GeolCarp_Vol57_No4_257

GEOLOGICA CARPATHICA, AUGUST 2006, 57, 4, 257—268

www.geologicacarpathica.sk

Conodont colour alteration related to a half-graben

structure: an example from the Mesozoic of the Mecsek and

Villány Hills area (Tisza Megaunit, Southern Hungary)

SÁNDOR KOVÁCS

, ERZSÉBET RÁLISCH-FELGENHAUER

and JÓZSEF BÓNA

Geological Research Group of the Hungarian Academy of Sciences, Department of Geology, Eötvös Loránd University,

Pázmány Péter sétány 1/c, H-1117 Budapest, Hungary; skovacs@iris.geobio.elte.hu

Hungarian Geological Institute, Stefánia u. 14, H-1143 Budapest, Hungary; ralisch@mafi.hu

Gorkij u. 37, H-7300 Komló, Hungary

(Manuscript received March 14, 2005; accepted in revised form December 8, 2005)

Abstract: The colour alteration of conodonts (CAI) is a good indicator of temperature increase with increasing burial
depth. A characteristic example is described herein from the Mecsek and Villány Zones of the Tisza Megaunit (or Tisia
Terrane), South Hungary. As well recorded by surface geological and borehole data in the area of the Mecsek and
Villány Hills, Middle Triassic conodont-bearing carbonates were buried there to very different depths during the
formation of a large half-graben structure from the Late Triassic to Early Cretaceous. In the Mecsek half-graben zone up
to 4300 m thick sediments (Upper Triassic atypical, “grey Keuper”, Lower Liassic coal-bearing Gresten Facies, Middle
Liassic to Lower Dogger “Fleckenmergel” or Allgäu Facies) were deposited until the end of the Bajocian. On the other
hand, coeval strata on the adjacent Villány Ridge zone are missing or reach only a maximum thickness of 90 m (Upper
Triassic Carpathian Keuper, Pliensbachian sandy, belemnitic limestone). The siliciclasic input ceased after the Bajocian,
but the Bathonian to Albian sedimentation and magmatism (Mecsek Zone: Bathonian to Berriasian pelagic carbonates
and some cherts, Berriasian to Barremian/Aptian alkaline rift-type basalts and associated volcaniclastic conglomerates
with some carbonates; Villány Zone: shallow water carbonates up to Early Albian, flysch-type sediments in the Late
Albian) essentially did not result in a further difference between the overburden of the two zones. The investigated
conodonts of the Mecsek Zone (deriving from the depocenter of the half-graben zone) became dark grey to blackish
coloured (CAI = 4), whereas those of the Villány Zone preserved their original yellowish white colour (CAI = 1). Neither
the Early Cretaceous volcanism in the Mecsek Zone, nor the additional tectonic overburden resulting from middle—Late
Cretaceous northward thrusting in the Villány Zone influenced the colour of conodonts. We also made comparison of
these CAI values with previously published vitrinite reflectance data from the study area and from some other parts of
Hungary. However, the few existing data from the Villány Zone would indicate an excessively high temperature related
to the CAI = 1 value, implying the necessity of new and more numerous measurements from this region.

Key words: Mesozoic, South Hungary, Tisza Megaunit, Mecsek and Villány Zones, half-graben, colour alteration
(CAI), conodonts.

Introduction

Since the basic work of Epstein et al. (1977), the colour al-
teration index of conodonts (CAI) has been extensively
used in many parts of the world to determine the thermal
maturity of conodont-bearing (Upper Cambrian to Upper
Triassic) sedimentary sequences (Table 1). This method is
based on the fact, that conodont elements built of an
amorphous variety of apatite (francolite; Pietzner et al.
1968) contain a minor amount of organic matter, which
becomes progressively coalified with increasing tempera-
ture. In context with the coalification of the organic matter
contained, the originally yellowish white or “honey
coloured” (CAI = 1; see below) conodont elements become
darker and darker, and finally black in colour (CAI = 5). This
colour alteration was experimentally tested and correlated
with palynomorph colour alteration and vitrinite reflec-
tance data by Epstein et al. (op. cit.). The Colour Alteration
Index (CAI) is based on the alteration of colour during labo-
ratory testing (heating conodonts in electronic furnace un-

der atmospheric conditions) and observation of the colour
of conodonts from natural collections.

At further heating the blackened conodonts become grey

(CAI = 6), then opaque white (CAI = 7) and finally crystal
clear or glassy (CAI = 8). This reverse path of colour alter-
ation is discussed in more details in Sudar & Kovács (2006).

However, Árkai & Kovács (1986) and Kovács & Árkai

(1987) observed partly different colours between yellow-
ish white (CAI = 1) and black (CAI = 5) in NE Hungary.
Correlating with illite crystallinity (KI – Kübler index)
indices and other metamorphic petrological parameters,
they concluded, that all the colour changes to complete
blackening (CAI = 5) took place below 200 ºC tempera-
ture. On the other hand, the reverse path of colour
changes (from CAI = 5 to 8), under conditions of regional
dynamothermal metamorphism, not only changed the
colour of conodonts, but they became recrystallized and
usually deformed, too.

