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Early Carnian anoxic event as recorded in the southernTethyan margin, Tunisia: an overviewMohamed Souaa

a Entreprise Tunisienne d’Activités Pétrolières, Immeuble Zouila, 1001 Tunis, TunisiaPublished online: 28 Oct 2014.

To cite this article: Mohamed Soua (2014) Early Carnian anoxic event as recorded in the southern Tethyan margin, Tunisia: anoverview, International Geology Review, 56:15, 1884-1905, DOI: 10.1080/00206814.2014.967315

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Early Carnian anoxic event as recorded in the southern Tethyan margin, Tunisia: an overview

Mohamed Soua*

Entreprise Tunisienne d’Activités Pétrolières, Immeuble Zouila, 1001 Tunis, Tunisia

(Received 22 July 2014; accepted 15 September 2014)

Considerable attention has been given to the Carnian (Late Triassic) Pluvial and Reingraben events associated with organic-rich shale accumulation in the Germanic basin, Alps, southern Appenines as well as in northwestern Tethyan margins. Lessinterest has been shown to the southern Tethyan portion represented by the northern margin of Africa, including Tunisia.Tunisian basins represented by the Tellian domain, Tunisian trough, the ‘Dorsale’, and the North–South Axis (NOSA)belong to the southern Tethyan margin, where northern and central areas record the early Carnian anoxic event within anextensive carbonate platform. This short-lived (~2 million year) period of anoxia ranges within the Aonoides to Austriacumammonite zones, and corresponds in Tunisia generally to intermediate to shallow water environments marked by organic-rich black limestone, dolomite, and shale. Interestingly, toward the south, the dysaerobic conditions in the Jeffara–southernDahar basin appear to have prevailed locally also in the early Carnian. Here we review evidence of early Carnian anoxia inTunisia based on the analysis of more than 17 Triassic sections and wireline logs from several petroleum exploration wellspenetrating the black dolomites, limestones, and shales. In addition, biostratigraphic and complete geochemical reviewshave been undertaken from published papers and unpublished internal reports to better assess this important and promisinghydrocarbon source interval.

Keywords: early Carnian anoxic event; Carnian Pluvial Event (CPE); black shale; Late Triassic; Tunisia

1. Introduction

1.1. Triassic tectonic setting

The Carnian stage (Late Triassic) witnessed several globaltectonic events that coincidedwith the break-up of the Pangeasupercontinent (Ziegler 1988; Veevers 1994, 2004; Withjacket al. 1998; Golonka and Ford 2000; Golonka 2002). Theseincluded the closure of the Palaeotethys and the collision ofthe drifting Cimmerian plates with Cathaysia, resulting in theIndosinian orogeny (Golonka 2007) (Figure 1). Beginning inPermian–Triassic time, the Cimmerian fragments driftednorthward from Gondwana toward Laurasia to close thePalaeotethys (Şengör 1984; Zonenshain et al. 1990;Sengör and Natalin 1996; Golonka 2004) as the NeotethysOcean opened behind them (Figure 1) (Burollet et al. 1978;Jongsma et al. 1985; Turki 1985; Morgan et al. 1998;Guiraud 1998; Touati and Rodgers 1998; Grasso et al.1999; Soussi 2000; Soua and Tribovillard 2007). Thenorthward drifting of Cimmerian fragments was the con-sequence of Carboniferous–earliest Permian rifting, whichaffected the northern Gondwana margin. According toseveral authors (e.g. Lawver and Gahagan 1993; Scoteseand Langford 1995; Golonka 2007), the existence of sub-duction zones during the Permian–Triassic period resultedin the so-called Pangean Rim of Fire. Alternatively, duringthe Early Triassic, the western Pangean margin was char-acterized by separation of North America and Gondwana

