GeolCarp_Vol57_No6_483

GEOLOGICA CARPATHICA, DECEMBER 2006, 57, 6, 483—494

www.geologicacarpathica.sk

Introduction

For more than 30 years geochronological investigations
have been carried out on Miocene volcanic and
volcaniclastic rocks of the Styrian Basin, which is part of
the western Pannonian Basin System (Fig. 1). The Neo-
gene Pannonian Basin System belongs to the western
Paratethys with a distinct stratigraphy different from the
Mediterranean area. Since the early days, several at-
tempts have been made in order to establish a precise
Paratethyan timescale by means of correlation of
biostratigraphical, geochronological and geomagnetical
investigations (e.g. Steininger & Bagdasarjan 1977; Vass
& Balogh 1989; Rögl 1996). These early radiometric in-
vestigations focused on the occurrence of lava and
subvolcanic rocks from stratovolcanoes. Tuffs were
rarely investigated because of their widespread alter-
ation, which did not allow the preparation and concen-
tration of sufficient unaltered material for analysis.
During the last decades, however, analytical procedures
made great progress and now geochronological results
with a higher precision can be achieved on smaller
samples. This advanced approach made possible not
only dating of lava and subvolcanic rocks, but of tuffs as

Ar/

Ar dating of Miocene tuffs from the Styrian part of the

Pannonian Basin: an attempt to refine the basin stratigraphy

ROBERT HANDLER

1,2,*

, FRITZ EBNER

, FRANZ NEUBAUER

, ANA-VOICA BOJAR

and SIEGFRIED HERMANN

Division General Geology and Geodynamics, Department Geography and Geology, University Salzburg, Hellbrunner Str. 34,

A-5020 Salzburg, Austria

Present address:

Forstinger+Stadlmann ZT-OEG, Achenpromenade 14, A-5081 Anif, Austria; * ro-handler@aon.at

Department of Applied Geosciences and Geophysics, University of Leoben, Peter-Tunner-Str. 5, A-8700 Leoben, Austria

Institute for Earth Sciences, Geology and Paleontology, Karl-Franzens University Graz, Heinrichstr. 26, A-8010 Graz, Austria

GeolithConsult, Frauentalerstr. 51, A-8530 Deutschlandsberg, Austria

(Manuscript received September 19, 2005; accepted in revised form June 22, 2006)

Abstract: In this study we present new

Ar/

Ar age data obtained from volcaniclastic material intercalated within

shallow-marine to neritic sediments of the Styrian part of the Miocene Paratethyan Sea, which allow a better control of
the sedimentation. At Retznei quarry, a volcaniclastic layer has been deposited in erosional patches above a consolidated
rhodolite limestone (“Leitha Limestone”) of the siliciclastic/marine Badenian Weissenegg Formation. Ar-release plots of
a biotite bulk-grain concentrate (30 grains) and a concentrate of three sanidine crystals (0.5—1 mm) display fairly flat
release-patterns with minor fluctuation in the low-energy gas-release steps. From the statistical point of view the biotite
concentrate yielded a high-precision plateau age of 14.21 ± 0.07 Ma, the three sanidine crystals yielded a plateau age of
14.39 ± 0.12 Ma. The radiometric ages obtained match the biostratigraphic record (Upper Lagenide Zone). A drill-core
recovered from the well Hörmsdorf, exposes several sand-dominated horizons and two layers of crystal tuff, of the
Karpatian Eibiswald Formation. Ar-analyses of one single biotite grain ( > 1 mm) from the hanging-wall, another biotite
single-grain from the lower tuff both display slightly disturbed Ar-release spectra. However Ar-plateau ages of
15.08 ± 0.09 Ma and 15.22 ± 0.17 Ma, respectively, have been obtained. Volcaniclastic rocks from Pöls are intercalated
within the Florian Formation, for which previous authors suggested a Early Badenian age (16.4—ca. 15 Ma according to
Rögl 1996). A concentrate of two clear sanidine crystals (0.5—1.0 mm), yielded a perfect Ar-plateau recording an age of
15.75 ± 0.17 Ma, which is more precise than previously published K-Ar results.