Nevertheless, the usefulness of conodonts for tempera-

ture estimations in case of burial and contact metamor-

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KOVÁCS, RÁLISCH-FELGENHAUER and BÓNA

phism has been proven by numerous papers (see, for ex-
ample, Armstrong & Strens 1987; Nowlan & Barnes 1987;
Belka 1990; Nöth 1991; Königshof 1992; Pondrelli 2000
for some more detailed studies).

In this contribution we present conodont colour alter-

ation data from a Mesozoic half graben zone (Mecsek
Unit), in which conodonts became dark grey to blackish
(CAI = 4), whereas on the adjacent ridge zone (Villány
Unit) they remained unaltered (CAI = 1). We attempted to

correlate these CAI values with published vitrinite reflec-
tance data from both units (Laczó 1982, 1984; Horváth et
al. 1982), as was done by Epstein et al. (1977) and
Königshof (1992). Comparisons from some other parts of
Hungary, where both CAI and published vitrinite reflec-
tance data are available, are also included.

Geological setting

The Mecsek and Villány Hills in SE Transdanubia

(Southern Hungary, western vicinity of the Danube River;
Figs. 1, 2) form surface outcrops of the northern zones of the
Tisza Megaunit (or Tisia Terrane): of the Mecsek (– North-
ern Great Plain) Zone in the most external position and of
the Villány (– Bihor) Zone. East of the Danube River
they are known only from boreholes in the pre-Neogene
basement of the Great Plain, the latter having much larger
surface exposures in the Bihor Unit of the Northern
Apuseni Mts, Romania (Bleahu et al. 1994).

The Mesozoic formations of the Mecsek and Villány

zones were laid down on a Variscan basement composed
mainly of synkinematic granitoids and associated medium
grade metamorphics, showing close correlation with the
Moldanubian Zone of the European Variscides (Buda
1996; Buda et al. 2004; Klötzli et al. 2004). In the area of
the Mecsek and Villány Hills, a fault-controlled basin
came into existence in late Variscan time, in which up to
3500 m thick Upper Carboniferous to Upper Permian

Fig. 1. Location of the Mecsek and Villány Hills, South Hungary, in the major tectonic framework of the SW part of the Pannonian area.
1 – Ophiolitic complexes representing the continuation of the Vardar Zone; 2 – Units related the southern (Apulian) margin of Neo-
tethys; 3 – Units related to the northern (European) margin of Neotethys. Shaded areas indicate surface outcrops.

Table 1: Conodont CAI (Colour Alteration Index) values and cor-
responding temperatures as calibrated by experimental testing, and
their correlation with vitrinite reflectance data and burial depth in
the Appalachian Basin, USA (compiled after Epstein et al. 1977).

259

CONODONT COLOUR ALTERATION RELATED TO A HALF-HRABEN STRUCTURE (SOUTHERN HUNGARY)

molasse sediments and associated acidic volcanics were
accumulated. Other areas of the Hungarian part of the
Tisza Megaunit were exposed to erosion during this time
(Barabás & Barabás-Stuhl 1998).

The Alpine sedimentary cycle began both in the

Mecsek and Villány Zones (at that time they were not
separated) with Lower Triassic “Buntsandstein”-type
redbeds (Jakabhegy Sandstone Formation; Barabás-Stuhl
1993). (For thickness data of both zones see Fig. 3). Ma-
rine transgression commenced in the Early Anisian, with
the partly evaporitic Patacs and Hetvehely Formations,
corresponding to the Germanic “Röt” (Barabás-Stuhl, op.
cit.). In the late Early to early Middle Anisian a strongly
bioturbated limestone sequence (Lapis Limestone For-
mation), corresponding to the Germanic “Wellenkalk”
(Török 1993) was deposited. Deeper-water conditions
prevailed in the late Middle Anisian (late Pelsonian), in-
dicated by micritic limestones with frequent slump phe-
nomena. These limestones from both areas are included

in the Zuhánya Limestone Formation (Rálisch-
Felgenhauer & Török 1993) and contain the conodonts
considered below. Equivalents of the Late Anisian,
evaporitic “Middle Muschelkalk” are missing in both ar-
eas; this interval seems to correspond to a hiatus (Konrád
1998; Kovács & Rálisch-Felgenhauer 2005). The carbon-
ate ramp environment was re-established in the Ladinian,
represented by a rather monotonous dolomite sequence
(Csukma Dolomite Formation). During the Early and
Middle Triassic times the Mecsek and Villány Zones be-
longed to the same depositional domain, which was not
yet differentiated. The total thickness of the Anisian and
Ladinian carbonate successions of both areas are not sig-
nificantly different: 850—1250 m in the Mecsek Hills,
and ca. 800 m in the Villány Hills (Nagy & Rálisch-
Felgenhauer in Bleahu et al. 1994). From these the
Ladinian cover of the Zuhánya Limestone Formation in
the Mecsek (Kozár Limestone Member, Kán Dolomite
Member, Kantavár Formation) is 50—300 m, whereas in

