GeolCarp_Vol63_No2_139

www.geologicacarpathica.sk

GEOLOGICA CARPATHICA

, APRIL 2012, 63, 2, 139—148 doi: 10.2478/v10096-012-0011-9

Introduction

Acidic  tuffs  are  widely  recognized  throughout  the  Middle-
and  Upper  Miocene  in  the  eastern  part  of  the  Carpathian-
Pannonian area, especially in and around the Transylvanian
Basin (Mârza & Mészáros 1991). Only within the Badenian
deposits  of  the  Transylvanian  Basin,  at  least  three  different
widespread acidic tuffs have been recognized and described
(Răileanu  1959;  Mârza  &  Mészáros  1991).  The  Dej  Tuff
(Pošepny 1867) is the most prominent of them, and it is the
only one predating the intermediate calc-alkaline volcanism
of the Eastern Carpathians and the Apuseni Mts.

This easily recognizable lithological unit consisting of al-

ternations of numerous reworked tuff layers and fine-grained
siliciclastic sediments is also known as the upper part of the
“Dej  Tuff  Complex”  (Moisescu  &  Popescu  1967)  or  as  the
Dej Formation (Popescu 1970). However, its most frequently
used  denomination  in  Romania  and  the  neighbouring  coun-
tries  is  the  Dej  Tuff,  the  term  introduced  by  Pošepny  in
1867,  after  the  north-western  Transylvanian  town  of  Dej,
where it was first time described in detail.

Although the Early Badenian age of the Dej Tuff is widely

accepted,  based  on  planktonic  foraminifera  (e.g.  Popescu
1970; Mészáros &  uraru 1991; Popescu & Cioflica 1973),
and  calcareous  nannoplankton  (e.g.  Mészáros  &  Filipescu
1991;  Mészáros  et  al.  1991;  Mészáros  &  uraru  1991;
Mărun eanu et al. 2000), its precise age is still not well con-
strained on the grounds of radiometric dating. Since the Dej
Tuff is widely dispersed throughout the whole Transylvanian
Basin  and  its  closer  or  broader  surroundings,  its  value  as  a
regional marker horizon would be significantly increased by
the application of more accurate dating techniques. Calibra-
tion of basin evolution models is impossible without precise-

On the age of the Dej Tuff, Transylvanian Basin (Romania)

ALEXANDRU SZAKÁCS

1,2

, ZOLTÁN PÉCSKAY

, LÓRÁND SILYE

, KADOSA BALOGH

DANIELA VLAD

and ALEXANDRINA FÜLÖP

Sapientia University, Department of Environmental Sciences, Matei Corvin St. 4, 400112 Cluj-Napoca, Romania; szakacs@sapientia.ro

Institute of Geodynamics “Sabba S. Stefanescu”, Romanian Academy, Romania; szakacs@k.ro

Institute of Nuclear Research, Hungarian Academy of Science, Debrecen, Hungary; pecskay@namafia.atomki.hu

Babe -Bolyai University, Department of Geology, Cluj-Napoca, Romania; lorand.silye@ubbcluj.ro

Shell Canada Energy, Calgary, Alberta, Canada

North University, Baia Mare, Romania

(Manuscript received March 3, 2011; accepted in revised form September 30, 2011)

Abstract: The Dej Tuff is an important stratigraphic marker in the Transylvanian Basin. However, its Early Badenian age
is known only on biostratigraphical grounds so far. A number of radiometric dating techniques including K-Ar, Ar-Ar
and fission-track have been used in order to constrain more precisely its age, allowing the calibration of the Transylvanian
Basin’s evolutionary models. Although individual dating methods could not provide a unique, reliable and accurate
radiometric age, comparison and evaluation of multiple methods gives 14.8—15.1 Ma as the most likely formation age
of the Dej Tuff.

Key words: Badenian, Transylvanian Basin, radiometric dating, tephrochronology, explosive volcanism, rhyolite tuff.

ly defined and dated marker horizons within the sedimentary
sequences of the basin fill. This paper aims to constrain the
age  of  the  Dej  Tuff  based  on  data  obtained  with  various
radiometric  dating  methods,  such  as  K-Ar,  Ar-Ar  and  fis-
sion-track  (FT)  supplementing  the  updated  biostratigraphic
age constraints.

General features of the Dej Tuff

Occurrence

The Dej Tuff represents one of the most prominent felsic

(acidic) tuff “layers” of the whole Carpathian-Pannonian re-
gion. Its classical outcrop area is the northwestern and north-
ern  border  of  the  Transylvanian  Basin.  It  is  also  exposed
along the southeastern border of the Silvania and Baia Mare
basins, which are small marginal sub-basins belonging to the
great  Pannonian  Basin  System.  The  lithological  and  chro-
nostratigraphic equivalents of the Dej Tuff are known under
various names, namely Per ani Tuff at the southeastern mar-
gin  of  the  Transylvanian  Basin  (Rado  et  al.  1980);  Slănic
Tuff along the outer Carpathian bend (Murgeanu et al. 1968)
and  Govora  Tuff  at  the  southern  tip  of  the  Southern  Car-
pathians. Furthermore, drillhole and subsurface data strongly
suggest that the Dej Tuff forms an almost continuous litho-
stratigraphic  unit  inside  the  Transylvanian  Basin  (Mârza  et
al. 1991; Krézsek & Filipescu 2005; Krézsek & Bally 2006),
frequently used as a marker horizon in early gas exploration
works  (e.g.  Ciupagea  et  al.  1970).  Taking  into  account  the
geographical distribution of all these “tuffs”, and their litho-
and biostratigraphic as well as petrographic features, it is ap-
parent that they form a well-defined unique lithological enti-

