Regional Diagenetic Patterns in the

St. Peter Sandstone: Implications for

Brine Migration in the Illinois Basin

By Janet K. Pitman, Martin B. Goldhaber, and Christoph Spöetl

EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

Jennie L. Ridgley, Project Coordinator

U.S. GEOLOGICAL SURVEY BULLETIN 2094–A

A multidisciplinary approach to research studies of

sedimentary rocks and their constiutents and the

evolution of sedimentary basins, both ancient and modern

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997
For sale by U.S. Geological Survey, Information Services

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Any use of trade, product, or firm names in this publication is for descriptive purposes only and

does not imply endorsement by the U.S. Government

Library of Congress Cataloging-in-Publication Data

U.S. DEPARTMENT OF THE INTERIOR

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY

Gordon P. Eaton, Director

Pitman, Janet K.

Regional diagenetic patterns in the St. Peter Sandstone : implications for brine migration in the Illinois Basin / by Janet K. Pitman, Martin B. Goldhaber, and Christoph Spöetl.

p. cm.—(Evolution of sedimentary basins—Illinois Basin ; A) (U.S. Geoloigcal 

Survey bulletin ; 2094)

Includes bibliographical references.

Supt. of Docs. no. : I 19.3:2094–A

1. Diagenesis—Illinois Basin. 2. Geology, Stratigraphic-Ordovician. 3. Saint Peter 

Sandstone. I. Goldhaber, Martin B. II. Spöetl, Christoph. III. Title. IV. Series

V. Series: U.S. Geological Survey bulletin ; 2094.

QE75.B9 no. 2094–A

[QE571]

[557.3 s—dc21 97–10644

[552′.5] CIP
III

CONTENTS

Abstract ........................................................................................................................... A1

Introduction..................................................................................................................... 1

Geologic and Tectonic History ....................................................................................... 1

Depositional Framework................................................................................................. 2

Methods........................................................................................................................... 2

Detrital Mineralogy......................................................................................................... 3

Sedimentary Diagenesis.................................................................................................. 4

Mineral Authigenesis .............................................................................................. 4

Carbonate Cements ......................................................................................... 4

Secondary Quartz............................................................................................ 7

Authigenic Potassium Feldspar....................................................................... 8

Clay Minerals.................................................................................................. 9

Anhydrite ........................................................................................................ 9

Other Phases.................................................................................................... 9

Mineral Dissolution................................................................................................. 9

Time and Temperature of Major Diagenetic Events............................................... 10

Evolution of Pore Fluids ................................................................................................. 12

Composition of Precipitating Waters...................................................................... 12

Origin of Precipitating Fluids ................................................................................. 12

Conclusions..................................................................................................................... 14

Acknowledgments........................................................................................................... 15

References Cited ............................................................................................................. 15

FIGURES

1. Map showing location of the Illinois and Michigan Basins and isopach map showing regional thickness of

St. Peter Sandstone in the Illinois Basin ................................................................................................................... A2

2. North-south cross section of lower Middle Ordovician (Whiterockian) through Lower Devonian (Ulsterian) 

rocks in Illinois ......................................................................................................................................................... 3

3. Graphical representation of mineral paragenesis in the St. Peter Sandstone ............................................................ 5

4. Maps showing regional distribution of authigenic mineral cements and porosity in the St. Peter Sandstone.......... 6

5. Graph showing stable isotopic composition of carbonate cements in the St. Peter Sandstone ................................ 8

6. Maps showing regional variations in the mean oxygen-isotope compositions of dolomite in the 

St. Peter Sandstone.................................................................................................................................................... 8

7. Burial-history curve of the St. Peter Sandstone showing timing of major diagenetic events in the 

Fairfield subbasin, southern Illinois .......................................................................................................................... 11

8. Graphs showing relationship between the oxygen isotopic composition of pore water, temperature of mineral 

precipitation, and measured range in δ18O ratios of carbonate cements in the St. Peter Sandstone ......................... 13

9. Map showing paleoflow directions in the Illinois Basin area ................................................................................... 15
A1

 Regional Diagenetic Patterns in the St. Peter Sandstone:

Implications for Brine Migration in the Illinois Basin

By Janet K. Pitman,1 Martin B. Goldhaber ,2 and Christoph Spöetl3

ABSTRACT

Diagenetic minerals and alteration patterns in the

Ordovician St. Peter Sandstone, Illinois Basin, record varied

hydrologic and chemical conditions during the basin’s long

and complex geologic history. Major diagenetic events modifying the St. Peter Sandstone include (1) mechanical compaction, (2) early K-feldspar overgrowth and dolospar

precipitation, (3) burial quartz, dolospar, anhydrite, and calcite cementation, and (4) carbonate-cement and K-feldspar

grain dissolution. Radiometric age dates of authigenic

K-feldspar and illite in combination with the reconstructed

burial history of the St. Peter reveal that early-diagenetic

K-feldspar and dolospar precipitated at shallow to moderate

depths in the Devonian, whereas late-diagenetic quartz,

dolospar, anhydrite, and calcite formed during deep burial in

the Late Pennsylvanian to Early Permian. Stable-isotope

geochemistry and fluid-inclusion paleothermometry suggest

that burial cements precipitated from saline fluids over a

wide temperature range. In the southern part of the basin,

burial cements preserve a record of diagenetic effects that

were in part controlled by fractures and hydrothermal-fluid

circulation. Baroque dolospar cementation is the most significant of these effects.

INTRODUCTION

The Middle Ordovician St. Peter Sandstone in the Illinois Basin (fig. 1) was examined to establish regional diagenetic patterns and to evaluate the possibility that the unit

acted as an aquifer for fluids that produced the zinc-lead-fluorine mineral deposits of the Mississippi Valley type (MVT)

adjacent to the basin. An important aspect of the study

involved deciphering the nature and timing of dolomitization

in relation to major hydrothermal brine migration and associated ore-forming events. The St. Peter Sandstone in the

Illinois Basin area previously has been studied regionally,

including evaluation of the detrital mineralogy and reservoir

quality (Odom and others, 1979; Hoholick and others, 1984);

however, potential mechanisms of diagenesis and the timing

of diagenesis have received little attention until recently

(Pitman and Spöetl, 1996). The diagenetic minerals preserved in Ordovician sandstones are indicators of the types

of fluids that migrated through the basin. Therefore, knowledge of the age and origin of the authigenic assemblage and

its regional distribution pattern places constraints on the

nature and timing of diagenetic fluids and the extent of fluid

migration in the basin. The origin and timing of mineral

alterations also provide insight into the role of hydrothermal

ore-forming fluids in porosity reduction and enhancement

and the timing of petroleum migration and entrapment.

GEOLOGIC AND TECTONIC HISTORY

The Illinois Basin is an elongate structure whose

north-south-trending axis lies immediately west of the

LaSalle anticline in Illinois (fig. 1). The basin is bounded on

the northwest by the Mississippi River arch, on the northeast

by the Kankakee arch, and on the south by the Cincinnati

arch, the Pascola arch, and the Ozark uplift. Basin strata

extend across Illinois into western Indiana and western Kentucky. The deepest part of the basin is situated near the

Rough Creek graben in western Kentucky where more than

7,000 m of sediment are preserved. Overall, the stratigraphy,

structure, and tectonic history of the basin are well known

because the basin has been extensively explored for hydrocarbon resources and is adjacent to MVT deposits.

The Illinois Basin initially was a broad intracratonic

embayment that formed as a result of rifting in the early Paleozoic (Burke and Dewey, 1973; Ervin and McGinnis, 1975).

