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O RI G I N A L P A P E R
Stratigraphic and geochemical study of the organic-rich blackshales in the Tarcau Nappe of the Moldavidian Domain
(Carpathian Chain, Romania)
Habib Belayouni Angelida Di Staso Francesco Guerrera Manuel Martn Martn
Crina Miclaus Francisco Serrano Mario Tramontana
Received: 8 November 2006/ Accepted: 23 June 2007 / Published online: 4 September 2007
Springer-Verlag 2007
Abstract An integrated stratigraphic analysis has been
made of the Tarcau Nappe (Moldavidian Domain, EasternRomanian Carpathians), coupled with a geochemical study
of organic-rich beds. Two Main Sequence Boundaries
(Early Oligocene and near to the OligoceneAquitanian
boundary, respectively) divide the sedimentary record into
three depositional sequences. The sedimentation occurred
in the central area of a basin supplied by different and
opposite sources. The high amount of siliciclastics at the
beginning of the Miocene marks the activation of the
foredeep stage. The successions studied are younger
than previously thought and they more accurately date the
deformation of the different Miocene phases affecting the
Moldavidian Basin. The intervals with black shales
identified are related to two main separate anoxic episodes
with an age not older than Late Rupelian and not beforeLate Chattian. The most important organic-rich beds cor-
respond to the Lower Menilites, Bituminous Marls and
Lower Dysodilic Shales Members (Interval 2). These
constitute a good potential source rock for petroleum, with
homogeneous Type II oil-prone organic matter, highly
lipidic and thermally immature. The deposition of black
shales has been interpreted as occurring within a deep,
periodically isolated and tectonically controlled basin.
Keywords Carpathian Chain Modavidian Basin
Tarcau Nappe Stratigraphy Black shales
Geodynamic evolution
Background and aim
The organic-rich black shales of the well-known Menilite
Member (Popov et al. 2002; Curtis et al. 2004), outcrop
throughout the Carpathian Chain (Romania, the Ukraine,
Poland, and Slovakia) and constitute typical marker beds
within the Tarcau Nappe (Moldavidian Domain, Romanian
Carpathian Chain; Sandulescu et al. 1995). These have
been documented by several authors for their organic
content in relation to petroleum production and basinevolution (Koltun1992; Roore et al.1993; ten Haven et al.
1993; Lafargue et al. 1994; Kruge et al. 1996; Bessereau
et al. 1997; Rospondek et al. 1997; Koltun et al. 1998;
Koster et al. 1998a; 1998b; Kotarba and Koltun 2006;
Stefanescu et al.2006). Specifically, it was inferred that the
Menilite black shales and their lateral equivalent facies
occurring throughout the Carpathian Chain represent the
signature of a major anoxic event which had developed
during the Oligocene (Kruge et al. 1996, Koster et al.
H. Belayouni
Depart. de Geologie, Univ. Tunis, 2092 Tunis, Tunisia
A. Di Staso
Dip. di Scienze della Terra, Univ.Napoli Federico II,
Largo San Marcellino 10, 80138 Napoli, Italy
F. Guerrera M. Tramontana
Ist. Scienze della Terra, Univ.Urbino Carlo Bo,
Campus Scientifico, 61029 Urbino, Italy
M. Martn Martn (&)Dpto. Ciencias de la Tierra y del Medio Ambiente,
Univ. Alicante, Campus San Vicente,
San Vicente del Respeig, 03080 Alicante, Spain
e-mail: [email protected]
C. Miclaus
Dep. Geologie-Geochimie, Univ. Al. I. Cuza,
B-dul Carol I, nr. 20A, 700505 Iasi, Romania
F. Serrano
Dpto. Geologia, Univ. Malaga, Campus De Teatinos,
29071 Malaga, Spain
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DOI 10.1007/s00531-007-0226-7
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1998a; 1998b; Nagymarosy 2000; Curtis et al. 2004;
Puglisi et al. 2006) and which had led to the deposition of
particularly good potential petroleum source beds.
The Menilite black shales are also of a great significance
as organic-rich marker beds within the Oligocene silici-
clastic succession, because precise indications, such as the
physico-chemical conditions of the depositional environ-
ment, the origin of the sedimentary materials and thegeological basin evolution, could be deduced from the
geochemical study of their organic content (Hunt 1979;
Demaison and Moore 1980; Tissot and Welte 1984; Be-
layouni et al. 1990; Curtis et al.2004).
The present study focuses on these typical black shale
intervals and Menilites lithofacies (Oligocene p.p.-Early
Miocene in age), outcropping in the Tarcau Nappe (Mold-
avidian unit) mainly for their relevance to petroleum
exploration in the south-eastern Carpathian Chain. A review
of the stratigraphy (litho- and biostratigraphic approach)
also with sequence-stratigraphy tools was carried out in
order to update the traditional literature and to confirm theorganic-matter intervals. In fact, the traditional stratigraphy
published does not apply modern criteria to the subdivision
of the successions studied and therefore needs to be revised.
In addition, special interest is paid to the main events in
the Oligo-Early Miocene evolution of the Tarcau Nappe
deposits. This analysis has performed out taking into
account the current palaeogeographic models on the
Tethyan and para-Tethyan domains. The origin of anoxic
deposits is discussed to clarify the controversy concerning
the roles of bottom-water anoxia or high organic produc-
tivity in shallow waters in the control of black shale
deposition.