(western Africa and western Europe), which is believed tohave continued as the Late Triassic extensional phase(Bryan et al. 1977). In addition, some authors (e.g.Macdonald et al. 2003) stress the Late Triassic initiationof continental extension in isolated areas such as SouthAmerica. Triassic evaporate-related basalts and tuffs(ophites) in Tunisia have been reported by Kurtz (1983),who considered them as tholeiitic continental basalts(according to their Ti-enrichment trend, K2O, Rb, and Srcontents) with slightly transitional alkaline character. Herelated these ophites to the widespread basaltic volcanismassociated with the break-up of Pangea; since then, con-tinental tholeiitic basaltic activity related to this episodehas been reported from the Argana valley of the Moroccanwestern High Atlas (Beauchamp 1988; Aït Chayeb et al.1998; Tourani et al. 1998) and in the Khemisset basin ofthe Moroccan Meseta (e.g. Et-Touhami 1994). Additionaloccurrences are reported from the eastern USA, Guinea,Mali, and Mauritania (Weigand and Ragland 1970;Biju-Duval et al. 1977; Bertrand and Coffrant 1977;Laubscher and Bernoulli 1977; Manspeizer et al. 1978;Bertrand et al. 1982).

1.2. Triassic sedimentary facies

The break-up of Pangaea as outlined above continuedthrough the Norian stage (Late Triassic; Ziegler 1982;

*Email: elmohology@yahoo.frPresent address: Saudi Arabian Oil Company (Saudi Aramco), Dhahran, Saudi Arabia.

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Veevers 1994, 2004; Withjack et al. 1998; Golonka andFord 2000; Golonka 2002, 2007). During the EarlyTriassic, the separation of North America from Africacontinued through the Late Triassic extensional phaseduring which continental red beds were deposited inevolving continental rifts (Olsen 1997; Withjack et al.1998; Laville et al. 2004). During the immediately pre-ceeding Carnian stage, in the central and western HighAtlas (Morocco), red fluvial braided conglomerates andplated sandstones were deposited in WSW–ENE-trendingrift systems (Petit and Beauchamp 1986; Benaouiss1990; Benaouiss et al. 1996; Tourani et al. 1998;Courel et al. 2003). This is true also for clastic deposits

that prevailed over the whole area between NorthAmerica and Europe (Golonka and Ford 2000). TheCarnian continental rift system is represented by redbeds and fluvial sequences, which are characteristic ofearly stage rift grabens (Ziegler 1988; Withjack et al.1998; Kutek 2001; Golonka 2007). To the north, andabout the same time (Middle–Late Triassic), the CentralEuropean Triassic basins witnessed deposition of threedifferent facies: (1) Buntsandstein, essentially siliciclas-tic; (2) Muschelkalk, essentially marine carbonates; and(3) Keuper, essentially mixed siliciclastics, carbonates,and evaporites (Köppen and Carter 2000; Kutek 2001;Golonka 2007).

SicilyTurkey

Apulia

Arabia

Mad

agas

car

India

Lhasa

Australia

South America

North

America

Iberia

Tisa

PANGEA

Laurasia

Greenland

Tunisia

PANTHALASSA

OCEAN

Oceanic spreading center and transform faults

Subduction zone

Thrust fault

Normal fault,

Transform fault

Mountain ranges

Landmass

Carbonate with possible OM

Shallow sea carbonatesDeep ocean basinTunisia

Sicily

TurkeyApulia

Arabia

Mad

ag

ascar

India

Iberia

Tisa

Laurasia

Tunisia

Africa

Southern

America

N Am

erica

EuropePALAEOTETHYS

NEOTETHYS

Cimmerian platesNEOTETHYS

PALAEOTETHYS

0

30N

0

30S

0

Cimmeria

n plates

A

B

EXPLANATION

Limit of Carbonate depositionCarbonate deposition with possible organic-rich sedimentation

Plates position as of Late Triassic (Carnian at 224 Ma)

Figure 1. Palaeogeographical map for the Carnian at 224 Ma (Late Triassic). (A) Global reconstruction. (B) Late Triassic regionalstructural, palaeoenvironment, and lithofacies map of the future Mediterranean region; asterisk: Tunisia (adapted from Golonka 2007,modified).