From Late Cambrian through the Middle Ordovician, sediment accumulated in the depocenter of the basin overlying

the rift complex, which encompassed the intersection of the

Reelfoot rift and the Rough Creek graben (fig. 1). During the

remainder of the Paleozoic, the basin experienced multiple

1U.S. Geological Survey, Mail Stop 939, Box 25046, Denver, Colorado 80225. 2U.S. Geological Survey, Mail Stop 973, Box 25046, Denver, Colorado 80225. 3Institut für Geologie und Paläontologie, Universität Innsbruck,

Innrain 52, 6020 Innsbruck, Austria.
A2 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

ILLINOIS

BASIN

MICHIGAN

BASIN

Arkoma Basin

Ouachita fold belt Black

Warrior

Basin Appalachian

Mountains

Mississippi Reelfoot

Embayment

Pascola arch

Rough Creek

graben

Fairfield

subbasin Ozark

dome

Kankakee arch

Wisconsin arch

LaSalle anticline

2

9

3

4

5 6 7

10 11

8

 ORE DISTRICT

1 Upper Mississippi Valley

2 Tri State

3 Central MO Pb/Zn

4 Barite

5 Viburnum Trend

6 Old Lead Belt

7 IL/KY Fluorspar

8 Central KY Pb/Zn

9 North AK Pb/Zn

10 Central TN Pb/Zn

11 East TN Pb/Zn

1

rift

0 100 Kilometers

Illinois

Missouri

Indiana

Kentucky

Arkansas Tennessee

1 00

200

200

20 0

100

0

3 00

100

0







Outcrop >300 ft A B

periods of tectonic deformation in response to the Taconic,

Acadian, and Ouachita orogenies. In the Mississippian and

Pennsylvanian, uplift of the LaSalle anticline occurred across

east-central Illinois causing the St. Peter Sandstone in that

area to be exposed and folded (Kolata and Nelson, 1991).

Later, in Pennsylvanian and Permian time during the Ouachita orogeny, compressional stresses near the intersection of

the Reelfoot rift and Rough Creek graben led to extensive

folding, faulting, and igneous intrusion at the southern margin of the basin (Nelson, 1991). Major ore-forming events

related to MVT mineralization (Plumlee and others, 1995)

also occurred on the southern tectonic flank of the basin and

were associated with Ouachita tectonism in the Permian (270

Ma; Pan and others, 1990; Snee and Hayes, 1992; Chesley

and others, 1994). Isotopic dating indicates the MVT mineralization at the north end of the basin also occurred in the

Permian (Brannon and others, 1993). The Illinois Basin

attained its present structural configuration following the

uplift of the Pascola arch in the late Paleozoic and early

Mesozoic (Kolata and Nelson, 1991). The arch is now covered by Late Cretaceous and Early Tertiary siliciclastics of

the Mississippi Embayment (Kolata and Nelson, 1991).

DEPOSITIONAL FRAMEWORK

The St. Peter Sandstone is a laterally continuous unit

that extends across the entire Illinois Basin, reflecting the

broad, relatively flat topography that existed during the

Ordovician in the southern Midcontinent (fig. 2). The

sequence thins southward, and varies from about 10 to 200 m

thick, although typically it averages 30–60 m thick (Willman

and others, 1975). Within the basin, Ordovician sandstones

are buried at depths less than 1,000 m at the shallow northern

margin in the vicinity of the Upper Mississippi Valley

(UMV) ore district and more than 2.5 km in the Fairfield

basin, a major subbasin in southern Illinois. In the Fairfield

subbasin, the St. Peter Sandstone unconformably overlies the

Everton Dolomite and underlies the Joachim Dolomite and

Dutchtown Limestone; in northern Illinois, the St. Peter

unconformably rests on the Prairie du Chien Group (Willman

and others, 1975; see fig. 2). Within the Illinois Basin proper,

the St. Peter comprises three members. The basal Kress

Member locally contains chert conglomerate in a sandstone

matrix. In some areas it consists of red and green shale and

argillaceous sandstone (Buschbach, 1964). The middle Tonti

Member generally forms the main body of the St. Peter and,

except in western Illinois, is composed of fine-grained,

well-sorted, porous and permeable sandstone (Templeton

and Willman, 1963). The upper Starved Rock Member is

mainly a well-sorted, medium-grained sandstone. This sandstone sequence, interpreted to be an offshore bar deposit

(Willman and others, 1975), is confined to a belt trending

northeast-southwest across the basin.

METHODS

Samples representative of the St. Peter Sandstone were

collected from 113 wells throughout the Illinois Basin in

Figure 1. A, Location of the Illinois and Michigan Basins, major structural features, and principal ore-forming districts in the Midcontinent region. B, Isopach map (modified from Kolata, 1991) showing regional thickness variations in the St. Peter Sandstone in

the Illinois Basin (contour interval in feet).
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A3

S top of Tippecanoe Sequence N

0 50 km

300 m

0

Backbone Limestone

Grassy Knob Chert - Bailey Limestone

St Clair Limestone

Moccasin Springs Formation

Sexton Creek

Limestone

Maquoketa Shale Group

Racine Formation

Galena Group

Platteville Group

top of Sauk Sequence

Joachim

Dolomite

Dutchtown

Everton

Dolomite

Clear Creek Chert

Kankakee Formation

Pitman

Figure 2

St. Peter Ss

IL

IN

Limestone

Illinois and Indiana. The sample suite covers the range of

present subsurface depths, ~50–3,300 m, and spans the

thickness of the formation. The deepest samples (at ~ 3,300

m) are from the Fairfield subbasin. Individual sandstone

samples were studied using a combination of techniques,

including optical petrography, scanning electron microscopy

(SEM), cathodoluminescence (CL) microscopy, electron

microprobe analysis, fluid-inclusion microthermometry, and

stable-isotope analysis.

Standard thin sections of sandstones were impregnated

with blue-dye epoxy to identify porosity and stained to facilitate mineral identification. Staining with Alizarin red-S and

potassium ferricyanide permitted Fe-free carbonate minerals

to be distinguished optically from Fe-bearing carbonate minerals, and sodium cobaltinitrite stain aided in the identification of K-feldspar. Detailed point counts (300 counts per

section) were performed on representative sandstones in

each well to compare with previous studies; relative mineral

abundances were determined visually in the remainder of the

sandstones. CL microscopy was employed to qualitatively

evaluate carbonate paragenesis in carbonate-bearing samples. A cold-cathode Technosyn 8200 Mk II model operating

at ~20 kV, 500–600 mA and ~7 Pa pressure was used in the

analysis. SEM was also routinely employed to identify mineral phases in back-scattered-electron mode and using an

energy-dispersive X-ray analytical system. In addition,

growth relationships of individual cements were studied on

freshly broken sandstones in secondary-electron mode.

Fluid-inclusion-microthermometric measurements were performed on doubly polished wafers of a few selected samples

using a U.S. Geological Survey-type gas-flow-heating/freezing system calibrated against known melting point standards

(analyst, T.J. Reynolds, Fluid Inc., Englewood, Colo.).

Homogenization temperatures (Th) were not pressure corrected. Representative samples of calcite and morphologically distinct dolomite (dolomicrospar, planar dolospar,

baroque dolospar) were analyzed isotopically in the Branch

of Petroleum Geology (analyst, Augusta Warden), U.S. Geological Survey, Denver, Colo.. Carbon and oxygen isotope

ratios were obtained by a timed-dissolution procedure based

on different reaction rates for chemically distinct carbonate

phases (i.e., calcite vs. dolomite; Walters and others, 1972).

To prevent contamination by CO2 from organic matter during acid digestion, kerogen was isolated from the samples.

Upon reaction with phosphoric acid, CO2 gas evolved in the

first hour was attributed to calcite and CO2 gas evolved after

several hours was assigned to dolomite. Isotope analyses on

evaporite sulfur was carried out by Global Geochemistry

Corp. using standard techniques. Sulfide was dissolved in

acid, reprecipitated as BaSO4 and combusted to produce SO2

gas. All carbon and oxygen isotope results are reported as the

per mill (‰) difference relative to the Peedee belemnite

(PDB) standard using the delta (δ) notation. Sulfur isotope

data are reported relative to the Canyon Diablo Troilite standard. Data reproducibility is precise to ± 0.2 ‰.

