GEOLOGICA CARPATHICA, 50, 5, BRATISLAVA, OCTOBER 1999
365372
ILLITE CRYSTALLINITY AND VITRINITE REFLECTANCE
IN PALEOZOIC SILICICLASTICS IN THE SE BOHEMIAN MASSIF
AS EVIDENCE OF THERMAL HISTORY
EVA FRANCÙ
1
, JURAJ FRANCÙ
1
and JIØÍ KALVODA
2
1
Czech Geological Survey, Leitnerova 22, 658 69 Brno, Czech Republic; francu@cgu.cz
2
Department of Geology, Masaryk University, Kotláøská 2, 611 37 Brno, Czech Republic; dino@sci.muni.cz
(Manuscript received December 9, 1998; accepted in revised form June 22, 1999)
Abstract: The thermal maturity of Paleozoic rocks in the SE part of the Bohemian Massif is characterized by clay
minerals and organic matter. The expandability of illite-smectite (S in I-S), illite crystallinity index (IC) and reflec-
tance of (R
r
) were measured and their regional distribution was evaluated. The mutual correlation of IC and R
r
from
diagenesis to very low-grade metamorphism is compared with the published data and used to distinguish data with
more reliable paleogeothermal information from those affected by other factors. In the SE part the Paleozoic units
have illite-smectites with an expandable component of 1535 % S. The reflectance values (R
r
of 0.551.1 %) are in
good agreement with the expandability and suggest the oil window range of catagenesis with paleotemperature close
to 100 °C. In the NNW part of the area the clays contain no expandable layers in illite. The illite crystallinity (IC of
0.360.24
∆
°2
Θ
) and vitrinite reflectance (R
r
from 3.17 to 5.23 %) indicate very low-grade metamorphic conditions
with probable maximum paleotemperatures of 240300 °C. The systematic change in both clay and organic param-
eters shows the gradual decrease in thermal exposure towards the front of the Variscan orogenic zone in the S and SE
and suggest extensive erosion in the NNW.
Key words: Paleozoic, thermal history, vitrinite reflectance, expandability, illite crystallinity, I-S.
Introduction
The illite crystallinity index (IC), expandability of illite-smec-
tite (S in I-S) and vitrinite reflectance (R
r
) are widely used pa-
rameters of thermal alteration of sedimentary rocks. Smectite
to illite evolution is a typical diagenetic reaction with a gradu-
al decrease of expandable layers (% S), increase of illite layers
in I-S, and progressive ordering (reichweite R0 and R > 0)
followed by growth of illite crystals (rodoñ & Eberl 1984).
The Al-Si substitution in the tetrahedral sheet, the increase of
the layer charge and irreversible potassium fixation in the in-
terlayer play the decisive role in illitization (Moore & Rey-
nolds 1997). These processes occur in temperature range from
50 to 300 °C covering diagenesis and very low-grade meta-
morphism. The understanding of the structural changes during
smectite to illite conversion benefited from computer simula-
tion of X-ray diffraction profiles (Reynolds & Hower 1970).
The illite crystallinity index characterizes the structural
evolution of illite, mainly the increasing size of coherently
diffracting domains and decreasing lattice distortion. It is
based on the shape of the first (001) peak of illite (Weaver
1960; Kübler 1964; Weber 1972). The fullwidth at half max-
imum (FWHM) expressed in
∆
°2
Θ
is used by most authors
as IC the illite crystallinity index (Kübler 1967; Árkai
& Lelkes-Felvári 1993). A similar parameter is evaluated for
chlorite using 001 and 002 peaks (Árkai et al. 1995). The
correlation of the IC of illite and chlorite shows an earlier
narrowing of the chlorite peaks than that of illite during late
diagenesis and very low-grade metamorphism (Árkai 1991).
Organic matter in sedimentary rocks (kerogen) is a sensi-
tive indicator of thermal stress in the range of 50300 °C
(Bostick 1979; Robert 1988). Coalification (thermal maturi-
ty) depends on the total thermal history of the host rocks, that
is both temperature and exposure time (Waples 1985). Vitrin-
ite reflectance (R
r
) is the best established parameter of organ-
ic matter which can be measured in most black shales and
slates. Its application is limited by the absence of terrestrial
plant debris in pre-Devonian or purely marine rocks.