Geological setting
The Romanian Carpathian Chain (Fig. 1) belongs to the far
larger fold-and-thrust belt extending from Gibraltar to
Indochina and its most peculiar feature is the double-arc
shape. The Carpathians are the result of Tethys Ocean
closure during Cretaceous and Miocene convergence
events. Two main periods of compressional deformation
can be recognized in the Romanian Carpathians
(Sandulescu1988): (a) the Cretaceous period, during which
the Dacide (Inner Carpathians: Inner, Middle, Outer Da-
cides and Marginal Dacides only in the southern
Carpathians) and Transylvanide Units, were built up; (b) a
younger period (Miocene) during which the Moldavide
Units (Outer Carpathians: Teleajen, Macla, Audia, Tarcau,
Vrancea and Pericarpathian Nappes) were built up.
The Romanian Carpathians formed in response to the
Triassic-Tertiary evolution of three continental blocks: (1)
Tisza (Inner Dacides), (2) Dacia (Median Dacides), (3)
Eastern European-Scythian-Moesian Platforms. These
blocks were separated by two oceanic domains: (a) the
Vardar-Mures Ocean, between Tisza and Dacia blocks,
from which originated the Transylvanide/Pienide Units; (b)
the Ceahlau-Severin Ocean, between the Dacia block and
the external platforms from which Outer Dacide Units
developed (Radulescu and Sandulescu 1973; Sandulescu
1975, 1980, 1984, 1988; Csontos and Voros 2004). TheVardar-Mures Ocean, a branch of Tethys Ocean
(Sandulescu 1984, 1988), opened in Triassic and closed
during Cretaceous compressional events. The Ceahlau-
Severin Ocean, another branch of Tethys Ocean, opened in
Jurassic, devolved during the Early Cretaceous (Sandule-
scu 1980, 1984), and closed in the Miocene. In the inner
part of this basin, above the oceanic crust (basalts and basic
tuffs of intraplate type), only the Middle Jurassic-Early
Cretaceous black flysch were deposited. This basin might
be an extension of the Silesian Basin (Golonka et al. 2006)
or of the Magura Ocean of the Western Carpathians
(Csontos and Voros 2004). Badescu (2005) considers thisbasin as merging into the Vardar-Mures Ocean; conse-
quently, the Dacia block should represent a pinching-out
ribbon microplate. From the internal part of the Ceahlau-
Severin Ocean, the Outer Dacides originated (Ceahlau and
Severin Nappes) while the external part evolved into the
Moldavidian Units, belonging to the Outer Carpathians
(Fig.1) (Golonka et al. 2006). These consist of Early
Cretaceous-Miocene deposits, deformed during Miocene
tectonic events.
The Moldavides (Fig. 1) are sedimentary cover nappes
and represent the most important part of the eastern Car-
pathian Flysch Zone. From inside to outside, Teleajen (or
Convolute Flysch), Macla, Audia, Tarcau (studied in this
paper), Vrancea (or Marginal Folds) and Pericarpathian
Nappes are recognizable. The Moldavidian Units (exclud-
ing the Pericarpathian Nappe) are made up mainly of
Cretaceous to Miocene flysch-type deposits containing, at
different levels, shaly, pelagic or bituminous (black-shale)
deposits. The siliciclastics were supplied from two main
opposite sources: an external (cratonic) area, characterized
by green schists of the Central Dobrogea type (Grasu et al.
1999,2002), and an internal (orogenic) area represented by
the already-stacked internal units of the eastern Carpathi-
ans (Middle and Outer Dacides). An intermediate mixed
deposition area (Moldovita Group) is represented by a
stratigraphic succession consisting of an alternation of
Kliwa-type quartzarenites (supplied from an external cra-
tonic source) and Tarcau type litharenite (from an internal
orogenic source). This suggests that the Moldovit a sedi-
mentation area represented a basin depocentre as was well-
documented by Guerrera et al. (1993, and references
therein) for the Maghrebian Chain, where similar succes-
sions are known as Mixed Successions.
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Two anoxic events can be recognized in the Moldavi-
dian succession and their black-shale-type deposits
represent the main potential oil source rocks in the Car-
pathian realm (Popov et al. 2002; Stefanescu et al. 2006;
Kotarba and Koltun 2006). The first event, corresponding
to the so-called Oceanic Anoxia Events is Lower Creta-
ceous. The second, Oligocene-Early Miocene in age, is
probably controlled by different factors such as: the iso-
lation of the Paratethys basin from the Mediterranean area
after the collision between Africa and Eurasia plates during
Oligocene (Rogl1999); the global climatic changes which
began since the Middle Eocene (Pomerol and Premoli Silva
1986; Sotak et al. 2002); and sea-level fluctuations. The
two main anoxic events were separated by well-oxygenated
conditions with deposition of variegated shales, marls, and
greygreen shales known in the whole Moldavides
(Sandulescu 1984, 1988; Kotarba and Koltun 2006;
Stefanescu et al. 2006). The most important hydrocarbon
source rocks accumulated in Carpathian realm during the
Oligocene-Early Miocene anoxia are known as Menilite
facies or Menilite member (Popov et al.2002), consisting
of black-shale deposits such as: dysodilic shale, bituminous
marls, and menilite. At the end of NP23 was the maximum
isolation of Paratethys when the marker black cherts were
accumulated (Nagymarosy 2000 in Sotak 2001; Rogl
1999). These anoxic conditions were interrupted from time
to time, as in the NP 24 interval (Rogl1999).