DETRITAL MINERALOGY

Detrital framework constituents in the St. Peter Sandstone consist of quartz and minor feldspar, lithic fragments,

and matrix material (Dapples, 1955; Odom and others, 1979;

Hoholick and others, 1984). Overall, the sandstones are texturally and compositionally mature, and moderately to well

sorted. Based on the classification scheme of Folk (1974),

Figure 2. North-south cross section of lower Middle Ordovician (Whiterockian) through Lower Devonian (Ulsterian) rocks in Illinois

(Tippecanoe sequence of Kolata, 1991).
A4 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

the St. Peter Sandstone comprises predominantly quartzarenites and subarkoses.

Quartz constitutes the major proportion of the detrital

fraction in the sandstones. Most grains are monocrystalline

fragments that are subrounded to well rounded in shape and

fine to medium grained in size. Grain contacts generally are

concave to convex, although long and sutured contacts are

common in some more deeply buried sandstones. Secondary

overgrowths tend to be minor and are best developed in sandstones in the deep, southern part of the basin.

Detrital feldspar is a common constituent in shallow-buried sandstones in the northern part of the basin. On

the basis of (bright yellow) staining by sodium cobaltinitrite,

most feldspar is potassium rich, although rare sodic varieties

are present locally. Potassium feldspar is distributed as

twinned and untwinned fragments that vary in their morphology and degree of alteration. In the northern part of the basin,

K-feldspar comprises subangular to subrounded, silt to very

fine sand-size grains that commonly display optically continuous overgrowths. Along the LaSalle anticline, K-feldspar

grains and grain overgrowths show the effects of dissolution.

In more deeply buried sandstones to the south, detrital

K-feldspar grains and authigenic overgrowths are rare except

in dolomicrospar where tiny K-feldspar grains devoid of

overgrowths are abundant.

Other constituents in the sandstones include illitic

matrix and lithic grains, mainly finely crystalline chert, polycrystalline quartz, and granitic fragments. Matrix material,

which is present in varying amounts, has been replaced by

authigenic K-feldspar, clay minerals, and carbonate.

SEDIMENTARY DIAGENESIS

MINERAL AUTHIGENESIS

The St. Peter Sandstone displays a variety of diagenetic

minerals, most notably multiple generations of carbonate

cement, potassium feldspar and quartz overgrowths, anhydrite, and illite. The relative timing of these diagenetic

events is depicted in figure 3. Other authigenic minerals in

the sandstones include minor barite, hematite, illite, chlorite

and kaolinite, and traces of sphalerite and fluorite. Mechanical compaction, chemical compaction (including styolitization and pressure solution), and dissolution textures are

features common in some parts of the St. Peter. Spatial variations in authigenic mineral abundance and reservoir quality

are shown in figure 4. It is noteworthy that mineral abundances vary on a basin-wide scale and that differences in

mineral content can be found over short intervals within

individual wells. Overall, the distribution of cements is similar to previously reported authigenic mineral patterns

(Hoholick and others, 1984), although locally there are some

differences in the occurrence of K-feldspar, quartz, and

calcite compared to earlier work (see fig. 4). In the southern

part of the basin, there is a broad geographic zonation of

late-diagenetic quartz, dolomite, calcite, and anhydrite.

Authigenic mineral development is more restricted in the

shallow, northern part of the basin. Potassium feldspar and

illite are geographically widespread, whereas dolomite tends

to be localized. Primary and secondary porosity is well preserved in the northern portion of the basin but is not uniformly distributed.

The range in carbon and oxygen isotopic composition of

individual carbonate phases in the St. Peter is illustrated in

figure 5 and the basinal distribution pattern of oxygen-isotope ratios of dolomite cements is shown in figure 6. Figure

5 reveals that there is significant compositional overlap

between various carbonate phases. In figure 6, regional variations in the δ18O of dolomite, in part, correspond to major

structural trends in the basin. The stable-isotope geochemistry of authigenic carbonate (i.e., dolomite) together with the

carbonate-mineral paragenesis provide insight into the processes and possible fluid sources responsible for burial dolomitization. In the following sections, the petrographic

characteristics, relative chronology, and geographic distribution of the main diagenetic constituents are discussed, and

stable isotopic compositions together with limited microthermometric data are presented.

CARBONATE CEMENTS

On the basis of optical properties and crystal size, four

major varieties of authigenic carbonate—dolomicrospar, planar dolospar, baroque dolospar, and poikilotopic calcite—

have been identified in the St. Peter Sandstone. Variations in

total dolospar abundance and calcite contents are shown in

figure 4. Finely crystalline (4–25 μm) dolomicrospar is a

locally abundant cement (2–10 percent, locally 20 percent by

volume) in the northern and central portions of the basin.

Minor dolomicrite (crystal size ≤ 4 μm) also was observed.

Textural evidence indicates that dolomicrospar formed early,

before most other mineral cements; however, dolomicrospar

samples commonly exhibit clear-cut evidence of recrystallization, indicating that this early carbonate phase underwent

diagenetic alteration. Dolomicrospar is composed of individual planar crystals and, less commonly, interlocking nonplanar crystals of nonferroan dolomite. Some dolomicrospar

samples reveal distinct microfacies features, including dark,

micritic, rounded components and patches, and peloidal and

mottled textures. Other samples show rare elongate features

that strongly resemble root casts known from calcretes and

related carbonates (e.g., Esteban and Klappa, 1983; Wright,

1990). Silt-size, subrounded, detrital K-feldspar crystals

devoid of authigenic overgrowths commonly are inclusions

within dolomicrospar. Pyrite, distributed as individual cubes

(5–30 μm), or as clusters of euhedral crystals, are associated

with the mineral. Stable isotopic compositions of
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A5

Mechanical compaction

Pressure solution

Illite

Hematite coatings

Quartz overgrowths

Pyrite

K-feldspar overgrowths

Dolomicrospar

Planar dolospar

Baroque dolospar

Calcite

Fracturing

Carbonate dissolution

Anhydrite

Chlorite

Bitumen

*

Late

*

*

recryst.

Pitman

Figure 3

dolomicrospar (45 analyses) are depleted in δ13C (–0.5 to

–9.2 ‰ PDB) and δ18O ratios (–1.4 to –6.8 ‰ PDB) (fig. 5).

The lightest δ18O ratios are in the southern part of the Michigan Basin and along the north-south-trending LaSalle anticline (see fig. 6).

A large fraction of dolomite (2–10 percent, locally

15–20 percent) in the central and southern parts of the Illinois Basin and a small component of dolomite in the northern portion of the Illinois Basin and southern part of the

Michigan Basin is planar dolospar (referred to as “planar-e”

by Sibley and Gregg, 1987). In well-cemented sandstones,

planar dolospar postdates mechanical compaction and comprises equant crystals that form an interlocking mosaic with

little or no intercrystalline porosity. Some planar dolospar

locally contains abundant solid inclusions, giving rise to a

cloudy appearance in transmitted light. Rare detrital K-feldspar grains and authigenic pyrite also may be included

within planar dolospar. In many samples, there is evidence

that a significant fraction of planar dolospar is a replacement

of dolomicrospar. Coarse (20–300 μm) euhedral crystals of

planar dolospar commonly float in a groundmass of dolomicrospar, which suggests that planar dolospar grew at the

expense of dolomicrospar. Other samples display euhedral,

inclusion-poor planar dolospar crystals protruding into open

pores, which clearly indicate they are neoformed. In the

southern part of the basin, planar dolospar coexists with

authigenic quartz, calcite, and anhydrite and may be overgrown by a later generation of ferroan dolomite. Stable δ13C

and δ18O isotope ratios of 24 samples of planar dolospar fall

between –6.5 to –0.9 ‰ (δ13C) and –9.2 to –1.2 ‰ (δ18O)

and are similar to the values that characterize dolomicrospar

and baroque dolospar (fig. 5). This range in the data in part

arises from mixing of end-member carbonate phases in

varying proportions. Light δ18O ratios of planar dolospar

occur in the southern Michigan Basin, and δ18O-depleted

mixtures of planar and baroque dolospar are found along the

LaSalle anticline and in the Fairfield subbasin, close to

Hicks dome and the Illinois-Kentucky Fluorspar district

(fig. 6).