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However, comparison with the Carnian global palaeo-geographical map shows that an extensive area ofcarbonate platforms characterized the Neotethys andPalaeotethys margins (Dercourt et al. 1993; Sengör andNatalin 1996; Golonka 2007) (Figure 1B), extending as faras Arabia (Alsharhan and Magara 1994). Carnian sedi-mentation was influenced by monsoonal activity (Parrish1999) at the same time as enhanced extension related tothe break-up of Pangea (Golonka 2007). Enhanced volca-nic activity (Kurtz 1983), which increased CO2 emissioninto the atmosphere (Hornung et al. 2007), is thought to beresponsible for the well-known Carnian Pluvial Event(CPE), which was caused by enhanced rainfall and riverrun-off through the shelves as a consequence of the inten-sification of the hydrological cycle due to global warming(Hornung et al. 2007). As a consequence of these tectonicand climatic changes, organic-rich black shale accumula-tion occurred in the Tethyan realm during the earlyCarnian.

1.3. Brief history of previous investigations

Most of the previous investigations on Tunisian Triassicrocks have been structural studies focused on diapirs andthrusts (e.g. Perthuisot 1981; Turki 1985). The Carniancarbonate shelf and its organic richness were noted bySoussi et al. (1996). Kamoun et al. (2001) published apalaeogeographic review of the Triassic system ofTunisia. The organic-rich Carnian environment of deposi-tion and its sedimentation model have not been evaluatedin the context of the surrounding Mediterranean geologi-cal data.

This paper reviews and focuses on the accumulation oforganic-rich strata within the extensive early Carnian car-bonate platform in Tunisia besides enhancing our under-standing of the role of anoxic deposits in generatingpetroleum systems in Tunisia and adjoining regions onthe northern Gondwana margin.

2. Geological background

2.1. Regional setting

During Early Carnian time, Tunisia was situated in thenorthern margin of Gondwana. This time was character-ized by the intensification of the break-up of the Pangeasupercontinent (Figure 1), which began in Permian-Triassic time when the Cimmerian plates dislocatedfrom the northern African margins and began to driftnorthward toward Laurasia to close the Palaeotethys(Celâl Şengör 1984; Zonenshain et al. 1990; Sengörand Natalin 1996; Golonka 2004) and consequentlyopen the Neotethys Ocean (Burollet et al. 1978;Jongsma et al. 1985; Turki 1985; Morgan et al. 1998;

Guiraud 1998; Touati and Rodgers 1998; Grasso et al.1999; Soua and Tribovillard 2007). This approximatelyN–S to NE–SW motion was accompanied by the forma-tion of E–W to NW–SE normal faults associated withhalf-graben systems formed on the N-dipping slope at thesouthern Tethyan passive margin. This passive marginevolution did not include a panoply of facies transectand important thickness variation during the earlyCarnian (Figures 2 and 3). However, large carbonateplatforms developed along the Tunisian shelf, includingorganic-rich and organic-lean black to dark grey lime-stones and dolostones, and even laminated black shales.This reflects the regional palaeogeographical depositionalpattern of the Neotethys and Palaeotethys margins(Figure 1B; Dercourt et al. 1993; Philip et al. 1996;Sengör and Natalin 1996; Philip 2003; Golonka 2007).These carbonates grade progressively to clastic andmixed clastic as well as to evaporitic series in theSaharan Atlas of Algeria toward the east (Baird 1963;Achab 1970; Busson 1971; Reyre 1973; Mami 1980,1989; Mami and Bourmouche 1994; Aït Ouali andNedjari 1994; Askri et al. 1995; Aït Salem 1992;Courel et al. 2003) and then grade into clastic red bedstoward the central and western High Atlas of Morocco(Cousminer and Manspeizer 1977; Mattis 1977; Biron1982; Salvan 1984; El Youssi 1986; Beauchamp 1988;Manspeizer 1988; Benaouiss 1990; Benaouiss et al.1996; Tourani et al. 1998; Oujidi et al. 2000; Courelet al. 2003).

