origin of illite

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For permission to copy, contact Copyright Clearance Center at www.copyright.com 2001 Geological Society of America1092

GSA Bulletin; August 2001; v. 113; no. 8; p. 10921104; 12 figures; 5 tables.

Origin of illite in the lower Paleozoic of the Illinois basin:

Evidence for brine migrations

Georg H. Grathoff*Department of Geology, Portland State University, Portland, Oregon 97207-0751, USA

Duane M. MooreIllinois State Geological Survey, 615 East Peabody Drive, Champaign, Illinois 61820, USA

Richard L. HayDepartment of Geosciences, University of Arizona, Tucson, Arizona 85721, USA

Klaus WemmerInstitut fur Geologie und Dynamik der Lithosphare, Goldschmidtstrasse 3, Universitat Gottingen, D-37077 Gottingen, Germany

ABSTRACT

In the lower Paleozoic of the Illinois Ba-

sin, three illite polytypes are found: 2M1

of

detrital origin, and 1Md

and 1M of diage-

netic origin. Illite polytype quantification of

detrital 2M1

illite and diagenetic 1Md

and

1M illite, combined with K-Ar age dating,

allows extrapolation to apparent detrital

and diagenetic illite ages. Kinetic modeling

of smectite illitization, combined with the

calculated age of illitization, can evaluate

different origins of illite. The diagenetic il-

lite in the lower Paleozoic of the Illinois Ba-sin is interpreted not to have formed solely

by burial diagenesis but mainly during

multiple brine events.

The Upper Ordovician Maquoketa

Group contains diagenetic illite (dominant-

ly 1Md

with minor 1M) with an extrapolat-

ed age of 360 m.y. (356377 m.y.) and

formed from smectite at temperatures of

50100 C. This age falls within the span of

dates for illite/smectite (I/S) in K-bentonites

from the Upper Mississippi Valley and is

interpreted to be a combined result of illi-

tization by burial diagenesis and either ahydrothermal brine from the southern and

deeper part of the basin or a K-rich brine

from the Michigan Basin, Upper Mississip-

pi Valley area, or Forest City Basin.

In Ordovician and Cambrian shale part-

ings and sandstone older than the Maquo-

keta Group, the diagenetic illite (1Md

in

shale and 1M in sandstone) has an age of

*E-mail: [email protected].

300 m.y. and formed at temperatures140 C. This late Paleozoic age falls with-

in the range of illites from sandstone in the

Upper Mississippi Valley and K-bentonites

of the Appalachian Basin; it coincides with

the Alleghany orogeny and is interpreted as

having formed by gravity-driven flow from

the uplifted Alleghanian-Ouachita orogenic

belt that drove hot (140 C) fluids

through the Illinois Basin.

Keywords: Illinois basin, illite, K/Ar, kinet-

ics, Maquoketa Group, polytypes.

INTRODUCTION

Understanding the nature, origin, and age of

illite helps unravel the depositional and dia-

genetic history of sedimentary basins. The

burial diagenetic reaction of smectite to illite

has been studied in many basins, including the

U.S. Gulf Coast (Hower et al., 1976), Paris

Basin (Mathieu and Velde, 1989), and central

Poland (Srodon and Eberl, 1984). The distri-

bution of illite and smectite in the Illinois Ba-

sin (for geologic setting see Fig. 1) is differentfrom that of these basins. Hughes et al. (1987)

noted that illite in the Upper Ordovician Ma-

quoketa Group is mainly the 2M1

illite poly-

type, whereas the older Ordovician and Cam-

brian sandstone and carbonate contained

principally the 1M illite polytype (for stratig-

raphy see Fig. 2). This observation is opposite

of what would be expected if illite formed

solely by burial diagenesis of smectite. If the

2M1

illite is detrital in origin, like most 2M1

illite in shale including ours, the change co-incides with the change in the source area.

Bailey et al. (1962) recognized that illite in

shale is a mix of detrital mica, its weathering

products, and diagenetic illite. Later workers

concluded that the detrital mica or illite is of

the 2M1

polytype, whereas the 1M and 1Md

are diagenetic. Five observations support

these conclusions. First, as grain size decreas-

es, the amount of2M1illite polytype decreases

in proportion to the 1M and 1Md

illite poly-

types (Pevear, 1992; Grathoff et al., 1998).

Second, the ages of the progressively smaller

size fractions decrease (Hower et al., 1963;Pevear, 1992; Grathoff et al., 1998). Third, the

2M1polytype is clearly established as the most

stable of these three polytypes (Yoder and

Eugster, 1955; Velde, 1965; Weaver and

Broeckstra, 1984). Fourth, bentonites that

have undergone diagenesis to I/S or illite, or

both, may contain biotite as phenocrysts, but

contain no detrital 2M1

dioctahedral illite; all

I/S and illite in bentonite are 1M and 1Md,

unless they have been changed by metamor-

phism. Fifth, additional support for the con-

clusion that1Mand 1Mdare diagenetic comes

from the observation that illite with morphol-ogy indicating that it grew in the pore space

of clean sandstone is exclusively 1M(Pevear,

1999).

Others who have attempted to date illite in

shale (Hurley et al., 1961, 1963; Bailey et al.,

1962; Hower et al., 1963; Zhao et al., 1997;

Grathoff and Moore, 1996) have recognized

detrital and diagenetic components with the

detrital age older than the stratigraphic age

and the diagenetic age younger. It has not been


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Geological Society of America Bulletin, August 2001 1093

ORIGIN OF ILLITE IN THE LOWER PALEOZOIC OF THE ILLINOIS BASIN

Figure 1. Geologic setting of the Illinois Basin showing locations of the cores that were

sampled, La Salle anticlinorium, Hicks Dome, and surrounding structures. For explana-

tions of the core abbreviations, core information, and sample interval, see Table 1.

Figure 2. General stratigraphic column ofthe lower Paleozoic rocks of the Illinois Ba-

sin. From Kolata (1991) and Willman et al.

(1975).

universally recognized that all size fractions

contain some detrital material, but Pevear

(1992), Girard and Barnes (1995), and this

study show that even the 0.2 m size frac-

tion contains some detrital 2M1 polytype.

Gharrabi and Velde (1995) suggested that

diagenetic illite in the Illinois Basin formed

solely by burial diagenesis. However, another

explanation could be that the hot Upper Mis-

sissippi Valley ore-bearing fluids proposed to

have migrated through lower Paleozoic sedi-

ments in the Illinois Basin could also have

formed some or all of the illite (e.g., Bethke,

1985). The Upper Ordovician Maquoketa

Group, the oldest low-permeability unit that is

present uniformly across the basin, may have

confined these long-range brine migrations.

Today, the Maquoketa Group is a confining

layer or hydrologic boundary to groundwater

flow in northern Illinois and southern Wiscon-

sin (Siegel, 1990) and most likely would have

behaved as a confining layer in the past as

well. Long-range brine migrations through Pa-

leozoic strata below the Maquoketa Group are

thought to be responsible for producing re-

gional K metasomatism (Hay et al., 1988), Pb-

Zn ore deposits (Heyl et al., 1959; Bethke,

1985; Bethke and Marshak, 1990), and petro-

leum migrations (Bethke et al., 1991). Potas-

sic alteration in the U.S. Midcontinent has

produced widespread authigenic K-feldspar

and illite (Hay et al., 1988; Matthews, 1988;

Liu, 1997). Although the areal extent is still

unknown, the potassic alteration (authigenic

K-feldspar and illite) occurs in all units older

than the Upper Ordovician Maquoketa Group,

from the Precambrian through the Ordovician

(for review, see Matthews, 1988; Liu, 1997).

