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GEOLOGICA CARPATHICA, JUNE 2007, 58, 3, 263—276

www.geologicacarpathica.sk

Mineralogy and geochemistry of Upper Miocene pelitic

sediments of the Zagorje Basin (Croatia): implication for

evolution of the Pannonian Basin

ANITA GRIZELJ

1

, DARKO TIBLJAŠ

2

 and MARIJAN KOVAČIĆ

1

1

Croatian Geological Survey, Sachsova 2, P.O. Box 268, 10000 Zagreb, Croatia;  anita.grizelj@hgi-cgs.hr;  mkovacic@geol.pmf.hr

2

Department of Geology, Faculty of Science, University of Zagreb, Horvatovac bb, 10000 Zagreb, Croatia;  dtibljas@public.srce.hr

(Manuscript received November 29, 2005; accepted in revised form October 5, 2006)

Abstract: According to X-ray powder diffraction analyses of Upper Miocene pelitic sediments from the Zagorje Basin
all of the samples contain same mineral species, but in significantly different quantities. Calcite is a dominant component
in most of the samples (31—74 wt. %), clay minerals (18—50 wt. %), quartz (5—21 wt. %) and feldspars (1—5 wt. %) are
less abundant, while dolomite and pyrite are present only in a few samples. Among clay minerals, in  < 2  m insoluble
residue fraction, smectite, illite, chlorite and kaolinite were determined. The dominant constituents in 0.09—0.16 mm
fraction, as determined by optical microscopy, are quartz, feldspars, rock fragments (chert, quartzite and schists) and
micas. Chlorite, limonite, pyrite, garnet, tourmaline, zircon, epidote and staurolite are present as heavy minerals. Pelitic
sediments within older (Upper Pannonian) investigated sediments are, in accordance with the mineral composition of
insoluble residue and CaCO

content, classified as marls, while those in the younger (Lower Pontian) sediments are silty

marls. The observed gradual decrease in carbonate content, and simultaneous increase of clayey-silty component, going
from older to younger deposits, is the result of the gradual increase of terrigenous influence. Mineral composition of marls
together with elemental ratios critical of provenance (SiO

2

/Al

2

O

3

, K

2

O/Na

2

O, Eu/Eu*, La/Sc, Th/Sc, Th/Co, Th/Cr and

La/Co) and source rock discrimination diagrams (Fe

2

O

3

-K

2

O-Al

2

O

and La-Th-Sc), point out that source rocks were

from the Upper Crust and remarkably felsic in nature. The chemical composition of the sediments and modal composi-
tion of silt-size fraction indicate Alpine provenance of the clastic material.

Key words: Upper Miocene, Zagorje Basin, chemical composition, clay minerals, marl, X-ray diffraction.

Introduction

The composition and provenance of the detrital material,
which was deposited during Late Miocene in the Croatian
part of the Pannonian Basin System (PBS), were studied in
detail by Šćavničar (1979), Šimunić & Šimunić (1987),
Kovačić (2004) and Kovačić et al. (2004), Kovačić & Gri-
zelj (2006). However, due to the fact that their investiga-
tions were focused on medium- to coarse-grained clastic
sediments, the petrology, mineral composition and prove-
nance of Upper Miocene pelitic sediments from the Croat-
ian part of the PBS is still obscure.

On the contrary, numerous investigations dealt with

mineral composition, provenance and diagenesis of pelitic
layers within the Hungarian part of the PBS. These investi-
gations (Varsányi 1975; Viczián 1975, 2002; Tanács &
Viczián 1995) showed that pelitic sediments contain illite,
smectite, illite/smectite, chlorite and kaolinite. According
to Viczián (2002) in the Great Hungarian Plain clay miner-
al composition was basically the same: a polymineral de-
trital clay mineral suite displaying slight systematic
regional differences, with no significant differences in
composition of different stratigraphic horizons in the time
span from Late Pannonian to Late Pliocene—Pleistocene.
Detrital clay minerals were transported from neighbouring
Carpathian and Alpine areas. Sub-basins may differ in de-
gree of disorder and quantitative proportion of clay miner-

als and quantitative relations of other phases like calcite,
dolomite, quartz and feldspars depending on relatively
permanent source areas and transport direction.

The aim of the work was to investigate mineral and

geochemical characteristics of Upper Miocene pelitic sed-
iments from the Zagorje Basin, a sub-basin located in the
south-western marginal area of the PBS, in the north-west-
ern part of Croatia (Fig. 1). Investigations were performed
on the samples collected from two localities, Pušća and
Kupljenski Hruševec (Fig. 2). The stratigraphic attribu-
tions given in Fig. 2, which are used in the article, have
doubtful chronostratigraphic value due to the fact that age
determination of the sediments was based on their fossil
content.