A small fraction of dolomite (< 5 percent ) in sandstones predominantly from the southern portion of the Illinois Basin (Fairfield subbasin) has optical properties

characteristic of baroque (or saddle) dolospar (equivalent to

“nonplanar dolomite” of Sibley and Gregg, 1987). Crystals

are coarsely crystalline, have slightly to moderately curved

crystal faces, and display broad sweeping extinction.

Locally, saddle dolomite forms ferroan overgrowths on

planar dolospar crystals. The development of sweeping

extinction seems to be unrelated to the formation of ferroan

overgrowths because incipient sweeping extinction also

occurs in some of the larger planar dolospar crystals. In

addition, there is no correlation between the development of

sweeping extinction and the stoichiometry of baroque

dolospar. Petrographic observations show that baroque

dolospar postdated quartz overgrowths and predated anhydrite, which indicates that it formed during deep-burial

diagenesis. The isotope ratios of 7 baroque dolospar samples, –7.5 to –3.4 ‰ (δ13C) and 7.5 to –4.6 ‰ (δ18O), span

the range of values for planar dolospar and represent mixtures of planar dolospar cores and ferroan baroque dolomite

rims (fig. 5). A few samples of baroque dolospar from

northern Illinois contain two-phase fluid inclusions. In one

Figure 3. Mineral paragenesis in the St. Peter Sandstone. Diagenetic phases marked by

an asterisk are largely confined to the northern part of the basin; relative thickness of bar

reflects significance of diagenetic event. Recryst., recrystallized.
A6 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

sample, very small (< 3 μm) primary fluid inclusions

located at the boundary between planar dolospar and

baroque dolospar overgrowths displayed consistent liquid-to-vapor ratios and yielded homogenization temperatures from 110°–115°C. However, most of the inclusions on

growth features in planar and baroque dolospar had significantly lower temperatures, ~65°C. Final ice melting temperatures of all inclusions were consistently less than –20°C,

which suggests formation from highly saline pore waters

with compositions > 20 weight percent NaCl equivalent.

Hydrocarbon-bearing fluid inclusions were not observed in

the samples studied.

Calcite in the St. Peter Sandstone occurs in minor

amounts (< 5 percent by volume average) in the southwest

part of the basin (fig. 4). The mineral is distributed as a

poikilotopic, nonferroan to slight ferroan, pore-filling

cement; less commonly, it occurs as a framework grain

replacement. Locally, calcite shows evidence of leaching.

Figure 4 (above and facing page). Regional distribution of authigenic mineral cements and porosity in the St. Peter

Sandstone. A, Illite; B, K-feldspar; C, Secondary quartz; D, Total dolomite; E, Anhydrite; F, Calcite; G, Secondary

porosity. Mineral abundances reflect combined early- and late-diagenetic phases.

1.0

0.5

1.0

1.0

2.0

1.0

1.5

1.0

2.0

2.0

0.5

0.5

0.5 0.5

1.0

0.5

1.0

1.5

1.5

1.0

1.0

1.5

1.5

1.5

2.0

2.0

0.5

0.5

0.5

0.5

0.5

0.5

1.0

1.0

3.0

1.0

1.0

2.0

1.0

Illite K-Feldspar

Secondary

Quartz

Total

Dolomite

A B

C D

1.0

1.0

1.5
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A7

In some sandstones, calcite contains trace amounts of

micrometer-size fluorite and sphalerite. It is noteworthy

that this fluorite- and sphalerite-bearing calcite occurs in

areas where trace fluorine anomalies in carbonate rocks are

highest (Rowan and Goldhaber, 1996). Calcite in sandstones postdates quartz cement as well as baroque dolospar

and other authigenic mineral phases; thus, it appears to be

one of the latest cements to have formed in the St. Peter.

Carbon and oxygen isotopes of 19 calcite samples vary

from –3.1 to –6.6 ‰ (δ13C) and –6.5 to –10.6 ‰ (δ18O),

respectively. The δ18O ratios of calcite tend to be lighter

than the δ18O ratios of burial dolospar cements, whereas

the δ13C ratios of calcite are within the range of dolomite

cements (fig. 5).

SECONDARY QUARTZ

Authigenic quartz, formed as syntaxial overgrowths on

detrital grains, occurs in minor amounts (< 5 percent by

volume average), predominantly in the southern part of the

basin (fig. 4). The majority of sandstones examined lacked

3.0

2.0

3.0

0.5

0.5

1.0

1.5

1.0

0.5

1.5

3.5

3.0

3.0

2.5

3.0

2.5

2.5

3.5

E F

G

Anhydrite Calcite

Secondary Porosity Explanation

3.0

Contour value

 Contour value 

 1 

 2 

 3 

 4 

 0 100 200 KILOMETERS

 0-1

 2-10

 11-25

>25

Mineral abundance or 

porosity (in percent)
A8 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

-12 -10 -8 -6 -4 -2 0

-10

-8

-6

-4

-2

0

δ 18O (‰, PDB)

Calcite

Planar

dolospar δ 13 C (‰, PDB)

Dolomicrospar

Baroque

dolospar

Pitman

Figure 7

2

Marine

dolomite

Omaha

dome

Hicks

dome

MICHIGAN

BASIN

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞ ➞

➞ ➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞ ➞

➞

➞

➞

➞

➞

➞

➞ ➞

➞

➞ ➞

➞

➞ ➞

➞ ➞

➞ ➞

LaSalle

anticline

-3.5

-4.7

-5.9

-3.4

-3.5

-5.1

-3.1

-2.5

-1.7 -2.6

-3.5 -3.8

-5.1

-2.4

Ozark

dome

 Fairfield

subbasin

Mississippi River arch

Sangamon

arch

Wisconsin

arch

Kankakee arch

Cincinnati arch

ILLINOIS

BASIN

Mississippi

Valley

district

Rough Creek graben

-3

-3

-3

-5

-5

-3.2

-3.4

-3.7

-2.8

-2.4

-3

A

Anticline

Syncline

Monocline

➞

➞

➞ ➞ ➞

Fault; ticks on downthrown side

EXPLANATION

Cryptoexplosive or impact structure

Core hole

Isocontour (δ O) 18 -6

-5.2

Ozark

dome

Fairfield

subbasin

Mississippi River arch

Sangamon

arch

Wisocnsin

arch

Kankakee arch

Cincinnati arch

Omaha

dome

Hicks

dome

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞ ➞

➞ ➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞

➞ ➞

➞

➞

➞

➞

➞

➞

➞ ➞

➞

➞ ➞

➞

➞ ➞

➞ ➞

➞ ➞

LaSalle

anticline

-7.0

-3.8

-4.4 -4.6

-2.6 -4.3

-2.4

-6.5

-8.9

MICHIGAN

BASIN

ILLINOIS

BASIN

Mississippi

Valley

district

Rough Creek graben

-6.6

-8

-9.2

-6.1

-6.4

-6

-4

-6

-4

B

authigenic quartz or displayed incipient and incomplete

overgrowths; however, the deeper portions of some wells

(below ~1,500 m) contain abundant quartz cement (as much

as 20 percent). Textural relations indicate that quartz formation postdated K-feldspar authigenesis but predated burial

dolospar precipitation. Sandstones that exhibit porosity

enhancement often contain quartz overgrowths that are corroded or have residual voids with partially dissolved mineral

cements. Quartz cement also may occur immediately adjacent to millimeter- to centimeter-thick zones showing extensive pressure solution, which suggests that at least some

silica was sourced from chemical compaction of framework

grains. Primary fluid inclusions in quartz overgrowths from

one well in the Fairfield subbasin yielded homogenization

temperatures of 90°–110°C. Fluid compositions based on ice

melting temperatures are highly saline, ~15 weight percent

NaCl equivalent. No hydrocarbon-bearing fluid inclusions

were observed.