This study is the first to date illite in shales

in the Illinois Basin. In this paper, we delimit

the nature and origin of illite in the lower Pa-

leozoic strata of the Illinois Basin by using

illite polytype quantification (Grathoff and

Moore, 1996), combined with dating by K-Ar

age (Grathoff et al., 1998). These results show

that both multiple brine migration events and

burial diagenesis are responsible for illitiza-

tion in the Illinois Basin.

GEOLOGIC SETTING AND SAMPLE

LOCATIONS

The Illinois Basin began as a failed rift dur-

ing the breakup of a supercontinent during

Early and Middle Cambrian time and was

closed off at its southern end by the uplift of

the Pascola Arch some time after the Late

Pennsylvanian and before the Late Cretaceous

(Fig. 1; Kolata and Nelson, 1991). It is bound-

ed by the Kankakee Arch to the northeast and

the Cincinnati Arch to the east. It is now anoval depression, deepest in the south and shal-

lowing northward, containing Cambrian

through Permian sedimentary rocks noncon-

formably overlying Precambrian granite and

rhyolite. The source area for sediments older

than the Maquoketa Group was the Canadian

shield and, to a minor extent, the Ozark dome

to the southwest (Kolata and Graese, 1983).

With onset of the Taconic orogeny, the source

area changed to an easterly Appalachian

source for the Upper Ordovician (Cincinna-

tian) Maquoketa Group. During the Alleg-

hany-Ouachita orogeny, the Illinois Basin wassubjected to intrusion, mineralization, and

compressional stresses related to major reac-

tivation of faults and structural activity (Beth-

ke and Marshak, 1990; Buschbach and Kolata,

1991; Kolata and Nelson, 1991; Nelson, 1995;

Marshak and Paulsen, 1996). In southern Il-

linois, the Hicks dome developed ca. 270 Ma

(Snee and Hayes, 1992; Zartman et al., 1967).

The Upper Mississippi Valley Pb-Zn ores and

the fluorspar district of southern Illinois are

also dated at 270280 Ma (Chesley et al.,

1994; Brannon et al., 1992; Zartman et al.,

1967). See Leighton et al. (1991) for more de-

tails on all other geologic aspects of the Illi-

nois Basin.

The Maquoketa Group is a clastic wedge

that progrades westward from the Taconic up-

lands. The sediments of this major transgres-

sive cycle were deposited within a shallow

epicontinental sea at about latitude 1020S

and have a constant thickness of about 60 m

across the Illinois Basin. They thicken to 300

m in eastern Indiana and to about 1800 m in

Pennsylvania (Witzke, 1980; Kolata and

Graese, 1983). The Maquoketa has a total or-

ganic carbon content (TOC) of up to 9.5%


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1094 Geological Society of America Bulletin, August 2001

GRATHOFF et al.

TABLE 1. LOCATIONS OF THE SAMPLED CORES, THE SAMPLED INTERVAL, THE NUMBER OFSAMPLES COLLECTED, AND THE UNITS SAMPLED

Core ISGS Illinois Location Number of Sampled interval Units sampledcore county samples (m) (from to)

number

CL C228 Clinton Sec. 3, T. 2 N., R. 3 W. 60 999.01234.5 MAQ SPSG C7608 Fulton Sec. 5, T. 8 N., R. 1 E. 43 174.0242.7 MAQ GALIQ C3904 Iroquois Sec. 11, T. 26 N., R. 13 W. 29 186.01010.1 MAQ MSS

SS35648

W C2740 White Sec. 27, T. 4 S., R. 14 W. 7 1827.62252.4 MAQ and KNXUPH3 C12996 Stephenson Sec. 7, T. 28 N., R. 6 E. 3 570.0644.1 MSS

Total 142

Notes: ISGSIllinois State Geological Survey, MAQMaquoketa Group, SPSSt. Peter Sandstone, GALGalena Group, MSSMt. Simon Sandstone, KNXKnox Group.

Figure 3. Illite crystallinity (IC) plotted against depth for the 1 m size faction (CL

core) collected from XRD traces (glycol solvated). The solid symbols represent shale and

shale partings, and the open symbols represent carbonates.

Figure 4. IR (Srodon, 1984) of the 1m size fraction (Clinton County core) collected

from XRD traces from oriented aggregates is plotted against depth. IR is the intensity

ratio of the air-dried illite 001/003 peaks, divided by the glycolated 001/003 peak ratio.

The solid symbols represent shale and shale partings, and the open symbols represent

carbonates. Asterisk denotes a sample that contains large amounts of corrensite (chlorite/

smectite), therefore increasing the error of the IR calculation.

(Hatch et al., 1991) and is a good source rock

for petroleum (Guthrie and Pratt, 1994; Guth-

rie, 1996).

Cambrian and Ordovician rocks below the

Maquoketa Group consist of the Galena-Plat-

teville Group carbonates, St. Peter Sandstone,

the thick Knox Supergroup carbonates, and

the Mt. Simon Sandstone (Fig. 2). The Cam-brian Mt. Simon Sandstone, at the base of the

Paleozoic section, and the Ordovician St. Pe-

ter Sandstone are the two main aquifers in the

basin. See Willman et al. (1975) and Kolata

(1991) for more details on the Illinois Basin

stratigraphy.

The samples analyzed in this study were

collected from five cores within the basin (Ta-

ble 1; Fig. 1). We sampled different lithologic

units, concentrating on shale and shale part-

ings. Other than the Maquoketa Group, we

sampled the Ordovician Galena Group, Platte-

ville Group, Joachim Formation, St. PeterSandstone, and Everton Formation, as well as

the Cambrian Eau Claire Formation and Mt.

Simon Sandstone. The Maquoketa Group

samples from the G core are from a subset

studied for organic geochemistry and carbon

isotopic compositions (Guthrie, 1994; Guthrie

and Pratt, 1994; Guthrie, 1996). See Appendix

1 for analytical methods.

RESULTS

Illite Crystallinity Index, Intensity Ratio

(IR), and (001) Decomposition

Illites in rocks of the lower Paleozoic in the

Illinois Basin contain few expandable inter-

layers, on the basis of X-ray diffraction

(XRD) patterns of oriented mounts exhibiting

minimal illite peak shifts after glycolation and

no low-angle reflection (28 2 , CuK ra-

diation). Three methods were used to deter-

mine the percentage of smectite: full width at

half maximum (FWHM) of the 001 reflection,

known as illite cr ystallinity index (IC) (Ku-

bler, 1964), Jan Srodons intensity ratio (IR)

(Srodon, 1984, Srodon and Eberl, 1984), and

peak decomposition of the 001 reflection

(Lanson and Champion, 1991; Velde et al.,

1986).