The main aim of the paper was determination of the

mineral composition of pelitic sediments and reconstruc-
tion of their provenance based on their mineral and chemi-
cal characteristics. Correlation of the results with those
obtained in the neighbouring, Hungarian part of the PBS
is planned.

Geological setting

The Miocene deposits of the Zagorje Basin belong to

the south-western marginal belt of the PBS. The PBS,
which belongs to the Central Paratethys, comprises several

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264

GRIZELJ, TIBLJAŠ and KOVAČIĆ

smaller sub-basins separated by relatively shallowly laid
basement rocks (Royden 1988). It is surrounded by the
Alps, Carpathians and Dinarides (Fig. 3).

The basin evolved during the Tertiary simultaneously

with the Alpine-Carpathian orogen, as a result of back-arc
extension due to continental rifting and collision of the
European and African plates.

In the paleogeographical sense the PBS occupied major

part of the Central Paratethys, a sedimentation area which
was several times connected and separated from the world
seas during its evolution. The PBS was definitely isolated
from other parts of Paratethys at the end of Middle Mi-
ocene, what resulted in formation of the brackish Pannon-
ian Lake (Rögl & Steininger 1983; Rögl 1998). During
the Late Miocene, because of clastic systems prograda-
tion, the area of the lake was gradually diminished while
land areas simultaneously enlarged (Bérczi & Phillips
1985; Pogácsás et al. 1988; Mattick et al. 1998; Juhász &
Magyar 1992; Vakarcs et al. 1994; Kovačić et al. 2004;
Kovačić & Grizelj 2006).

Sedimentation of the Upper Miocene deposits in the in-

vestigated part of the Zagorje Basin shown on Fig. 2, start-
ed by, usually continuous, deposition of Lower Pannonian
Croatica Beds, thin-bedded clayey limestones and calcite-
rich marls, on the Sarmatian beds in a littoral zone of the
low-salinity lake.

The Upper Pannonian Banatica Beds, represented al-

most exclusively by marls with equal shares of clayey and
carbonate components, were continuously deposited on
the Croatica Beds within the deepened sedimentation ba-
sin (Šikić et al. 1978, 1979; Kovačić 2004). They were

continuously covered by Lower Pontian marly layers.
Marl deposition was occasionally interrupted by deposi-
tion of sandy and silty detritus originating from the basin
margin. Such deposition resulted in the Abichi Beds, alter-
nating beds of sands, silts and silty marls. Due to further
increase of clastic material input and simultaneous de-
crease of basin subsidence, the sedimentation basin became
shallower, so in the Upper Pontian Rhomboidea Beds,
sandy-silty sediments, were deposited in a shallow lake.

The Upper Miocene deposits are overlain by Pliocene

siliciclastic deposits accumulated in small fresh-water
lakes, swamps and rivers (Pavelić et al. 2003).

Materials and methods

Thirteen samples of massive marls of homogeneous

structure were investigated. According to fossil mollusc
and ostracod associations samples 1 to 6 are of Late Pan-
nonian age, while samples 7 to 13 are of Early Pontian age.

The distribution of major and trace elements in the clay

size fraction is strongly influenced by rock mineral com-
position (Refaat 1993), therefore analyses were performed
on insoluble residue and its clay size fraction ( < 2  m).
Samples were prepared by carbonate fraction dissolution
by acetic acid with an ammonium acetate (1 mol · dm

—3

)

buffer of pH 5 (Jackson 1956) followed by separation of
the < 2  m fraction by normal gravity settling according
to Stoke’s law.

X-ray powder diffraction (XRPD) patterns were recorded

on random and oriented mounts of air dried material, and

Fig. 1. Geological sketch-maps of Hrvatsko Zagorje and Mt. Medvednica with locations of Pušća (Puš-I) and Kupljenski Hruševec
(KuH-I) outcrops (based on the 1:300,000 Geological Map of the Republic of Croatia, Institute of Geology, Zagreb).

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UPPER MIOCENE PELITIC SEDIMENTS OF THE ZAGORJE BASIN (CROATIA)

Fig. 2. Lithostratigraphical column of the Upper Miocene succession of the Zagorje Basin
(1 : 2,000) (after Kovačić 2004) with lithological columns recorded on Pušća and Kupljenski
Hruševec outcrops (1 : 500).

after glycol treatment, heating to
400 ºC and 550 ºC (Starkey et al.
1984) using a Philips  vertical X-
ray goniometer (type X‘Pert)
equipped with Cu tube and
graphite crystal monochromator,
with the following experimental
conditions: 40 kV, 40 mA, prima-
ry beam divergence 1/4º, contin-
uous scan (step 0.02 º2 /s). In
order to distinguish kaolinite and
chlorite several additional tests
were used, namely the XRD pat-
tern was recorded after boiling
the sample in 2 N HCl (Starkey et
al. 1984) as well as after dimeth-
yl-sulfoxide (DMSO) treatment
(Calvert 1984). Quantitative anal-
ysis was performed according to
Schultz (1964).