Many authors applied analytical data of clay minerals and
organic matter in regional studies and related them to pale-
otemperatures in sedimentary basins and orogenic belts
(Pearson & Small 1988; Robert 1988; Francù et al. 1990;
Pollastro 1990 and 1993; Underwood et al. 1993; rodoñ
1995). Thermal alteration of both rock components is irre-
versible during uplift and temperature drop. During weather-
ing, however, the clay minerals undergo illite-to-smectite
breakdown which erases the paleo-thermal signature.
Regional setting
The surface geology of the studied area consists of the Pa-
leozoic of the Drahany and Zábøeh Uplands (Drahanská and
Zábøeská vrchovina), Lower Miocene and Pliocene of the
366 FRANCÙ, FRANCÙ
and KALVODA
Carpathian Foredeep, and the Tertiary of the Carpathian Fly-
sch Belt. The outcrop and subcrop map of the Paleozoic with
simplified names of regional units and overlain boundaries of
the surface units is in Fig. 2. The sample location numbers re-
fer to Table 1 where more detailed lithostratigraphy is given.
The Paleozoic units of the eastern Bohemian Massif are re-
garded as a part of the Rhenohercynian and Subvariscan
Zone (Franke in Dallmeyer et al. 1995) of the Variscan oro-
genic belt (Fig. 1). The Bruno-Vistulian crystalline basement
consolidated during the Cadomian orogeny is overlain in some
places by Lower Cambrian siliciclastics (Vavrdová 1997), one
occurrence of Silurian shales and limestones, and widespread
Middle Devonian basal clastics. The stratigraphic profile
continues upwards with Devonian to Lower Carboniferous
carbonates (Macocha, Jesenec, and Líeò Fms.) and pre-fly-
sch siliciclastics (Stínava-Chabièov and Ponikev Fms.). The
Variscan synorogenic flysch (Culm) of Lower Carboniferous
(Viséan) age (Protivanov, Rozstání, and Myslejovice Fms.)
covers most of the outcrop and subcrop surface of the Paleo-
zoic in this region. The Upper Carboniferous molasse sedi-
ments in the east (Ostrava and Karviná (?) Fms.) are the up-
permost units of the Paleozoic and represent the Subvariscan
Zone (Dvoøák 1995).
The Mírov Unit (Otava & Sulovský 1997) is a separate
tectonic block adjacent to the NW of the Drahany Upland
and includes mostly siliciclastics, probably of Devonian age
(Mohelnice Fm.).
Table 1: Geological and analytical characteristics of the studied samples.
Location
Locality
Depth
Age
Formation
Tectonic
Fraction FWHM
% EXP
R
r
R
max
R
min
in map
(m)unit
mm
IC (°2Q)
mean
(%)(%)(%)
1
Mezihoøí
0
D2.3
Mohelnice
Mírov unit
< 2
0.36
0
3.89*
5.41
0.85
1
Mezihoøí
0
D2.3
Mohelnice
Mírov unit
< 0.2
0.53
0
3.89*
5.41
0.85
2
KDH 8A
125.5
D3.C1
Macocha
carbonates
< 2
0.24
0
4.84*
5.78
2.97
2
KDH 8A
171.6
D3.C1
Macocha
carbonates
< 2
0.24
0
5.23*
5.95
3.79
3
KDH 5
158.5
D3.C1
Ponikev
Pre-flysch Paleozoic
< 2
0.34
0
4.24*
5.51
1.69
3
KDH 5
229.4
D3.C1
Ponikev
Pre-flysch Paleozoic
< 2
0.36
0
4.86*
6.00
2.58
4
KDH 1
11
C1
Protivanov
Variscan flysch belt
< 2
0.32
0