The IC index of the Maquoketa Group shale

(1 m size fraction) has an average of 0.53

2 (0.03); the underlying Ancell Group

shale partings show a larger IC index, with an

average of 0.85 2 (0.04) (Fig. 3). The IR

of the Maquoketa has an average of 1.24

(0.10), and the Ancell shale partings have a

larger IR, averaging 1.39 (0.14) (Fig. 4). As-

suming similar iron and water content per

smectite layer, the larger IR of the underlying


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Figure 5. Different size fractions of the G 261 sample showing an increase in detrital2M

1with increasing size fraction. The 12 m and 24 m size fractions are not shown

here but contain 40% and 50% detrital 2M1illite, respectively. With increasing percentage

of 2M1

illite, the illite hump (1Md

illite) decreases in area. Dashed lines show the position

of 1M(trans-vacant) illite polytype-specific peaks, and triangles indicate peaks specific to

the detrital 2M1

polytype; Qquartz; FK-feldspar; Aanatase.

units indicates that these units contain more

smectite. Using NEWMOD (Reynolds,

1985) modeling, both units contain more than

95% illite in illite/smectite (I/S), and illite of

the Maquoketa Group contains 2% less

smectite than illite of the underlying Ancell

Group. The results of the decompositionmethod of Lanson and Champion (1991) and

Velde et al. (1986) show that the d-spacing for

the Maquoketa Group I/S peak is 10.65 A and

for the underlying units 10.90 A, indicating

that illite of the underlying units contains

more smectite than that of the Maquoketa

Group. Use of a graph in Velde et al. (1986)

shows that the Maquoketa Group contains

91% illite in I/S and the Ancell Group 88%

illite in I/S.

All three methods indicate that the lower

Paleozoic strata of the Illinois Basin contain

at least 88% illite in I/S and that the Maquo-keta Group contains about 3% more illite than

that of the underlying Ancell Group. This is

opposite to what would be expected if increas-

ing smectite illitization with depth were the

only explanation for the origin of these illites,

but it can be explained using data from ran-

dom powder XRD.

Random Powder X-Ray Diffraction (XRD)

and Illite Polytype Quantification

Illite in all lower Paleozoic shales and shale

partings throughout the basin is mainly com-

posed of the 1Md

polytype. The only differ-

ence is that the Maquoketa contains a higher

percentage of 2M1

illite than the older under-

lying units. On average, the 1 m size frac-

tion of the Maquoketa from the CL core con-

tains 68% 1Md, 23% 2M

1, and 9% 1M,

whereas the shale partings below the Maquo-

keta contain 90%1Md, 5%1M, and 5%

2M1. Illite in Mt. Simon Sandstone is mainly

the 1M polytype. The Glenwood Formation

sandstone illite is a mixture of the 1Mand1Md

polytypes.

The 2M1

polytype increases in abundance

with increasing size fraction (Fig. 5). Sample

G 261shows an increase from 10% 2M1

in

the 0.2 m size fraction to 30 % 2M1in the

0.51 m size fraction to 50% 2M1in the 2

4 m size fraction. Polytype1Mddecreases in

abundance with increasing grain size, and the1Mpolytype remains about constant.

In summary, the Maquoketa Group contains

a higher percentage of detrital 2M1illite (23%

2M1

in the 1 m size fraction) than the un-

derlying units (5% 2M1

in the 1 m size

fraction). Because2M1illite contains no smec-

tite layers, only 77% of the illite (percent 1Md

percent 1M) in the Maquoketa Group con-

tains smectite interlayering, compared to

95% for the underlying units; this finding

explains the higher percentage of smectite in

the units underlying the Maquoketa Group.

Associated Minerals

Illite in the Maquoketa Group is associated

with chlorite, quartz, calcite, dolomite, fluor-

apatite, pyrite, and small amounts of K-feld-

spar. Chlorite was present in all samples ex-

cept at the base in the Parrish (G) core, where

chlorite disappears and fluorapatite appears.Illite in the underlying units is associated

with chlorite, chlorite/smectite (including cor-

rensite), dolomite, calcite, quartz, anhydrite,

and abundant monoclinic K-feldspar. The

monoclinic K-feldspar is commonly euhedral

and authigenic in origin (Gruner, 1937; Matth-

ews, 1988). It decreases in abundance with de-

creasing grain size in the clay size fractions

and is usually absent from the 0.2 m size

fraction. Chlorite was generally less abundant

in units below the Maquoketa Group. Mixed-

layered chlorite/smectite including corrensite

was found in the Ordovician Joachim Dolo-mite of the Ancell Group in the CL core. Thin

layers of anhydrite are abundant in the interval

with the chlorite/smectite.

K-Ar Dating

The extrapolated K-Ar dates of the diage-

netic illite in the Maquoketa Group are close

to 360 Ma, and the underlying unites give

dates of close to 300 Ma (Table 2). K-Ar dates

of three different size fractions of two Ma-

quoketa Group samples (CL 3391 and G 26

1) indicate that the Maquoketa Group 2M1 il-lite has a common detrital age of 520 m.y.

(Fig. 6). Assuming this detrital age of 520

m.y. for all Maquoketa Group samples, the re-

sulting apparent diagenetic ages range from

356377 m.y., averaging around 360 m.y. The

exceptions are G 261 and G 27, which were

deposited at the contact with the underlying

Galena Group and have an apparent diagenetic

age of 321 m.y., 40 m.y. younger than the

other Maquoketa samples (Fig. 6). It is as-

sumed that the 2M1

illite is the only detrital

K-bearing component and that the 1M and

1Md

illite are both diagenetic (for reasoning,

see Discussion). A second assumption is that

the extrapolation to detrital and diagenetic

end-member ages is a straight line; this is a

good assumption as long as the detrital and

diagenetic ages are not too far apart, and the

K content of the two illites is similar, which

is the case for our samples. For more discus-

sion on the extrapolation, see Grathoff et al.

(1998).

The dated samples from below the Maquo-

keta (Table 2) also have a common apparent

diagenetic age, 293307 m.y., averaging about

300 m.y. The age for the Eau Claire shale (400


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GRATHOFF et al.

TABLE 2. K/AR DATES FROM THIS STUDY, INCLUDING SIZE FRACTION (s.f.), PERCENT DETRITAL ILLITE (2M1), MEASURED K/AR DATE, AND THECALCULATED DATE USING THE AMOUNT OF DETRITAL COMPONENT AND ITS AGE (520 Ma, FROM FIG. 6)

Sample Age Depth Core Unit Lith. s. f. K/Ar age Error Ma K2O 40Ar* 40Ar* %2M1 Diagenetic Diagenetic age

(ft) (m) (m) (Ma) (2) (wt%) nl/g STP (%) polytype (Ma)

IQM Ord 690 210 IQ MAQ sh 0.2 391 8.1 6.88 96.75 97.51 10% 1Md 1M} 377CL 3391 Ord 3391 1034 CL MAQ sh 0.2 380 8.3 6.51 88.74 97.14 12% 1Md 1M} 365CL 3391 Ord 3391 1034 CL MAQ sh 0.5 407 9.2 5.77 84.94 97.35 20% 1Md 1MCL 3391 Ord 3391 1034 CL MAQ sh 2 441 9.1 4.60 74.04 98.61 52% 1Md 1M