Fig. 2. Geological sketch map of the Mecsek and Villány areas, with location of the boreholes mentioned in the text and on Table 2, and with
the sites of conodont studies. 1 – Variscan granitoids; 2 – Permian continental molasse sequence; 3 – Mesozoic in general; 4 – Neogene
and Quaternary; 5 – Boreholes mentioned in the text and in Table 2; 6 – Sites of conodont and limestone microfacies studies; 7 – Strike-
slip fault (Mecsekalja fault); 8 – Thrust front; 9 – Anticline; 10 – Syncline. Abbreviation: Misina gksc = Misina geological key section.

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KOVÁCS, RÁLISCH-FELGENHAUER and BÓNA

the Villány area (Csukma and Templomhegy Dolomite
Members) it is ca. 370 m.

Separation of the Mecsek and Villány Zones began in

the early Late Triassic, probably along the Mecsekalja
(= “Foot of Mecsek”) fault zone, which acted originally as
a late Variscan sinistral strike-slip zone (Szederkényi 1996
and in Kovács et al. 2000) and is active even nowadays.
The future Mecsek Zone became a southward deepening
half-graben zone, whereas the adjacent Villány (– Bihor)
Zone on the South remained an elevated ridge. This sharp
separation persisted throughout the rest of the Alpine sedi-
mentary cycle, till the beginning of middle Cretaceous
tectogenesis. Although the ENE-ward continuation of the
two zones is fairly well known from borehole data (Bérczi-

Makk 1986; Bérczi-Makk et al. 1996), precise thickness
data are known only from the Mecsek—Villány area.
Therefore we restrict the brief review on their development
only to this area.

The Mecsek half-graben was clearly depicted and pre-

cisely documented by Nagy (1969, 1971), but the term
“half graben” was first used for it by Szente (1992).

In the initial stage of the Mecsek half-graben zone first

the specific Kantavár Formation (black marls with vitrite
seems, a possible CH-source rock, in 50—150 m thickness)
was deposited in a brackish to fresh-water environment
(Monostori 1996), which is already missing in the Villány
Zone. Subsidence was accelerated during the deposition of
the Late Triassic Karolinavölgy Sandstone Formation: an
up to 600 m thick grey, fluviatile and lacustrine sandstone
sequence deriving from a northerly lying metamorphic-gra-
nitic provenance (Nagy 1968). Subsidence became espe-
cially intense in the Early Jurassic, when the up to 1200 m
thick paralic coal-bearing sequence of “Gresten Facies” ac-
cumulated (cf. Figs. 4—5), followed by an up to 2500 m
thick “spotty marl” (Fleckenmergel, Allgäu Facies) succes-
sion till the end of Bajocian (Galácz 1984). The siliciclastic
detritus derived from the same provenance, as during the
Late Triassic (Nagy 1969).

During the same time interval, only the 0—40 m thick Up-

per Triassic Carpathian Keuper-type sediments and 0—50 m
thick Pliensbachian sandy, belemnitic limestones were de-
posited on the Villány Ridge. In most parts of the Villány
Unit, however, the whole Upper Triassic to Middle Jurassic
sequence is missing (see Haas 2001, for latest review).

In the Bathonian the Tisia Terrane was separated from

its northerly lying continental hinterland due to the
Penninic rifting and the sedimentation pattern was basi-
cally changed: the siliciclastic input into the Mecsek
Zone ceased and ammonitico rosso-type pelagic marls
were deposited, followed by Callovian to Berriasian pe-
lagic cherty limestones, siliceous limestones and some
radiolarites, with a maximum thickness of 180 m (see in
Haas 2001 and references therein; Haas & Péró 2004).

On the Villány Ridge a less than 1 m thick iron oolite

bank, famous for its fossils, was formed in the Callovian.
The Upper Jurassic is represented by an up to 300 m thick
limestone succession, first of basinal, than of platform fa-
cies (Haas, op. cit.).

During the Early Cretaceous intense alkaline rift-type ba-

saltic volcanism took place in the Mecsek Zone, with a
paroxism during the Valanginian—Hauterivian (Harangi
1994). The thickness of the pyroclastics and lava flows is
about 150 m in the SE Mecsek, nearest to the Misina con-
odont locality, but reaches up to 1000 m on the North in the
Kisújbánya Basin, where the eruption centre can be sup-
posed (Bilik 1996; Császár 1998). On the slopes of the
partly emerged volcanoes up to 300 m thick volcaniclastic
conglomerates and some marls and limestones were accu-
mulated. The Mecsek area seems to have been a dryland
from the Albian, partly from the Aptian onward; no
younger Mesozoic and Paleogene formations are known
in the Transdanubian part of the Mecsek Zone, except
some Turonian red marls (Császár 1998, 2003).