3.17*
4.09
1.32
4
KDH 1
24.5
C1
Protivanov
Variscan flysch belt
< 2
0.32
0
3.56*
4.46
1.75
4
KDH 1
76.2
C1
Protivanov
Variscan flysch belt
< 2
0.30
0
4.51*
5.63
2.26
5
Sloup-oùvka
0
D2-C1
paleocarst
carbonates
< 2
0.32
< 4
-
-
-
6
Ostrov u Macochy
0
C1v
Rozstání
Variscan flysch belt
< 2
0.44
< 4
1.79
-
-
7
Jedovnice
0
C1v
Rozstání
Variscan flysch belt
< 2
0.55
< 4
1.61
-
-
8
Køtiny
0
C1v
Rozstání
Variscan flysch belt
< 2
0.48
< 4
2.42
-
-
9
Ochoz, Nové Dvory
0
C1v
Rozstání
Variscan flysch belt
< 2
0.48
< 4
1.92
-
-
10
Mokrá - LV 10
44.7
C1v
Myslejovice
Variscan flysch belt
< 0.2
0.709
(ChC)
< 4
-
-
-
10
Mokrá - LV 10
44.7
C1v
Myslejovice
Variscan flysch belt
< 2
0.637
(ChC)
< 4
-
-
-
11
Mokrá - LV 9
125.6
C1v
Myslejovice
Variscan flysch belt
< 0.2
1.21
< 4
1.57
-
-
11
Mokrá - LV 9
125.6
C1v
Myslejovice
Variscan flysch belt
< 2
1.33
< 4
1.57
-
-
12
Mokrá - LV 6
68.7
D2-C1
Líeò
carbonates
< 0.2
1.09
< 4
1.38
-
-
12
Mokrá - LV 6
68.7
D2-C1
Líeò
carbonates
< 2
1.46
< 4
1.38
-
-
13
Mokrá - LV 11
41.8
D2-C1
Líeò
carbonates
< 0.2
1.36
< 4
1.55
-
-
14
Hády (Dungle) 0
D2-C1
Líeò
carbonates
< 2
0.44
< 4
2.01
-
-
15
Uhr-13
2563
C1
Myslejovice
Variscan flysch belt
< 0.2
1.65**
13
0.55
-
-
16
Dam-1
2791
C2-1
Ostrava
Variscan Foredeep
< 0.2
1.98**
32
0.63
-
-
16
Dam-1
3021
C2-1
Ostrava
Variscan Foredeep
< 0.2
1.42**
35
0.68
16
Dam-1
3021
C2-1
Ostrava
Variscan Foredeep
< 2
1.91**
-
0.68
16
Dam-1
3592
C1v
Myslejovice
Variscan flysch belt
< 0.2
2.28**
24
0.75
-
-
16
Dam-1
3592
C1v
Myslejovice
Variscan flysch belt
< 2
1.68**
-
0.75
-
-
16
Dam-1
3872
D2-C1
Macocha
carbonates
< 0.2
2.26**
19
0.80
-
-
17
Nìm-2
3796
C2-1
Ostrava
Variscan Foredeep
< 0.2
1.98**
23
0.80
-
-
17
Nìm-2
3864
C2-1
Ostrava
Variscan Foredeep
< 0.2
2.16**
25
0.85
-
-
17
Nìm-2
4243
C2-1
Ostrava
Variscan Foredeep
< 0.2
1.92**
23
0.95
-
-
17
Nìm-2
5338
D2-C1
Macocha
carbonates
< 0.2
2.14**
-
1.10
-
-
17
Nìm-2
5338
D2-C1
Macocha
carbonates
< 2
1.11**
-
1.10
-
-
* recalculated R
r
from R
max
and R
min
** IC values have just approximate meaning
Fig. 1. The Bohemian Massif (BM) and its position in the
Variscan orogenic zones of central and NW Europe (modified after
Chaloupský 1989). The grey areas are Variscan massifs. The posi-
tion of the studied area is indicated by the rectangle.
ILLITE CRYSTALLINITY AND VITRINITE REFLECTANCE 367
Fig. 2. Outcrop and subcrop map of the SE part of Moravo-Sile-
sian Paleozoic (modified after Dvoøák 1995 with supplements of
Otava, personal com.). The geological subdivision is simplified
and each unit includes several formations. Sample numbers refer
to the more detailed description in Table 1.
Samples
Surface and borehole core samples of Devonian and Carbon-
iferous sedimentary rocks were collected to characterize the re-
gional trend of thermal maturity from N to S (Fig. 2 and Ta-
ble 1). Lithologically they are mostly siltstones, shales or slates
(occasionally quartzified), and shale interlayers in limestones.