140

SZAKÁCS, PÉCSKAY, SILYE, BALOGH, VLAD and FÜLÖP

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

ty over an area of at least 15,000 km

in the eastern part of

the Carpathian-Pannonian region (Fig. 1).

Stratigraphy

The ca. 2000 m thick Badenian sedimentary pile of the

Transylvanian Basin (Krézsek & Filipescu 2005) consists of
(see Fig. 2; Krézsek & Filipescu 2005; Filipescu 2001 and
references therein):

1) the Lower Badenian (Praeorbulina glomerosa Biozone)

Ciceu-Giurge ti Formation composed of conglomerates and/
or gravels, covered by an alternation of sandy marls and thin
tuff layers;

2) alternation of tuffs and marls ( = Dej Tuff of Pošepny

1867) covering the previous unit, and correlated to the Orbu-
lina suturalis/Globorotalia bykovae and Globoturborotalita
druryi/Globigerinopsis grilli Biozones (Lower—Middle
Badenian);

3) the Middle Badenian evaporites: gypsum in the western

border of the basin (Cheia Formation) and salt (Ocna Dejului
Formation) in its middle part;

4) the Upper Badenian (Velapertina Biozone) deep-marine

siliciclastics (Pietroasa Formation).

The Dej Tuff was re-interpreted and re-named several times

due to its lithological heterogeneity and the presence of some
minor  tuff  layers  within  the  conglomerates  below  it,  causing
nomenclatural  confusions.  Moisescu  &  Popescu  (1967)
grouped  the  above  mentioned  first  three  lithological  entities
into the “Dej Tuff Complex”, and named the Dej Tuff as the
“Dej  Tuff  level”  within,  whilst  Popescu  (1970)  noticed  the
lithological  heterogeneity  and  therefore  proposed  the  “Dej
Beds” ( = Dej Formation) name for it, and described it as such.

It is beyond the scope of this article to fully discuss the no-

menclature problems one may encounter in the relevant liter-
ature. We will use the original name Dej Tuff (Pošepny
1867) for the lithostratigraphic entity composed by the alter-
nation of tuffs and marls, because it should be considered as
a valid name, being in accordance with the International
Stratigraphic Guide (see Salvador 1994).

Lithology

The Dej Tuff is actually extremely variable in thickness

(from  a  few  meters  to  116 m)  and  composition.  In  the  out-
cropping  area  (north-western  Transylvanian  Basin)  a  large
number of tuff layers alternate with siliciclastic deposits con-
sisting  of  non-volcanic  or  mixed  material,  mostly  marls.
Three types of lithofacies have been recognized. They alter-
nate  in  a  rather  regular  manner  (Szakács  2000):  (1)  meters
thick  coarse,  sand  to  pebble-sized,  volcaniclastic  deposits,
including coarse lapilli tuff, (2) meters thick massive to strati-
fied and/or graded tuff layers, and (3) centimeter to decime-
ter  thick,  coarse-  to  medium-grained  tuff  layers  alternating
with  fossiliferous  marls.  They  represent  (1)  subaqueous  de-
bris flow deposits and redeposited pyroclastic flow deposits,
(2) high-density turbidites and (3) low-density turbidites, re-
spectively. The erosional base is often visible at the bottom
of  type  1)  deposits,  which  commonly  show  lenticular  mor-
phology and cannot be correlated over even short distances.
One may easily recognize Bouma sequences within the tuff
layers  deposited  from  low-density  turbidites.  Lithofacies
types commonly succeed each other according to the above
order,  from  bottom  to  top,  in  characteristic  facies  associa-
tions. Such facies associations may be repeated 3 to 6 times.

Fig. 1.  Occurrence  of  felsic  Mi-
ocene tuffs in the Pannonian-Car-
pathian region in outcrops (black)
and  in  drillings,  covered  by
younger sediments (hatched). The
dotted-line  frame  shows  the  area
of  Fig. 3.  The  main  tectonic  ele-
ments  (thrusts  and  faults)  are
shown.

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ON THE AGE OF THE DEJ TUFF, TRANSYLVANIAN BASIN (ROMANIA)

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

Fig. 2.