W 6100 Ord 6100 1859 W MAQ sh 0.2 362 9.5 5.63 72.69 94.76 ** 1Md 1M (362)G-19 Ord 660 201 G MAQ sh 0.2 369 9.9 7.19 94.75 96.59 8% 1Md 1M} 356G-251 Ord 759 231 G MAQ sh 0.2 377 7.8 6.12 82.50 96.60 13% 1Md 1M} 356G 261 Ord 774 236 G MAQ sh 0.2 340 7.2 6.92 100.68 97.46 10% 1Md 1M} 321G 27 Ord 775 236 G MAQ sh 0.5 372 9.7 6.57 87.43 96.79 25% 1Md 1MG 261 Ord 774 236 G MAQ sh 2 403 9.7 6.95 83.85 96.77 42% 1Md 1MIQGL Ord 1202 366 IQ GLW ss 0.5 364 7.7 7.68 99.76 96.81 b.d. ?CLJ Ord 3969 1210 CL JOA sh 0.2 307 4.1 6.78 73.04 95.55 b.d. 1Md 307CLJ Ord 3969 1210 CL JOA sh 24 474 9.7 7.27 127.04 98.04W7508 Ord 7508 2288 W KNX sh 0.2 345 8 8.14 99.68 97.80 17% 1Md 1M} 303IQE Cam 3243 988 IQ EC sh 0.2 400 9.2 7.35 109.14 98.64 b.d. 1Md ?IQE Cam 3243 9 88 IQ EC sh 24 669Ma 13.8 8.53 222.64 98.95 10%IQMTS Cam 3367 1026 IQ MSS ss 0.5 293 9.3 8.81 90.42 98.95 b.d. 1M 293Duf fin et al. ( 1989) Cam 1900 579 UPH3 MSS ss 1 254 (5) 11% 1M

Notes: b.d.below detection limit (5%), MAQMaquoketa Group, MSSMt. Simon Sandstone, JOAJoachim Formation, KNXKnox Group, ECEau Claire Group,GLWGlenwood Formation.

*Too little sample to quantify the polytypes.

Figure 6. Three size fractions of the Maquoketa from two different cores (CL and G);

both samples show a common extrapolated detrital age for the Maquoketa Group at 520

Ma.

m.y., 0.2 m) may be a mixed age reflecting

minor amounts (not detectable by XRD) of

very old (1 b.y.) mica or K-feldspar; the 2

4 m size fraction gives a K-Ar age of 669

m.y. and contains illite and K-feldspar.

Kinetics

The smectite to illite reaction is a function

of temperature (Hower et al., 1976; Boles and

Franks, 1979; Pytte and Reynolds, 1988), K

concentration (Altaner, 1985, 1989; Huang etal., 1993), and time (Velde and Vasseur,

1992). We tested three models for the origin

of diagenetic illite: (1) pure burial diagenesis

(Fig. 7), (2) pure brine migrationhydrother-

mal at 100 C and K-rich at 50 C (Table 3),

and (3) a combination of burial diagenesis and

brine migration (Fig. 8). The kinetic calcula-

tions were compared with 90% illite in I/S and

the diagenetic illite age of 360 m.y. for the

Maquoketa. Our illitization age was calculated

using a method similar to that described in

Elliott and Matisoff (1996). It is the sum of

the product of the fraction of illite that formed

at each time step (10 m.y.) and the date of that

time step. For example, if 2% illite of the total

87% illite in I/S formed between 250 and 240

Ma, the contribution to the calculated date for

this time step would be 0.02 245 Ma. The

summed calculated age is then normalized

(sum/0.87) to the final percentage of illite

(87% illite in I/S).

The first model assumes that illite formed

solely by burial diagenesis. The calculations

of the two end members, early (at 240 Ma)

and late (at 10 Ma) erosional stages, produce

83%92% illite and a calculated time of illi-

tization of 301311 Ma (Fig. 7, A and B).

Both calculations use a geothermal gradient of

30 C/km (Cluff and Byrnes, 1991). Smaller

geothermal gradients (20 C/km) produce too

little illite in I/S (67%82%). Both the fast

and slow erosion models assume slow burial

(8.5 m/m.y.) from the Ordovician to the Penn-

sylvanian (450300 Ma) and fast burial (30

m/m.y.) up to the Permian (300250 Ma). The

fast burial is based on the 1.5 km of additional

sedimentary overburden postulated by Dam-

berger (1991) and others. If 1.5 km of rock is

not added, only 72% illite in I/S is formed. If

we assume that 20% of the illite is inherited,

as Hower et al. (1976) interpreted for the U.S.

Gulf Coast, the kinetic and age calculations

show that no additional illite is being pro-

duced and that the inherited illite must be old-

er than the detrital 520 Ma 2M1

illite (Fig.

7C). We therefore conclude that even if some

1Md

illite is inherited, which cannot be ruled

out, burial diagenesis alone cannot account for

both the highly illitic I/S and the age of the

1Md

illite.


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Figure 7. The pure burial diagenesis kinetic model. The model assumes an additional

1.5 km of overburden, deposited during the Pennsylvanian and Permian, as suggested

by Damberger (1991). The difference between A and B is the time of erosion. Erosion

in A occurred within the first 10 m.y. after maximum burial. Erosion in B has occurred

within the past 10 m.y. In model C, 20% of the K that formed the illite is assumed to

be old, 520 m.y. In A and B: 1slow burial to 1.5 km, 2fast burial to 3.0 km. In A,

3erosion of 1.5 km, 4isothermal (at 1.5 km). In B, 3isothermal (at 3 km), 4

erosion of 1.5 km.

TABLE 3. TIME REQUIRED TO PRODUCE 90% AND 80% ILLITE IN I/S USING THE KINETIC MODEL OFHUANG ET AL. (1993) WITH DIFFERENT K CONCENTRATIONS AND TEMPERATURES

200 ppm 1000 ppm 2000 ppm 2000 ppm 10,000 ppm 20,000 ppm100C 100 C 100 C 50 C 50 C 50 C(m.y.) (m.y.) (m.y.) (m.y.) (m.y.) (m.y.)

90% illite in I/S 17.3 3.5 1.7 601 120 6080% illite in I/S 7.7 1.6 0.77 267 53 27

The pure brine migration models assume

that all the illitic I/S was formed in the Ma-

quoketa Group during a hydrothermal or K-

rich event at 360 Ma. Oxygen isotope data and

organic maturity indices indicate a tempera-

ture range of 50 to 100 C for the Maquoketa.

Therefore, for the hydrothermal calculations

we assumed a maximum temperature of 100C, and for the K-rich brine event we assumed

a temperature of 50 C. For the hydrothermal

calculations (100 C), K pore-fluid concen-

trations must exceed 2000 ppm to form highly

illitic I/S over a short period (1 m.y.). For

the K-rich event calculations, to form 90% il-

lite in I/S at 50 C, K pore fluid concentra-

tions must be extremely high (20 000 ppm),

and the reaction time is very long (60 m.y.)

(Table 3). Our conclusion is that a hydrother-

mal event or a low-temperature K-rich brine

event alone is unlikely to produce the highly

illitic I/S in the Maquoketa.The mixed models are constructed to sim-

ulate illitization by burial diagenesis with ei-

ther a hydrothermal or K-rich brine event (Fig.