Methods
Clay mineral analysis
The clay fraction was separated from the powdered rocks
after elimination of cements such as carbonates, organic mat-
ter and iron-manganese oxyhydroxides (Jackson 1975).
Grain size fractions < 2
µ
m and < 0.2
µ
m were obtained by
sedimentation and centrifugation. Oriented slides were anal-
ysed by X-ray diffraction both air dry and glycolated (EG, 10
hrs. at 60 °C, then 2 hrs. at 20 °C). Philips diffractometer PW
1830 (generator) and PW 3020 (goniometer) wereused with
0.02° step from 2 to 30
∆
°2
Θ
. Peak fitting was applied to
substract the ramp background and determine the individual
peak positions in the composite peaks and to evaluate the full
width at half maximum (FWHM). The Ir index (rodoñ &
Eberl 1984) defined as the ratio (001/003 air-dry)/(001/003
EG) was measured and calculated to characterize trace
amounts of expandable layers in highly illitic materials. Ex-
pandability of illite-smectite was evaluated from peak posi-
tions using the plots of rodoñ & Eberl (1984) and NewMod
simulations (Reynolds 1985). The illite crystallinity index
(IC) was measured as FWHM of the 001 basal reflection
(Kübler 1967) or the chlorite (002) crystallinity index (ChC)
when illite was absent (Weaver et al. 1984; Árkai 1991).
Organic matter
The measurements of reflectance were carried out in oil on
polished surfaces of rocks using Leitz Wetzlar MPV2 micro-
scope-photometer, objective 50
×
and Leitz standards of 1.26
and 5.42 %. The reflectance was measured in non-polarized
and plane-polarized light (R
r
, R
max
and R
min
). In order to ob-
tain a single parameter for the entire range of thermal maturi-
ty, the R
max
and R
min
values of particles with higher bireflec-
tance were recalculated to random reflectance using the
equation R
r
= (2*R
max
+R
min
)/3 (Teichmüller et al. 1998). The
measured organic particles (macerals) were petrographically
identified as vitrinite, liptinite, inertinite or redeposited ma-
terial following Teichmüller et al. (1998) and only the data of
indigenous vitrinite or vitrinite-like macerals were used in
further evaluation of the thermal history.
Results
The clay fraction of the studied series of rocks includes il-
lite, chlorite, kaolinite and mixed-layer minerals. Illite-smec-
tite (I-S) has expandability of 035 % S and illite crystallini-
ty index (IC) ranges from 0.24 to 2.28
∆
°2
Θ
. Vitrinite
reflectance (R
r
) increases from 0.55 to 5.23 %. These values
(Table 1) cover thermal alteration from burial diagenesis to
very low-grade metamorphism. The data are divided into
four groups of samples characterized by typical features in
the XRD patterns which represent subsequent phases of the
shale-to-slate evolution (Figs. 37 and 10).
The first group of data comprises Paleozoic sediments
with the lowest thermal alteration. It is documented by the
XRD patterns (Fig. 3) of an air-dry and glycolated clay (frac-
tion < 2
µ
m) of an Upper Carboniferous shaly limestone with
a low amount of detrital silt and clay. The IC index of 1.68
∆
°2
Θ
is too high to be used as a measure of illite crystal size.
The shape and position of I-S peaks change significantly af-
ter glycolation and indicate a mixture of detrital, that is in-
herited illite and newly formed expandable I-S.
The expandable component is better characterized by an
analysis of the very fine clay fraction (< 0.2
µ
m) of the same
368 FRANCÙ, FRANCÙ
and KALVODA
sample where the detrital illite is eliminated. Glycolation re-
sults in a split of broad 001 and 002 illitic peaks into pairs of
rectorite-type peaks with expandability of 24 % S and R1 or-
dering (Fig. 4). Such clay parameters and vitrinite reflec-
tance of 0.551.1 % are typical of the burial diagenetic phase
of thermal alteration. Regionally these rocks occur in the S
and SE where boreholes encountered the Devonian and Car-
boniferous below the nappes of the Western Carpathians at
depths of 2.85.4 km.