Key words: Miocene, Paratethys, Pannonian Basin, volcanism,

Ar/

Ar step-heating.

well, as significantly less material – sometimes only one
grain – is required to obtain perfect results.

The

Ar/

Ar method of dating seems to be the most

suitable tool for dating young volcanic and especially
volcaniclastic rocks, because of the following reasons:
(1) It is not necessary to correlate isotopic analysis of
the volcanic phenocrysts with analysis from whole-rock
samples, as is necessary, for instance, in Rb-Sr dating.
(2) Because of the higher abundance of potassium com-
pared to trace elements such as uranium, errors obtained
from Ar-isotopic analyses are much lower. The errors for
zircon fission-track (FT) dating generally range be-
tween 5—10 % of the age, whereas in ideal cases errors
of less than 1 % can be obtained from

Ar/

Ar analy-

sis. (3) Compared to the conventional K-Ar method of
dating,

Ar/

Ar analysis allows an insight into the Ar-

isotopic distribution of the grain, namely to see whether or
not the isotopic composition within the grain(s) is dis-
turbed or not. Thus, alteration of the volcanic minerals, as
well as loss or incorporation of extraneous

Ar-compo-

nents can be detected by Ar-release spectra in combina-
tion with inverse isochron plots, and therefore data
obtained by means of the

Ar/

Ar method are much

more reliable compared to conventional K-Ar data.

484

HANDLER, EBNER, NEUBAUER, BOJAR and HERMANN

First attempts to correlate radiometric ages of tuffs with

paleontological and/or stratigraphic investigations have
been carried out at the very beginning of this millennium
by the Leoben working group (Ebner et al. 2000, 2002).
Unfortunately, in the early phases of investigations less at-
tention has been paid to the

Ar/

Ar technique, and fis-

sion-track dating of volcanic zircons was their choice for a
tephrachronological attempt (e.g. Sachsenhofer et al.
1998; Ebner et al. 2000, 2002). A first attempt to correlate

Ar/

Ar geochronology with

O changes of pectinid

and brachiopod shells and global climate change has been
carried out in the uppermost rhodolite limestone (“Leitha
Limestone”) within the Badenian Weissenegg Formation
exposed in the Retznei quarry (Bojar et al. 2004).

In this pilot study, we report new data from few Miocene

tuffs collected from two surface exposures and a drill hole.
The ages show a potential for refining the Paratethyan
time-scale calibration by

Ar/

Ar dating of unaltered

phenocrysts of tuffs.

Miocene stratigraphy of the Styrian Basin

A short summary of Miocene formations of the Styrian Ba-

sin is given in Fig. 2. A detailed description of the evolution
of the Styrian Basin has been provided by Ebner &

Sachsenhofer (1991). Additionally, more recent studies on
the stratigraphy and paleogeography have been published by
Rögl (1996, 1998, 2001) and Gross (2000). Sedimentation
within the western Styrian Basin started with terrestrial, flu-
viatile-limnic Ottnangian—Karpatian sedimentary formations,
which are followed by shallow-marine, mostly siliciclastic,
neritic formations. Within the uppermost Karpatian and
lower to middle sectors of the Badenian formations, a few ma-
jor stratovolcanoes or shield volcanoes, are intercalated. The
most important ones are those of Weitendorf and
Gleichenberg, and the huge, buried volcanic bodies of
Walkersdorf and Mitterlabill, the latter forming the western
continuation of the Gleichenberg volcano. Thin, heavily al-
tered, tuffs and bentonite horizons are widespread in shallow-
marine sedimentary rocks (Ebner 1981). Stratigraphically
upwards along intra-basin swells, the Weissenegg Formation
with the “Leitha Limestone” as the hanging-wall member be-
long to the Lagenide Zone of Badenian age.

Previous geochronology on Miocene volcanic and

volcaniclastic rocks in the Styrian Basin and

adjoining areas

Geochronological analyses of volcanic and volcani-

clastic rocks have a long tradition in the Styrian Basin. A

Fig. 1. Simplified map of the Styrian Basin indicating sample locations and position of Miocene volcanoes as possible sources.