AUTHIGENIC POTASSIUM FELDSPAR

Authigenic K-feldspar is a common but volumetrically

minor constituent (< 5 percent by volume average) in the

northern part of the basin and along the LaSalle anticline in

east-central Illinois (fig. 4). K-feldspar occurs as clear,

euhedral, non-luminescent overgrowths on subrounded,

fine-grained detrital cores that luminesce bright blue. Where

Figure 5. Stable isotopic composition of carbonate cements in

the St. Peter Sandstone. Data used to construct the diagram are reported in Pitman and Spöetl (1996). The approximate composition

of dolomite in isotopic equilibrium with Ordovician seawater is

taken from Lohmann and Walker (1989).

Figure 6. Regional variations in the mean oxygen-isotope compositions of dolomite in the St. Peter Sandstone. A, δ18O ratios in dolomicrospar. B, δ18O ratios in planar and baroque dolospar. Note

similarity in isotopic pattern of dolospars on a basin-wide scale. Major structural features taken from Nelson (1991).
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A9

abundant, the overgrowths generally are untwinned and

continuous around detrital grains. Authigenic K-feldspar

also occurs as rhombs, relict cement, and subhedral to anhedral aggregates replacing illitic matrix. The cement and

aggregate crystals commonly lack detrital cores (cf. Odom

and others, 1979). Based on electron microprobe analysis, no

distinct compositional differences exist between grain and

feldspathitized overgrowths, which suggests that the detrital

cores may have been diagenetically altered at the time of

overgrowth precipitation. K-feldspar overgrowths predate

planar dolospar and appear to be as early as dolomicrospar.

In wells along the LaSalle anticline, K-feldspar overgrowths

often display a skeletal morphology indicative of dissolution, whereas the detrital cores are unaltered. In the same

samples, K-feldspar cement is distributed as patches, suggesting that it also might be dissolution controlled.

CLAY MINERALS

Authigenic clay minerals in the St. Peter Sandstone

comprise minor illite (as much as a few percent, locally as

high as 10 percent; fig. 4) and rare kaolinite. Illitic clay minerals comprise two generations, an early grain-rimming

illite, and a later stage fibrous-filamentous illite. The micromorphology of these two generations suggests they are both

probably illite/smectite mixed-layer clays. Illite grain coats

are composed of webby to flakey growths oriented tangential

to framework grain surfaces. In the northern and central portions of the basin, early illite is ubiquitous and is commonly

distributed between framework grain contacts—this indicates that it formed before mechanical compaction and

quartz overgrowths. Late-stage fibrous illite, in contrast,

tends to be scarce and occurs as delicate lath-shaped projections and as pore bridges. Locally, it fills secondary intergranular pores caused by carbonate dissolution, which

suggests a close association between carbonate leaching and

illite precipitation. In some sandstones, pressure solution is

highly variable (even on a millimeter scale) and clearly was

promoted by illitic grain coatings (detrital and authigenic) as

well as finer grain sizes of detrital constituents (Stackelberg,

1987).

Minor diagenetic kaolinite, distributed as randomly oriented, euhedral, pseudohexagonal crystals, occurs as a pore

fill in sandstones near recharge zones in the northern part of

the basin. The origin of the voids occupied by kaolinite is

uncertain, although textural observations suggest that primary as well as secondary pores contain the mineral.

ANHYDRITE

Anhydrite is a late diagenetic mineral phase that occurs

sparsely in the Fairfield subbasin (fig. 4), although textural

features in sandstones suggest it was formerly more widespread (T. Shaw, Unocal Corp., 1994, oral commun.). The

mode of occurrence of anhydrite ranges from small scattered

patches to large euhedral crystals that preferentially replace

mineral cements, including planar dolospar, baroque

dolospar, and quartz. Anhydrite also replaces calcite,

although, in most samples, the paragenesis of anhydrite and

calcite is ambiguous.

OTHER PHASES

Miscellaneous constituents in sandstones consist of

finely crystalline iron oxide, rare lithic fragments of chert,

polycrystalline quartz, granite, and matrix material.

Iron-oxide cement (hematite) forms a brownish-red matrix

that predates quartz in some sandstones near the northern

edge of the basin. Matrix often shows the effects of replacement by authigenic clay and K-feldspar. Solid bitumen is

observed in a few samples in the deeper part of the basin and

is intergrown with ferroan planar dolospar cement.

MINERAL DISSOLUTION

The amount and distribution of porosity (primary and

secondary) in the St. Peter Sandstone varies regionally and

with decreasing burial from south to north across the basin

(fig. 4) (see also Hoholick and others, 1984). In the northern

part of the basin, sandstones buried to a shallow depth contain large, well-developed intergranular and intragranular

pores that locally exceed 20 percent by volume of the rock.

Porosity loss due to compaction generally is minor. Two

types of secondary porosity, leached framework grain overgrowths and partly dissolved diagenetic cements, are

observed in the sandstones. Dissolution of framework grains

mainly affected authigenic overgrowths on detrital K-feldspar. Several stages ranging from minor corrosion to complete dissolution (i.e., moldic pores) are observed, but, in

most samples, K-feldspar overgrowths show only partial

leaching. Sandstones that contain leached K-feldspar overgrowths commonly display K-feldspar cement distributed as

patches, suggesting its occurrence also might be dissolution

controlled. Secondary pores devoid of cement, or that contain remnant carbonate grains with etched serrated boundaries, also occur. Textural features suggest that some of the

preserved porosity probably is of primary origin, although

the exact amount is difficult to quantify. Quartz grains in

sandstones near the outcrop often contain well-developed

euhedral overgrowths that project into pores, which strongly

suggests that the porosity is primary. However, in porous

sandstones, it is difficult to determine if pore-fill cement was

dissolved or if cementation never occurred. Thus, it is
A10 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

possible that the amount of primary porosity may be volumetrically greater than can be documented texturally.

In the southern part of the basin, more deeply buried

sandstones display substantial porosity reduction due to

mechanical compaction and mineral cementation; porosity

varies from 0 to 10 percent and averages 5 percent. At depths

of ~2,000 m, preserved porosity is exclusively of secondary

origin caused by the dissolution of preexisting cement,

mainly dolospar and possibly calcite. In well-cemented sandstones at these depths, high pre-cement porosities (20 percent

average) indicate that only minor to moderate amounts of

mechanical compaction occurred prior to emplacement of

authigenic cement. Burial cements, principally dolospar and

quartz, and minor calcite and anhydrite occlude intergranular

pores, which accounts for the substantial reduction in porosity. Partly dissolved carbonate cement and evidence for

repeated episodes of carbonate dissolution occur locally in

some of the deeper sandstones, although the exact timing of

these dissolution events is not well constrained petrographically. In dolospar-cemented samples, solution features such

as discontinuous (corrosive) contacts between successive

carbonate phases and relict cement in pores are common,

suggesting that carbonate dissolution, burial dolomitization,

and calcite precipitation were closely spaced diagenetic

events.

TIME AND TEMPERATURE OF MAJOR 

DIAGENETIC EVENTS

The generalized paragenetic sequence of cementation

and dissolution events that affected the St. Peter Sandstone is

shown integrated with a decompacted burial curve constructed for the Fairfield subbasin in southern Illinois (fig. 7).

Data and assumptions used to generate the curve (i.e.,

heat-flow conditions and stratigraphic configurations) are

reported in Pitman and Spöetl (1996). The curve depicts periods of subsidence, changing subsidence rates, uplift and erosion, maximum burial depth, and paleothermal conditions.

Radiometric ages of early-authigenic K-feldspar, and earlyand late-diagenetic illite in Ordovician sandstones and

K-bentonites adjacent to the basin (authigenic minerals

within the Illinois Basin have not been dated) were used to

constrain the timing and depths of formation of burial-related

quartz, dolospar, and anhydrite. The age of MVT mineralization discussed above (~270 Ma, Early Permian) is shown on

the curve for comparison. In the southern part of the basin,

burial reconstruction indicates that the St. Peter was more

deeply buried and experienced higher temperatures in the

past. According to the reconstruction, the St. Peter attained

its maximum burial depth and temperature (3,300 m and

140°C) in the Late Pennsylvanian and Early Permian when

igneous activity and major ore-forming events were taking

place in the region.