2.2. Geological domains and Triassic lithostratigraphybackground

Detailed lithostratigraphic description of Triassic rocks ingeneral and Carnian sections in Tunisia has been providedby several authors (e.g. Peybernes et al. 1993; Soussi et al.1996; Kamoun et al. 2001; Mehdi et al. 2009). Generally,the Carnian carbonates are described within four structuraldomains from northern to southern Tunisia (Figure 4). Tothe north, the main structures are represented by theTelliandomain or unit (including the Ichkeul–Haireshstructures), where the Triassic rocks are characterizedmainly by metamorphic and dolomitized carbonates gen-erally assigned to the Bechateur and Ras El KoraneFormation (see later in the text, Table 1; Kamoun et al.2001), and by the Northern Atlas structures, which arerepresented in this area by the ‘Dorsale’ range andTeboursouk unit, characterized mostly by thrusts andTriassic extrusive structures (Salaj 1978; Perthuisot 1981;Turki 1985; Chandoul et al. 1993; Peybernes et al. 1993).In the central part of the country, near the ‘North–SouthAxis’ (NOSA) (Figure 4), the Triassic is dominated byevaporites. The area is affected by halokinesis structures

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alternating generally with continuous stratigraphic series(Burollet 1956; Ben Ferjani et al. 1990; Chandoul et al.1993; Soussi et al. 1996; Mehdi et al. 2009). Southwardalong the North–South Axis, the Chott domain, which isrelated geologically to the southern Gulf of Gabes (Figure4), is represented by a transitional Triassic series (Mzoughiet al. 1994; Buratti et al. 2012).

Finally, farther south, in the so-called SaharanPlatform and the Dahar–Jeffara domain, the Triassic seriescomprises siliciclastic-dominated beds, represented bycontinuous sandstone strata (Kamoun et al. 2001;Arfaoui and Montacer 2004).

3. Material and methods

The review of the early Carnian anoxia occurrence inTunisia is based mainly on an analysis of more than 17Tunisian Triassic sections and wireline logs from severalpetroleum exploration wells penetrating black dolomite,limestone, and shale levels (Figure 2). Gamma-ray ampli-tudes, spontaneous potential, and resistivity logs havebeen analysed in different studies, which will be citedwithin the text. An isopach map has been compiled forthe early Carnian (Figure 3) based on this dataset andother available data (e.g. Burollet 1973; Salaj 1978; Ben

Figure 2. Facies distribution of the early Carnian Black Limestones–Dolostones unit of the Rhéouis Formation and age-equivalent unitsin Tunisia and related organic-richness (TOC), maturity (Tmax), depth, and thickness; facies zones modified after Kamoun et al. (2001);Transect A–A′ in the Gulf of Tunis refers to the regional seismic section of Figure 6; section and well codes are shown in Table 1.

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Ferjani et al. 1990; Chandoul et al. 1993; Mzoughi et al.1994; Soussi et al. 1998; Kamoun et al. 2001; Mehdi et al.2009; Buratti et al. 2012). In addition, a palaeogeographi-cal map of the petroleum source rock has been constructedto evaluate the controlling mechanisms on the depositionof the laminated black limestone levels. Additionally,fieldwork was carried out in central (Jebel Rhéouis) andsouthern (Jebel Sidi Toui) Tunisian basins where Triassicblack shales crop out in order to collect more information.Geochemical exploration tools are represented by totalorganic carbon (TOC) and pyrolysis and have beenachieved earlier, namely by Soussi et al. (1998) as wellas Arfaoui and Montacer (2004). These data were

compiled with biostratigraphy and palynology, and refer-ences will be cited within the text.

4. Early Carnian anoxic event

4.1. Distribution and characteristics of the organic-richCarnian strata

In central Tunisia, Bedir (1995) used seismic stratigraphicdata to conclude that Carnian sediments in the subsurfaceof the Sidi Aich–Souinia area (Sidi Ali Ben Aoun) andBen Kheir (BK-1) structures (Figure 5) consisted of alarge laminated carbonate platform, which is termed

Figure 3. Thickness map of the early Carnian Black Limestones–Dolostones of the Rhéouis Formation and age-equivalent units inTunisia. Illustrated isopachs are simplified and short-distance thickness; thickness and codes of sections/wells are shown in Table 1.