485

Ar/

Ar DATING OF MIOCENE TUFFS FROM THE STYRIAN PART OF THE PANNONIAN BASIN

summary of previous radiometric data on volcanic and
volcaniclastic rocks is provided in Table 1.

One problem regarding old ages is, that in the early days

of geochronology other

K-decay constants than today

were used. Therefore, for a detailed and refined calibration
of the stratigraphy these “old” data have to be recalcu-
lated with the “new” decay constants suggested by the
IUGS Subcommission on Geochronology (Steiger & Jäger
1977). Unfortunately, in some publications analytical de-
tails, such as the decay constant used for age-calculation,
or the abundances of

Ar*, or the

Ar ratio were not

reported, and, therefore, these analyses cannot be recalcu-
lated to “modern” decay constants.

Thirty years ago Lippolt et al. (1975) was the first to

publish K-Ar ages on Miocene volcanics from various
sampling sites in Styria and the adjoining Lavant Valley
in Carinthia. These first results outlined the importance
and the possibility to combine geochronological data
with results from paleontological investigations in order
to establish a stratigraphic scheme for this area.

In the following (see also Table 1) we briefly summarize

the age constraints on Miocene volcanic and volcaniclastic

rocks mainly to show possible sources for the tuff layers
under consideration and to allow comparison with previ-
ous results.

Gleichenberg/SE-Styria

The earliest age reported for the Gleichenberg volcano

was presented by Lippolt et al. (1975), but unfortunately
only as a “preliminary result” without any analytical de-
tails. Therefore, the age of “ca. 14.6 Ma” (Lippolt et al.
1975) cannot be recalculated. Steininger & Bagdasarjan
(1977) published four ages obtained from two samples of
trachyandesite sampled in the quarry “Klause” north of
Gleichenberg. Their results, as above calculated with the
“old” decay constants, range between 15.4—17.5 Ma. Re-
calculation of their analytical results yielded ages of 15.4
and 15.8 Ma for the hanging-wall and 15.4 and 16.8 Ma
for the lower part of the quarry.

A further attempt to date the Gleichenberg volcano has

been carried out by Kolmer (1980), who calculated a Rb-Sr
errorchron age from four whole-rock samples of different
volcanic rocks collected at various localities around

Fig. 2. Sedimentation, facies distribution, volcanism, and position of the samples analysed. Radiometric ages, Central Paratethyan stages,
and lithostratigraphic formations according to “The Stratigraphic Table of Austria” (Piller et al. 2004). Biozonations follow Steininger
& Wessely (2000), which is based on earlier work by Berggren et al. (1995), Steininger et al. (1996) and Steininger (1999).

487

Ar/

Ar DATING OF MIOCENE TUFFS FROM THE STYRIAN PART OF THE PANNONIAN BASIN

Gleichenberg. The rather obscure result was an age of
22.97 ± 1.93 Ma.

On the northern flank of the Gleichenberger Kogel stra-

tovolcano near Gossendorf the trachyte/trachyandesite is
completely converted to opal, alunite, and clays due to
post-volcanic hydrothermal activity. For this alteration
product, the so-called “Styrian Trass” a K-Ar whole-rock
age of 13.2 ± 1.0 Ma was determined by Balogh et al.
(1994).

Weitendorf/SW-Styria

Circa 3.5 km ESE of Weitendorf, near Wildon, a

shoshonite is excavated in an open-pit quarry. There, vol-
canic rocks are intercalated within fine-clastic sediments.
Occurrences of fossil-bearing marly clay can be found as
intercalations within, as well as both in the hanging- and
footwall of the volcanic body. This interesting and rather
rare situation suggests that this occurrence is an ideal site
for paleontological-geochronological correlations. How-
ever, the age and nature of this volcanic body have been a
matter of debate, due to contrasting observations and in-
terpretations (see Ebner & Gräf 1977, and Krainer 1987 for
summaries of older literature). On the basis of fritted mar-
gins and thermally induced bleaching effects, slickensides
in the footwall and local steep inclinations of adjacent
sediments, as well as the presence of local pillow-struc-
tures, today’s most accepted theory is that the volcanic
body intruded as a sill into the wet marine sediments near
the hydro-/lithosphere boundary (Krainer 1987).