8). A 1-m.y.-long hydrothermal event at 360

Ma of 100 C (K concentration of 2000

ppm), combined with burial diagenesis, pro-

duces 87% to 93% illite in I/S with an age of

347 to 359 m.y. A K-rich brine event (20 000

ppm K, 50 C) at 360 Ma, combined with

burial diagenesis, produces 85% to 93 % illite

in I/S with an age of 329 to 344 m.y. Both

the hydrothermal event and K-rich brine alone

produce about 80% of the illite; the rest isproduced by burial diagenesis. Burial diagen-

esis of smectite is needed to form 90% illite.

We assume for the K-rich brine that the total

reaction time was about 16 m.y. until the K

was depleted enough to slow down the reac-

tion, and kinetics of burial diagenesis took

over. (Table 4)

Oxygen Isotopic Composition

The 18O of three samples were analyzed,

two from the Maquoketa Group (G 19, 0.2

m, 17.6; CL 3391, 0.2 m, 18.1) and

one from the Mt. Simon sandstone of the IQ

core (IQMTS, 0.5 m, 15.3). The illiti-

zation temperatures of 50100 C for the Ma-

quoketa Group and 95145 C for the under-

lying Mt. Simon Sandstone were calculated

using the fractionation equation of Savin and

Lee (1988). We assumed a fluid composition

06 SMOW (standard mean ocean water)

and adjusted the 18O values of the Maquoketa

Group for detrital 2M1

illite. The heavy iso-

topic composition of the brines (0) was

based on fluid-inclusion data of dolomite in

the St. Peter Sandstone (Pitman and Spotl,


7/13


GRATHOFF et al.

Figure 8. The mixed-brine burial kinetic model. The kinetic model assumes a 100 C brine

for 1 m.y. for model A and 20 000 ppm K for 16 m.y. for model B. The assumptions for

the burial part are the same as in Figure 7. 1slow burial to 0.4 km, 2hydrothermal

event (in A) and K-rich (in B), 3fast burial to 3 km, 4erosion of 1.5 km, 5isothermal

(1.5 km).

1996) indicating high salinities (1520 wt%

NaCl) for the Illinois Basin. In support of this,

we found anhydrite in the Ancell Group, a

diagenetic mineral that forms in highly saline

environments. For the upper limit 6 was

used, on the basis of data from Clayton et al.

(1966) and Stueber and Walter (1991), who

analyzed deeper formation brines from the Il-linois Basin. The 18O values of the Maquo-

keta Group illite were adjusted according to

the amount of detrital2M1illite, assuming that

2M1

illite forms at higher temperatures with

lower 18O values [e.g., 10.413.3 for

muscovite (2M1) from metasedimentary strata

of southeastern New York (Garlick and Ep-

stein, 1967)]. Assuming the range of

10.413.3 for the 2M1 illite (10%), the

diagenetic illite (90%) changes from 17.6

18.1 to about 18.419.0 .

Morphology

The illite morphology for three samples of

almost pure diagenetic illite [CL core Maquo-

keta Group (CL 3391, 0.2 m), CL core Jo-

achim Formation (CLJ, 0.2 m), IQ core

Mt. Simon Sandstone (IQMTS, 0.5 m)]

was examined using an atomic force micro-

scope. On the basis of these three samples,

illite in the Maquoketa Group is morphologi-

cally distinct from that of illite in the units

older than the Maquoketa Group. The Maquo-

keta Group (0.2 m size fraction) contains

no laths, only very thin flakes (12 nm thick).The Ancell and Mt. Simon samples contain

primarily laths that are relatively thick (130

200 A).

DISCUSSION

The diagenetic illite in the Maquoketa

Group and that of the underlying units are of

different origin and did not form by burial dia-

genesis alone but were formed mainly during

two brine induced events, one 360 m.y. ago

and one 300 m.y. ago. This conclusion is

based on the different apparent ages of illiti-

zation, kinetic modeling, different illite poly-

types, different oxygen isotopic compositions,

and different morphologies.

An assumption made throughout the dis-

cussion is that the 1Mand 1Md

illites in both

the Maquoketa and the older underlying units

are diagenetic and that the apparent age of the

extrapolated pure (1Mand 1Md) end-members

is not contaminated by detrital illite. Aronson

and Hower (1976) inferred the presence of de-

trital K and Ar in U.S. Gulf Coast shale. How-

ever, they did not determine or quantify the

illite polytypes. A possibility is that small

amounts of 2M1

illite account for the detrital

K and Ar. For our samples, it cannot be ruledout that the illite classed as diagenetic con-

tains some detrital admixture. However, the il-

litization age determined by our kinetic cal-

culations starting with 80% smectite (Fig. 7C)

shows that the detrital K must be older than

the detrital2M1illite (520 m.y.) to account for

the extrapolated K-Ar age. We therefore think

we are justified in assuming that the starting

composition was close to 100% smectite and

that detrital 1Mand 1Mdillite is insignificant.

Timing of Illitization

The timing of illitization in lower Paleozoic

shales and shale partings of the Illinois Basin

is similar to some extent to the timing of il-

litization in lower Paleozoic K-bentonite and

sandstone of eastern North America and the

Upper Mississippi Valley (Fig. 9). Three

groups of illitization dates can be recognized

(Fig. 10). These are the same as the three ep-

isodes of illitization that Lee and Aronson

(1991) first reported for the Upper Mississippi

Valley: 215230 Ma, 300310 Ma, and 340

360 Ma.

The oldest group is mid-Paleozoic, around

360 Ma, and occurs only in the northern part

of the U.S. Midcontinent, such as in the Ma-quoketa Group within the Illinois Basin (this

study), the Upper Mississippi Valley (Hay et

al., 1988; Lee and Aronson, 1991) and the

Michigan Basin (Girard and Barnes, 1995).

The second group is late Paleozoic, 250

310 Ma, and interpreted to be associated with

the Alleghany-Ouachita orogeny and possibly

with the Upper Mississippi Valley ore depo-

sition. These illitization dates are apparent

throughout a much wider area, from the Ap-

palachian Basin, through the Illinois Basin,

northeastern Missouri, to the Forest City Ba-

sin in Iowa (Fig. 10). The Upper Mississippi

Valley ore deposits were dated at 270 5 Ma

(Brannon et al., 1992) and the Illinois-Ken-

tucky fluorspar district at 277 16 Ma (Ches-

ley et al., 1994), and the Hicks dome devel-

oped around 270 Ma (Snee and Hayes, 1992;

Zartman et al., 1967).

The third group (215230 Ma) has been

documented only in northwestern Illinois,

close to the Upper Mississippi Valley ore de-

posits (Lee and Aronson, 1991; Duffin et al.,

1989). Duffin et al. (1989) reported three dates

on illite in the Cambrian Mt. Simon Sandstone

in the Upper Mississippi Valley as 214, 254,


8/13



TABLE 4. SUMMARY OF KINETIC MODELING RESULTS AND MEASURED RESULTS

I in I/S formed Calculated Extrapolated K-Ar(%) ill itization age illitization age

(Ma) (Ma)

Solely burial diagenesis and early erosion 83 311Solely burial diagenesis and late erosion 92 301Hydrothermal and early erosion 87 359Hydrothermal and late erosion 93 347K-rich brine and early erosion 85 344

K-rich brine and late erosion 93 329Burial diagenesis with 20% 520-m.y.-old K and early erosion 83 350Bu rial d iag en es is with 2 0% 52 0-m.y.-o ld Ka nd late e ro sion 9 2 33 6Maquoketa Group diagenetic illite 90 360

Figure 9. Map of the K/Ar dates of diagenetic illite of Cambrian and Ordovician sediments

in and close to the Illinois Basin. For the dates in the Illinois Basin (inside the dashed

line), dates above the horizontal lines are from the Maquoketa Group and dates below

are from the underlying units. References: 1This study; 2Duffin et al. (1989); 3

Elliott and Aronson (1993); 4Hay et al. (1988); 5Girard and Barnes (1995); 6Duffin

(1990). Modified from Nelson (1995).

and 271 Ma. They concluded that the illite and

K-feldspar formed in multiple episodes of K

metasomatism. Our XRD results of the 254

Ma sample shows that it contains 11% 2M1

illite (Table 2). When we adjust the age for

the presence of this detrital 2M1, we see that

the diagenetic age of the 1M illite must be

much younger, possibly as young as 214 m.y.If the detrital2M

1illite has an age of 577 m.y.,

the resulting diagenetic age would be 214 m.y.