The second and third groups of samples (Fig. 5) represent
highly illitic material with illite peaks slightly changing their
shape after glycolation. Chlorite is common and sometimes
even more abundant than illite (Fig. 6). In some samples a shift
of the chlorite peak after glycolation suggests the presence of an
expandable component in the chlorite. Little or no material of
the < 0.2
µ
m grain size can be separated from the samples in a
non-destructive way, most probably due to crystal growth. The
estimated amount of smectite layers in illite is < 4 % and the il-
lite crystallinity index is 1.460.44
∆
°2
Θ
. The chlorite crystal-
linity index (ChC) is 0.70.6
∆
°2
Θ
. The clay parameters and
vitrinite reflectance of 1.42.4 % found in different stratigraphic
Fig. 3. The XRD pattern of coarser clay fraction (< 2
µ
m) of a
shaly limestone, Damboøice-1 (3592 m). The sample is enriched in
detrital illite (Id) and chlorite (C) which are associated with illite-
smectite (I-S).
Fig. 4. The XRD pattern of the fine clay fraction (< 0.2
µ
m) of a
shaly limestone, Damboøice-1 (3592 m). Detrital illite is eliminated.
Fig. 5. The XRD pattern of the Hády outcrop sample (V Dungli
Quarry), fraction < 2
µ
m. I illite, C chlorite, Q quartz.
Fig. 6. The XRD pattern from the Mohelnice outcrop, fraction < 2
µ
m.
Fig. 7. The XRD pattern of KDH-8A borehole, 171.6 m, fraction
< 2
µ
m.
units of the Drahany Upland and Mírov Unit correspond to ther-
mal alteration of the late diagenetic phase.
The last group of samples is characterized by almost non-
expanding illitic mineral (Fig. 7) with narrow and sharp peaks
ILLITE CRYSTALLINITY AND VITRINITE REFLECTANCE 369
and IC values of 0.240.36
∆
°2
Θ
. This type of minerals oc-
curs in the Konice area and is associated with high vitrinite re-
flectance (R
max
= 46, R
r
= 3.175.23 %) and bireflectance
(R
max
R
min
= 2.163.82 %) equivalent to the metaanthracite
rank. Both parameters suggest that these rocks experienced
very low-grade metamorphic conditions.
Correlation between illite crystallinity
and vitrinite reflectance
The illitization stage and coal rank are primarily controlled
by thermal history (rodoñ & Eberl 1984; Robert 1988). Ex-
amination of their mutual correlation is an important step in
the data reliability assessment. The cross-plot of the illite
crystallinity index (IC) and mean random vitrinite reflec-
tance (R
r
) shown in Fig. 8 reviews the earlier published data
(Duba & Williams-Jones 1983; Underwood et al. 1991,
1993; Todorov et al. 1992; Henrichs 1993).
Other authors characterize the illite crystalinity and coalifi-
cation rank by Hb
rel
and R
max
. To convert Hb
rel
to IC readers
need to know the value of peak width at half maximum
(FWHM) of the quartz standard (Hb
rel
= [Hb (001) illite/Hb
(100) quartz]*100). Conversion of R
max
to R
r
requires R
min
values (see methods above). The published data where the
FWHM of quartz and R
min
are missing or the R
r
and IC values
are given only as ranges (Wolf 1975; Kish 1987; Teichmüller
et al. 1979) are, therefore, not included as references in Fig. 8.
The diagram in Fig. 9 summarizes the diagenetic and
metamorphic zones with their boundary values (Teichmüller
et al. 1979; Robert 1988; Kish 1983, 1991). The general
trend shows decrease of vitrinite reflectance (R
r
) with in-
creasing IC index. The data below this trend (Todorov et al.
1992) represent measurements of authigenic vitrinite and re-
deposited detrital illite. The data which would plot above this
trend (Figs. 8 and 9, upper right corner) do not represent con-
sistent evidence of thermal history. Such a combination of
the R
r
and IC values may occur when the following materials
are measured:
1. redeposited organic matter from more metamorphosed
rocks associated with authigenic illite-smectite;
2. preserved organic matter and disaggregated illite in sec-
ondary illite-smectite in weathered black slates;
3. graphitized organic matter (or metaanthracite) associat-
ed with poorly aggraded illite in rocks from the contact meta-
morphic zones of igneous bodies (Árkai, personal communi-
cation 1999).