Fig. 3. Simplified Lower Triassic to Lower Cretaceous lithos-
tratigraphy and thickness data of the Mecsek and Villány Zones.
1 – continental siliciclastics (“Buntsandstein”, “Keuper”); 2 – eva-
porites (“Röt”); 3 – shallow water carbonates; 4 – pelagic car-
bonates;  5  – coal-bearing sequence (Gresten facies); 6 – spotty
marl (“Fleckenmergel”) sequence (Allgäu facies); 7 – flysch-type
sequence;  8 – sandy limestone (in Villány Zone); 9 – alkaline
basalts;  10 – volcaniclastic conglomerate; 11 – conodonts.
(Modified after Haas, unpublished.)

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CONODONT COLOUR ALTERATION RELATED TO A HALF-HRABEN STRUCTURE (SOUTHERN HUNGARY)

Fig. 4. Thickness and direction of sediment transport of the Liassic coal-bearing sequence (Gresten facies) in the Mecsek area (after Nagy
1969). 1 – granitoid erosional terrain; 2 – carbonatic erosional terrain; 3 – abrasional conglomerate (exposed); 4 – actual western
boundary of the coal measures; 5 – isohypses; 6 – postulated isohypses; 7 – current direction; 8 – direction of sediment transport from
the granitoid terrain.

In the Villány Zone, after an emerged period, Barremian

to Lower Albian Urgon-type platform-type platform carbon-
ates were accumulated, reaching 500 m in thickness on the
South, but only 30 m in the northern units (Császár 2003).
Sedimentation ended in Albian with flysch-type marly and
siliciclastic formations with a maximum thickness of 300 m
(Császár 1998).

Summarizing the above described sedimentary (and

magmatic) evolution, the Middle Triassic carbonates, em-
bracing the late Middle Anisian conodont-bearing Zuhánya
Limestone Formation, had been buried by an up to 4300 m
thick succession by the Bathonian in the Mecsek half-gra-
ben, whereas on the Villány Ridge only by 0 m to maxi-
mum 90 m thick sediments. Additional burial took place

during the Late Jurassic to Early Cretaceous with a maxi-
mum thickness of 330 m in the southern part of the Mecsek
area (where the Misina conodont-bearing locality can be
found), but over 1000 m on the North in the Kisújbánya Ba-
sin area. At the same time in the Villány area a maximum of
630 m thick additional burial can be calculated to the
North, but up to 1000 m to the South (Császár, op. cit.).
These practically did not change the enormous difference
which had developed between the two zones by the
Bathonian.

This difference was somewhat compensated during the

middle to Late Cretaceous tectogenesis, when the north-
ward thrusting of the Villány Mesozoic sequences took
place. However, the resulting tectonic overburden did not
influence the colour of the conodonts even in the northern
foreland of the hills with the conodont locality Peterd 1
borehole (at 612.2 to 523.5 m depth; Bóna 1976; Kovács
et al. 2005), which was overthrust at least by the five
thrust sheets recognizable in the surface outcrops.

Conodonts and their colour alteration indices (CAI)

Mecsek Hills

Conodonts studied from the Mecsek Hills derive from

the Misina geological key section, located just before the
U-shaped sharp curve of the motor road leading from the
town Pécs to the Misina Hill panoramic point (Rálisch-
Felgenhauer 1986). The artificially cleaned section ex-
poses the higher part of the Bertalanhegy Limestone
Member (crinoidal-brachiopodal grey, nodular, marly
limestone) and the lower part of the overlying Dömörkapu
Limestone Member (grey micritic limestone with yellow

Fig. 5. Tentative cross-section through the Mecsek half-graben
and Villány Ridge zones, referring to the Late Triassic and earliest
Jurassic time (after Nagy 1969).

262

KOVÁCS, RÁLISCH-FELGENHAUER and BÓNA

dolomite mottling). Both lithostratigraphic units contain
the same, fairly rich conodont fauna consisting of
Gondolella hanbulogi (Sudar et Budurov), G. bifurcata
(Budurov et Stefanov) and G. bulgarica (Budurov et
Stefanov) (in decreasing order of freqeuncy). From the
lowermost part of the key section a Schreyerites?
binodosus ammonoid specimen was described by Detre
(1973). Ramiform elements are very rare. According to this
fauna, the whole section belongs to the uppermost part of
the Pelsonian Substage, namely to the binodosus Zone.
Some more details about the section and the conodont
fauna can be found in Kovács & Papšová (1986), whereas
a detailed one has recently been given by Kovács &
Rálisch-Felgenhauer 2005.

All conodont elements are uniformly dark grey coloured

(CAI = 4 according to Epstein et al. 1977), except in part
the tips of the denticles on the carina, which may have re-
mained whitish.