IR spectra were recorded on

Perkin Elmer Spectrum One in-
strument. Approximately 0.03 g
of < 2  m fraction of sample in-
soluble residue was mixed and
homogenized with 0.2 g of KBr
in an agate mortar. The mixture
was pressed at 5 tons. The spectra
were recorded over the range
4000—450 cm

—1

, but the conclu-

sion about the presence of ka-
olinite was based on spectrum
appearance in the OH-stretching
region (Russell & Fraser 1994).

Chemical analyses of insoluble

residue and its  < 2  m mineral
fraction were performed in ACME
Analytical Laboratories LTD,
Vancouver, Canada. Major ele-
ments content was determined by
inductively coupled plasma emis-
sion spectroscopy (ICP-ES), while
trace elements were measured on
an inductively coupled plasma
mass spectrometer (ICP-MS). For
both of the methods most of the
elements were analysed after
melting of samples with lithium-
metaborate (LiBO

2

), while noble

and base metals were analysed
from solutions prepared by disso-
lution of samples in aqua regia.
The content of CaCO

3

 was deter-

mined by EDTA (ethylenediamine
tetra acetic acid) titration.

Composition of the sandy-silty

fraction of samples was deter-
mined on a polarizing micro-
scope by quantitative modal

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266

GRIZELJ, TIBLJAŠ and KOVAČIĆ

analysis of the 0.09—0.045 mm mineral fraction after re-
moval of calcite by 4 % cold hydrochloric acid.

Results

The mineral composition of the insoluble residue and

clay mineral content of its <2  m fraction are shown in
Tables 1 and 2 respectively. The mineral composition of
the whole rock (Table 3) was calculated on the basis of the
mineral composition of its insoluble residue and CaCO

3

content, and accordingly rocks were classified after Konta
(1973). Table 4 shows the modal composition of 0.09—
0.045 mm fraction obtained by polarizing microscope.
Fig. 4 shows the XRD pattern of untreated sample 3 (A), in
comparison with patterns recorded after ethylene glycol
treatment (B) and heating at 550 ºC (C), while Fig. 5 gives
its IR spectra. An absorption band at 3698 cm

—1

 is typical

of kaolinites (Russell & Fraser 1994). Kaolinite presence
was also confirmed by DMSO treatment. Fig. 6, on powder
pattern of sample 2, shows a peak shift from 7.15 to
11.2 Å, as a result of kaolinite swelling with DMSO.

Results of the chemical analyses of insoluble residue

and <2  m fraction are shown in Tables 5 and 6.

By comparison of the main element content of insoluble

residue and its clay fractions, it is evident that the whole
insoluble residue is enriched in Si, Na, K, Mn and Ti. On
the other hand Al, P and loss on ignition (LOI) are en-
riched in clay fraction, while Ca, Mg and Fe are present in
similar concentrations in the whole residue and its fine-
grained fraction. The elevated Si, as well as Na and K con-
centrations in the whole insoluble residue are in
accordance with the results of the XRPD and modal analy-
ses by polarizing microscope that revealed the presence of
quartz and feldspars respectively. The higher Ti concen-
trations are most probably due to the rutile that was deter-
mined in the silty fraction of the insoluble residue. The
elevated contents of Al and LOI in clay fraction could be
correlated with clay minerals, in which Al is one of the

Fig. 3. Tectonic sketch of the Pannonian Basin and its surround-
ings with location of the Zagorje Basin (marked with arrow). Af-
ter Royden (1988).

Table 1: Quantitative mineral composition (in wt. %, + – traces)
of insoluble rock residue. The analyses were made by XRPD ac-
cording to the procedure described by Schultz (1964).

Table 2: Semi-quantitative content of clay minerals in the  < 2  m
fraction of insoluble rock residue. The analyses were made by XRPD
according to the procedure described by Schultz (1964). **** – domi-
nant (60—100 %), *** – abundant (30—60 %), ** – considerable
(10—30 %), * – subordinate (1—10 %), + – traces.

Fig. 4.  XRD pattern of untreated sample 3 (A), in comparison
with patterns recorded after ethylene glycol treatment (B) and
heat treatment at 550 

ºC (C).

main components, and the water could be structural, pore
or adsorbed on particle surfaces (Weaver & Pollard 1973;
Newman & Brown 1987).

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UPPER MIOCENE PELITIC SEDIMENTS OF THE ZAGORJE BASIN (CROATIA)

Table 3: Quantitative mineral composition (in wt.%, + – traces) and classification of pelitic sediments according to Konta (1973).