From the footwall sediments, Ebner & Gräf (1977) and

Fenninger & Wassermann (1982) described a rich marine
macro-fauna, which suggests a Badenian age (Upper
Lagenide Zone). Krainer (1987) found similar heavy-min-
eral assemblages in the foot- and hanging-wall mudstones,
and a rich micro-fauna in the hanging-wall sediments, and
interpreted his data to support a model of either emplace-
ment of the shoshonite as a sill within the sediments, or at
least rapid coverage of the shoshonite with marine sedi-
ments. In any case, a Badenian age (Upper Lagenide Zone)
is well constrained by paleontological data.

The first geochronological results regarding Styrian vol-

canic rocks were obtained from the Weitendorf quarry.
Lippolt et al. (1975) reported an age of 15.2 ± 0.9 Ma for
K-Ar analyses performed on the 200—500 µm size-fraction
of a whole-rock sample. Unfortunately, the authors did not
give any analytical details, and therefore it is difficult to
judge the reliability of this datum, or to recalculate the age.
Steininger & Bagdasarjan (1977) published seven ages
obtained from two samples. Their results range between
16.0—18.2 Ma calculated with the “old” decay constants.
Recalculation of their analytical results yielded ages still
within a large range of 16.1—17.51 Ma for the hanging-wall
and 15.8—16.6 Ma for the lower part of the quarry.

The most recent age data have been obtained by Balogh

et al. (1994), who reported a series of K-Ar whole-rock
data from various areas within the Styrian Basin. They ob-
tained ages of 14.0 ± 0.7 Ma for the shoshonite/latite ex-
posed in the Weitendorf quarry.

Summing up, the ages reported span over a period of

4 Myr (see Table 1) and range between 14.0 Ma (Balogh
et al. 1994) and 18.0 Ma (Steininger & Bagdasarjan 1977).

Kollnitz near St. Paul/Carinthia

In the Lavant Valley of southeastern Carinthia a basalt

was quarried near the village Kollnitz. The first ages were
obtained by Lippolt et al. (1975), who reported ages of
14.9 ± 0.9, 17.0 ± 4.2 and 19.6 ± 2.2 Ma, the latter two from
a contaminated basalt. Therefore, the youngest age of
14.9 ± 0.9 Ma is interpreted as geologically significant.
However, due to the lack of detailed analytical data a re-
calculation of the age with recently accepted decay con-
stants is not possible.

Pauliberg and Oberpullendorf/Burgenland

Balogh et al. (1994) reported K-Ar whole-rock ages of

10.5 ± 1.0 Ma for an alkali-basalt (Dobosi et al. 1991) from
the hanging-wall part of the quarry at Pauliberg, and ages
of 11.0 ± 0.5, 11.7 ± 0.4, and 12.3 ± 1.1 Ma for the diabase
at the base of the quarry. Based on interpretations of “clas-
sical” Ar-isochron plots (i.e.

Ar/

Ar vs. K/

Ar), these

authors suggest an age of ca. 11.5 Ma as the “most prob-
able” age for the volcanics from Pauliberg.

A K-Ar whole-rock age of 11.1 ± 1.2 Ma is reported for

an olivine-tholeiitic basalt from the lower levels in the old
quarry of Oberpullendorf (Balogh et al. 1994). Despite the
fact that extraordinarily high amounts of gas have been
obtained from this sample, which might indicate excess-Ar
components, the authors interpret this age as geologically
meaningful, because it is within the same range as ages
obtained for the nearby Pauliberg volcanics.

Eastern Pohorje Mts/Slovenia

Sachsenhofer et al. (1998) reported an apatite fission-

track age of 14.6 ± 1.8 Ma for a dacite dyke from the area
of Vuzenica, Slovenia. Because the tracks show very mi-
nor shortening, these authors conclude that the depth of
emplacement was shallow or exhumation took place soon
after volcanic activity. Vitrinite reflectance and FT data
on detrital apatite from adjoining Tertiary sedimentary
rocks, however, indicate a thermal overprint with a mean
age of 14.4 ± 2.3 Ma, which is within error to the age re-
ported from the dacite. Therefore, the age reported for the
Vuzenica dyke can only be regarded as a minimum age for
the volcanic activity.