Villány Hills

The conodonts involved in the present study derive

from the Zuhánya Limestone Formation of the drill-core
section Peterd 1, from the depth interval 612.2 to 523.5 m.
They were first described and illustrated by Bóna (1976).
A taxonomical revision is given by Kovács et al. (2005).
The fauna consists of the same taxa and indicates the same
age, as in the Mecsek Hills.

Two samples investigated from the Zuhánya quarry,

type locality of the formation, yielded some, mostly bro-
ken conodont elements.

Conodont elements from both localities are yellowish

white (CAI = 1 according to Epstein et al. 1977).

Vitrinite reflectance data

A great number of vitrinite reflectance data (altogether

179) in connection with coal petrological and oil-gas
prognostic research were published by Laczó (1982, 1984)
from Triassic (mainly Upper) and Jurassic (mainly Liassic)
sedimentary rocks of the Mecsek region (Table 2). Unfor-
tunately, on the other hand, only 4 samples were investi-
gated from the Lower and Middle Triassic of the Villány
region and none from the Upper Triassic to Middle Juras-
sic deposits, which are very reduced or missing there. As
we had no opportunity to carry out new investigations by
this method, we extended our comparisons to other areas
of Hungary, from where both CAI and published vitrinite
reflectance data are available (see Table 3).

Except the somewhat anomalously high (Ro = 1.54)

value of a single Lower Triassic sample from the Mecsek
region, they show practically the same (Ro = 1.37—1.39)
values for the Lower—Middle Triassic of both regions in
spite of the enormous difference in their burial depth, that
can be calculated for the end of the Early Cretaceous.

Samples from the Upper Triassic sandstones of two bore-

holes at Pécs nearest to the Mecsekalja fault zone (Pécs-28
and -57) also show similar values (Ro = 1.32 and 1.36, re-

Table 2: Vitrinite reflectance data published by Laczó (1982, 1984)
from the Mecsek and Villány areas, mentioned in the text. For loca-
tion of the boreholes see Fig. 2.

Table 3: Vitrinite reflectance and conodont CAI data from other re-
gions (Transdanubian Range, Rudabánya Mts) mentioned in the
text, after Laczó (1984) and Árkai & Kovács (1986). CAI data
from borehole Zsámbék 14 are after Kristan-Tollmann et al. (1991)
and from borehole Bakonyszűcs 3 after unpublished investigations
by S. Kovács. Note: in the Szőlősardó 1 borehole vitrinite reflec-
tance data are given in R

max

(

) in Árkai & Kovács (1986).

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CONODONT COLOUR ALTERATION RELATED TO A HALF-HRABEN STRUCTURE (SOUTHERN HUNGARY)

spectively; n = 5 in both), such as from the Liassic of two
other nearby located boreholes (Hosszúhetény-16 and
Hidas-64; Ro = 1.33 in both; n = 4 + 8). The Liassic of the
northwesterly located Komló-142 borehole shows some-
what lower values: Ro = 1.17; n = 5. An interesting de-
crease in vitrinite reflectance was found in the sequence of
Máza-15 borehole located in the northern margin of
Mecsek Hills, where sediment thickness (and burial) was
less: Ro = 1.03, n = 10 in the Upper Triassic part of the se-
quence, decreasing to Ro = 0.90, resp. 0.83; n = 92 upward
in the Liassic coal-bearing part (see Table 2).

It is worth mentioning, that in black coals from two

mines at Komló considerably smaller values were mea-
sured: Ro = 0.87; n = 16 from the Zobák shaft and

Fig. 6. Position of the oil zone at the end of the Triassic and in the Early Cretaceous in the Mecsek Zone, and at the end of the Permian and
in the Early Cretaceous in the Villány Zone, respectively (after Horváth et al. 1982, slightly modified). 1 – Pre-Alpine metamorphics;
2 – Continental conglomerates and sandstones (Upper Paleozoic); 3 – Continental siliciclastics (Lower and Upper Triassic) and major
sandstone horizons Lower Jurassic; 4 – Siltstones (Triassic); 5 – Shales (Triassic, Jurassic); 6 – Marls of the Allgau (Fleckenmergel) fa-
cies in the Lower Jurassic; 7 – Calcareous marls, marly limestones; 8 – Dolomitic marls; 9 – Dolomites; 10 – Limestones; 11 – Major
coal measures; 12 – Probable source rocks; 13 – Probable confining horizon. Horizons: 1—2 – top of the oil zone (1 – at 60

ºC) and

its base (2 – at 135

ºC) in the Early Cretaceous; 3—4 – top of the oil zone (3 – at 60 ºC) and its base (4 – at 135 ºC) at the end of the

Triassic (Mecsek area) or at the end of the Permian (Villány area), respectively.

Ro = 0.81; n = 12 from the Kossuth shaft. These can be ex-
plained by the more distal setting in relation to the
depocenter of the half-graben structure, and, consequently,
by the smaller burial depth. Unfortunately, Middle Trias-
sic conodont-bearing limestones are not known in this re-
gion.