The evolution of thin-skinned Moldavidian Nappe
stacking is well documented (Sandulescu 1984, 1988;
Roure et al. 1993; Ellouz and Roca 1994). Three com-
pressional deformations of Moldavidian Basin occurred in
Early (Old Styrian), Middle (New Styrian), and Late
(Moldavian) Miocene. Locally, folding of Pericarpathian
Nappe deposits occurred also in the Pleistocene in Carpa-
thian Bend Area, known as the Wallachian tectonic event
(Sandulescu1988).
Fig. 1 Geological and tectonicsketch map of the eastern
Carpathians with cross section
(after Badescu2005, modified)
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As a consequence of Miocene tectonic events, a fore-
deep basin (autochthonous molassic basin, Fig. 1) started
to develop in front of the evolving orogenic belt in which
SarmatianQuaternary molasses deposits were accumu-
lated partly over the outer part of the deformed Moldavides
and partly over the foreland represented by platforms of
different ages (Fig.1).
The foreland of the East Carpathians is represented byplatforms of different ages (Fig.1) and it includes, in a
sector close to the Black Sea, the so-called North Dobrogea
Orogen, representing a Cimmerian folded belt. This belt is
made up of deformed Palaeozoic crystalline and sedimen-
tary units, Triassic and Jurassic sedimentary and magmatic
rocks (with Triassic intra-plate ophiolites; Sandulescu and
Visarion2000).
The main palaeogeographic events recognized in the Tar-
cau Nappe during the Oligocene p.p.-Early Miocene are also
similar to those have been pointed out in the Maghrebian
Flysch Basin (North Africa) during the siliciclastic sedimen-
tation (two main opposite internal and external sources areaswith an intermediate mixed succession) of the foredeep
stage (Guerrera et al.1993,2005and references therein).
Lithostratigraphy
TheTarcau Nappeis characterized by different, sometimes
heteropic, lithofacies defined by Dumitrescu (1948, 1952)
and Dumitrescu et al. 1962. This nappe corresponds with
Skole Unit in Poland and Skiba Nappes in the Ukraine
(Oszczypko2004) and is the largest among the Modavide
Units (Fig. 1).Two source areas supplied different types of sands, and
therefore, beginning with Eocene, in the Tarcau Nappe
sedimentation area the so-called Lithofacies were differ-
entiated (Bancila1958; Ionesi1971; Grasu et al.1999): the
Tarcau-Fusaru Lithofacies, the Tazlau-Moldovita Lithofa-
cies (mixed Lithofacies), and the Doamna-Kliwa
Lithofacies. The internal source supplied mainly litho-
feldspathic sands rich in micas (Tarcau-Fusaru Sandstone),
while the external one mainly quartzose sands (known
throughout the Carpathian Basin as Kliwa Sandstone).
The three main different successions recognised by the
traditional stratigraphy of the Tarcau Nappe have beennamed here as (from west to east) the Tarcau, Moldovita,
and Kliwa Groups (Tables1, 2, 3). Their upper portions
Table 1 Lithostratigraphy and adopted terminology of the Tarcau Group (log 1, Tarcuta River, Neamt area, 1,280 m thick; 44 samples),Vinetisu Fm (log 2, Rachitis River, Neamt area, 230 m thick, 4 samples) and correlation with the traditional literature; modified after Badescu
2005
and
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the sample L1/2 (Plate 1, 13) suggests an age not older
than Late Rupelian (NP 23 of Martini 1971 = CP 18 of
Okada and Bukry 1980) (L 1/4). The rare microfauna
identified include only agglutinated foraminifers: Ast-
rorhicidae, Glomospirella, Reophax, Cyclammina and
Trochammina.The overlying Ardeluta Member displays similar mi-
crofacies, but representative levels of the highest part (L1/
9) show an increase in carbonates containing Rupelian
planktonic-foraminifer assemblages composed by Globi-
gerina eocaena Gumbel, G. corpulenta Subbotina,
G. increbescens Bandy, G. ampliapertura Bolli, G. gala-
visi Bermudez, G. venezuelana Hedberg, G. ouachitaensis
Howe and Wallace, G. praebulloides Blow, Globorotalo-
ides suteri Bolli, and Catapsydrax dissimilis (Cushman
and Bermudez). The absence of the Early Rupelian
Pseudohastigerinaspecies, on the one hand, and the fail-
ure to find Neogloboquadrina opima(Bolli), on the other,could indicate that Ardeluta Member, at least partially, is
N1/P20 Blows zone (Blow1969), Early, Late Rupelian in
age.
The silexitic levels interlayered in the Bituminous Marls
Member (L1/15) do not show significant features of
probable biosiliceous skeletons in origin. Above these
levels, all the samples collected from the Lower Dysodilic
Shales Members have been found free of the microfauna,
but the occurrence of Helicosphaera recta (Plate1, 69)
andTriquetrorhabdulus carinatus(Plate1, 45 and 1314)
in sample L1/52 indicates an age not older than Chattian
(NP25 of Martini 1971= CP19b of Okada, Bukry 1980)
for the lowermost part of the Lower Dysodilic Shales
Member with arenites.
Also the samples from the Fusaru Fm are usually azoic,but a specimen such as Globoquadrina dehiscens (Chapman,
Parr and Collins) (Plate1, 20) has been found in the Pelitic-
arenitic Member (L1/66). If this specimen is not reworked,
the level would then be Early Miocene in age and the Fusaru
Fm sedimented during the Aquitanian p.p. at least.