The mineral paragenesis in the context of burial reconstruction can be deduced from isotopic age dates and suggests a multistage precipitation history that commenced after

the emplacement of authigenic K-feldspar and continued

through the formation of late-stage illite and calcite. Radiometric dates reported for authigenic K-feldspar in Ordovician strata consistently yield Devonian ages that average

~400 Ma with a large uncertainty (Krueger and Woodard,

1972; Marshall and others, 1986; Elliott and Aronson, 1987,

1993; Barnes and others, 1992; Hay and others, 1988; Lee

and Aronson, 1991; Hay and Liu, 1994). The ages suggest

that feldspar authigenesis in the St. Peter, assuming it is

coeval with the dated feldspars, took place during early

diagenesis at shallow to moderate burial depths (< 1,500 m;

see fig. 7).

The timing of illite precipitation in the St. Peter is more

difficult to constrain because the range of ages that has been

reported (360–215 Ma) varies widely (Hay and others, 1988;

Lee and Aronson, 1991; Elliott and Aronson, 1993). In the

Mississippi Valley area, two episodes of illitization have

been reported for Middle Ordovician bentonites: an early

Devonian–Mississippian episode dated at ~360 Ma north of

the Illinois Basin and a later Permian event on the west edge

and the east edge of the basin dated at ~265 Ma (Hay and

others, 1988; Lee and Aronson, 1991). In the UMV district,

illite-rich clays in the upper part of the St. Peter Sandstone

yield Mississippian ages of ~340 Ma (Lee and Aronson,

1991). This illite however, has been dated at ~310 Ma (Pennsylvanian) in the most mineralized part of the district and is

as young as 230–215 Ma (Triassic) on the Wisconsin arch

(Lee and Aronson, 1991). There is some evidence indicating

that early illite (~375–351 Ma) in the northern part of the

region comprises detrital and authigenic clay intergrowths

(R. Hay, Univ. of Illinois, 1995, oral commun.). If so, the

previously reported ages for this bulk illite may reflect the

older detrital value, in which case the actual ages of authigenic illite may be closer to those of the younger illites in the

region. In the St. Peter Sandstone, textural evidence indicates

two generations of illite—an early grain-rimming phase that

predated the growth of authigenic K-feldspar and a late

pore-fill phase that postdated the formation of quartz, burial

dolospar, and anhydrite. The early illite phase is more prevalent in the northern portion of the basin, as noted above,

although its age is uncertain. In the southern part of the basin,

late stage pore-fill illite is assumed to have formed in the late

Paleozoic at about the same time as the illites dated at 265

Ma or younger (see fig. 7). If so, then according to the

burial-history model, late diagenetic quartz, dolospar, calcite, and anhydrite, which are bracketed in time by early

authigenic K-feldspar and late-stage illite, also formed during the late Paleozoic after the rocks had undergone

significant burial (fig. 7). It is noteworthy that the age for

these burial diagenetic events is consistent with the timing of

main-stage ore formation in both the Illinois-Kentucky
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A11

pC OS M P P TJ K Pa/

E O M 0

2

4

6

8

10

12

14

16

600 500 400 300 200 100 0

DEPTH (feet x1000)

AGE (Ma)

C D R

1

2

3

4

25°C

50°C

100°C

125°C

150°C

Illite

MVT Event

K-spar

DEPTH (meters x1000)

75°C

 Quartz (T =90-110 h °C)

Illite Dolospar

Anhydrite

Calcite

Illite

Fluorspar district south of the basin and the UMV district

north of the basin.

There is some uncertainty about the age of calcite in

the St. Peter because the relationship between isotopically

dated calcites and calcite cements in sandstones is unknown.

In the UMV district, U-Pb systematics indicate that late

vein-filling calcite in Ordovician carbonate rocks (post

main-stage ore formation) formed at ~162 Ma (Middle

Jurassic), approximately 100 m.y. after MVT mineralization

(Brannon and others, 1993). There is presently no way to

correlate the age of this vein-fill calcite and the calcite

cement in the St. Peter. Moreover, there is no direct evidence that links calcite precipitation in the St. Peter to

vein-calcite development in superjacent carbonate strata.

However, localized micrometer-size fluorite and sphalerite

inclusions in calcite cement in the St. Peter (see earlier discussion) are consistent with calcite precipitation from fluids

enriched in ore-related constituents, suggesting that the minimum age of sphalerite/fluorite-bearing calcite is Permian,

which is in accord with the burial-history model.

In southern and central Illinois, solid bitumen intimately associated with (ferroan) planar dolospar and

baroque dolospar suggests that precipitation of late-diagenetic carbonate was closely related to migration of

hydrocarbons into reservoir sandstones. The time of hydrocarbon migration within the St. Peter is tentative; however,

hydrocarbon transport (and regional brine migration) presumably occurred during the Permian, assuming that the

Pennsylvanian New Albany Shale was the primary source of

oil (Barrows and Cluff, 1984).

Several lines of evidence indicate that burial diagenesis

in the St. Peter Sandstone occurred when the unit was at or

close to maximum burial in the Permian. Organic-maturation

kinetics (Sweeney and Burnham, 1989) were used in conjunction with the reconstructed burial history to model the

thermal conditions in the southern part of the Illinois Basin

(Pitman and Spöetl, 1996) (fig. 7). Predicted paleotemperatures (shown as isotherms on the burial curve) were calculated assuming that basement-derived (conductive) heat

flow was the major heat source during burial—the paleotemperatures do not take into account a potential heat source

related to advective heat transfer by migrating hydrothermal

fluids (e.g., Bethke, 1986; Garven and others, 1993; Rowan

and Goldhaber, 1996). Silicate-mineral age dates (i.e.,

K-feldspar and illite) in the context of the thermal history

suggest that burial diagenesis in the St. Peter occurred over

a temperature range of ~65°–140°C (see fig. 7). The minimum temperature of diagenesis, ~65°C, corresponds to the

Devonian age assigned to K-feldspar overgrowth precipitation; whereas the maximum (calculated) temperature of

diagenesis, 140°C, conforms to the timing of late-diagenetic

illite, regarded to be Permian. Fluid-inclusion homogenization temperatures indicate that secondary quartz precipitated

at temperatures as high as 110°C. Thus, dolospar (and anhydrite) postdating quartz overgrowths may have precipitated

at temperatures between 110°–140°C, assuming that they

formed in a burial regime governed by conductive heat flow.

There is some evidence that suggests the modeled temperatures shown in figure 7 may be minimum values. Radiometric age dates on ores and igneous rocks (Snee and

Hayes, 1992; Chesley and others, 1994; Brannon and others,

1993) indicate that hydrothermal fluids involved in ore formation were migrating through the Illinois Basin during the

Permian (~270 Ma). According to fluid-inclusion studies,

the temperatures of these fluids in the Fluorspar and UMV

districts (~75°–180°C; McLimans, 1977; Richardson and

others, 1988) approximate the range of burial temperatures

(~65°–140°C) estimated for the St. Peter. However, the Fluorspar district ores are hosted in Mississippian-age carbonate

rocks, which in Permian time were at shallower burial depths

(by about 1.2 km) than the St. Peter Sandstone. The actual

peak temperatures experienced by the St. Peter in the Fairfield subbasin thus might have been somewhat hotter than

the temperatures predicted by the burial model, assuming

that the sandstones were conduits for hydrothermal fluid

flow.