Smrekovec volcano/Slovenia

An older period of magmatic activity is reported for the

Smrekovec volcano, Slovenia. Hanfland et al. (2004) re-
ported Oligocene/Early Miocene K-Ar ages obtained from
whole-rock analyses ranging between ca. 22 and 29 Ma.
Because of the intense hydrothermal alteration, the au-
thors suspect Ar-loss and interpret their ages as minimum
ages for volcanic activity.

488

HANDLER, EBNER, NEUBAUER, BOJAR and HERMANN

Previous geochronology on Styrian tuffs

From the locality Quellgraben near Pöls, two layers of

bentonite are intercalated within, and separated by ca. 1 m
thick marine sedimentary rocks of the Florian Formation.
These marls belong to the Lagenide Zone of the Lower
Badenian. K-Ar analysis on volcanic biotite has been car-
ried out by Balogh et al. (1994). For the lower sample
these authors report ages of 14.9 ± 0.7 Ma and 15.3 ± 0.6 Ma
(with the reported mean age of 15.1 ± 0.5 Ma). Biotite from
the second sample, which is 1 m in the hanging-wall,
yielded an age of 16.6 ± 0.6 Ma. It is interesting to note,
that the stratigraphic younger layer contains older biotite.
A significantly older K-Ar age of 17.5 ± 2.6 has been re-
ported for a concentrate of biotite from tuff-layer exposed
in the area around Köflach/Voitsberg (Balogh et al. 1994).
From the same area Ebner et al. (2000, 2002) reported zir-
con fission-track ages of 16.0 ± 0.7 Ma and 18.7 ± 0.9 Ma
which fit well with similar ages obtained by the same tech-
nique of 14.9 ± 0.7 Ma, 15.5 ± 0.8 Ma, and 17.1 ± 0.7 Ma
for tuffs deposited within the “Noric Depression” of Upper
Styria (Ebner et al. 2000, 2002), and another zircon fis-
sion-track age of 15.4 ± 0.5 Ma from a volcanogenic con-
taminated sandstone exposed near Ratsch in the
southwestern part of Styria (Ebner et al. 2002).

Sample location and description

Retznei (samples 1 and 2)

The “Retznei Zementwerk” quarry has been described

in detail by Friebe (1990, 1991), Fritz & Hiden (2001)
and Bojar et al. (2004). A first description of volcanic
rocks within the sedimentary succession exposed in the
quarry of Retznei can be found in Hauser (1951), who de-
scribes an “andesite” in the hanging-wall part of this
quarry. Friebe (1990) described two tuffite layers below
the above mentioned “andesite”. The lower tuffite is in-
tercalated in between a rhodolite limestone of the
Badenian Weissenegg Formation ( = “ Leitha Limestone”),
the younger was deposited above the limestone, at the
base of the sand-dominated top exposed in the quarry.
Both tuffite layers show a variable degree of detrital in-
flux. Still in the hanging-wall of the latter, a biotite-rich
volcanic layer, which was first described as an andesite by
Hauser (1951), is exposed. Today it is accepted that
Hauser’s andesite is a pyroclastic rock, which has been de-
posited in erosional patches above the consolidated lime-
stone. Gravitational slumping structures can be recognized
and are explained to be due to deposition of the tuff on the
slope of the reef flanks of the “Leitha Limestone”. In con-
trast to the two tuffite layers in the footwall of this layer,
no contribution of other detrital minerals has been recog-
nized within that layer. Therefore we believe that this
sample represents a real tuff rather than a tuffite. The tuff
itself is almost unconsolidated, inspite of local weathering
products, as indicated by the occurrence of montmorillo-
nite. Although weathered, it was possible to separate one

fresh biotite (sample 1 = AVB2 of Bojar et al. 2004) and
one fresh, coarse-grained sanidine (sample 2) concentrate.