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(T3). The general facies trend was complicated by a com-plex tectonic palaeorelief configuration, which resulted intilted blocks individualizing deep half-graben systemstoward the north (Turki 1985; Bedir 1995; Soussi et al.1996; Touati and Rodgers 1998; Kamoun et al. 2001;

Bedir et al. 2001; Bouaziz et al. 2002). This configurationresults in slight lateral thickness and facies variations ofthe early Carnian strata (Figure 3). Within the BK-1 wellstructure, Scallon (1980) and Gueinn (1980) describedlaminated organic-lean black dolomites alternating with

Figure 4. Simplified geological sketch map of Tunisia showing the Triassic extrusive structures and main structural domains discussedin the text.

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Table1.

Abb

reviationcodes,thickn

ess,litho

logy,totalorganiccarbon

(TOC,%)intervals,andreferences

ofTun

isianearlyCarnian

sections/wellsdiscussedin

thetext.

Cod

eSectio

n/well

Thickness

Lith

olog

yTOC(%

)Coo

rdinates

References

Bch

Bechateur

30m

White

limestone

–37

°18ʹ16

.67ʺ

N9°44

ʹ23.35ʺE

Salaj

andBajanik

(197

2);Biely

andRakus

(197

2);Kam

ounet

al.(200

1)Rk

Ras

ElKorane

–White

limestone

–37

°20ʹ05

.23ʺ

N9°40

ʹ12.17ʺE

Crampo

n(197

1);Kam

ounet

al.(200

1)UTQ-1

Utiq

ue-1

30m

Black

doloston

e,shale

0.13

–0.24

37°01ʹ57

.90ʺ

N10

°03ʹ43

.70ʺ

ESmaoui

(199

6)ROD-1

Raoued-1

20m

Black

doloston

e,shale

0.24

–2.86

37°10ʹ26

.66ʺ

N10

°25ʹ53

.25ʺ

EBou

faresandMekki

(198

4);Sou

a(200

7);

Sou

aandSmaoui

(200

8)CB-101

Cap

Bon-101

30m

Black

limestone,dolomites,shale

0.3–

2.1

36°37ʹ14

.00ʺ

N10

°40ʹ13

.00ʺ

ECou

ture

andMarti(196

5);Bon

nefous

(196

7);Burollet(197

3);Tou

atiet

al.

(199

5);Salaj

andMzoug

hi(199

7)Bl

JebelBaouala

6–8m

Black

limestone

–36

°53ʹ25

.72ʺ

N9°46

ʹ40.33ʺE

Perthuisot(198

1);Adil(199

3);Fou

rnet

etal.(199

4);Kam

ounet

al.(200

1)Ln

JebelLansarine

2m

Black

limestone

–36

°44ʹ59

.68ʺ

N9°37

ʹ51.88ʺE

Perthuisot(198

1);Kam

ounet

al.(200

1)Fk

JebelFkirine

––

–36

°12ʹ16

.47ʺ

N9°56

ʹ02.00ʺE

Salaj

andStranik

(197

0);Kam

ounet

al.

(200

1)Ch

JebelCheid

Verythin

––

36°20ʹ22

.57ʺ

N9°17

ʹ45.63ʺE

Perthuisot(198

1)KEA-5

Kou

diat

ElAlfa

Over50

mBlack

limestone,do

lomites,shale

0.6

35°30ʹ37

.44ʺ

N09

°16ʹ29

.56ʺ

EKhessibiandPacaud(196

8);Mehdi

etal.

(200

9)Rhs

JebelRhéou

is30

mBlack

limestone,do

lomites,shale

0.3–

7.5

34°55ʹ58

.16ʺ

N9°39

ʹ41.76ʺE

Burollet(195

6);Salaj

(197

8);Sou

ssiet

al.

(199

8);20

01);Kam

ounet

al.(200

1);

Mehdi

etal.(200

9),thiswork

BK-1

Belkh

eir-1

20m

Black

limestone,do

lomites,shale

0.37

–0.57

34°19ʹ25

.00ʺ

N09

°29ʹ44

.00ʺ

EScallo

n(198

0);Gueinn(198

0);Bedir

(199

5)DE-1

Degla-1

26m

Black

limestone,do

lomites,shale

0.1–

2.3

34°01ʹ55

.23ʺ

N10

°49ʹ43

.47ʺ

EMzoug

hiet

al.(199

2;19

94);Ha(200

9)DJM

-1Jerba-Mellita-1

20m

Black

limestone,do

lomites,shale

0.2–

0.89

33°51ʹ47

.20ʺ

N10

°46ʹ43

.66ʺ

EMzoug

hiet

al.(199

2;19

94);Burattiet

al.