Detailed

correlation

the

stratigraphic

position

the

Dej

Tuff

within

the

Badenian

succ

ession

Miocene

sediments

the

north-western

Transylvanian

Basin

with

summary

the

radiometric age data for the Dej Tuff. The light grey band show

s the most likely time interval of its genesis. Lithostratigrap

hy based on Filipescu (2001). Biozonation of planktonic foramin

fera

the

Transylvanian

Basin

after

Filipescu

Silye

(2008),

Pope

scu

(1975),

and

Popescu

Ghea

(1984),

the

later

drawn

here

cording

the

recalibration

Krézsek

Filipescu

(2005),

sta

dard

zonation

after

Wade

al.

(2011).

Calcareous

nannoplankto

zones

after

Martini

(1971).

Standard

chronostratigraphy

based

Lourens

al.

(2004a),

and

regional

chronostratigraphy

ac-

cording

Rögl

al.

(2008).

Ages

magnetic

chrons

are

from

Lourens

al.

(2004a,b)

with

corrections

according

Hüsing

al.

(2010).

Data

biostratigrahic

events

from

Lourens

al.

(2004b),

and

Berggren

al.

(1995).

142

SZAKÁCS, PÉCSKAY, SILYE, BALOGH, VLAD and FÜLÖP

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

These lithofacial features strongly suggest that the great bulk

of the Dej Tuff tephra has been reworked and redeposited into a
marine basin shortly after primary deposition (Szakács 2000).

Primary pyroclastic flow deposits are found in a single oc-

currence area (Măgura Ciceu Hill) as slightly welded ignim-
brites deposited in an underwater environment (Mârza &
Mirea 1991; Seghedi & Szakács 1991).

Petrography and mineralogy

Since it is widely redeposited, the Dej Tuff contains clasts of

both  volcanic  and  non-volcanic  origin  in  variable  amounts.
Components  of  magmatic  origin  include  vitric  clasts,  crystal
clasts  and  lithic  clasts  of  which  the  first  two  are  juvenile.
Coarse, lapilli-sized vitric clasts are pumice, while ash-sized vit-
ric  clasts  are  both  pumice  and  glass  shards.  Except  for  a  few
outcrops, these clasts are strongly transformed, being replaced
by an assemblage of mostly zeolite minerals. Massive, non-al-
tered  glass  fragments  (obsidian)  are  present  in  accessory
amounts. Slightly flattened pumice clasts are characteristically
present  in  the  primary  pyroclastic  flow  deposits  at  Măgura
Ciceu.  Crystal  clasts  of  juvenile  origin  include  quartz,  plagio-
clase  and  biotite  as  the  most  common  components.  Biotite  is
ubiquitous but it is frequently overprinted by subaqueous altera-
tion.  K-feldspar  and  amphibole  are  also  present  in  part  of  the
deposits,  while  the  presence  of  clinopyroxene  characterizes
certain levels of the sequence in a few outcrops. Zircon, apatite,
allanite and Fe-Ti oxides are common accessory minerals.

Petrochemistry

Whole-rock major element analyses on a few selected

samples  recalculated  on  a  volatile-free  basis  as  well  as  mi-
croprobe  analyses  on  unaltered  massive  glass  fragments  re-
vealed  the  basically  rhyolitic  composition  of  the  Dej  Tuff
(Szakács  2000),  which  is  in  contrast  with  the  traditionally
(e.g. Koch 1900; Vancea 1960) considered dacitic composi-
tion.  The  trace  element  distribution  clearly  shows  the  sub-
duction signature of the generating magma. Interaction with
crustal material is pointed out by Sr and Nd isotopic ratios.
Mineral chemistry indicates the origin of the rhyolitic mag-
ma through differentiation processes from more mafic melts
in zoned magma chambers (Szakács 2000).

Dating the Dej Tuff

Summary of biostratigraphic age data

The Dej Tuff is very poor in its macrofossil record. Chira

(1991) listed a number of 17 bivalves and 1 gastropod spe-
cies  from  the  collection  of  the  Paleontology-Stratigraphy
Museum  of  the  Babe -Bolyai  University  (Cluj-Napoca,
Romania) originating from the Dej Tuff. Due to their scarcity
and  their  stratigraphic  distribution,  these  fossils  are  almost
useless for biostratigraphic dating of their host deposits.

In contrast, the thin marls, or the marly tuffs, or sometimes

the tuff layers themselves contain calcareous nannoplankton,
planktonic foraminifera, and even ostracods.

The calcareous nannoplankton assemblages recovered from

the Dej Tuff were unanimously assigned to the NN5 Spheno-
lithus  heteromorphus  Zone (Martini  1971)  proving  the  Early
Badenian age of the tuff (e.g. Mészáros et al. 1991; Mészáros
& Filipescu 1991; Mészáros &  uraru 1991; Mărun eanu et al.
1999;  Vulc  &  Silye  2005).  The  planktonic  foraminiferal  as-
semblages  of  the  Dej  Tuff  were  assigned  in  classical  studies
(e.g.  Mészáros  et  al.  1991;  Mészáros  &  uraru  1991)  to  the
Orbulina suturalis/Globorotalia bykovae Biozone, which was
later  re-interpreted  by  Popescu  &  Ghe a  (1987),  Popescu
(2000)  and  re-calibrated  by  Krézsek  &  Filipescu  (2005).
Therefore it corresponds, in the current biozonations in use, to
the Orbulina suturalis/Globorotalia bykovae and Globoturbo-
rotalita druryi/Globigerinopsis grilli Biozones and are corre-
lated  to  the  M6,  and  partly  to  the  M7,  zones  of  Wade  et  al.
(2011). This also suggests a mostly Early Badenian biostrati-
graphic age for the Dej Tuff. The Early Badenian age of the
Dej Tuff was also confirmed, based on ostracods recovered in
the southern Baia Mare Basin, by Wanek & Clichici (1991).