Well Hörmsdorf (samples 3 and 4)

Near the village Eibiswald, a drilling project of the

Graz-Köflach-Railway (GKB) has been carried out (well
Hörmsdorf 3). The complete core with a diameter of ca.
7 cm has been sampled. The drilling reached the pre-Ter-
tiary basement at a final depth of 265 m. The basement is
overlain by the Eibiswald Formation for which a
Karpatian age is assumed. A lithostratigraphic description
is provided by Gruber et al. (2003) and summarized in
Fig. 3. The lower 70 cm of the core comprise massive,
coarse conglomerates of the Lower Eibiswald Formation.
These strata are interpreted as mass flows and turbidites. In
the hanging-wall, the massive conglomerates become lay-
ered with intercalations of more fine-clastic sedimentary
rocks. The basal succession contains a ca. 30 cm thick
layer of coal. A few meters above the coal-horizon several
sand dominated horizons and two layers of crystal tuff are
intercalated. From each of these layers one biotite concen-
trate has been prepared. Sample 3 has been sampled at the
basis of a well consolidated, ca. 10 cm thick layer of light,
not consolidated, laminated sands in the hanging-wall of
dark brown clay-silt material. The biotite grains from this
tuff layer are not altered and show no inclusions. Sample 4
has been prepared from a ca. 20 cm thick tuff layer a few
meters above sample 3. The largest biotite crystals in this
layer reach diameters up to 2 mm.

Pöls (sample 5)

Volcanic tuffs south of the village Pöls have been de-

scribed by Kopetzky (1957). Two different layers are inter-
calated within the shallow marine fine clastic Lower
Badenian Florian Formation which contains foraminifers
and rich mollusc faunas. From one of these tuff layers a
concentrate of clear, coarse-grained sanidine has been pre-
pared.

Analytical techniques

Mineral concentrates are packed in aluminium-foil and

loaded in quartz vials. For calculation of the J-values,
flux-monitors are placed between each 4—5 unknown
samples, which yield a distance of ca. 5 mm between adja-
cent flux-monitors. The sealed quartz vials are irradiated
in the MTA KFKI reactor (Debrecen, Hungary) for 16
hours. Correction factors for interfering isotopes have
been calculated from 10 analyses of two Ca-glass samples
and 22 analyses of two pure K-glass samples, and are:

Ar/

(Ca)

= 2.6025E-4,

Ar/

(Ca)

= 6.5014E-4, and

Ar/

(K)

= 1.5466E-2. Variations in the flux of neutrons

were monitored with DRA1 sanidine standard for which a

Ar/

Ar plateau age of 25.03 ± 0.05 Ma has been re-

ported (Wijbrans et al. 1995). After irradiation the minerals
are unpacked from the quartz vials and the aluminium-foil

489

Ar/

Ar DATING OF MIOCENE TUFFS FROM THE STYRIAN PART OF THE PANNONIAN BASIN

Fig. 3. Sequence of strata within the Eibiswald Formation includ-
ing the Hörmsdorf well with the two tuff layers dated (modified
after Gruber et al. 2003).

packets, and handpicked into 1 mm diameter holes within
one-way Al-sample holders.

Ar/

Ar analyses are carried out at the ARGONAUT-

Laboratory of the University of Salzburg using a UHV Ar-ex-
traction line equipped with a combined MERCHANTEK

UV/IR laser ablation facility, and a VG-ISOTECH

NG3600 Mass Spectrometer.

Stepwise heating analyses of samples are performed us-

ing a defocused ( ~ 1.5 mm diameter) 25 W CO

-IR laser

operating in Tem

mode at wavelengths between 10.57

and 10.63 µm. The laser is controlled from a PC, and the
position of the laser on the sample is monitored through a
double-vacuum window on the sample chamber via a
video camera in the optical axis of the laser beam on the
computer screen. Gas clean-up is performed using one hot
and one cold Zr-Al SAES getter. Gas admittance and

pumping of the mass spectrometer and the Ar-extraction
line are computer controlled using pneumatic valves. The
NG3600 is a 18 cm radius 60º extended geometry instru-
ment, equipped with a bright NIER-type source operated
at 4.5 kV. Measurement is performed on an axial electron
multiplier in static mode, peak-jumping and stability of
the magnet is controlled by a Hall-probe. For each incre-
ment the intensities of