Conodont alteration and vitrinite reflectance data

from other regions

For comparison we include two other areas from Hun-

gary, where both conodont CAI and vitrinite reflectance
data are available (Table 3). The colour of conodonts in

264

KOVÁCS, RÁLISCH-FELGENHAUER and BÓNA

the first case is similar to those of the Villány area, but in
the second case to those of the Mecsek area.

Triassic conodonts throughout the Transdanubian

Range show uniform CAI = 1 values, for example they do
not reflect any thermal overprint. Vitrinite reflectance data to-
gether with CAI data are available from two boreholes drilled
at the northwestern margin of the range. Reflectance data
from the Carnian Veszprém Marl Formation of Zsámbék 14
borehole shows Ro = 0.41; n = 10 (Laczó 1984). The spo-
radic conodont elements from here, with CAI = 1 value are
figured in Kristan-Tollmann et al. (1991). A Lower Tri-
assic siltstone sample from the Bakonyszűcs 1 borehole
show Ro = 0.80; n = 1 (Laczó, op. cit.). Late Anisian—
Ladinian conodonts with CAI = 1 value are known from
the nearby drilled Bakonyszűcs 3 borehole (unpublished
investigations of the first author).

Vitrinite reflectance data from the Szőlősardó Unit of

the Aggtelek-Rudabánya Mts, measured on the Anisian
Bódvarákó Formation and Carnian Szőlősardó Marl For-
mation of the drill-core section of Szőlősardó 1 borehole
show already anchizonal values (see Table 3; note, that re-
flectance data in Laczó 1984 are given in R

, whereas in

Árkai & Kovács 1986 in R

max

values). However, “illite crys-

tallinity” indices clearly still refer to the diagenetic zone:
KI (whole rock) = 0.536 ± 0.114 º 2 , n = 8; KI ( < 2 m)
= 0 .544 + 0.160 º 2 , n = 16 (Árkai, in Árkai & Kovács
1986) and conodonts also show CAI = 4—5 values (similar
to those of Mecsek Mts, with no sign of recrystallization
or deformation (see in Kovács & Árkai 1989: Pl. 2,
Figs. 5—6 and Pl. 4, Fig. 5). This controversy is explained
by Árkai & Kovács (op. cit.) by a short-time heating,
which did not reset the KI and CAI values.

Discussion

The Middle Triassic carbonates of the Mecsek area

reached the maximum 4300 m burial depth by the end of
the Bajocian (170 Ma), and stayed there until about the
end of the Albian (100 Ma), that is over a 70 Myr long pe-
riod. Applying the CAI method of Epstein et al. (1977)
and projecting the CAI = 4 value of the Mecsek area onto
the Arrhenius plot (Fig. 7), supposing this about 70 Myr
staying at the maximal burial depth, a paleotemperature of
about 200 ºC can be calculated. It is less, then estimated
by Horváth et al. (1982: Fig. 1) for the nearby located
Pécsbányatelep mining district, the succession of which
was deposited in (or near to) the depocenter of the half-
graben. Here the maximal burial depth, up to 4.5 km can
be counted for the top of the Middle Triassic carbonates at
the end of the Early Cretaceous. They counted with 5 ºC
geothermal gradient after Nagy (1969, 1971), with 20 ºC
surface temperature for that time. Accordingly, the top of
the Middle Triassic carbonates could be at the 245 ºC iso-
therm. At the same time, the top of the Liassic coal-bear-
ing sequence of Gresten-type, at 2.7 km depth, could be at
the upper limit of the oil-window, at 135 ºC (see Fig. 6
herein). The conodont-bearing upper Middle Anisian (up-
per Pelsonian) Bertalanhegy and Dömörkapu Limestone

Members beneath the 150—300 m thick Ladinian carbon-
ates could easily be at the 250 ºC isotherm. If we do not
count with this anomalously high 5 ºC/100 m “Pannonian”
geothermal gradient, but with the average 3 °C/100 m, and
with a 20 ºC surface temperature for the end of the Early
Cretaceous, then the resulting temperature would be only
155 ºC for 4.5 km depth. We should add to this point, that
in the non-metamorphosed units of Aggtelek-Rudabánya
Mts, NE Hungary, all CAI 5 values should have formed
below 200 ºC temperature according to “illite crystallin-
ity” (KI) data (Árkai & Kovács 1986).

The northern thrust unit of Mecsek Hills is characterized

by considerably less sedimentary thicknesses, reflecting
the more marginal position of its depositional area in re-
spect to the depocenter of the half-graben (cf. Figs. 2, 4, 5
and 6). Here the top of the Middle Triassic carbonates
could be only ca. 1300 m deep at the end of the Early Cre-
taceous and the upper temperature limit of the oil window
could be at their base (Horváth et al. 1982; Fig. 6 herein).
Unfortunately, we have no conodonts from this area.