However, the first occurrence (FO) of Orbulina suturalis

is 15.10 Ma (Berggren et al. 1995), which is earlier than the
last occurrence (LO) of Helicosphaera ampliaperta dated to
14.91 Ma (Lourens et al. 2004a,b), which suggest some cor-
relation problems at the base of the Dej Tuff (see also in
Chira & Bălc 2002), or simply the base of the Dej Tuff must
be correlated with the topmost part of NN4.

The biostratigrahic age of the Dej Tuff can be further con-

strained  based  on  the  LO  of  Sphenolithus  heteromorphus
dated to 13.65 Ma (Lourens et al. 2004a,b), and on the FO of
Orbulina suturalis dated to 15.10 Ma (Berggren et al. 1995;
Wade  et  al.  2011).  Although,  the  regional  biostratigraphic
markers  do  not  allow  a  more  accurate  location  within  this
1.45 Myr  interval,  this  interval  can  be  further  reduced  to
1.29 Myr  based  on  the  beginning  of  the  deposition  of  the
Badenian  salt,  dated  to  13.81 ± 0.08 Ma  (de  Leeuw  et  al.
2010).  The  Badenian  salt  as  a  lithostratigraphic  unit  is  re-
gionally  distributed  in  the  Central  Paratethys  and  it  covers
the Dej Tuff in the Transylvanian Basin.

Radiometric dating

Due to the inherent difficulties in obtaining accurate radio-

metric  ages,  a  number  of  different  dating  techniques  have
been  used,  their  results  then  compared  and  evaluated.  Tuff
samples  were  collected  from  different  occurrences  in  order
to  meet  the  requirements  of  these  methodologies.  Sampling
localities are shown in Fig. 3.

K-Ar dating

M e t h o d o l o g y: Measurement of K-Ar ages was performed

in the Institute of Nuclear Research of Hungarian Academy of
Sciences (ATOMKI), Debrecen. Part of each sample was pul-
verized  for  K  determination.  An  argon  extraction  line  and  a
mass  spectrometer,  both  designed  and  built  in  the  ATOMKI,
were  used  for  argon  measurement.  The  rock  was  degassed  by
high  frequency  induction  heating,  the  usual  getter  materials
(titanium  sponge,  getter  pills  of  SAES  St707  type  and  cold
traps)  were  used  for  cleaning  and  transporting  Ar.  The

143

ON THE AGE OF THE DEJ TUFF, TRANSYLVANIAN BASIN (ROMANIA)

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

Fig. 3. Sample locations of the Dej Tuff used for radiometric dating in the north-western Transylvanian Basin. 1 – Metamorphic basement
rocks on the basin margin, 2 – Pre-Lower Badenian sedimentary fill of the Transylvanian Basin, 3 – Lower Badenian sedimentary rocks
including the Dej Tuff, 4 – Post-Lower Badenian sedimentary fill of the Transylvanian Basin, 5 – Neogene intrusive magmatic rocks,
6 – Neogene volcanic rocks, 7 – Pleistocene-Holocene deposits, 8 – Sampling localities. Open square and open circles mark major cities
and towns.

Results

Precise radiometric dating of the Dej Tuff samples is chal-

lenging, because these rocks are the products of distal facies,
mainly fallout tephra, often reworked by various processes.

Three different occurrences of the Dej Tuff (Figs. 3, 4) have

been sampled and studied by conventional K-Ar and incre-
mental

Ar-

Ar dating. K-Ar analytical data are summarized

in  Table 1.  Whole-rock  samples  enclosing  accidental  litho-
clasts may not be completely outgassed of its pre-existing ra-
diogenic  argon,  even  at  high  temperatures,  therefore  only
monomineral fractions separated from the rhyolite tuffs have
been  dated.  Since  the  outcropping  rhyolite  tuffs  usually  lack
separable high K, fresh biotite, we paid attention to sampling

spike was introduced to the system from a gas-pipette before the
degassing was started. The purified Ar was directly introduced
into the mass spectrometer. The mass spectrometer was a 90°
magnetic sector type of 150 mm radius and was operated in the
static regime. Recording and evaluation of the Ar spectrum was
controlled by a microcomputer. Potassium was determined by
flame photometry with a Li internal standard and Na buffer.

The interlaboratory standards Asia 1/65, HD-B1, LP-6 and

GL-0 as well as atmospheric Ar were used for controlling and
calibration of analyses. Details of the instruments, the applied
methods and results of calibration have been described in more
detail elsewhere (Odin et al. 1982; Balogh 1985). K-Ar ages
were calculated using the constants proposed by Steiger & Jäger
(1977).