The Vinetisu Fm contains frequent pyrite grains and in
some levels (e.g. L2/4) most of these show spheroid mor-
phologies reminiscent of internal moulds of radiolarian
skeletons. Frequent carbonaceous or pyritized vegetal
remains also appear within this formation. These radio-
larian levels can be correlated with the well-known ones,
occurring in the lower Burdigalian silexites which appearfrequently in the flysch sediments from the Betic and
Maghrebian Chains (Lorez 1984; Guerrera et al. 1992).
From the nannofossils, the only fossiliferous sample (L2/1)
bears only reworked species.
In the Moldovita Mixed Group, only sample L3/1
(Linguresti Brown Marls Mb) contains nannofossils. The
occurrence ofReticulofenestra bisectaindicates a recorded
age of not older than the Bartonian (NP17 of Martini
1971= CP14b of Okada, Bukry 1980).
Table 3 Lithostratigraphy and adopted terminology of the Kliwa Group (log 4, left side of Moldova River, near Gura Humorului town, 460 mthick; 15 samples) and correlation with the traditional literature
and
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Fig. 2 Stratigraphy of the logs studied (Tarcau Nappe) and palaeog-eographic reconstruction of the Moldavidian Basin. Age, main and
secondary sequences, system tracts and organic-matter intervals are
also marked. Key: 1 massive arenites with conglomerates; 2
micaceous litharenites; 3 blackish bituminous shales; 4 micaceous
pelites; 5 marls, marly limestones, and limestones; 6 silicified
lithofacies: laminated bituminous shales, marls, arenites, etc.
(Menilites s.s. type); 7 well-stratified thin, brownish and bituminous
shales (Dysodilic type);8 quartzarenites;9 chaotic interval (slump);
10main unconformities;11 studied samples;TL TylawaLimestone
regional marker bed (brownish and thin, laminated bituminous marls,
clayey-marl beds); JL Jaslo Limestone regional marker bed
(brownish and thin laminated bituminous marls, clayey-marl beds,
some cm thick);MB 11 local arenitic marker-bed numbered
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In the Kliwa Group (log 4) some levels from the upper
part of the Lucacesti Fm contain relatively well-preserved
planktonic foraminifers (e.g. L4/5). The assemblages are
composed ofG. ampliapertura, G. increbescens, N. opima
(Plate1, 1719, respectively), G. eocaena, G. corpulenta,
G. tripartita, G. venezuelana, G. euapertura Jenkins, G.
ouachitaensis, G. praebulloides, and G. ciperoensis Bolli,
characterizing the N1/P20 zone (Blow 1969) of the latest
Rupelian. Accordingly, the occurrence of the nannofossil
Sphenolithus distentus(Plate1, 15, 16) in the same sample
suggests an age not older than Late Rupelian (NP23 of
Martini1971). In agreement with these data, the upper part
of the Lucacesti Fm is correlative to the Ardeluta Member
of the Tarcau Group.
The samples for nannofossil analyses were prepared
centrifuging after crushing and sodium-hypochlorite treat-
ment (C procedure in de Capoa et al. 2003) and the
slides were studied by light microscopy at 1250 magnifi-
cation. For turbiditic sediments such as the ones under
study, quantitative-analysis methods are unreliable, and
only the first occurrence of taxa allows a qualitative eval-
uation of age as not older than....
The recognized markers and biostratigraphic results are
listed in Table4 and the most significant calcareous
Plate 1 Significant biomarkersrecognised in the study
successions of the Tarcau
Nappe. Calcareous nannofossils
(all specimens 2,500):13
Sphenolithus distentus (Sample
L1/2);4 , 5 and 13 , 14
Triquetrorhabdulus carinatus
(L1/52);6, 7and 8, 9
Helicosphaera recta (L1/52);
10, 11 Reticulofenestra bisecta
(L3/1); 12 Discoaster
barbadiensis (L2/1);15 , 16
Sphenolithus distentus (L4/5).
Planktonic foraminifera (all
specimens 75):17
Globigerina ampliapertura
Bolli (L4/5); 18 Globigerina
increbescens Bandy (L4/5);19
Neogloboquadrina opima opima
(Bolli) (L4/5);20
Globoquadrina dehiscens
(Chapman, Parr and Collins)
(L1/66)
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nannofossils and Planktonic foraminifera species are
shown in Plate1.
Sequence stratigraphy
The correlation of the above-described successions and the
examination of the vertical and lateral facies evolution,
lead us to propose a sequential subdivision of the Tarca u
Nappe (middle area of the Moldavidian Basin), using
sequence-stratigraphy concepts and tools (Fig.2).
The identification of two main unconformities (Main
Sequence Boundaries), one at the Late Early Oligocene
(MSB1) and the second near to the Oligocene-Aquitanian
boundary (MSB2), allows the sedimentary record to be
divided into three depositional sequences (Fig. 2):
1. The lowermost sequence (S-1) is represented by the
Tarcau Sandstones Fm (Eocene ?), which has the
MSB1at the top.
2. The middle sequence (S-2) consists of Podu Secu to
Lower Dysodilic Shales Members and their lateral
equivalents, representing the Oligocene Depositional
Table 4 Biostratigraphicresults (calcareous nannofossils)
in the Carpathian logs
investigated
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Sequence separated from the overlying Miocene
succession by MSB2.