Ar,

Ar, and

Ar are

measured, the baseline readings on mass 35.5 are auto-
matically subtracted. Intensities of the peaks are back ex-
trapolated over 16 measured intensities to the time of gas
admittance either by a straight line or a curved fit. Intensi-
ties are corrected for system blanks, background, post-irra-
diation decay of

Ar, and interfering isotopes. Isotopic

ratios, ages and errors for individual steps are calculated
following suggestions by McDougall & Harrison (1999)
using decay factors reported by Steiger & Jäger (1977).
Definition and calculation of plateau ages has been car-
ried out using ISOPLOT/EX (Ludwig 2001). For inverse
isotope correlation diagrams regression algorithms of
York (1969) are used.

Ar/

Ar ages of Miocene tuffs

Analytical results from incremental heating experi-

ments of the biotite and sanidine samples are provided in
Table 2 and graphically presented as Ar-release spectra
in Fig. 4. In general all dated samples display well de-
fined plateau-ages. To allow control of the possible con-
tribution of excess-Ar components to the Ar-isotopic
system of the material analysed, inverse isotope correla-
tion diagrams have been included in Fig. 4. Although
seldom justified, because the probability of fit of the re-
gression is seldom more than 15 %, generally Model 1
solutions have been selected, because Model 2 regres-
sions assign equal weights to the individual data points
and ignore the individual data point errors. Despite the
discussion which type of regression model should be
used, it is worth noting, that the inverse isochron plots
do not display any sign of excess-Ar incorporation in
any of the analysed samples. In the following, the results
are discussed according to their sample location and
stratigraphic age.

Reztnei

One biotite (125—250 µm) bulk-grain concentrate of

30 grains (sample 1), and a concentrate of three large
(0.5—1.0 mm) sanidine crystals (sample 2) have been
analysed. Ar-release plots of both concentrates display
fairly flat release-patterns with minor fluctuation in the
low-energy gas-release steps. However, both samples dis-
play well defined plateau-ages, which are similar within
error. From the statistical point of view, biotite sample 1
yielded a high-precision age of 14.21 ± 0.07 Ma recorded
in steps 11—18 (fusion), which together comprise 77.3 %
of the

Ar released. Fluctuations in the first, low energy

gas-release steps are not matched by

Ar/

Ar variations.

491

Ar/

Ar DATING OF MIOCENE TUFFS FROM THE STYRIAN PART OF THE PANNONIAN BASIN

Table 2: Continued.

Sanidine sample 2, yielded 14.39 ± 0.12 Ma for steps 6—11
(fusion), together comprising 86.9 %

Ar released. The

staircase type pattern of the first five increments displayed
in the Ar-release pattern is matched by a slight decrease of
the

Ar/

Ar ratio. These first increments are interpreted,

therefore, as the result of degassing from a younger, less
retentive, mineralogical phase, probably the result of mi-
nor weathering, which has been reported from other layers
in this quarry.

Well Hörmsdorf

From the two tuff-layers recovered from the Hörmsdorf

drill-core, one biotite single-grain ( > 1 mm) each has been
analysed by step-wise heating technique. The biotite from
the hanging-wall (sample 3) displays a slightly disturbed
Ar-release spectrum as well, with higher ages reported in
the first two and the last increment. However, an Ar-pla-
teau-age of 15.08 ± 0.09 Ma is defined for steps 6—14 (to-
gether comprising 82.1 %

Ar released). The biotite

sample 4 from the lower tuff layer again yielded a slightly
disturbed Ar-release pattern. The first two increments
might indicate minor alteration of the sample, and are
matched by slightly higher

Ar/

Ar ratios. Therefore, we

interpret these ages at around 9 Ma to reflect degassing of
a younger, less retentive, more Ca-rich mineralogical
phase. However, for steps 4—12 (fusion), which together
comprise 67.5 % of the

Ar released, a plateau age of

15.22 ± 0.17 Ma can be calculated.