Fig. 5. IR spectrum of sample 3.

Table 4: Semi-quantitative mineral composition of 0.09—0.045 mm fraction determined by polarizing microscope. Op – opaque min-
erals, Dol – dolomite, Chl – chlorite, Tur – tourmaline, Zrn – zircon, Rt – rutile, Am – amphibole, Ap – apatite, Ep – epidote,
Zo – zoisite, Grt – garnet, Ky – kyanite, St – staurolite, Ttn – titanite, X – undetermined grains, Qtz – quartz, Pl – plagioclase,
Kfs – K-feldspar, S – rock fragments (chert, quartzite and schists), Ms – muscovite. *** – abundant (30—60 %), ** – considerable
(10—30 %), * – subordinate (1—10 %), + – traces.

The higher contents of Ca and Mg in samples 5, 7 and 9

are the consequence of the presence of dolomite (Table 1),
which can also contain some Fe. Mg and Fe are also
present in clay structures, they can replace Al in octahe-
dral sheet (Weaver & Pollard 1973; Newman & Brown
1987). Iron is bound to pyrite, limonite and eventually
present amorphous component, but also enters the struc-

Fig. 6. X-ray powder diffraction pattern of sample 2 after DMSO-
treatment.

ture of some transparent heavy minerals (garnets, epidote,
tourmaline) that are present in the silt fraction.

The major, as well as, the trace element content of the

analysed samples was compared with the values for Post-
Archaean Australian Shale (PAAS) (Taylor & McLennan
1985) (Fig. 7a). The Zagorje samples are explicitly en-
riched with LOI and depleted in MnO, CaO and Na

2

O.

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GRIZELJ, TIBLJAŠ and KOVAČIĆ

Table 5: Chemical composition of insoluble rock residue. Major elements (wt. %) and several trace elements, marked with star sign
(ppm) were determined by ICP-AES, and other trace elements (ppm, except Au in ppb) by ICP-MS. The CIA, chemical index of alter-
ation of Nesbitt & Young (1982), is calculated as [Al

2

O

3

/(Al

2

O

3

+ CaO* + Na

2

O + K

2

O)] 100, oxides are expressed in molar proportions,

CaO* is the amount of CaO in siliciclastic minerals only i.e. excluding carbonates and apatite.

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UPPER MIOCENE PELITIC SEDIMENTS OF THE ZAGORJE BASIN (CROATIA)

Table 6: Chemical composition of the  < 2  m fraction of insoluble rock residue. Major elements (wt. %) and several trace elements,
marked with star sign (ppm) were determined by ICP-AES, and other trace elements (ppm, except Au in ppb) by ICP-MS.

They have slightly lower contents of SiO

2

, TiO

2

 and K

2

O,

while Al

2

O

3

, FeO and MgO are present in similar concen-

trations as in the PAAS. Samples with dolomite (5, 7, 9)
are expectedly enriched with CaO and MgO in compari-
son to other samples and the PAAS. Comparison of trace
element content in whole insoluble residue and its clay
fraction showed that insoluble residue is enriched with:

Ba, Ce, Dy, Er, Eu, Hf, Ho, La, Lu, Nb, Nd, Pr, Sm, Ta, Tm,
Zr, Y and Yb (Tables 5 and 6). Relative to the PAAS, the
Zagorje samples contain comparable amounts of Sc, V, Cr,
Ni, Rb, Y, Nb and Th, however they are enriched with Zn,
Cu, Ga, and depleted in Co, Sr, Zr and Ba (Fig. 7b).

Barium as well as light rare earth elements (LREE: La,

Ce, Pr, Nd and Sm) in insoluble residue are associated with

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GRIZELJ, TIBLJAŠ and KOVAČIĆ

Fig. 7. Chemical composition of the Zagorje samples normalized to Post-Archaean Average Shale (PAAS) after Taylor & McLennan (1985).
a – plot of major element oxides (wt. %). b – trace elements (ppm) spidergram. In legend sample numbers and corresponding symbols are given.

feldspars; due to the similar ionic radii they can substitute
Ca and Na in feldspar structure (McLennan 1989; Deer et
al. 2001). Feldspars prefer LREE and bivalent Eu as a sub-
stitute for Ca (McLennan 1989; Prohić 1998). Dy, Ho, Er,
Tm, Yb, Lu and Y are most probably incorporated in gar-
nets and zircon which prefer heavy rare earth elements
(HREE) (Henderson 1996; Prohić 1998). Most probably,
Nb and Ta, which usually come together with HREE, are
related to heavy minerals, too. The analysed samples are
depleted in REE in comparison with the European Shale
composite (ES) (Haskin & Haskin 1966) (Fig. 8a and b).