On the other hand, the top of the Middle Triassic carbon-

ates of the Villány area was either on the surface or at a
maximum 90 m burial depth at the end of the Bajocian, and
only at maximum 1000 m burial depth at the end of the

Fig. 7. Projection on the Arrhenius plot (after Epstein et al. 1977
and Rejebian et al. 1987) the colour of the Mecsek conodonts
(CAI = 4) suggests about 200

ºC paleotemperature for the time of

the maximal burial, namely from the beginning of Bathonian till
the end of Albian (ca. 70 Myr). This estimation is somewhat high-
er, than what can be calculated from available vitrinite reflectance
data nearest to the conodont locality (cf. Fig. 2 and Table 2). The
Villány conodonts having CAI = 1 suggest accordingly a paleotem-
perature for this time even below 50

ºC (not shown on the figure).

265

CONODONT COLOUR ALTERATION RELATED TO A HALF-HRABEN STRUCTURE (SOUTHERN HUNGARY)

Fig. 8. Conodonts and fish remains with different CAI values from the two sides of the Mecsek half-graben zone. 1—11  –  Villány
Zone: 1 – Gondolella hanbulogi (Sudar et Budurov) subadult ontogenetic stage. Borehole Peterd 1, 606.0—612.0 m. 2 – Gondolella
bulgarica  (Budurov et Stefanov) subadult ontogenetic stage. Borehole Peterd 1, 610.0—612.2 m. 3 – Gondolella hanbulogi (Sudar et
Budurov) juvenile ontogenetic stage. Borehole Peterd 1, 610.0—612.2 m. 4 – Gondolella hanbulogi (Sudar et Budurov) medium onto-
gentic stage. Zuhánya quarry. 5 – “Hibbardella magnidentata” morphoelement of the Gondolella—apparatus. Borehole Peterd 1,
610.0—612.2 m. 6 – “Neohindeodella triassica” morphoelement of the Gondolella-apparatus. Borehole Peterd 1, 575.4 m. 7 – “Ozarko-
dina tortilis” morphoelement of the Gondolella-apparatus. Zuhánya quarry. 8 – “Ozarkodina tortilis” morphoelement (small form) of
the  Gondolella-apparatus. Borehole Peterd 1, 610.0—612.2 m. 9 – Thick fish tooth. Borehole Peterd 1, 523.5—527.2 m. 10 – Thin
fish tooth. Borehole Peterd 1, 589.5—598.0 m. 11 – Gnathoid fish scale. Borehole Peterd 1, 606.0—610.0 m. (Exact location of the
samples see in Kovács et al. 2005: Fig. 2). 12—21 – Mecsek Hills, geological key section at the road curve to Misina. 12 – Gondolella
hanbulogi  (Sudar et Budurov) adult ontogenetic stage. Sample Mecsek-T2, in Kovács & Papšová (1986). 13 – Gondolella hanbulogi
(Sudar et Budurov) adult ontogenetic stage. Sample Bh-11. 14 – Gondolella bifurcata (Budurov et Stefanov) adult ontogenetic stage.
Sample Bh-11. 15 – Gondolella bulgarica (Budurov & Stefanov) subadult ontogenetic stage. Sample Bh-11. 16 – Gondolella hanbu-
logi  (Sudar et Budurov) medium ontogenetic stage. Sample Mecsek T2. 17 – “Enantiognathus ziegleri” morphoelement of the Gon-
dolella-apparatus. Sample Bh-11. 18 – “Cypridodella muelleri” morphotype of the Gondolella-apparatus. Sample Mecsek-T2. 19 – Big
gnathoid fish scale. Sample Bh-6. 20 – Thin fish tooth. Sample Bh-7. 21 – Thick fish tooth. Sample Bh-6. All magnifications are
65 , except 19, which is 33 .

266

KOVÁCS, RÁLISCH-FELGENHAUER and BÓNA

Fig. 9. Textural types of the conodont-bearing Middle Triassic limestones hosting the conodonts shown on Fig. 8. 1—2 – Mecsek Hills,
Misina road curve geological key section: 1 – brachiopodal—crinoidal biomicrite, mudstone; sample Bh-1, 25 ; 2 – biomicrite, mud-
stone, with echinoderms and brachiopod shells; sample Bh-12, 25 ; (exact location of the samples see in Kovács & Rálisch-Felgenhauer
2005: Fig. 2). 3—4 – Villány Hills, Rigó quarry: 3 – ostracod-bearing microsparite, mudstone; bed No. 1, 25 ; 4 – microsparite with
argillaceous material (black) within the clasts; slump structure typical for the Zuhánya Limestone; bed No. 2, 12.5 . (Sample numbers of
3—4 are after unpublished documentation by Rálisch-Felgenhauer.) Note, that even the Mecsek samples yielding the conodonts with
CAI = 4 do not show any sign of even incipient foliation (compare with the texture of the least metamorphosed parts of the Bükk Mts, NE
Hungary, which yielded conodonts of the same CAI = 4 value, but weakly foliated; see in Sudar & Kovács 2006: Fig. 14.1).