Pöls

One concentrate (sample 5) containing two sanidine crys-

tals (0.5—1.0 mm) has been analysed. Results yielded a per-
fect plateau-type of the Ar-release pattern. For steps 4—10
(fusion), together comprising 91.9 %

Ar released, a pla-

teau age of 15.75 ± 0.17 Ma has been calculated. This age
fits perfectly with the previously published ages of
16.6 ± 0.6 Ma and 15.1 ± 0.5 Ma obtained from K-Ar dating
of biotite by Balogh et al. (1994).

Discussion

Generally no incorporation of excess Ar components is

indicated by Ar-release spectra nor isotope correlation
plots, and no significant loss of radiogenic Ar components
is indicated in the low-energy gas-release steps. Therefore

493

Ar/

Ar DATING OF MIOCENE TUFFS FROM THE STYRIAN PART OF THE PANNONIAN BASIN

the ages presented in this study are interpreted as reflect-
ing rapid cooling subsequent to volcanic eruptions.

Results from the Badenian tuffs in the hanging-wall of

the “Leitha Limestone” of the Weissenegg Formation ex-
posed in the quarry Retznei match the biostratigraphic
record (Upper Lagenide Zone) as described by Friebe
(1990) and correlate with published biostratigraphic data
such as that proposed by Rögl (1996).

The data from the Hörmsdorf well are of particular im-

portance for the age classification of coal occurrences in
the Styrian Basin. Three huge coal deposits exist along
the western margins of the Miocene Pannonian Basin. The
oldest is the Köflach-Voitsberg deposit, for which mostly
an Ottnangian, for uppermost sequences a Karpatian, age
is discussed (Steininger et al. 1998). The Eibiswald and
Wies coal-deposits are of Badenian age, with the
Eibiswald is in an intermediate position and the Wies de-
posit representing the uppermost and youngest deposit.
Based on mammal fauna, a Karpatian age (ca. 17.2—16.4 Ma
according to Rögl 1996) has been postulated for the coal
from Eibiswald (Mottl 1970). However, according to Rögl’s
(1996) stratigraphic correlation, the Mammal Zone MN6—5
covers an absolute age span from ca. 17.0—13.0 Ma, which
corresponds to nearly the whole Karpatian and Badenian
stages. Based on paleogeographic and stratigraphic rea-
sons Gruber et al. (2003) correlated the Eibiswald with the
lower coal occurrences exposed in the Hörmsdorf well.
Our new

Ar/

Ar ages which have been obtained from

two tuff-samples just a few meters in the hanging-wall of
this coal horizon, confirm the ideas of Gruber et al.
(2003), who argue for a younger, namely Badenian age
(16.4—13.0 Ma according to Rögl 1996) for the Eibiswald
coal and suggest a minimum age of ca. 15.22 ± 0.17 Ma
for the formation of the coal deposits.

As noted above the age of sediments in the area around

Pöls is well constrained by biostratigraphic data and re-
garded as Early Badenian (Kopetzky 1957). Our result of
15.75 ± 0.17 Ma fits perfectly with previously published
ages of 16.6 ± 0.6 Ma and 15.1 ± 0.5 Ma, which have been
obtained from K-Ar dating on biotite by Balogh et al.
(1994).

Conclusions

The new

Ar/

Ar ages presented in this study show the

potential to get high-precision ages of tuffs, provided
careful sample selection and preparation, although tuffs
are mostly altered. Because similar tuffs are widespread
not only in the Styrian Basin, but in a number of
intramontane basins as well (e.g. Ebner 1981), future work
in biostratigraphically well controlled sections could al-
low further refining of the Paratethyan time-scale calibra-
tion. Future work could also allow more precise
correlation of the wide marine area of the Pannonian Basin
with several small intramontane sedimentary basins, where
similar tuffs have been found in mostly terrestrial, lacus-
trine depositional environments (e.g. Sachsenhofer et al.
2000a,b; Neubauer & Unzog 2003 and references therein).

Acknowledgments:

The authors gratefully acknowledge

helpful reviews by Dr. J. Lexa, Prof. D. Vass, and one anony-
mous reviewer.

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