Insoluble residue enrichment with Zr and Hf is correlat-

ed with zircon; zircon usually incorporates approximately
50 % of Zr and several % of Hf present in the rock
(McLennan 1989). Chondrite normalized patterns are typ-
ical for the shale in general, with enrichment of LREE rel-
ative to HREE (Fig. 9).

The clay fraction is enriched with: Bi, Cs, Cu, Ga, Hg,

Pb, Rb, V and Zn (Table 6). Gallium could replace Al and
Si in tetrahedral sheets of clay minerals, while Cs and Rb
can be present as interlayer cations in some clay minerals
(Newman & Brown 1987). Cesium also substitutes potas-

Fig. 8. Rare earth element plots of Zagorje samples normalized to average European Shale (ES) after Haskin & Haskin (1966). a – samples
from Pušća outcrop. b – samples from Kupljenski Hruševec outcrop. In legend sample numbers and corresponding symbols are given.

Fig. 9.  Rare earth element plots of Zagorje samples normalized to
chondrite after Nakamura (1974). Sample numbers and corre-
sponding symbols are given in the legend.

sium in micas (Heier & Billings 1972). Copper and zinc
could replace magnesium, while vanadium could replace
aluminium in octahedral sheets of clay minerals (Newman

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UPPER MIOCENE PELITIC SEDIMENTS OF THE ZAGORJE BASIN (CROATIA)

& Brown 1987). Heavy metals, Hg and Pb, do not enter the
structure of clay minerals, but their elevated concentration
could point to the adsorption of these elements onto the
surface of clay particles. It is known that sediments with a
high specific area adsorb heavy metals by physical and
chemical sorption  (Siegel 2002). According to Ahrens &
Erlank (1969) elevated Bi content in the clay fraction
could indicate Bi for Ca substitution in silicates. The role
of organic matter could not be excluded since the total
carbon content is also elevated in the clay fraction.

Discussion

Mineral characteristics of pelitic sediments

The Upper Miocene marls of the Zagorje Basin, which

comprise marls collected from the localities Pušća and Ku-
pljenski Hruševec, were deposited in a brackish lake under
hot climate conditions, and there was no significant input
of coarse-grained terrigenous material (Kovačić 2004).
Clay minerals were deposited from suspension, while car-
bonate originated in the lake. The presence of calcite and
low-magnesium calcite in the marls indicate that the lake
salinity was low (Reading & Collinson 1996). Neverthe-
less, the presence of calcite could also be explained by di-
agenetic transformation of primarily deposited aragonite
or Mg-calcite into more stable calcite.

Comparison of calcite content in samples of different

ages (Pannonian—Pontian) is represented by box and whis-
kers diagrams in Fig. 10. Calcite content for Pannonian
samples from the Pušća outcrop varied from 49 to74 wt. %,
while in Pontian ones from Kupljenski Hruševec it was be-
tween 31 and 56 wt. %. The lower calcite content in the
Pontian samples could be the result of climate changes
that caused carbonate production decrement, or more like-
ly, it is due to intensified input, firstly, of clayey and later
of silty-sandy clastic detritus. Gradual increase of terrige-
nous influence is in good agreement with sediment type
change; most of the Pannonian age samples were classi-
fied as marl, while those of the Pontian age are silty marls.

In majority of the lakes clay minerals are of detrital ori-

gin and reflect the composition of source rocks (Chamley
1989; Weaver 1989). During the Late Miocene in the SW

Fig. 10.  Comparison of Pannonian and Pontian age samples on
the basis of CaCO

3

 (wt. %) content.

part of Lake Pannon deposition of different calcareous-
marly and sandy-silty sediments took place. According
to Kovačić & Grizelj (2006)  the sandy-silty detritus had
a terrigenous origin, most probably from the Eastern
Alps.

The results of the XRPD analyses of insoluble residues

(Tables 1, 2 and 3) revealed that samples differ primarily
by proportion of present mineral phases. All the analysed
samples contain quartz, feldspars, and clay minerals (illite,
smectite and chlorite, while the kaolinite is present only
in a few samples), but their concentration varies signifi-
cantly in particular samples. In several samples dolomite
and pyrite are also present (Tables 1 and 3, Fig. 11). It is
evident (Fig. 11) that clay mineral content is higher in
samples from the Pušća locality, while quartz and feld-
spars are more abundant in the Kupljenski Hruševec sam-
ples. The higher contents of quartz and feldspars in the
latter samples are in good agreement with their lithology;
they are determined as silty marls (Table 3).