In northern Illinois, maximum burial depths and temperatures of the St. Peter (including the south flank of the

Michigan Basin) are estimated to have been ~1,000 m and

Figure 7. Burial-history curve of the St. Peter Sandstone showing timing of major diagenetic events in the Fairfield subbasin,

southern Illinois. The interval was modeled kinetically with BASINMODTM using the maturation kinetics of Sweeney and Burnham

(1989). Moderate erosion (1,200 m) and variable heat flow (570 to

100 Ma = 38 mW/m2; 100 Ma to present = 55 mW/m2) were assumed in the model. Isotopic ages of early authigenic K-feldspar

and early and late diagenetic illite (age range shown as dashed line)

together with homogenization temperatures of secondary quartz

constrain timing of burial diagenesis; age of lead-zinc-fluorine mineralization (MVT event) is shown for comparison. Note that most

diagenesis took place when rocks were near maximum burial (see

text for discussion).
A12 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

~50°C (Cluff and Byrnes, 1991; Rowan and Goldhaber,

1996 and included references). A few dolospar samples in

these shallowly buried sandstones, however, have light δ18O

ratios (see fig. 6) that correspond to temperatures exceeding

those of maximum burial (see below). In addition, some

dolospar crystals in this area have fluid-inclusion crystallization temperatures (110°–115°C) that fall within the range

reported for ore and gangue minerals in the UMV district

(~80°–180°C; McLimans, 1977) and that are considerably

higher than can be readily explained by burial alone. The

combined burial-history curve and isotopic age dates

depicted for the deep Fairfield subbasin (discussed above)

suggest that the hot temperatures experienced by the St.

Peter Sandstone in the southern part of the basin coincided

with maximum burial as well as a hydrothermal fluid-flow

event in the Permian. Although deep burial alone was capable of generating elevated temperatures (>100°C) in southern Illinois, this mechanism cannot explain temperatures in

excess of 100°C in northern Illinois. Hydrologic modeling

has shown that hydrothermal fluid flow was capable of producing the elevated temperatures observed in the northern

portion of the study area (Bethke, 1986; Garven and others,

1993; Rowan and Goldhaber, 1996). Thus, in the absence of

Paleozoic igneous activity, we conclude that the fluids

involved in the diagenesis of the St. Peter in northern Illinois were, in part, related to the Permian hydrothermal flow

event that was responsible for ore formation in the UMV

district.

EVOLUTION OF PORE FLUIDS

COMPOSITION OF PRECIPITATING WATERS

Isotopic analyses of cements in the St. Peter Sandstone

suggest that pore fluids with variable isotopic composition

and (or) elevated temperature controlled the nature and

extent of authigenic mineral precipitation during burial

diagenesis. The δ18O values of carbonate-precipitating pore

waters that existed during burial were estimated using the

dolomite-water fractionation equation of Fritz and Smith

(1970) and the calcite fractionation equation of Friedman and

O’Neil (1977). Temperatures assumed in the calculations

(65°–140°C) were extrapolated from the burial-thermal

curve (fig. 7), constrained by isotopic ages of the silicate

mineral phases, K-feldspar and illite. The compositional

range of the hypothetical pore waters is shown in figure 8.

Based on the calculations, the δ18O values of dolomitizing

water varied from –3.3 to +12.7 ‰ (SMOW—standard mean

ocean water) during the course of burial diagenesis. The most

positive δ18O water compositions correlate with (late diagenetic) baroque dolospar, which precipitated near the estimated upper temperature limit (~100°–140°C); negative

δ18O water compositions correspond to (early diagenetic)

dolomicrospar, which precipitated at the lower temperature

limit (~65°C). Some calculated δ18O water compositions

overlap the range reported for Ordovician seawater (~–1 to

–3 ‰; Lohmann and Walker, 1989), indicating a potential for

an “original” source of marine water. However, the majority

of compositions are moderately to highly positive and exceed

seawater values, implying that most dolospar precipitated

from pore fluids elevated in temperature and (or) significantly enriched in δ18O. Based on the estimated timing of

calcite crystallization (fig. 7), there should not have been a

significant temperature change in the burial system between

the time of planar/baroque dolospar formation and calcite

precipitation. Thus, the composition of calcite-precipitating

water was in the range of +2.7 to +10.5 ‰ (SMOW), assuming that crystallization temperatures were similar to those of

quartz and baroque dolospar (~100°–140°C; fig. 8).

Geochemical and petrographic data provide strong evidence that the pore waters involved in burial cementation of

the St. Peter were brines similar in composition to Permian

ore-forming brines and modern Illinois Basin brines. The salt

content of fluid inclusions in quartz and carbonate cements

(~15–20 weight percent NaCl equivalent) indicates a saline

fluid source as does the occurrence of late-diagenetic anhydrite in deeper portions of the basin. Further, the range of

δ18Owater values estimated for the St. Peter (+3 to +10 ‰) is

similar to the δ18O ratios of modern Illinois Basin brines (~0

to +6 ‰ SMOW; Clayton and others, 1966; Stueber and

Walter, 1991) and is broadly comparable to the range of δ18O

ratios measured for fluid-inclusion waters from the UMV ore

district (–5.1 to +5.7 ‰; McLimans, 1977). According to

Stueber and Walter (1991), δ18O (and δD) ratios of deep Illinois Basin brines reflect varying degrees of water-rock interaction and (or) brine and meteoric water mixing. It is

noteworthy that the salinity of the brines responsible for

dolospar and quartz cementation in the St. Peter closely

approximates the salinity of the fluids involved in MVT mineralization (> 20 weight percent NaCl equivalent; McLimans, 1977; Richardson and others, 1988) and of modern

formation waters in the basin (~20 weight percent NaCl

equivalent; Stueber and Walter, 1991). Cl/Br values of

present-day formation waters (290±18) reflect a brine more

evaporated than seawater but undersaturated with respect to

halite (Stueber and Walter, 1991). Kesler and others (1995)

reached a similar conclusion regarding the composition of

ore-forming fluids in the Illinois Basin region. Thus, the Permian hydrothermal brines were similar geochemically to

modern, deep, Illinois Basin brines, although these modern

and ancient fluids are not likely to be related (Ranganathan,

1993).

ORIGIN OF PRECIPITATING FLUIDS

The spatial distribution pattern of early- and late-diagenetic cements in the St. Peter Sandstone provides a record of
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A13

-10 -5 0 5 10 15

δ18Owater (‰, SMOW)

0

40

80

120

160

200

Temperature ( °C)

-9.2‰

-1.4‰

-10.6‰

-6.5‰

-7.5‰

-4.6‰

-10 -5 0 5 10 15

δ18Owater (‰, SMOW)

0

40

80

120

160

200

Temperature ( °C)

Planar dolospar

Baroque

dolospar

Calcite

A B

the sources and migration pathways of fluids that passed

through the Illinois Basin during its burial history. Saline fluids from multiple sources internal and external to the basin

were potentially involved in sandstone diagenesis. However,

the relationship between late Paleozoic ground-water

recharge, brine evolution, and sediment diagenesis has not

been well constrained. On the basis of present-day formation-water data, Stueber and Walter (1991) concluded that

modern Illinois Basin brines contain residual (evaporated)

marine water that was not completely expelled or replaced

by meteoric water recharge. Kesler and others (1995) have

shown from fluid-inclusion studies of ore minerals in the

Fluorspar district and UMV district that the ore fluids did

indeed originate from evaporated seawater. Yet the relationship between modern and ancient brine is unclear because it

is highly unlikely that fluids as old as the Paleozoic would

have remained unaltered in a basin with a geologic and tectonic history as complex as that of the Illinois (Ranganathan,

1993). Within the Illinois Basin proper, a component of the

basin brines might have been influenced by sulfate evaporite

beds older than the St. Peter that underwent solution-precipitation reactions during burial. Occurrences of evaporite

minerals in Paleozoic rocks older than the St. Peter today are

sparse; nevertheless, peritidal environments promoted

penesaline conditions and the formation of evaporite deposits consisting of bedded gypsum and anhydrite in some parts

of the basin (T. Shaw, Unocal Corp., 1995, oral commun.).

Subsequent dissolution of these minerals may have produced

a sulfate-enriched fluid that conceivably could have

migrated into Ordovician sandstones during the Permian

fluid-migration episode. Textural relations clearly indicate

that anhydrite in Ordovician sandstones was a late diagenetic

phase—this precludes its formation from a pore fluid

deposited with the sediment. Thus, we conclude that sulfate-rich brines (i.e., marine-derived sulfate) must have been

imported into Ordovician strata when they were at or close

to maximum burial.

Contributions of brine from sources external to the

present Illinois Basin may have played an important role in

burial cementation of the St. Peter Sandstone (fig. 9).