(201

2St

JebelSidiTou

i8–

10m

Black

dolomites,shale,

sand

ston

e0.1–

332

°41ʹ08

.00ʺ

N11°15ʹ23

.50ʺ

EGlin

tzbo

eckel(195

6);Salaj

(196

9);Ben

Ferjani

etal.(199

0);Arfaoui

and

Mon

tacer(200

4),thiswork

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black to dark shale and marls (over 20 m) with TOC notexceeding 0.6% (Table 1, Figure 2). The stratigraphiccalibration has been subject to palynological investigationby Gueinn (1980), who described at 3812 m level an earlyCarnian palynomorph association dominated byTriadispora sp., ?Concavisporites sp., which could indi-cate a marine environment (Mostler and Scheuring 1974;Roghi et al. 2010) compared with the above associations.This is in good agreement with the seismic stratigraphicinterpretation of Bedir (1995).

Organic-rich early Carnian strata occur in many placesin northeastern and central Tunisia (Figure 2; Table 1; e.g.Utique, Gulf of Tunis, Cap Bon, North–South Axis) andare grouped into the Black Limestones–Dolostones levelof the Rhéouis Formation (Figure 5), which has beendefined by Burollet (1956). Equivalent terms are groupedin the Bechateur Formation (Kamoun et al. 2001), whichexhibit organic-poor facies in the Bechateur quarry (Salajand Bajanik 1972; Biely and Rakus 1972), or grouped inthe Mekraneb Formation in the Jeffara–Dahar area insouthern Tunisia (Bouaziz 1986) (Figures 4 and 5) andfinally grouped in the lower ‘Trias carbonate’ or AziziaFormation in the Saharan platform by petroleum geolo-gists. The thickness distribution of the Black Limestones–Dolostones level in Tunisia is illustrated in Figure 3(compiled from a host of references, e.g. Burollet 1956;Glintzboeckel 1956; Bonnefous 1967; Crampon 1971;Salaj and Bajanik 1972; Biely and Rakus 1972; Salaj1978; Perthuisot 1981; Ben Ferjani et al. 1990; Mzoughiet al. 1992; Adil 1993; Peybernes et al. 1993; Touati et al.1995; Bedir 1995; Soussi et al. 1998, 2001; Kamoun et al.2001; Arfaoui and Montacer 2004; Mehdi et al. 2009;Buratti et al. 2012).

The regional distribution of the five main early Carniancarbonate facies types is shown in Figure 2, which alsodistinguishes an organic-rich and an organic-poor BlackLimestones–Dolostones facies trends and white to yellow

carbonates. The organic-rich early Carnian deposits exist infour areas, being dolomites, limestones, or mixed litholo-gies (shale, limestone, dolomite), namely in the northeast(UTQ-1, ROD-1, and CB-101 wells), north-central Tunisia(Ln, Bl sections; KEA-5 well), central Tunisia (Rhs section,BK-1 well), central offshore Tunisia (the Gulf of Gabes:DE-1, DJM-1 wells), and exceptionally in southern Tunisia(Dahar-Jeffara, Jebel Sidi Toui) (see Figure 2 for location;Figure 5; Table 1). Unfortunately several neighbouringwells did not reach the Late Triassic strata (such as NA-101 in Central Tunisia and ZB-1 in the northern Chottsbasin, see Figure 2 for location). Rarely available well andoutcrop data confirm the organic-rich provinces of Soussiet al. (1998); however, the exact boundaries still remainunclear, especially for the Gulf of Tunis (Figure 2) whereROD-1 well recorded TOC values up to 2.86% (Figure 2;Table 1), Central Tunisia (and Eastern Algeria), Gulf ofHammamet (Pelagian block), and the neighbouring JebelRhéouis area, due to the non-availability of well data.However, geophysical evidence of the early Carnian blackshale distribution is shown in Figure 6 (A–A′ section),which exhibits black shale reflectors within possible half-graben systems and bordering normal faults. This NE–SWregional seismic line is located in the Gulf of Tunis (seeFigure 2 for location of seismic line and generated crosssection).