3. The uppermost Depositional Sequence (S-3) is repre-
sented by the Aquitanian-Burdigalian Fusaru Fm and
its lateral equivalents.
On the basis of the vertical evolution, and on the
recognition of the secondary sequence boundaries, a moredetailed subdivision showing the depositional system tracts
related to S-2 and S-3 have been attempted using the
minor-order sequence monitoring, consisting of a marly or
shaly sedimentation evolving upwards to coarse-grained
terrigenous deposits and topped by an unconformity.
Accordingly, the marly, bituminous marly and black-
shale facies recorded in the successions studied (also evi-
denced in Fig. 2) appear to be related to periods of relative
sea-level rise (Transgressive System Tract: TST) or rela-
tive high sea level (HST). Also, the appearance of a
terrigenous supply (sandstones and conglomerates) in
continuity with a non-terrigenous facies, have been inter-preted as periods of stable high relative sea level
(Highstand System Tract: HST). Special attention has been
also placed on the recognition of coarse terrigenous sedi-
mentation during relative sea-level falls, through the
identification of erosional surfaces (sequence boundary)
which indicate the presence of Lowstand System Tracts
(LST). The main results are the following.
The Oligocene Depositional Sequence (S-2) has been
divided into three minor sequences (S-2a, S-2b and S-2c),
all comprised of TST and HST and separated by two sec-
ondary sequence boundaries. The S-2a sequence is
composed of the Podu Secu Member (TST + HST) andPlopu Fm; the S-2b sequence is made up of the Ardelut a
Member (TST) and the Lower Menilites Member (HST)
and their lateral equivalents. Finally, the S-2c sequence is
composed of the Bituminous Marls and Lower Dysodilic
Shale Members (both TST), and the Lower Dysodilic
Shales Member (with sandstones) (HST).
The Aquitanian-Burdigalian Depositional Sequence
(S-3) has been divided into three minor depositional
sequences (S-3a, S-3b and S-3c) separated by two secondary
sequence boundaries. The S-3a sequence is composed by the
Arenitic (LST), the Dysodilic Shales, and the lowermost part
of the Pelitic-Arenitic Members (both TST) of the FusaruFm, the only complete minor sequence. The S-3b sequence
is made up of shales, marls, and limestones from the Pelitic-
Arenitic Member (TST + HST) of the Fusaru Fm. Finally,
the S-3c sequence consists of the Vinetisu Fm with sand-
stones and a slumping level at the base (LST), followed by
bituminous shales and arenites (TST). Our results agree with
those of Anastasiu et al. (1994), which focused only on the
external successions (Kliwa Group), but more accurately
with regard to the minor S-2b and S-2c sequences.
Geochemical analysis
Material and methods
To characterize the organic content of the black-shale
levels within the Tarcau Nappe, we studied 48 samples
(logs 1, 3 and 4). Some of the intervals defined below are
characterized by a low number of samples. Although thesesamples have been analysed two or three times for con-
firmation, we have been cautious in our interpretation.
The origin and thermal evolution of the organic matter
were estimated using a Rock-Eval II Plus instrument
(Espitalieet al. 1985a, b, 1986) equipped with a total organic
carbon (TOC) module. The results are expressed using
standard notations: S1 and S2 in milligrams of hydrocarbons
(HC) per gram of rock; S3 in milligrams of oxygenated
compounds (CO2) per gram of rock,Tmaxin C and the total
organic carbon (TOC) content in weight percentage (wt%).
The hydrogen index (HI = S2/TOC 100) and oxygen
index (OI = S3/TOC 100) are expressed in mg HC per gTOC, and mg CO2 per gram TOC, respectively.
Results
All the data are reported in Tables5, 6, 7, and Fig.3.
The TOC record, which usually reflects the quantity of
organic matter fossilised in the rock (Tissot and Welte
1984) as well as the Rock-Eval parameters (S1, S2, S3,
HI, OI and Tmax), register variable values in the samples
analysed:
Tarcau group (log 1)
The succession could be subdivided from bottom to top
into three separate intervals based on their relative organic
richness.
Interval 1 (0 to 170 m: samples L1/2 to L1/9), which
corresponds to the Podu Secu and Ardeluta Members,
exhibits generally low TOC amounts (TOC\ 0.83), except
for sub-interval (4590 m) represented by sample L1/5,
where the TOC value is up to 3.37% (Table 5). According
to the Rock-Eval parameters (Table6), the organic matteris of a highly oxidized residual nature (Type IV) except
again for the sub-interval (45-90 m: sample L1/5), which
shows an exceptional richness of lipidic material with a
genetic oil potential (GOP) up to 22.64 kg HC/ton, and HI
values up to 643 mg HC/gTOC.
Interval 2, which corresponds to Lower Menilites,
Bituminous Marls, Lower Dysodilic Shales Members,
extending from 170 to 430 m (samples L1/10 to L1/51), on
the contrary, registers relatively high TOC amounts with
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values fluctuating between 0.25% to 11.08%, HI values up
to 750 mgHC/g TOC (Fig.3) coupled with low OI values,
and good to excellent GOP values (from 1 to 62 kg HC/ton
rock). Such results indicate the presence within the shales
of Interval 2, of highly lipidic, well-preserved organic
matter. The Tmax values are typical of a thermally imma-
ture organic matter (Tmax\ 426C). Accordingly, this
interval could be considered as a good to excellent oil- andgas-prone, thermally immature source rock.