Fluid-flow models have postulated that tectonic compression, sediment loading, and topographic relief associated

with uplift are mechanisms that can transport warm fluid

through the interior portions of sedimentary basins (Cathles

and Smith, 1983; Bethke and others 1988; Bethke and Marshak, 1990; Garven and others, 1993). In the Illinois Basin,

there is compelling evidence for a short (~200,000 year),

intense, regional, hydrothermal brine-flow event during the

late Paleozoic that was initiated by tectonism at the southern

margin of the continent (Bethke, 1986; Bethke and others,

1988; Rowan and Goldhaber, 1996). Based on paleohydrologic reconstructions, deformation-induced gravity flow

drove hydrothermal brines northward in the basin. Brine

influx into the St. Peter in southern Illinois probably began

in the Late Pennsylvanian when Ordovician strata were near

maximum burial (~3,300 m) and continued through the

Figure 8. Relationship between the oxygen isotopic composition of pore water, temperature of mineral precipitation, and measured

range in δ18O ratios of carbonate cements in the St. Peter Sandstone. Note that pore-water δ18O compositions are reported relative to

SMOW, whereas δ18O ratios of carbonate cements are reported in PDB. A, diagram showing possible parent pore-water compositions

in isotopic equilibrium with planar dolospar (continuous lines) and baroque dolospar (dashed lines) using the fractionation equation of

Fritz and Smith (1970). For details about temperature assessment, see text. B, same diagram for calcite based on the relationship of

Friedman and O’Neil (1977). A 100°–140°C temperature window was assumed for late-stage calcite precipitation. See text for further

details.
A14 EVOLUTION OF SEDIMENTARY BASINS—ILLINOIS BASIN

Permian during the period that regional tectonism was most

intense. Ultimately, these fluids localized fluorine-lead-zinc

mineralization along the southern basin margin and have

been inferred to be responsible for the UMV district north of

the basin (Bethke, 1986; Goldhaber and others, 1994).

Hydrothermal fluid flow paths might have included uplifted

and extensively folded and faulted terrane associated with

the late Paleozoic Ouachita and Appalachian mountain belts

(Brecke, 1979; Farr, 1989; Garven and others, 1993). There

also is some evidence that a component of these fluids may

have been expelled from the Arkoma and Black Warrior

Basins, which underlie Mississippi Embayment sediments

(Rowan and Leach, 1989) (see fig. 9). These fold-and-thrust

belts, and foreland basins, are conduits for the flow pathways

because late Paleozoic tectonics in conjunction with Ouachita/Appalachian orogenesis significantly altered basin hydrodynamics.

In the absence of significant igneous activity, advective

fluid flow driven by topographic relief is an effective means

of transporting heat in the subsurface and, therefore, is a feasible model to explain many of the observed diagenetic and

geochemical trends in the St. Peter Sandstone. Anomalously

warm basinal brines (relative to burial temperature) focused

through aquifer sandstones are required to account for the

high-temperature saline inclusions in dolospar in the northern part of the basin, and they provide an explanation for the

systematically light δ18O values of burial dolospar near the

Fluorspar district in the deeper southern part of the basin. The

local focusing of hot fluids could also explain the

δ18O-depleted burial dolospar concentrated in Ordovician

sandstones along the LaSalle anticline in the east-central Illinois (fig. 6). As fluids moved updip into the basin, presumably they cooled, which, in turn, resulted in heavier δ18O

values of burial dolospar northward along the flow path.

Present ground-water flow in the Illinois Basin is from

northeast to southwest, and recharge zones are to the north

(Young, 1992). The modern regional flow patterns coincide

with extensive late-stage leaching of authigenic K-feldspar

and carbonate cement in and adjacent to the northern part of

the LaSalle anticline, suggesting that freshwater recharge

into the subsurface promoted mineral dissolution. The presence of hematite and kaolinite in the shallow subsurface further indicates there was a shift to more dilute and oxygenated

waters. 

CONCLUSIONS

The St. Peter Sandstone was diagenetically altered by

authigenic K-feldspar and illite, multiple generations of carbonate cement, secondary quartz, and anhydrite. Minor fluorite and sphalerite and dissolution features are also present.

On the basis of the reconstructed burial history constrained

by isotopic age dates of silicate minerals, precipitation of

deep-burial planar and baroque dolospar, calcite, and quartz

and anhydrite in southern Illinois probably occurred in the

Late Pennsylvanian to Early Permian when the rocks were at

or close to maximum burial. Deep burial suggests that elevated temperatures found in fluid inclusions were, in part,

related to conductive heat flow. However, regional variations

in the oxygen-isotope geochemistry of dolospar and occurrences of baroque dolospar north of the Illinois-Kentucky

Fluorspar district indicate that hydrothermal fluids associated with (Permian) ore-forming events also may have been

involved in dolomitization and contributed to elevated temperatures.

Carbonate-hosted lead-zinc deposits in the Midcontinent region are postulated to have formed from brines

expelled from tectonically active terrane south and east of

the present Illinois Basin (Richardson and Pinkney, 1984;

Leach and Rowan, 1986; Richardson and others, 1988;

Rowan and Leach, 1989; Leach and others, 1991; Garven

and others, 1993). According to isotopic and paleomagnetic

age dates of ore minerals (~270 Ma), these fluids migrated

in the Permian. Ordovician aquifer sandstones and adjacent

carbonate rocks were among the conduits for the passage

of these fluids. In shallowly buried sandstones in the northern portion of the basin near the UMV district, the temperature and composition of fluid inclusions in dolospar

(~110°–115°C; >20 weight percent NaCl equivalent) are

comparable to those of ore minerals (~80°–180°C, >20

weight percent NaCl equivalent; McLimans, 1977). The

similarity in temperature and salinity between ores and

dolomite cements is permissive evidence that the same

fluid-flow event was involved in mineralization and

cementation. Further, the elevated formation temperatures

for ore and burial dolomite formation are well above values

that could be achieved through simple burial and normal

crustal heat flow, which suggests that dolospar precipitation in the St. Peter was influenced by an advective hydrothermal heat source. In the deeper, southern part of the

basin, the oxygen isotopic compositions of burial cements

suggest that advective heat transfer along flow paths originally outside the basin coincided with elevated (conductive) heat flow within the basin. Evidence for an advective

hydrothermal fluid and heat-transport mechanism is a

decrease in δ18O ratios of planar and baroque dolospar

along structural features such as the LaSalle anticline and

the clustering of highly depleted δ18O values immediately

north of the Illinois-Kentucky Fluorspar district. The

δ18O-depletion trend along the anticline can best be

explained by the movement of gravity-driven hydrothermal

fluids northward through the structure. Thus, the evidence

provided in this study links many of the diagenetic events

in the St. Peter Sandstone to basin-scale and even

continental-scale tectonic events.
REGIONAL DIAGENETIC PATTERNS IN THE ST. PETER SANDSTONE A15

ILLINOIS

BASIN

MICHIGAN

BASIN

Arkoma Basin

Ouachita fold belt Black

Warrior

Basin Appalachian

Mountains

Mississippi

Embayment Reelfoot

Pascola arch

Rough Creek

graben

Fairfield

subbasin

Ozark

dome

Kankakee arch

Wisconsin arch

LaSalle anticline

➤

➤

➤

➤

➤

➤

➤

➤

➤

➤

➤

➤

rift

ACKNOWLEDGMENTS

This study was supported by the U. S. Geological

Surveys’ Evolution of Sedimentary Basins Program in

cooperation with the State Geological Surveys of Illinois,

Indiana, and Kentucky. We thank T.B. Shaw for providing

drill-core samples and V. Nuccio for generating a burial-history model. The authors are indebted to Robert Burruss and

Elisabeth Rowan for reviewing the paper and providing

helpful suggestions.

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Published in the Central Region, Denver, Colorado

Manuscript approved for publication March 7, 1997

Edited by Richard W. Scott, Jr.

Graphics prepared by George Garcia

Photocomposition by Norma J. Maes