tarcau nappe 2009 miclaus

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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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Int J Earth Sci (Geol Rundsch) (2009) 98:157176

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.

158 Int J Earth Sci (Geol Rundsch) (2009) 98:157176

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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)

Int J Earth Sci (Geol Rundsch) (2009) 98:157176 159

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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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