Therefore the Duffin et al. (1989) data might

indicate only two episodes of illitization, one

around 214 Ma and one around 271 Ma.

The K-Ar results from this study are inter-

preted to represent two basin-wide illitization

events, one at about 360 Ma affecting the Ma-

quoketa Group and one at around 300 Ma af-

fecting the underlying units. At the contact of

the Maquoketa Group and the underlying Ga-

lena Group, the diagenetic illite has an inter-

mediate age of 320 m.y. (Fig. 11). This raisesthe question, Why is the illite younger in the

stratigraphically older units? Four explana-

tions can be offered: (1) The dates were reset;

(2) during the 360 Ma event, illite was not the

geochemically stable phase to precipitate in

the underlying units; (3) the 360 Ma event

never reached the underlying units; (4) a pro-

portion of the Maquoketa Group 1Md

illite

contains detrital K.

The possibility that the ages in the under-

lying units were reset at 300 Ma is unlikely,

because the coarser size fractions, which con-

tain some2M1illite, are older and would prob-ably have been reset as well and because the

maximum temperature the sediments were ex-

posed to was significantly lower than the re-

setting temperature of at least 250 C (e.g.,

Hunziker et al., 1986; Wemmer and Ahrendt,

1997).

The second explanation, that the chemical

conditions were not right to precipitate illite,

is more likely. Authigenic K-feldspar formed

before 400 Ma in lower Paleozoic sediments

surrounding the Illinois Basin (Hay et al.,

1988, 1993; Duffin et al., 1989; Liu 1997).

Therefore, the chemical conditions might have

been such that K-feldspar was stable and not

illite until ca. 360 Ma. The 360 Ma event

could have supplied heat, K, or both to form

illite, but did not change the fluid chemistry

to make illite the stable phase in the under-

lying units.

The third explanation, that the 360 Ma

event might not have reached the underlying

units, is only possible if dense K-rich brines

are introduced into the rocks overlying the

Maquoketa Group, which served as a semi-

permeable membrane.

The fourth explanation, that the 1Md

illite

contains detrital K, is viable only if the orig-

inal smectite contained 20% or more old K

(illite layers), if the K retained an age older

than the detrital 2M1

illite of more than 520

m.y. (see kinetics results), and if the Ar was

retained during illitization.

Pitman and Spotl, (1996), reporting on the

St. Peter Sandstone, and Fishman (1997), re-

porting on the Mt. Simon Sandstone, found

two textural types of illite. One type coats

grains and the other one fills pores. Both types

of illite are diagenetic, according to Pitman

and Spotl, (1996) and Fishman (1997). The

pore-filling illite formed later and is much

more abundant. This raises the question

whether our samples and all Mt. Simon Sand-

stone samples that were K-Ar dated contain

both or just one type of diagenetic illite. We

were not able to detect two types of diagenetic

illite with XRD in the Mt. Simon Sandstone.

However, there are differences in ages of1Md

illite. The illite (0.2 m) in the Eau Claire

shale is of the 1Md

polytype and has a K-Ar

age of 400 m.y. This age could either be due


9/13


GRATHOFF et al.

Figure 10. Map showing the summary and interpretation of K/Ar dates of diagenetic illite of Cambrian and Ordovician sediments (sh

shale; sssandstones; bK-bentonites) in and close to the Illinois Basin. The 254 Ma date from Duffin et al. (1989) is from illite that

contains 11% 2M1illite, which is likely to be detrital. Therefore, the diagenetic illite is possibly as young as 214 m.y. and is not correlated

with the Alleghany orogeny.

to small amounts of contamination by unde-

tected K-feldspar and 2M1

illite or reflect the

age of the older type of illite that Pitman and

Spotl, (1996) and Fishman (1997) reported.

The K-Ar age of the illite in the Glenwood

Sandstone of 364 m.y. could reflect a mixture

of 300-m.y.-old 1MMt. Simon illite and 400-

m.y.-old 1Md

Eau Claire illite, because the

random powder XRD from the Glenwood

sandstone shows a mixture of 1Md

and 1M

illite.

The apparent detrital age of the Maquoketa

Group (520 m.y.) is not interpreted as a date

of an orogeny. The source of the 2M1

illite

is the Taconic uplands along the eastern mar-

gin of Laurentia and not the Precambrian Ca-

nadian shield. As a result of the Taconic

orogeny, metasedimentary strata, possibly

mica schist containing 2M1

muscovite, were

uplifted and eroded, and their detritus was

transported into the U.S. Midcontinent. The

520 m.y. age might be an average age, or it

might give the date of maximum burial of the

metasediments. The age is similar to the age

of the 662 m size fraction (540 m.y.) of

the Upper Ordovician Sylvan Shale deter-

mined by Hower et al. (1963). The Sylvan

Shale, present to the southwest of the Illinois

Basin, is equivalent to the Maquoketa Group

(Willman et al., 1975). The 520 m.y. age is

also close to the age of the coarse illite of

the St. Peter Sandstone in the Michigan Ba-

sin, dated at 544570 Ma (Girard and

Barnes, 1995).

Thermal Conditions of Illitization

The maximum temperature of illitization

was estimated three ways, from illite crystal-

linity, the presence of the 1Md

illite polytype,

and the oxygen isotopic composition of the

illite. These results were then compared with

fluid-inclusion data, organic maturity indices,

and conodont alteration index (CAI) data cited

in the literature.

Illite crystallinity index averages of the Ma-

quoketa Group (0.53 2 ) and the units older

than the Maquoketa Group (0.85 2 ) all lie


10/13



Figure 11. Summary stratigraphic column

of extrapolated K/Ar dates of diagenetic il-

lite in the Illinois Basin.

TABLE 5. RANGE OF TEMPERATURES DETERMINED FROM VITRINITE REFLECTANCE (R0) USING AGRAPH IN DAMBERGER (1991) AFTER BOSTICK ET AL. (1979)

Age R0 50 Ma* 200 Ma*(C) (C)

Herrin Coal Pennsylvanian 0.40.7 40100 3080New Albany Devonian 0.50.8 65110 5095Maquoketa Ordovician 0.60.8# 85110 7095

Note: The range in the vitrinite data does not include the bulls-eye close to Hicks dome, which has highervitrinite reflectance.