Early Cretaceous. Again assuming a 5 ºC/100 m geothermal
gradient, with 20 ºC surface temperature, as in the case of
the Mecsek area, 60 ºC temperature can be calculated for
this depth at that time. According to Horváth et al. (1982)
(see Fig. 6 herein) the Middle Triassic carbonate succession
of the Villány area could be within the oil window at the
end of the Early Cretaceous. The thickness of the Ladinian
dolomites overlying the conodont-bearing Zuhánya Lime-
stone reaches 370 m; counting 400 m of additional burial,
conodonts of the Villány area were affected by a max. tem-
perature of 90 ºC at that time, which is consistent with the
CAI = 1 value. Tectonic overburden resulting from middle—
Late Cretaceous N-ward thrusting did not change this value.

Vitrinite reflectance data from the Triassic of the

Transdanubian Range (Ro = 0.41, n = 10 in the borehole
Zsámbék 14; Ro = 0.80, n = 1 from borehole Bakonyszűcs 1)
are consistent with the CAI = 1 value. However, data known
from the Triassic of the Villány area (Ro = 1.37—1.39) are
the same, as in the Mecsek area and are anomalously high
for the CAI = 1 value. We can suppose two reasons for this

controversy: 1 – the vitrinite grains were redeposited;
2 – or a short-time local heating (like in the case of the
Szőlősardó Unit of Aggtelek-Rudabánya Mts; see above).
The second case could possibly be caused by a nearby lo-
cated alkaline basalt sill of Early Cretaceous age – however,
these are not known in the Mesozoic of the Villány Hills. In
the future it will be necessary to carry out more vitrinite re-
flectance determinations from the Villány Hills area.

Conclusions

1 – The present study confirms, that observation of

conodont colours up to the value CAI 5, in conditions of
burial diagenesis, is a useful method to reveal thermal his-
tory of sedimentary basins. However, combined applica-
tion with other methods (vitrinite reflectance, “illite
crystallinity”, etc.) is higly recommended; anomalous val-
ues given by one or other method can be eliminated in
this way. On the other hand, above CAI 5, in conditions

267

CONODONT COLOUR ALTERATION RELATED TO A HALF-HRABEN STRUCTURE (SOUTHERN HUNGARY)

of regional dynamothermal metamorphism, where fluid
pressure also plays a decisive role, the colour of con-
odonts cannot be used for direct paleotemperature deter-
minations (cf. Árkai & Kovács 1986; Kovács & Árkai
1987, 1989; Sudar & Kovács 2006).

2 – The necessity of application of different methods

for paleotemperature estimations is also underlined by dif-
ferent vitrinite reflectance and overburden thickness data
related to the same CAI values in different regions, as in
the Appalachians, USA (Epstein et al. 1977 and Table 1
herein), in the Rheinisches Schiefergebirge, Germany
(Königshof 1992) or in our case.

3 – In the studied area, Middle Triassic carbonates of

the Mecsek half-graben zone had been buried in the
depocenter by a sediment pile up to 4.3 km thick by the
boundary of Bajocian/Bathonian. On the other hand,
those of the Villány Ridge were either subareally exposed
during this period, or buried only by several 10 m (max.
90 m) thick Upper Triassic to Middle Jurassic sediments.
The colour of conodonts (CAI = 4 in the Mecsek Zone, but
only CAI = 1 in the Villány Zone) characteristically reflect
this enormous difference in burial depth.

4 – Late Jurassic—Early Cretaceous additional overbur-

den (max. 1000 m in both zones) did not change this dif-
ference; the Middle Triassic rocks remained at the
maximal burial depth for about 70 Myr, till the end of the
Early Cretaceous.

5 – The Early Cretaceous alkaline basaltic volcanism

did not influence the colour of the studied conodont as-
semblages.

6 – Albian flysch sedimentation marks the beginning

of tectonic activity in the Villány Zone. Tectonic overbur-
den resulting from N-ward overthrusting in this zone did
not influence the colour of conodonts. Unload resulting
from the formation of the large SW Mecsek antiform in the
Mecsek Zone likely began at the same time (Wein 1967),
or even in the Aptian (Császár 2003).

Acknowledgments:

Preparation of the present contribu-

tion was supported by the Hungarian National Research
Fund, grants No. T 029654 and T 037595 (S.K.). Mária
Hámor-Vidó is thanked for her kind help in evaluation of
vitrinite reflectance data, Ágnes Siegl-Farkas and György
Don for helping in colour photographing for Fig. 8. Our
thanks are are extended to Dezső Simonyi, who prepared
Fig. 8, and to Károly Szoldán for preparing all other fig-
ures and tables. Péter Árkai (Budapest), Fritz Ebner
(Leoben) and Ján Mello (Bratislava) reviewed the earlier
version of the manuscript: their critical notes and helpful
sugesstions are kindly acknowledged herein.

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