Kentucky Stone Co., Quarry Rd., Upton, KY40. Core, Hardin Co., C-117, Kentucky Geological Survey Core Facility, Lex-

ington, KY41. Outcrop, Cecelia Quarry, Larry Glass Paving, S.R. 62, 3 miles west of Ce-

celia, KY42. Core, Breckenridge Co., C-118, Kentucky Geological Survey Core Facility,

Lexington, KY43. Outcrop, Stephensburg Quarry (abandoned), on dirt road 2 miles north of

Stephensburg, KY44. Outcrop, Irvington Quarry, Kentucky Stone Co., S.R. 477, 2 miles northwest

of Irvington, KY45. Outcrop, Battletown Quarry, Kosmos Cement Co., S.R. 228, 0.5 miles north-

west of Battletown, KY46. Outcrop, Cape Sandy Quarry, Pit 1, Mulzer Crushed Stone Inc., 4 miles east

of Alton, IN47. Core, Harrison Co., C-339, Indiana Geological Survey Core Facility, Bloo-

mington, IN48. Core, Perry Co., C-303, Indiana Geological Survey Core Facility, Blooming-

ton, IN


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49. Outcrop, Tower Quarry, Mulzer Crushed Stone Co., 2 miles north of Leav-enworth, IN

50. Outcrop, roadcut at milepost 99 on I-64, 7 miles west of Corydon, IN exit51. Outcrop, Corydon Quarry, Corydon Crushed Stone Inc., 2.5 miles northwest

of Corydon, IN52. Outcrop, Robertson Quarry, Robertson Crushed Stone Inc., S.R. 64, 1 mile

west of Depauw, IN56. Core, Crawford Co., C-626, Indiana Geological Survey Core Facility, Bloo-

mington, IN

57. Core, Orange Co., C-338, Indiana Geological Survey Core Facility, Bloo-mington, IN60. Core, Orange Co., C-651, Indiana Geological Survey Core Facility, Bloo-

mington, IN62. Outcrop, W. Paoli Quarry, Cave Quarries Inc., U.S. 150, 3.5 miles northwest

of Paoli, IN63. Core, Martin Co., C-214, Indiana Geological Survey Core Facility, Bloo-

mington, IN64. Outcrop, Mitchell Quarry, Rogers Group, S.R. 60, 5 miles west of Mitchell, IN

65. Core, Lawrence Co., C-340, Indiana Geological Survey Core Facility, Bloo-mington, IN

67. Core, Martin Co., C-797, Indiana Geological Survey Core Facility, Bloo-mington, IN

68. Outcrop, Sieboldt Quarry, Rogers Group, county roads, 5 miles north of Oo-litic, IN

69. Core, Greene Co., C-487, Indiana Geological Survey Core Facility, Bloo-mington, IN

70. Core, Monroe Co., C-672, Indiana Geological Survey Core Facility, Bloo-

mington, IN71. Outcrop, Bloomington Quarry, Rogers Group, Oard Rd., 4 miles west ofBloomington, IN

73. Core, Clay Co., C-444, Indiana Geological Survey Core Facility, Blooming-ton, IN

74. Outcrop, Putnamville Quarry, Kentucky Stone Co., S.R. 343, 1 mile west ofCloverdale, IN

75. Core, Putnam Co., C-105, Indiana Geological Survey Core Facility, Bloo-mington, IN

DISCRIMINATION OF LOCAL AND GLOBAL EFFECTS ON UPPER MISSISSIPPIAN STRATIGRAPHY illinois basin

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