*Refers to effective time of maximum burial. Effective time is the time that the sediment was within 15 C ofthe maximum temperature. To determine the temperatures, we used Damberger (1991) after Bostick et al. (1979).

Data from Cluff and Byrnes (1991).Data from Hasenmueller and Corner (1994).#Estimated by Guthrie (1994) using Rock-Eval and Tmax data.

in the diagenesis zone of Weaver and Broeck-

stra (1984) (0.42 2 ). The temperatures

did not reach the anchizone, which begins

with an illite crystallinity index of 0.42 2

and ends with the start of the epizone at 0.25

2 . In addition to determining the illite crys-

tallinity index, Weaver and Broeckstra (1984)

defined the end of the diagenesis zone and be-ginning of the anchimetamorphic zone as the

first occurrence of authigenic 2M1

illite. Illite

of2M1type in the Illinois Basin is inferred to

be detrital because the K-Ar age of this illite

is older than the depositional age. Type 2M1

illite forms at temperatures of about 200250

C (Velde, 1965), and if the diagenetic reac-

tion sequence from 1Md

to 1Mand finally to

2M1from the laboratory experiments of Velde

(1965) holds true for shales, the high concen-

tration of 1Md

illite in the Illinois Basin can

be used as another indicator of temperature

(200C).The temperature of illitization was further

constrained by means of oxygen isotope data

indicating that the illite in the Maquoketa

Group formed at 50100 C and that the illite

from the underlying Mt. Simon Sandstone

formed at 95145 C. Organic maturity indi-

ces all indicate low temperatures as well. The

organic matter for the Maquoketa Group is

immature to early mature (Guthrie, 1994). Vi-

trinite reflectance values (R0) for a Pennsyl-

vanian coal are 0.40.7 (Cluff and Byrnes,

1991), for the New Albany shale are 0.50.8

(Hasenmueller and Comer, 1994), and for theMaquoketa Group are 0.60.8 (Guthrie,

1994) (Table 5). The reflectance values for the

Maquoketa Group were estimated using Rock-

Eval data and Tmax

(Guthrie, 1994). The re-

flectance data for the Herrin Coal and the New

Albany shale do not include higher reflectance

associated with the intrusion of the Hicks

dome. Vitrinite reflectance is a function of

temperature and effective time of heating.

Damberger (1991) (after Bostick et al., 1979)

estimated temperatures by using effective time

of heating, which refers to the time the sedi-

ment was within 15C of the maximum tem-

perature. The resulting temperatures for the

Pennsylvanian to the Upper Ordovician Ma-

quoketa Group section seem to be no higher

than 110C, if we assume 50 m.y. of effec-

tive time of burial (Table 5).

For Ordovician carbonates underlying the

Maquoketa Group, unpublished CAI data

from the central and northern part of the Illi-nois Basin indicate indexes between 1 and 2

(Rodney D. Norby, Illinois State Geological

Survey, 1996, personal commun.); these de-

crease toward the north, indicating tempera-

tures of50 to 140 C (Harris, 1979).

Illite crystallinity, illite polytype, and oxygen

isotope data agree with the interpretation of the

published data all indicating that maximum

temperatures in the Maquoketa Group were be-

tween 50 and 100 C and that the units under-

lying the Maquoketa Group formed at higher

temperatures, but not higher than 140 C.

Evaluation of Origins of Diagenetic Illite

Diagenetic illite in the Maquoketa Group

consists of highly illitic I/S with an extrapo-

lated age of 360 m.y. Is the origin of diage-

netic illite due to the long burial time, as in-

terpreted by Gharrabi and Velde (1995), to a

brine event (hydrothermal or low-T K-rich),or some combination of the above?

Our results indicate that only a combination

of burial diagenesis and a hydrothermal or a

K-rich brine event can explain both the 360

m.y. age of the 1Mand 1Mdillite and the per-

cent illite in I/S in the Maquoketa Group.

Gharrabi and Velde (1995) interpreted their I/

S quantification data as being caused solely by

burial diagenesis, with the addition of 1.5 km

of additional overburden, eroded after the

Permian. Our kinetic modeling indicates that

highly illitic I/S can be a product of several

origins: burial diagenesis alone, a hydrother-mal event, a K-rich brine event, or a combi-

nation of the three. However, determining the

calculated age of illite from kinetic modeling

and comparing these data with observed age

data can distinguish whether burial diagenesis

alone or a combination of processes formed

the illite. Currently, we are not able to distin-

guish which process, the hydrothermal or low-

T K-rich brine, is more likely.

The hydrothermal event from the south

could have been triggered by fault move-

ments, for which Nelson and Marshak (1996)

found evidence during the Devonian. In ad-dition the Ozark dome, the Sangamon Arch,

and the Sparta shelf were also uplifted during

the Devonian (Kolata and Nelson, 1991). Mar-

shak and Paulsen (1996) suggested that in the

U.S. Midcontinent, reactivated fault zones act-

ed as fluid conduits to drive fluids from the

basement into shallower sediments, depositing

ore bodies. A similar mechanism, combined

with gravity-driven regional brine migrations,

could have caused the 100 C hot fluids to

migrate from the deeper part of the basin into

the shallower and younger sediments, heating

the Maquoketa Group to 100 C for 1 m.y. or

less (Fig. 12).

The hypersaline brines flowing from the

north into the Illinois Basin could have come

from the Michigan Basin, the Forest City Ba-

sin, or the Upper Mississippi Valley area. The

heavy hypersaline brine flowed into the Illi-

nois Basin on top of the Maquoketa Group

(Fig. 12). Movement of these brines could

have been caused by uplift of the Wisconsin

and/or Kankakee arches. Both arches were up-

lifted during the Silurian and Devonian (Nel-

son, 1995; Kolata and Nelson, 1991). These

gravity-driven brines would flow into the Si-


11/13


GRATHOFF et al.

Figure 12. Model of two different brines migrating though the Illinois Basin during the

deposition of the New Albany shale (360 Ma). The filled arrows point toward the fluid

flow direction for the hydrothermal brines. A representative fault is depicted by the half

arrows, which point toward the direction of fault movement. The driving force would be

the uplift of the Ozark dome, the Sangamon Arch, or the Sparta shelf. The white arrows

point toward the fluid flow direction for the K-rich brines, which migrated from north of

the Illinois Basin into the basin on top of the Maquoketa Group. The driving force would

be the uplift of the Kankakee arch or the Wisconsin arch.

lurian and Devonian carbonates in the Illinois

Basin and migrate downward, owing to the

density of the brine. The Maquoketa would

serve as an aquitard to these dense hypersaline

brines. The brines would slowly flow through

the Maquoketa Group, forming illite from

smectite and depleting the brine of K.

The illite in the underlying units with a K-Ar age of300 m.y. is likely to have formed

by means of a hydrothermal event, which

originated in the Alleghany-Ouachita orogenic

belts. The reasons for our interpretation are (1)

the temperature of illitization is higher, 140

C, reducing the time to form 90% illite from

pure smectite to less that 0.5 m.y. with K con-

centrations of 200 ppm (we used an equation

of Huang et al., 1993); (2) the age of the illite

is similar to other dated illite in the U.S. Mid-

continent and the Appalachian basin, and it

corresponds to the Alleghany-Ouachita orog-

eny; and (3) the morphology of the illite islaths, suggesting direct precipitation.