Lab #

Sample #

Locality

Dated

fraction

K (%)

rad

(mcm3g)

Ar rad

(%)

K-Ar age

(Ma)

Obs.

4698 TD105

Măgura Ciceu

biotite

6.00

2.757 10

–6

18.1

11.78 ± 0.94

Ar loss, minimum age

4700 TD134-I-Bi1 Pâglişa biotite

2.48

1.233 10

–6

23.3

12.76 ± 0.83

altered, minimum age

4699 TD134-I-Bi2 Pâglişa biotite

2.48

1.109 10

–6

13.0

11.47 ± 1.24

altered, minimum age

4701 TD134-III-Bi1 Pâglişa biotite

3.78

1.698 10

–6

23.5

11.52 ± 0.68

altered, minimum age

4702 TD134-III-Bi2 Pâglişa biotite

5.14

2.958 10

–6

41.6

14.74 ± 0.67

treated

4056 301FA

Romană biotite

6.26

3.765 10

–6

56.0

15.40 ± 0.63

W Gutâi Mts

Table 1: K-Ar dating of Dej Tuff samples.

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GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

the most suitable samples. For instance, samples were collect-
ed  from  two  different  levels  of  the  Pâgli a  occurrence,  and
measurements  were  performed  on  two  biotite  separates  from
each  sample  (Table 1,  Fig. 4).  Mineral  separation  has  been
made on the basis of microscopic inspection of thin-sections.
Strongly altered rock specimens were eliminated. All samples
were  first  crushed  and  sieved.  Biotite  and  feldspar  crystals
were  separated  from  the  250—315 µm  grain-size  fraction.
Magnetic separator and heavy liquids were used for obtaining
mineral concentrates. For the sake of improving the purity of
the mineral fractions, additional hand-picking was used under
a binocular microscope.

Nevertheless, there is a spread in the measured ages which is

uncorrelated with biostratigraphic ages in some instances. Con-
sequently, some K-Ar ages can be considered as “apparent
ages”, but others are in accordance with the geological data.

Evidence for the role of syn- and/or post-depositional

changes was found in the variety of biotite chemistry (see the
K concentrations in Table 1). As a consequence, low K contents
(less than 4 %) and substantial atmospheric Ar concen-
trations (about 80 %) of the biotites result in low

rad

tot

ratios.  This  hampers  the  precision  of  K-Ar  age  determina-
tions.  According  to  our  experience,  after  cleaning  biotite  in
methyl  alcohol  (no. 4702)  the  atmospheric  Ar  content  de-
creased and the K concentration increased. We suppose that
this simple treatment partially removed the very fine-grained
clay  minerals  which  are  presumably  located  at  the  grain
boundaries  of  biotite.  For  the  sake  of  detecting  possible

rad

loss, or the presence of excess Ar, plagioclase mineral

fractions (no. 4701 and no. 4699) were also measured by
conventional K-Ar dating.

In spite of the highly consistent K-Ar ages obtained on

biotites no. 4701, no. 4699 and no. 4698 (11.5 Ma—11.8 Ma)
we do not consider them as formation ages, rather we assume
that they are the consequence of some rejuvenation process-
es resulting in secondary effects. However, it is worth men-

tioning that a similar age (11.9 ± 0.7 Ma) has been determined
on biotite separated from the Căline ti Tuff, exposed in the
Oa  Mts, NW Romania (Pécskay et al. 1995a). Thus, this age
can  reflect  a  geological  event,  which  has  strongly  affected
the rocks under investigation. Fischer & Steiger (1988) stud-
ied  the  influence  of  lithification  on  K-Ar  ages.  They  ob-
served that decrease of the K-Ar ages occurs with the degree
of lithification and is presumably correlated with it. Grant et
al.  (1984)  arrived  at  similar  conclusions.  They  suggest  that
diagenetic  events  should  have  caused  substantial  chemical
change  leading  to  loss  of  previously  accumulated

rad

hence rejuvenated K-Ar ages.

The most reliable K-Ar ages (14.74 ± 067 Ma and

15.40 ± 0.63 Ma)  have  been  determined  on  the  biotites
(no. 4036 and no. 4702, respectively) with the highest K con-
tent  and  with  the  highest  radiogenic  Ar  percentage.  Conse-
quently,  the  eruption  of  the  Dej  Tuff  cannot  have  occurred
earlier than the Early Badenian or later than the Middle Bad-
enian.  Similar  ages  were  reported  for  some  rhyolite  tuffs
within the Pannonian Basin (Pécskay et al. 1995b).

On the basis of lithostratigraphic data it is possible to dis-

tinguish various tuff sequences (Szakács 2000) with similar
radiometric ages, but the time interval of the volcanic activi-
ty that produced these volcanic products cannot be clearly
defined because the experimental error surely overlaps with
the life span of the volcanism.