Interval 3 corresponds to the lower part of the Fusaru
Fm (Dysodilic Shales and Pelitic-Arenitic p.p. Members),
extending from 430 m to 884 m (samples L1/56 to L1/
61), and registers (Fig. 3) low TOC amounts with values
of less than 0.45%, low HI values (\65 mg HC/gTOC),
coupled with high OI values (up to 333 mg CO2/gTOC)
and very low GOP values (\0.45 mgHC/g rock). Such
results indicate highly oxidized conditions for the
depositional environment of the sediments within this
interval.
Moldovita mixed group (log 3)
The variation in the TOC amounts (Table6, Fig.2, 3)
along the succession measured in this log, allow it to besubdivided into two separate intervals, which from top to
bottom are:
Interval 2 (equivalent of the log 1) corresponds to the
Lower Menilites (not older than Late Rupelian), Bitumi-
nous Marls and Lower Dysodilic Shales (not older than
Late Chattian) Members, extending from 0 to 200 m
(samples L3/1 to L3/9), where the TOC amounts are sig-
nificantly high, especially at the base, with values of up to
13.96%. This interval, related to Interval 2 of Log 1,
Table 5 Lithostratigraphy, TOC and Rock Eval pyrolysis data, of the Tarcau Group of the Tarcau Nappe (log 1); particularly organic-richintervals are indicated in italics
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displays all the characteristics of a well-preserved organic
matter, highly lipidic and thermally immature, as indicated
by the high HI values (250 to 740 mg HC/gTOC), the
particularly high (2.5 to 110 kg HC/ton rock), genetic oil
potential (GOP), the low OI values (\39 mg CO2/g TOC)
and the low Tmax values (\436C). These characteristics
Table 6 Lithostratigraphy, TOC and Rock Eval pyrolysis data, of the upper part of the Moldovita 129.0Mixed Group of the Tarcau Nappe (log3); particularly organic-rich intervals are indicated in italics
Table 7 Lithostratigraphy, TOC and Rock Eval pyrolysis data, of the Kliwa Group of the Tarcau Nappe (log 4); particularly organic-richintervals are indicated in Italics
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are typical of a good potential oil-and gas-prone, thermally
immature source rock.
Interval 3 (equivalent of the log 1) corresponds to the
Moldovita Fm p.p. (Aquitanian p.p.; samples L3/10 to L3/
14). Here, the TOC amounts registered are lower than those
registered along the first underlying interval, but never-
theless remain relatively significant with values up to
1.69%. The Rock-Eval parameters (Table6, Fig.3) also
show significant HI and GOP amounts up to 400 mg HC/g
TOC and 7.25 kgHC/ton rock, respectively. Such values
Fig. 3 Geochemical results ofthe studied samples from logs 1,
3 and 4 marked according to the
stratigraphic intervals
recognized
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characterize this interval, especially, at its base, as a good
oil-prone source rock, thermally immature (Tmax\ 428C)
and deposited under anoxic conditions.
Kliwa group (log 4)
The succession extending from 0 to 400 m, including alarge unexposed interval, could be subdivided into four
lithologic intervals based on their variation in organic
richness. These intervals are, from bottom to top:
Interval 1, corresponds to the Lucacesti Fm p.p.
([26 m thick, not older than Late Rupelian; samples L4/3
to L4-4) and shows (Table8 and Figs.2, 3) very low
amounts of TOC (\0.07%), very low HI (\70 mg HC/g
TOC) and GOP (\0.38 kg HC/ton). Such values indicate
that the sediments have been deposited under strongly
oxic conditions and very little organic matter has been
preserved;
Interval 2, 55 m thick (samples L4/6 to L4/11) is, onthe contrary, highly organic rich (Table 7, Figs. 2,3) with
considerable TOC amounts (up to 10%), relatively high
HI values (235 to 490 mg HC/gTOC), coupled with low
OI values (\145 mg CO2/g TOC), particularly high GOP
(2.550 kg HC/ton rock), and low Tmax values (\430C).
Such results, indicating the presence of a highly lipidic,
well-preserved organic matter, characterize this interval as
having good oil- and gas-prone, thermally immature
source rock. Corresponding to the Lower Menilites (not
older than Late Rupelian), Bitumonous Marls, and Lower
Dysodilic Shales p.p. Members (not older than Late
Chattian), this interval is related to Interval 2 of log 1 of
the same age (anoxic episode), with which it correlates
very well.
Interval 3, ranging between 105 and 155 m (samples L4/
12 to L4/13), corresponds to the Lower Dysodilic Shales
Member p.p. It includes organic-poor sediments with TOC
amounts of less than 0.28% and with an organic content
depleted in lipidic compounds, as indicated by the low HI
(\221 mg HC/g TOC) and GOP ([0.83 Kg/ton ) values
(Fig. 3). This interval could be related toInterval 3 of log 1,
corresponding to a highly oxic depositional environment;
Interval 4, corresponds to the Kliwa Fm p.p. ([60 m;
sample L4/15). This interval includes numerous shaly thin
beds, highly organic rich, with TOC amounts of around
7%. The GOP and HI Values are particularly high
(23.52 kg HC/ton and 316 mg HC/g TOC, respectively),
thus attesting that these sediments contain well-preserved
highly lipidic organic matter, and hence are an excellent
oil- and gas-prone source rock (Fig.3). This source rock is
thermally immature, as indicated by the low Tmax values
(Tmax\ 431C).