A comparison with the palaeogeographic map (Figure 2)suggests that the distribution of the organic-rich BlackLimestones–Dolostones level is restricted to medium to shal-low water depth representing possible impingement of anoxygen minimum zone (OMZ) onto the northern margin ofAfrica (Figure 7), which may record the Carnian anoxicevent documented within the southern Apennines, Pobasin, southern Alps (Gorno Formation in centralLombardy [organic-rich lower part], Cassian Dolomites[organic-rich lagoonal series], Cassiano Formation[organic-rich shale and limestone succession], and Calcare

230

240

228

237

241

Tuvalian

JulianCordevolian

Longobardian

FassanianLad

inia

nC

arn

ian

Mid

dle

Late

Tri

assic

Southern Tunisa

JeffaraKirchaou-Sidi Toui(BMT-1, St-3 wells)

Dahar-Tebaga Jerba-Gulf ofGabes (DJM-1, DE-1 wells)

Central Tunisia

Gafsa(BK-1 well)

North-South Axis(Jebel Rhéouis)

Sbiba-Kodiat El Halfa (KEA-5 well) Jebel Fkirine Jebel Cheid,

Baouala-LansarineIchkeul - Hairech

Bechateur-Ras El Korane

Gulf of tunis-Cap Bon-Utique(ROD-1, CB-101,UTQ-1)

Northern Tunisia

Northern Atlas Tellian

Ras

Ham

ia

Myophoria Lst

Kirchaou

Sidi Stout UnconformityMekraneb

Touareg

Rehach

Mhira

Kef El Aneba

Tebaga,

Malaab high

Myophoria Lst

Unité 4

Unité 5

T2

T3

Uni

té 3

Rhé

ouis

Black Lst

Lower Siltstones

Lower Gypsum

MiddleGypsum

Yellow LstUpper Siltstones

Black dolomites

Hairech

Black dolomites and Lst

Bec

hate

ur

Hiatus

Hiatus Hiatus

Gulf of GabesTriassic System

Ladinian – Carnian

236

234

232

238

Figure 5. Lithostratigraphic and chronostratigraphic reconstruction of the Ladinian–Carnian time span in different basins of Tunisiadiscussed in the text (Tellian units, Tunisian trough, Dorsale, North–South Axis and the Chott basin, Jeffara-Dahar, etc.) (modified fromKamoun et al. 2001). Black bands represent organic-rich interval. Note that carbonates are dominating in the northern portion of thecountry and that no deposition has been recorded in the Tebaga–Malaab high in southern Tunisia (hiatus).

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del Predil [organic-rich basinal succession]; Stefani andBurchell 1993), and Germanic basin within the RaiblGroup (Hornung et al. 2007).

The Black Limestones–Dolostones level is a few tens ofmeters thick (2 to 30 m, Table 1; Figures 2 and 3) in the‘Dorsale’, the NOSA, Gulf of Tunis, and Cap Bon areas witha maximum thickness of over 50 m recorded by Mehdi et al.(2009) in KEA-5 in Central Tunisia (Figure 6). In the

northern Jebel Rhéouis (Figure 2), Oued El Bhima, andOued El Forka localities, the lower part of the RhéouisFormation is represented by micritic fine-grained grey todark limestone with a somewhat coarser texture interbeddedwith finely laminated black dolomites and black shales(20 m–30 m). Generally, the Black Limestones–Dolostoneslevel consists of a regular alternation of dark-coloured shalesand laminated carbonates and sublithographic black and

ROD-1NW SE

Triassic

CretaceousJurassic

Pliocene to recentL. Miocene-Oligocene-EoceneOligocene

Sandstones

Cretaceous

Jurassic

Pre-Triassic rocks

MGT-1GMT-1CAR-1ROD-1RAJA-1

OligoceneEocene-Palaeocene

Miocene

Triassic

Triassic

NW SE

10 km

approximate position of the early Carnian Black limestones and

black shale level

(projected)