Ar-Ar dating

M e t h o d o l o g y : Biotite separated from sample TD-105,

showing the highest K content and the least signs of alter-
ation of all Dej Tuff samples, has been chosen for Ar-Ar dat-
ing after it was dated previously by the K-Ar method.

The sample was irradiated in the 229/3 position (out of the

centre of the core) of the nuclear reactor of the Atomic Ener-
gy Research Institute of Physics, Budapest, along with inter-
laboratory  standard  biotite  LP-6.  Samples  were  wrapped  in
Al foil and placed in a cylindrical container made of 0.5 mm
thick  Cd.  The  Cd  container  was  sealed  hermethically  in  an
Al  canister.  The  distribution  of  the  integrated  fast  neutron
flux  was  monitored  by  Ni  foils  placed  beside  the  samples.
The irradiation parameter was J = 1.277 10

—3

for the sample.

Ar extraction was performed in a resistance heated molyb-

denum furnace. The temperature was controlled by a Pt—PtRd
thermocouple. The furnace was connected to the Ar purifica-
tion line used for K-Ar dating. The sample was heated for 50
minutes at each temperature step. Procedural system blanks
(atmospheric composition) were measured before degassing
for different temperature steps, they were increasing from
10

—9

STP to 10

—8

STP at 1400 °C. The particulari-

ties of the experimental method used in this work are de-
scribed by Balogh & Simonits (1998).

Evaluation of Ar-Ar age spectrum

A disturbed age spectrum has been obtained (Table 2,

Fig. 5). The ages gradually increase with increasing tempera-
ture, and the spectrum obtained is similar to a diffusive loss
profile. The oldest age of 13.7 ± 1.2 Ma at the highest tem-

Fig. 4.  Synthetic  lithological  column  of
the  Dej  Tuff  at  Pâgli a  showing  sample
collection  levels  (black  squares).  The  to-
tal thickness of the column is ca. 35 m.

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ON THE AGE OF THE DEJ TUFF, TRANSYLVANIAN BASIN (ROMANIA)

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

perature gives a lower limit for the time of volcanic activity
producing  the  dated  sample-rock  and  the  lowest  age  of
7.04 ± 0.54 Ma is the oldest age limit of the age of the main
secondary post-depositional event, most likely a fluid inter-
action effect, that released part of the radiogenic argon from
sites  near  the  grain-boundary  of  biotite  and  from  the  phase
boundaries  between  elementary  crystallites  (subcrystals)
within the biotite grains.

Few Ar-Ar ages are available for Miocene tuffs in the

Carpathian-Pannonian Basin. An Ar-Ar age on biotite from
the ignimbrite at Ipolytarnóc has been published by Hámor
et al. (1987).

This spectrum is also disturbed and shows the irregulari-

ties characteristic for heterogeneous samples which are com-
posed of micrometer-sized mineral phases differing in age
and mineral composition. Unpublished results from altered
biotites from the rhyolite tuffs at Ipolytarnóc show that high-
er K concentration is not a convincing argument against loss
of

rad

. For example, a zero age has been measured in bio-

tite of 4.3% K concentration.

Few Ar-Ar ages are available for Miocene rhyolite tuffs,

exposed  in  the  Carpathian  Pannonian  Basin.  Ar-Ar  fusion
ages  obtained  from  biotite  and  plagioclase  have  been  pub-
lished by Hámor et al. (1987) and Pálfy et al. (2007). Middle
Badenian  tuff  outcropping  in  South  Poland  gave
13.81 ± 0.08 Ma (de Leeuw et al. 2010), and a highly similar
age has been determined for the Bochnia tuffites (13.76 Ma,
Bukowski et al. 2010).

FT dating

Since both K-Ar and Ar-Ar dating failed to result in accu-

rate age determination of the Dej Tuff, we attempted to use
the  FT  method  on  zircon  grains.  Seven  samples  have  been
measured,  five  of  them  from  outcrops  in  the  north-western
Transylvanian Basin and two from the south-eastern margin
of  the  Silvania  Basin  (marginal  sub-basin  of  the  great  Pan-
nonian Basin) (Fig. 3, Table 3).

M e t h o d o l o g y : The analyses were done in the Mineral

Separation  Laboratory  and  Fission  Track  Laboratory  of  the
Free University of Amsterdam on 7 samples from 7 different
outcrops  (Vlad  1998).  The  external  detector  method  has
been used on a number of single zircon grains. Monomineral
zircon  grain  separates  of  > 80 %  purity  have  been  obtained
from each sample by crushing, pulverizing, sieving, separa-
tion using heavy liquids and final magnetic separation. Zir-
con  grains  were  embedded  in  Teflon.  Grinding  of  mounted
grains was done with carbide abrasive papers with adhesive-
backing and a HANDIMET grinder. Smooth surfaces of the
grains resulted from polishing with a 6 micrometers diamond
polishing  compound  (METADI  II)  and  with  a  diamond
grain-size of 1 micrometer. A Buchler POLIMET machine
(6  micrometers) and a KENT MK 2A machine (1 microme-
ter), both with polishing cloth disks with activated adhesive
were used. Eutectic melt of KOH + NaOH was used for etching.
The mounts were covered with a flake of low-uranium mus-

Step

Ar (% cumulative) Temperature (

C) Age

(Ma)

30.5

400

7.04±0.57

48.5

645

9.70±1.00

84.5

860

12.7±0.50

100 990

13.7±1.20

Table 2: Ar-Ar dating (sample Td-105).