Discussion
The present interdisciplinary study provides a better
stratigraphic resolution, an original sequence-stratigraphy
analysis and a geochemical characterization of the highly
diffuse black shales of the three important successions of
the Tarcau Nappe. The integration of the data presented
above allows the discussion of certain topics: (a) sedi-mentary framework and evolutionary model of the
Moldavidian Basin; (b) significance of geochemical data;
(c) origin of black shales and (d) palaeogeographic sketch
during the Oligocene-Early Miocene.
Sedimentary basin framework and evolutionary model
Previous authors pointed out that in the external part of the
Moldavidian Basin the Eocene-Lower Miocene sedimen-
tation was complex because of different input. For this
reason in the Tarcau Nappe, the sedimentation was differ-entiated in the so-called Lithofacies (Bancila 1958; Ionesi
1971; Grasu et al.1999): the TarcauFusaru Lithofacies, the
TazlauMoldovita Lithofacies (mixed Lithofacies), and the
DoamnaKliwa Lithofacies. In the present paper, a litho-
stratigraphic revision has been proposed, applying more
modern criteria for the subdivision of the successions taking
into account the traditional stratigraphy which can be con-
sidered an updating of the previous literature (cfr. Tables 1,
2 , 3). In particular, some regional marker beds such as
Tylawa Limestone (not older than Late Rupelian) and
Jaslo Limestone (not older than Late Chattian) appear to
correlate with those considered by Melinte (2005).
A new subdivision of the study successions, according
to sequence-stratigraphy criteria, has also for the first time
been proposed, enabling the recognition of the main and
secondary depositional sequences and related system tracts.
Despite the difficulties (rare, badly preserved, and
reworked fauna) encountered in the biostratigraphic
determination, new integrated data (foraminifera and cal-
careous nannofossils) provided better dating of the
stratigraphic intervals (cfr. Tables 1, 2 , 3). The new inte-
grated biostratigraphical approach appears more consistent
for correlations (stratigraphic units, regional marker-beds
and peculiar geochemical features) of the successions
reconstructed in different sectors (internal, intermediate,
and external) of the Tarcau Nappe (Moldavidian Basin).
In the sector examined in the Romanian eastern Car-
pathians the Tarcau Nappe is not well exposed; however
the five reconstructed successions appear to be sufficient to
recognize lateral relationships within the framework of a
simple model of the transversal of the Moldavidian Basin
(Fig.2).
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In the Moldavidian Basin, two main opposite source
areas are recognizable. The Tarcau Group succession was
fed by the Middle and possible Outer Dacides while the
Kliwa Group succession was fed by the foreland. The
Moldovita Mixed Group originated from the interfinger-
ing of the two different supplies (cfr. palaeogeographic
sketch in Fig.2). The vertical sedimentary evolution
allows us to recognize three stages related to the main
depositional sequences defined. The lowermost sequence
(S-1) is represented by the Tarcau Sandstones Fm, whichhas the MSB1 at the top; the middle sequence (S-2)
consists of Podu Secu to Lower Dysodilic Shales Mem-
bers and their lateral equivalents, representing the
Oligocene Depositional Sequence separated from the
overlying sequence by MSB2; the uppermost depositional
sequence (S-3) is represented by the Aquitanian-Burdi-
galian Fusaru Fm and its lateral equivalents. This vertical
evolution shows an upward increase in the terrigenous
supply (foredeep stage), especially from MSB2. This
abrupt change in the sedimentation has been interpreted
(Fig. 2) in the same way as for other Tethyan basins
developing in this period (Guerrera et al. 1993, 2005 andreferences therein), as being related to a tectonic inver-
sion from oceanic opening of the basin (drifting) to
continental convergence (foredeep).
Significance of geochemical data
As regards the black-shale deposits that characterize dif-
ferent stratigraphic intervals of the study successions,
already known in the literature (as discussed above in the
Background and aim section) of the Carpathian Chain,
we have carried out a new geochemical characterization of
the main layers in order to estimate more accurately the
amount of the organic matter and also to recognize the
relationships between anoxic facies and the depositional
environment.
The geochemical study performed on the samples col-
lected from the successions within the Tarcau Nappe
enabled the identification of four interval with black shales.A shaly inteval (Interval 2) is particularly rich in a highly
lipidic, well-preserved organic matter (Table8).
The TOC amounts and the Rock-Eval parameters reg-
istered in this interval throughout the study sections lead us
to consider this unit as a good potential thermally immature
source rock. It is difficult, based on Rock-Eval and TOC
analysis alone, to draw definitive conclusions on the
organic-matter type; however the high S2 and HI values
(Tables5,6), and results for similar materials reported by
several authors (Koster et al. 1998a,b; Curtis et al. 2004)
lead to the conclusion that this source rock is mostly of
Type II (i.e. oil and gas prone).In another respect, in relation to the palaeoenvironment
depositional realm, our results clearly indicate that the
latter was highly favourable to the preservation of the
organic matter (high S2 values coupled with high HI and
low OI values). Here also our data are limited to conclude
definitively whether the depositional environment of this
source rock is oxic or anoxic, although all the registered
values from the Rock-Eval pyrolysis study suggest a highly
anoxic depositional environment. This observation,
Table 8 Synthetic correlation of the main organic-matter intervals (TOC and Rock Eval pyrolysis data) of the study successions within theTarcau Nappe
Key: Interval 1 TOC\ 0.83, Rock-Eval: Type IV. Such values indicate that the sediments must have been deposited under strongly oxic
conditions and no organic matter had been preserved;Interval 2TOC between 0.2511.08%, thermally immatureTmax\ 430C, Type II: lipidic
oil-prone source rock, thermally immature deposited in a highly anoxic environment;Interval 3 TOC up to 1.69%, results indicate a highly
oxidized conditions for the environment and, especially at its base, as a good oil-prone source rock, thermally immature (Tmax\ 428C),deposited under anoxic conditions;Interval 4TOC around 7%, attesting that these sediments are an excellent oil-prone source rock, sedimented
under highly anoxic conditions
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however, confirms the results and conclusions pointed out
through several previous studies conducted on equivalent
lateral facies from other countries along the Carpathian
chain (Koltun 1992; Roore et al. 1993; ten Haven et al.