All our measured samples (Fig. 10) plot within the shaded
belt of good correlation in Fig. 9.
Regional distribution of the paleothermal signature
The distribution of the diagenetic-to-metamorphic alter-
ation based on the illite crystallinity index is shown in the
outcrop and subcrop map of the studied Paleozoic (Fig. 11).
Fig. 8. Review of the published data on diagenesis/metamorphism
(1 Todorov et al. 1992; 2 Underwood et al. 1993; 3 Un-
derwood et al. 1991; 4 Henrichs 1993; 5 Duba & Williams-
Jones 1983). Our data plot within the dashed line envelope.
Fig. 9. Generalized relationships between illite crystallinity (IC)
and random vitrinite reflectance (R
r
). The boundaries of the meta-
morphic zones are given according to Teichmüller et al. (1979),
Robert (1988), Kisch (1983).
Fig. 10. Cross plot of two paleothermal indicators in Paleozoic sam-
ples from the southeastern part of the Bohemian Massif. Different
symbols represent the partial areas. The open symbols (fraction
< 0.2
µ
m) linked with black symbols (< 2
µ
m) belong to the same
raw samples. The arrow indicates the estimated equivalent illite
crystallinity derived from the chlorite crystallinity index (ChC).
370 FRANCÙ, FRANCÙ
and KALVODA
The Carboniferous strata below the West Carpathian over-
thrust have diagenetic expandable illite-smectite and organic
maturity equivalent to the oil generation zone of diagenesis
(c.f. Francù et al. 1989; Pollastro 1990; Pereszlényi et al.
1993, 1997; Milièka et al. 1994; Masaryk et al. 1995). The
present results of clay analysis support the earlier conclu-
sions based only on vitrinite reflectance (Dvoøák & Wolf
1979; Dvoøák 1989; Krejèí et al. 1994). The expandability
values correspond to the maximum burial temperatures of
80130 °C observed in other basins (Francù et al. 1990)
which is also well constrained by the vitrinite reflectance of
R
r
= 0.551.1 %.
In the SE part of the Drahany Upland (e.g. Mokrá quarry)
the clay and organic data (ChC of 0.64, R
r
of 1.381.57 %)
suggest a late diagenetic phase (dry gas zone) with an esti-
mated paleotemperature of 130170 °C.
In the central Drahany Upland the illitic material in the
< 2 µm fraction has an expandable component of < 4 % S. The
IC ranges from 0.32 to 0.55
∆
°2
Θ
and vitrinite reflectance (R
r
)
from 1.9 to 2.4 %. These data are typical of late diagenetic
conditions with temperatures of 170200 °C (Bostick 1979;
Underwood et al. 1991, 1993).
Even higher thermal maturity is observed in the Mírov
Unit (Mohelnice Fm.) NW of the Drahany Upland (Table 1,
Figs. 10 and 11).
In the Konice window the vitrinite reflectance is of the
metaanthracite rank and the illite crystallinity is in the very
low-grade metamorphic range. From comparison with simi-
lar data (Milièka et al. 1991; ucha et al. 1994) or supported
also by fluid inclusions (e.g. Robert 1988; Frey et al. 1980;
Frey 1986) it may be estimated that the evaluated Variscan
flysch and pre-flysch sediments were buried at a temperature
of 240300 °C.
Conclusions
Illite crystallinity and vitrinite reflectance in the Paleozoic
sedimentary rocks of the SE Bohemian Massif show a rather
broad but clear correlation belt. The data suggest a close link
between the maximum paleotemperature and position within
the Variscan orogen. High temperature exposure, deep burial
and significant erosion probably occurred in the inner part of
the Variscan thrust and fold belt which is now situated in the
NW part of the Drahany Upland.
Intermediate paleo-thermal signature is observed in the
central and SE part of the Drahany Upland and in the Mírov
Unit (Zábøeh Upland).
Low thermal maturity gives evidence of shallower burial
in the frontal Rhenohercynian and Subvariscan zones now
buried under the nappes of the Outer Carpathians.
Acknowledgements: The authors wish to express their
many thanks to J. Otava, J. Dvoøák, P. Müller, and J. rodoñ
for helpful suggestions and to V. ucha and P. Árkai for the
revision of the manuscript.
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