Sample

Sample locality

FT age* (Ma)

Central age** (Ma) Mean age** (Ma)

Obs.***

North-western Transylvanian Basin
CEP1 Cepari 13.79

1.92

13.82 13.95 15

CG1 Ciceu-Giurgeşti 14.34

1.92

14.39

14.80

TD-137-3 Coruş 13.94

1.58

13.95

14.34 20 H

TD-142 Jichişul de Sus

15.11 ± 1.50

15.41

16.05

TD-144-2B Bobâlna

14.86

1.74 15.14

15.78

Average 14.41

1.73

14.54 14.98

Silvania Basin
TD-148-2C Benesat

13.32

1.56 14.40

15.65

TD-149-5D Ciolt

12.31

1.38 13.10

13.96

General average

13.95 ± 1.66

14.32

14.93

* – Calculated acc. to the Berekening external detection method, ** – Calculated according to the radial projection method, *** – Population homoge-
neity; H – homogeneous, NH – not homogeneous, n – number of zircon grains measured.

Table 3: Fission-track dating of the Dej Tuff using zircon grains.

Fig. 5. Ar-Ar spectrum of sample TD-105.

146

SZAKÁCS, PÉCSKAY, SILYE, BALOGH, VLAD and FÜLÖP

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

covite and were irradiated in a nuclear reactor. A standard of
known age and a glass dosimeter with known uranium con-
centration  was  included  in  the  sample  package  during  irra-
diation.  For  age  calculation  the  zeta  approach  was  used,
according  to  the  formula  developed  by  Price  &  Walker
(1963) and Naeser (1967).

Analytical results and interpretation

Fission tracks have been counted on a variable number (15

to 30) of zircon grains in each sample. The analytical results
are given in Table 3. The mean ages, obtained using the radi-
al projection method, range from 13.96 to 16.05 Ma, with an
average of 14.93 Ma. This value is consistent, within analyt-
ical  error,  with  the  K-Ar  age  of  the  Valea  Romană  sample
(15.4 ± 0.63 Ma)  and  the  134-III-Bi2  Pâgli a  sample
(14.74 ± 0.67 Ma). Moreover, it is also consistent, within an-
alytical  error,  with  the  FO  of  Orbulina  suturalis  dated  to
15.10 Ma  (Berggren  et  al.  1995),  whilst  the  calculated  cen-
tral  ages  (14.54 Ma,  and  14.32 Ma),  fit  well  within  the
13.65 Ma  and  15.10  time  interval  constrained  according  to
the  LO  of  Sphenolithus  heteromorphus,  and  the  FO  of
Orbulina suturalis.

Discussion and conclusions

Different radiometric methods have been used in order to

accurately determine the age of the Dej Tuff. Although all
methods applied yielded results which are generally consis-
tent with Early Badenian dating of the Dej Tuff on biostrati-
graphic grounds, none of them is able alone to constrain its
age to higher accuracy.

Monomineral samples have been measured by K-Ar and

Ar-Ar methods. Although biotite is the most suitable mineral
for this kind of dating, the biotite in the Dej Tuff is altered
by fluid interaction during burial and diagenesis, thus show-
ing argon loss and recording events which are younger than
the eruption and emplacement of tephra. Thus, K-Ar dating
resulted in minimum ages for most of the samples. However,
one sample (TD-134-III-bi2, Table 1), which has been spe-
cially

treated

for

measurement,

yielded

age

(14.74 ± 0.67 Ma) close to the eruption age of the Dej Tuff.
Such an age partially overlaps with the age (15.40 ± 0.63 Ma)
obtained for the rhyolitic ignimbrites in the western Gutai
Mts (sample 301FA, Table 1) which can be regarded as the
proximal facies of the Dej Tuff (Szakács & Fülöp 2002).

A minimum age (13.7 ± 1.2 Ma) has also been obtained by

applying the Ar-Ar method to one sample. Although it is not
interpretable alone, the minimum age obtained using this
method is consistent with the K-Ar ages. There is a limited
overlap of the Ar-Ar age, including error-bars, with the age
of the most reliable K-Ar age of the Pâgli a samples.

One sample (TD105) has been measured by both the K-Ar

and  Ar-Ar  methods.  The  K-Ar  age  of  the  sample
(11.78 ± 0.94 Ma,  Table 1)  is  well  within  its  Ar-Ar  age  spec-
trum  (7.04—13.7 Ma).  We  consider  that  the  K-Ar  age  repre-
sents a post-depositional event related to the burial history of
the  tuff  sequence,  while  the  Ar-Ar  spectrum  step  at

13.7 ± 1.2 Ma (Fig. 5) is interpretable as the minimum
formation age.