According to Chamley (1989) and Weaver (1989) illite

and smectite constitute the dominant species in detrital
supply of freshwater lake, and are as frequently encoun-
tered as quartz. Chlorite, kaolinite and mixed-layers are
variously present, depending on the lake location. A gen-
eral correspondence exists between the mineral composi-
tion of most freshwater lakes and average clay mineralogy
of rocks and soils in the surrounding drainage basins.
Chlorite, as a mineral that is not resistant to chemical
weathering, is an important detrital component in areas
with reduced chemical weathering (Weaver 1989). Such
areas are characterized by colder climate, steeper relief, in-
tensive erosion and faster transport. In the Pannonian and
Pontian such conditions existed in the Alps, that is in the
young mountain belt, the probable source region of detri-
tal material, including chlorite, which was transported to
Lake Pannon. Lower concentrations of chlorite in pelitic
sediments in comparison to contemporaneous sandy-silty
sediments, which were deposited to the north of investi-
gated area (Kovačić 2004), is most probably the result of
their alteration. This alteration was facilitated by the hot,
humid climate that existed in the south-western part of the
Pannonian Lake in Late Miocene times (Pantić 1986;
Planderová et al. 1992).

Fig. 11. Mineral composition of insoluble rock residue.

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GRIZELJ, TIBLJAŠ and KOVAČIĆ

Smectite and illite are more abundant than kaolinite

when the water salinity is higher, as interpreted from the
Gulf of Mexico  example by Brooks & Ferrell (1970).
Weaver (1989) thinks the same, but he recognized some
exceptions such as the coastal Snuggedy Swamp. During
the Late Miocene the PBS was separated from marine
realm, and as a consequence, till the end of the Miocene,
there was a lake area of oligohaline (Pannonian) and mio-
haline (Pontian) salinity characteristics (Steininger et al.
1988).

According to Chamley (1989) chlorite, illite and irregu-

lar mixed layered I/S originate directly from less weath-
ered intrusive and metamorphic rocks, while poorly to
medium-crystallized smectites are the result of weathering
in mild humid areas with locally intensified volcanic ac-
tivity. No evidence of volcanic activity was discovered in
the investigated sediments, but such evidence exists for
the Upper Miocene for other parts of the PBS (Balogh et
al. 1983; Szabó et al. 1992; Pamić et al. 1995). Conse-
quently, smectite occurrence could be correlated with vol-
canic activity in other parts of the basin, owing to the fact
that during the eruptions volcanic material could be trans-
ported far away. Nevertheless, smectite could also origi-
nate from volcanic material that is redeposited from the
older sediments as well as by weathering of other rocks.
Part of the illite present in the analysed samples is proba-
bly redeposited, and the other part could be authigenic,
but the presence of smectite indicates that diagenetic
changes were not pronounced.

Hungarian geologists have described a similar mineral

composition of pelitic sediments. Viczián (2002) claims
that in the Great Hungarian Plain the mineral composition
was, in principle, the same: polymineral detrital clays dis-
play slight, systematic regional differences, with no signif-
icant differences in composition of various stratigraphic
horizons within the Late Pannonian to Late Pliocene-
Pleistocene time span. Based on XRD, thermal and some
chemical analyses he determined the dominant clay min-
erals in pelitic sediments of the Great Hungarian Plain: in
the whole rock samples these are smectite, I/S, illite and
chlorite, and in  < 2  m fraction ternary-mixed-layered il-
lite/smectite/chlorite and kaolinite with different degrees
of ordering, that are absent in some samples. According to
him clay minerals are essentially detrital, derived from
various areas of the surrounding Carpathians and Alps. He
compared several sub-basins and concluded that they dif-
fer in degree of ordering and abundance of particular clay
and other minerals such as calcite, dolomite, quartz and
feldspars, in dependence with constancy of source area
and transport direction. Similarly, Varsányi (1975) deter-
mined montmorillonite, kaolinite, mica, chlorite, amor-
phous material, quartz, potassium and sodium feldspars in
the < 10  m fraction of samples of Early and Late Pannon-
ian age from boreholes in the southern part of the Great
Hungarian Plain.

When the composition of the silt fraction of insoluble

residue (Table 4) is compared with the composition of
contemporaneous sandy-silty sediments (Kovačić & Gri-
zelj 2006), it is evident that they are very similar: in both

cases quartz and rock fragments are dominant constitu-
ents, and garnets and epidote are prevailing transparent
heavy minerals. The similarity of composition suggests
the same provenance of the detritus, and according to
Kovačić & Grizelj (2006), most probably the Alpine. Ob-
served differences in granulometric properties and quanti-
ty of clastic detritus are the result of distance of deposition
area from the coast, since marls are deposited in more dis-
tal environments in comparison to sandy-silty sediments.