1993; Lafargue et al. 1994; Kruge et al. 1996; Bessereau
et al. 1997; Rospondek et al. 1997; Koltun et al. 1998;
Koster et al. 1998a,b, Curtis et al. 2004).
This interval 2 (corresponds to the Lower Menilites,Bituminous Marls and Lower Dysodilic Shales Members in
logs 1 and 3; and to Lucacesti Sandstones Fm p.p., Lower
Menilites, Bituminous Marls and Lower Dysodilic Shales
p.p. Members in log 4) is not older than Late Rupelian-Late
Chattian.
Within this succession, two separate anoxic episodes
seem to have developed over time, thus generating two
separate highly organic-rich black-shale intervals, which
have been documented as excellent petroleum source rocks
(Fig. 2, 3; Tables5, 6,7, 8).
Thefirst anoxic episodeoccurred at the top of the PoduSecu Member (not older than Late Rupelian) docu-
mented only in the internal Tarcau Group.This episode is
expressed through nearly 40 m of a succession made up
of highly organic rich sandstones and arenaceous shales.
Thesecond anoxic episode, which developed later (not
before the Late Chattian) characterizes three lithostrati-
graphic units: Lower Menilites, Bituminous Marls and
Lower Dysodilic Shales Members (Fusaru, Moldovita
and Kliwa Groups). This second anoxic episode
partially corresponds to the episode defined by Puglisi
et al. (2006) in their study of the Early Oligocene
menilite facies (Tarcau Nappe), as the intervalincluding the uppermost part of the lower turbidite
system and the entire succession of the Basin Plain
System, even if this age must be considered out-of-date.
Origin of black shales
The origin of marine black shales is strongly debated and
discussed. Two opposite hypotheses have been proposed
(Amieux 1980; Demaison and Moore 1980; Herbert and
Fischer 1986; Belayouni et al. 1990, 2003; Fiet 1998;
Ettensohn2001; Varentsov et al.2003; Schieber2004and
references therein): bottom-water anoxia versus highorganic productivity in shallow water. The first case
implies long time periods, while the second case involves
cyclic phases but not longer in time. In these latter time
periods, some authors have suggested that normal organic
productivity was widespread and always sufficient to form
black-shale deposits and that other factors may be equally
compelling such as the availability of repositories where
organic matter was preserved (Ettensohn 2001). Such
depositional conditions may be basins generated in periods
with tectonic stress where a setting of sediment starvation
takes place due to geographic isolation, great depth, and
increased nutrient influx. Such context has been docu-mented in North America during paroxysmal tectonic
periods, and the results show that all black shales are
recorded in tectonic basins during Palaeozoic and Jurassic
times with plate assembly or disassembly (Ettensohn2001;
Schieber 2004). In the study area, the above interdisci-
plinary and integrated data seem to be sufficient to propose
an origin related to deep-water with anoxic conditions. The
palaeogeographic models (Rogl1999) for the area shows a
basin bad connected with the Indian ocean and bounded by
continental domains in the assembly phase. The lithofacies
belong to deepwater realms and the fossil assemblage
indicates also deep bathymetries. The evolution of the
basin studied indicates a foredeep evolution, and the dis-
tribution of black shales belong to relative sea-level periods
(TST or HST) as indicated by the sequence stratigraphy.
Palaeogeographic sketch
The palaeogeographic evolution from Eocene to Miocene
in the study area is closely related to the end of the Tethyan
Ocean and the birth of the Paratethys and Mediterranean
Seas (Rogl1999). The northward drift of the continents of
India and Australia caused the end of the Tethyan Ocean,
changing the previous relict Mesozoic palaeogeography
(Debelmas et al.1980; Popov et al.1993; Rusu et al.1996;
Scotese et al. 1988). From this time on, to the east, the
Indian Ocean was born, while to the western (between
Europe and Africa) the Mediterranean Sea began to open
(Guerrera et al.2005). Part of Central Europe consisted of
an archipelago with minor continental domains surrounded
and covered by the so-called Paratethys Sea (Fig. 4). This
area, where the Carpathian Chain begins to rise by an
Atlant
icOcean
ParatethyanSea
Indian
Ocean
AFRICA
Iberia
EURASIA
AlboranBlock
Mesomediterran
ean
Microplate
Continental domainsFuture Carpatian Chain
Oceanic domainsMoldavidian basin
Anatolia
Block
Persia-Tibet
Block
Arabia
Block
AdriaBlock
Fig. 4 Palaeogeographic sketch of the Paratethyan Sea betweenAfrica and Eurasia plates during the late Oligocene-Early Miocene
(from Rogl1999and Guerrera et al2005, modified)
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