CONCLUSIONS

The diagenetic illite in the lower Paleozoic

of the Illinois Basin did not form by burial

diagenesis alone, but was formed mainly dur-

ing two events, one at 300 Ma and one at 360

Ma. This conclusion is based on comparing

analytical results (percentage of smectite in il-

lite/smectite and extrapolated K-Ar ages of the

diagenetic illite) with kinetic modeling results

(percentage of smectite in I/S and calculatedage of illitization). The late Paleozoic 300 Ma

event might have been associated with the Al-

leghany-Ouachita orogeny. During the

Alleghany-Ouachita event, the Maquoketa

Group served as an aquitard to brines that

formed the illite in the underlying units. The

360 Ma event that formed most of the diage-

netic illite in the Maquoketa Group might

have been caused by either a hydrothermal

event (100 C) from the southern and deeper

part of the Illinois Basin, possibly due to uplift

of the Ozark dome, Sangamon Arch, and

Sparta shelf, or a K-rich brine from north of

the Illinois Basin (Michigan Basin, Upper

Mississippi Valley, Forest City Basin) induced

by uplift of the Wisconsin Arch or Kankakee

Arch.

The Upper Ordovician Maquoketa Group

contains less smectite than the underlying

units, which is opposite of what would be ex-

pected if burial diagenesis of smectite were

the only explanation for the origin of the illite.

This anomaly can be explained using illite po-

lytype quantification. The Maquoketa Group

contains an average of 23% detrital 2M1illite

(1 m size fraction), which has no smectite

interlayers and forms larger crystallites than

the diagenetic 1M and 1Md

illite. The under-

lying units contain on average less than 5%

detrital 2M1

illite (1 m size fraction).

Therefore, the Maquoketa Group contains

only apparently less smectite.The temperature of illitization in the Ma-

quoketa Group was 50100 C. For the units

underlying the Maquoketa Group, the temper-

ature of illitization was higher, but not as high

as 140 C. The illite in the units underlying

the Maquoketa Group formed at higher tem-

peratures, which were caused by uplift and

subsequent flushing of warmer brines from the

deeper part of the basin.

APPENDIX 1. METHODS

For X-ray diffraction (XRD) analysis of orientedaggregates, we separated the 0.2m and1 msize fractions, using a centrifuge, and collected asize fraction with a Milliporefilter and transferredit onto a glass slide (Moore and Reynolds, 1997).Then FWHM of the 001 reflection, known as theillite cr ystallinity index (Kubler, 1964), Jan Sro-dons IR (Srodon, 1984; Srodon and Eberl, 1984),and peak decomposition of the 001 reflection weredetermined. IC is a function of crystallite size (or,more appropriately, X-ray diffracting domain size)and smectite content. The IR is a function of thesmectite content, the number of water layers in theair-dried samples, and the iron content. Selectedsamples were sent to Bruce Veldes laboratory atthe Ecole Normale Superieure, Paris, where Dr.James Matthews decomposed the XRD traces in the

region of the illite 001 peak, using the same methodas Lanson and Champion (1991) and Velde et al.(1986). Their method decomposes the illite 001peak (air-dried treatment) into an illite peak, a micapeak, and an I/S peak. The d-spacing of the inter-preted I/S peak component is proportional to the

smectite content of the I/S. The larger the d-spacingis, the larger the smectite content in the I/S.

For random powder XRD, we separated the 0.2m, 0.20.5 m, 0.51 m, 12 m, and 24 msize fractions using a centrifuge, flocculated thesize-separated suspensions, dialyzed the suspen-sions, and then evaporated them. Chlorite and car-bonate were removed using 1 M HNO

3. The organic

material was removed using household bleach (so-dium hypochlorite) (1 part bleach to 2 parts distilledwater). These treatments were also followed by di-alysis and evaporation.

Using the method described in Grathoff andMoore (1996), we quantified the amount of eachillite polytype from all the units studied. An ex-ample of the method is shown in Figure A1, where

an experimental pattern of the Maquoketa Groupsample G 261, 0.20.5 m size fraction, is plottedtogether with the mixed WILDFIRE-calculatedpattern (Reynolds, 1993). XRD data were obtainedusing a Scintag theta-theta diffractometer with aDMS operating system and operating conditions of40 kV and 30 mA. This diffractometer uses CuK

radiation, a liquid N2-cooled germanium solid state

detector, two Soller slits, a 2 mm divergence slit,and a 0.5 mm slit at the detector.

The K-Ar age, oxygen isotopic composition, andmorphology of illites were determined for selectedsamples. K-Ar dating of the illites was obtained atthe Institut fur Dynamik der Lithosphare at theGeorg August Universitat, Gottingen, Germany. Fordetails of the analytical procedures, see Wemmer


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Figure A1. Experimental and calculated trace of the 0.20.5 m size fraction of G 261,

Maquoketa Group sample (G core) taken at a depth of 232.2 m. Dashed lines show the

position of1M(trans-vacant) illite polytype-specific peaks, triangles indicate peaks specific

to the detrital 2M1 polytype; Qquartz; Aanatase.

and Ahrendt (1997) or Grathoff et al. (1998). Ox-ygen isotopic composition was determined in Sam-uel Savins laboratory, Case Western Reserve Uni-versity, by Robert Ylagan. The morphology wasexamined with an atomic force microscope at theBeckman Institute, University of Illinois, with thehelp of Javier Cuadros.

For the kinetic modeling of smectite illitizationthrough burial diagenesis, we used a computer pro-gram developed by Bill Benzel, Marathon Oil Co.,Littleton, Colorado, and Stephen Altaner, Depart-ment of Geology, University of Illinois; the programis based on the kinetic equation of Pytte and Reyn-

olds (1988). For smectite illitization in hydrother-mal and K-rich brine systems, we used the kineticequation of Huang et al. (1993). Kinetic parametersfor both equations were the same as Elliott and Ma-tisoff (1996), who compared these kinetic models.For the Pytte and Reynolds equation, the input var-iables were a fifth-order reaction, activation energyof 30 000 cal/mol, and a frequency factor of 90 000/s. For Huang et al.s (1993) third-order equation,the input variables were an activation energy of 28000 cal/mol, a frequency factor of 80 800/s, and Kconcentrations of 1000, 2000, and 20 000 ppm. Oth-er assumptions included (1) a K/Na ratio for Pytteand Reynolds (1988) that was controlled by K/Nafeldspar equilibrium; (2) a burial rate of 8.5 m/m.y.[from Gharrabi and Velde (1995) for a core about

32 km south of the CL core; Fig. 1]; (3) a geother-mal gradient of 30 C/km (Cluff and Byrnes, 1991);and (4) that an additional overburden of 1.5 km waseroded after the Permian [based on Damberger(1991), Cluff and Byrnes (1991) and Gharrabi andVelde (1995), using, respectively, coal-seam mois-ture, kinetic modeling of organic maturation, andkinetic modeling of smectite illitization].

ACKNOWLEDGMENTS

This research was part of Georg Grathoffs Ph.D.thesis, completed at the University of Illinois at Ur-bana/Champaign and supported in part by the Illi-nois State Geological Survey and by a researchgrant from the Clay Minerals Society. We thank

Elizabeth Burton, Eric Daniels, and W. CrawfordElliott for their review comments on ourmanuscript.

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