Geochemical characteristics of the pelitic sediments

The degree of chemical weathering of the source rocks

can be constrained by calculating the chemical index of
alteration (CIA=[Al

2

O

3

/(Al

2

O

3

+CaO*+Na

2

O+K

2

O)] 100,

oxides are expressed in molar proportions, CaO* is the
amount of CaO in siliciclastic minerals only i.e. excluding
carbonates and apatite) of Nesbitt & Young (1982). Ac-
cording to Nesbitt & Young (1982) the CIA values for the
average shale is 70—75, and for illite and montmorillonites
they are 75—85. The calculated CIA values are similar for
both outcrops and vary from 77 to 80 (Table 5) and sug-
gest a high intensity of chemical weathering. CIA value
for PAAS is 69. This value together with the spider dia-
gram (Fig. 7a) enables the conclusion that Zagorje sam-
ples contain more clay minerals than PAAS.

Intense weathering of the source rocks can be also de-

duced from the triangular Al

2

O

3

-(CaO+Na

2

O)-K

2

O plot

(after Nesbitt & Young 1982, 1984) on which data for av-
erage PAAS and Upper Crust (UC) given by Taylor &
McLennan (1985) and North American Shale Composite
(NASC) given by Gromet et al. (1984) are shown for com-

Fig. 12. Al

2

O

3

-(CaO + Na

2

O)-K

2

O plot of Zagorje samples (after

Nesbitt & Young 1982, 1984) in comparison with the data for Post-
Archaean Australian Shale (PAAS) and Upper Crust (UC) given by
Taylor & McLennan (1985) and North American Shale Composite
(NASC) data given by Gromet et al. (1984). Small dots are ideal-
ized mineral compositions of plagioclase, K-feldspar, kaolinite,
muscovite, illite and smectite and numbers denote compositional
trends of initial weathering profiles of average granodiorite (1) and
average granite (2) (from Nesbitt & Young 1984).

background image

273

UPPER MIOCENE PELITIC SEDIMENTS OF THE ZAGORJE BASIN (CROATIA)

Fig. 13. Fe

2

O

3

-K

2

O-Al

2

O

plot of the Zagorje samples in compari-

son with the data for Post-Archaean Australian Shale (PAAS) data
given by Taylor & McLennan (1985) and North American Shale
Composite (NASC) data given by Gromet et al. (1984).

Fig. 14.  a – Th ver-
sus Sc plot for the
Zagorje samples. Th/
Sc ratios near unity are
typical of upper conti-
nental crust (UC) deri-
vation and Th/Sc ratio
near 0.6 suggest a
more mafic compo-
nent.  b – Cr versus
Th plot for the Zagor-
je samples, shown in
comparison to the Up-
per Crust (UC) and
Post—Archaean Austra-
lian Shale (PAAS) after
Taylor & McLennan
(1985).

weathering of first-cycle material (Barshad 1966). The av-
erage ICV value for the Zagorje samples is 0.67 and sug-
gests relative compositional maturity indicating recycled
or intensely weathered first cycle sediment. Another index
of pelitic sediments composition is the ratio K

2

O/Al

2

O

3

.

The ratio K

2

O/Al

2

O

3

 can be used as an indicator of the

original composition of ancient pelitic sediments (Cox et
al. 1995). Ordered from high to low values, the K

2

O/Al

2

O

3

ratios of minerals are: alkali feldspars —0.4 to 1, illite —0.3,
other clay minerals – nearly 0 (Cox et al. 1995). Pelitic
sediments with ratios of K

2

O/Al

2

O

3

 greater than 0.5 sug-

gest a significant quantity of alkali feldspar relative to
other minerals in the original rocks, those with ratios of
K

2

O/Al

2

O

3

 less than 0.4 suggest recycling of pelitic sedi-

ments (Cox et al. 1995). The Zagorje samples have an av-
erage K

2

O/Al

2

O

ratio of 0.23 (Table 5), suggesting

minimal alkali feldspar relative to other minerals in the
original pelitic sediments. The average ratio of SiO

2

/Al

2

O

3

=2.84 for the Zagorje samples (Table 5) suggests a rela-
tively lower amount of quartz in relation to PAAS (SiO

2

/

Al

2

O

3

=3.32) and NASC (SiO

2

/Al

2

O

3

=3.83), in accordance

with the already discussed CIA values, spider diagram
(Fig. 7a) and Al

2

O

3

-(CaO+Na

2

O)-K

2

O diagram (Fig. 12).

The elemental ratios of Eu/Eu*, La/Sc, Th/Sc, Th/Co,

Th/Cr and La/Cr indicative of source rock composition
(Cullers 2000) are in the range of those derived from si-
licic rather than basic source rocks (Table 5).