On the basis of the Ar-Ar step degassing spectra (Fig. 5) it

can be stated that the biotite was affected by extensive alter-
ation during post-depositional processes. The alteration
products often do not retain

rad

quantitatively. As a con-

sequence, the K-Ar age of the same biotite mineral fraction
cannot be regarded as a real geological age, since it has been
rejuvenated.

FT ages obtained on zircon grains show a higher disper-

sion, but the average mean age value (14.93 Ma) is close,
within analytical error, to the most reliable K-Ar ages, as
well as to the biostratigraphic age constraints based on dated
FO or LO of biostratigraphic marker species.

A summary of radiometric ages, together with their error

bars, obtained during this study, along with the summary of
current biostratigraphic constraints, is presented in Fig. 2. It
is obvious that most of the age values – and their related er-
ror-bars – obtained with different radiometric dating meth-
ods  overlap  in  two  domains,  at  14.75—15 Ma  and  at
11.4—12 Ma. The clustering of data in the 14.75—15 Ma age
domain reflects, in our opinion, the formation age of the Dej
Tuff as a whole, as it was previously suggested by Szakács et
al.  (2000).  Practically  the  same  age  (i.e.  15.1 ± 0.5  Ma)  has
been obtained recently on zircon crystals from the Dej Tuff by
Nicolescu  &  Mârza  (2010)  by  using  the  higher-resolution
(U-Th)/He dating method. Since reworking of loose volcanic
material occurs shortly after eruption and primary deposition,
“formation  age”  includes  both  the  eruption  and  reworking
processes. Furthermore, these age data show no clues concern-
ing  the  number  of  eruptive  and  related  secondary  deposition
events,  although  a  succession  of  three  eruptions  has  been
inferred  from  compositional  sequences  (Szakács  2000).  The
whole  story  would  have  occurred  within  a  time  interval  of  a
few hundred thousand years, well within the   analytical error
bars.  Thus,  eruption  sequentiality  is  not  resolvable  with  the
currently used radiometric dating methods.

The radiometric ages around 15 Ma correspond to the Mid-

dle Langhian age, or early Middle Miocene on the standard
stratigraphic scale of Lourens et al. (2004a), and can be corre-
lated to the Early Badenian regional age (e.g. Rögl et al. 2008).

It is worth mentioning, that about half of the K-Ar ages

obtained  for  the  Dej  Tuff  are  clustered  in  the  11.4—12 Ma
time  interval.  Moreover,  the  K-Ar  age  (11.9 ± 0.7 Ma,
Pécskay  et  al.  1995a)  of  the  Căline ti  Tuff  in  the  Oa   Mts
(NW from the Tansylvanian Basin), considered Badenian on
biostratigraphic  grounds,  also  fall  into  this  time  interval.
This fact probably bears some geological meaning. We inter-
pret these ages as reflecting a post-depositional event in the
thermal  history  of  the  sedimentary  pile  including  the  Dej
Tuff. Sanders (1998) inferred a ca. 3.5 ± 0.5 km burial depth
and  a  corresponding  temperature  of  ca.  80 ± 10 °C  for  one
Dej  Tuff  sample  from  Cepari,  based  on  apatite  FT  data.
These burial temperatures are not high enough to “reset” the
K-Ar isotopic clock. However, a long time exposure to these
temperatures can result in a partial loss of radiogenic argon
by diffusion, hence younger analytical ages are obtained.

Processes related to long-term fluid-mineral contact and

pore-fluid expulsion may also have resulted in Ar loss,

147

ON THE AGE OF THE DEJ TUFF, TRANSYLVANIAN BASIN (ROMANIA)

GEOLOGICA CARPATHICA

GEOLOGICA CARPATHICA, 2012, 63, 2, 139—148

which  in  turn  “rejuvenated”  the  rocks.  Certain  tectonic
events  may  trigger  such  processes.  The  last  major  conver-
gence and deformation event occurred ca. 11 Ma ago in the
Outer  Carpathians  (Săndulescu  et  al.  1981;  Săndulescu
1988) with a likely echo in the “back-arc” realm, such as the
Transylvanian  Basin.  Thus,  the  “rejuvenation”  of  the  Dej
Tuff around 11.5 to 12 Ma may be related to a major tectonic
event in the Carpathians.

Acknowledgment:  The  Ar-Ar  dating  was  supported  by  the
Hungarian  Science  Fund  (OTKA)  Project  No. 0299897.
Prof. Dr. Paul A.M. Andriessen is thanked for the excellent
discussions and ideas on the Fission Track project, as well as
for the laboratory support at the Vrije Universiteit in Amsterdam,
The  Netherlands.  Discussions  with  Mariana  Mărun eanu,
Gheorghe Popescu and Sorin Filipescu greatly helped under-
standing  biostratigraphical  issues  related  to  the  Dej  Tuff.
Natália Hudáčková and Jaroslav Lexa are acknowledged for
their  careful  reviews  of  the  manuscript  and  insightful  com-
ments and suggestions of improvement.

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