Plots of the Th/Sc and Cr/Th of the marl samples are

shown in Fig. 14, along with the continental and mafic
signatures in terms of Th/Sc and Cr/Th ratios. Th as an in-
compatible element is enriched in silicic rocks, while Sc
and Cr are compatible elements that are enriched in mafic
rocks. The plot positions show a high affinity towards a fel-
sic component. The marl samples were also plotted on a La-
Th-Sc diagram (Fig. 15) together with data for UC, PAAS,
Oceanic Crust (OC) and Bulk Continental Crust (BCC)
from Taylor & McLennan (1985). Our samples plot in a fair-
ly narrow region of the Upper Crust (UC) and Post—
Archaean Australian Shale (PAAS) fields.

parison (Fig. 12) together with idealized compositions of
plagioclase, K-feldspar, kaolinite, illite and muscovite and
trends of initial weathering profiles of granite and grano-
diorite (Nesbitt & Young 1984). Most of the samples plot
close to the Al

2

O

3

-K

2

O boundary and along the grano-

diorite weathering trend, implying that source rocks un-
derwent intense weathering. The marl samples plot closer
to PAAS than to UC or NASC.

On the triangular Fe

2

O

3

-K

2

O-Al

2

O

3

 diagram (Fig. 13),

the Zagorje samples plot in the same region as PAAS and
NASC. Cox et al. (1995) proposed the Index of Composi-
tional Variability (ICV) as a measure for characterizing the
differences between samples of different provenance. ICV
is defined as (Fe

2

O

3

+K

2

O+Na

2

O+CaO+MgO+MnO+TiO

2

)

/Al

2

O

3

 and measures the abundance of alumina relative to

the other major cations in the rock or mineral. Non-clay
silicates contain a lower proportion of Al

2

O

3

 than do clay

minerals, therefore they have higher ICV. Because miner-
als show a relationship between resistance to weathering
and ICV, the ICV may be applied to pelitic sediments as a
measure of compositional maturity. Pelitic sediments with
abundant clay minerals tend to have ICV’s less than one
and characterize tectonically quiescent or cratonic envi-
ronments (Weaver 1989) where sediment recycling is ac-
tive, but may also be produced by intensive chemical

background image

274

GRIZELJ, TIBLJAŠ and KOVAČIĆ

Conclusions

1 – XRPD analyses of marl insoluble residue revealed

that most of the samples contain the same mineral species,
however particular minerals are present in different quanti-
ties in different samples. Clay minerals content varied
from 18 to 50 %, quartz content from 5 to 21 %, and feld-
spar content from 1 to 5 %. Several samples contained
small quantities of dolomite and pyrite. Pannonian age
marls, from the Pušća outcrop, contain higher amounts of
clay minerals and calcite than Pontian marls, from Kupl-
jenski Hruševec outcrop, that are richer in quartz and feld-
spars.

2 – The <2  m insoluble residue fraction of all samples

contains the following clay minerals: smectite, illite, chlo-
rite; while kaolinite is present only in a few samples.

3 – The dominant constituents in the 0.09 to 0.16 mm

fraction are quartz, feldspars, rock fragments (chert, quartz-
ite and schists) and micas. Chlorite, limonite, pyrite, gar-
net, tourmaline, zircon, epidote and staurolite are present
in small amounts.

4 – Sediments were classified on the basis of CaCO

3

content and insoluble residue mineral content; most of the
Pannonian age samples were classified as marls, while the
samples of the Pontian age are silty marls.

5 – Variations in chemical composition of pelitic sedi-

ments are in perfect agreement with the variations in sedi-
ment mineral composition. The CIA values of the
investigated pelitic sediments (77—80) and the average
ICV of 0.67 indicate relatively compositionally mature
and intensely weathered first cycle sediments. Major and
trace element composition, elemental ratios critical of
provenance (SiO

2

/Al

2

O

3

, K

2

O/Na

2

O, Eu/Eu*, La/Sc, Th/

Sc, Th/Co, Th/Cr and La/Co) and source rock discrimina-
tions diagrams (Fe

2

O

3

-K

2

O-Al

2

O

and La-Th-Sc), suggest

Fig. 15. La-Th-Sc plot for the Zagorje samples in comparison
with the data for Post-Archaean Australian Shale (PAAS), Upper
Crust (UC), Bulk Continental Crust (BCC) and Oceanic Crust (OC)
from Taylor & McLennan (1985).

that the source rocks of these pelitic sediments were from
the Upper Crust and remarkably felsic in nature.

6 – The chemical composition of the sediments and

modal composition of the silt-size fraction indicate Alpine
provenance of the clastic material.

Acknowledgments:  Investigations in this paper represent
a part of the Project: Geological Map of the Republic of
Croatia, financed by the Ministry of Science, Education
and Sports of the Republic of Croatia. The authors are
grateful to Dražan Jozić for IR-spectrum recording.  We
would like to thank the journal reviewers for their careful
revision of the manuscript.

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