MEDITERRANIAN SEA

10 km

0

1000

2000

3000

4000

5000

6000

TW

T (

ms)

A

B

Figure 6. Regional geoseismic section in the Gulf of Tunis: Early Triassic half-graben systems controlling the deposition of earlyCarnian strata on the Tunisian shelf in the present Gulf of Tunis. Note the Triassic structural style. (A) Regional N–S seismic line in theGulf of Tunis with projected wells, (B) interpreted seismic line. The position of the early Carnian strata is only approximated. Forlocation, see Figures 2 and 3. Interpretation is based mainly on the existing well data and on the NW–SE regional seismic section.

Tebaga – Malaab High

Sea level

Permian successionMekraneb

Jebel

Sidi Toui

Gulf of Gabes

DE-1, DJM-1

Unit 5 T3

BK-1

NS Axis

(Jebel Rhéouis)

Kodiat El Alfa

(KEA-5)

Jebel Fkirine

(Dorsale)

Jebel Cheid

Jebel Baouala-

Lansarine

Cap Bon, Gulf Of Tunis, Utique(CB-101, ROD-1, UTQ-1)

Bechateur, Ras Korane

Hairech, IchkeulBlack

limesto

ne

NWSE

NWSE

NESW

Carnian Pluvial event

and monsoonal circulation system

Oxygen minimum zone expansion

Early Triassic and palaeozoic rocks

Monsoonal circulation system

Figure 7. Proposed depositional model for the early Carnian organic-rich sedimentation, in the Tunisian shelf, which assumeimpingement of an OMZ. Organic-rich early Carnian strata are deposited within half-graben systems, organic-lean strata may occurbeneath the OMZ. Note that this model compiles the regional structural framework (enhanced distensional movements) associated withthe regional pluvial event (increased riverine influx, enhanced run-off, as well as high nutrition). In addition, probable increased humidityrelated to enhanced CO2-emission associated with tholeiitic volcanic activity (based partly on Kurtz 1983; Simms and Ruffell 1989;Barrett 1998; Lüning et al. 2004; Brumsack 2006; Soua and Tribovillard 2007; Hornung et al. 2007).

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greyish limestones and marls with mean TOC values of up to3% (Soussi et al. 1996; p. 262) (Figure 2). This sequenceoverlies generally bioclastic beds (gastropods, echinoderms)interbedded with radiolarian-bearing shale levels and stro-matolitic dolomites. A single 7.5% of TOC value wasreported by Soussi et al. (1998) from the Oued El Forka(northern Jebel Rhéouis). The vertical distribution of lime-stone versus marl in the different localities depends on theposition of the palaeoshelf/slope (Figure 7). Generally therewas more carbonate deposition in proximal settings (Rhs,CB-101, UTQ-1) and contrarily it becomes marl-dominatedin distal settings (DE-1, DJM-1). Particularly, in the Rhssection and the surrounding area, Soussi et al. (1998) notedthat the TOC values in calcareous and laminated beds rich inbrachiopods (~2 m thick) are locally higher than in thesurrounding marl/carbonate alternations and varyingbetween 0.3% and 3% of TOC (Figure 8).

The TOC ranges vary significantly in different local-ities (Figure 2). Typical ranges observed include (1)spiky profile as well as sudden increases and decreasesof values with significantly higher range of TOC withinthe early Carnian interval in the type section of JebelRhéouis, (2) good TOC potential represented by intervalsranging between 0.2% and 3% (e.g. ROD-1, CB-101,DE-1, and Jebel Sidi Toui), (3) poor TOC intervals ran-ging between 0.2% and 0.6% (e.g. BK-1, DJM-1, KEA-5), and (4) very poor TOC content between 0.1% and0.25% (e.g. UTQ-1, which flowed in turn over 96% ofCO2) (Figure 2; Table 1). A close inspection of theRhéouis spiky TOC pattern (Figure 8) in combinationwith typical alternations of dark shales, organic-richlimestone, and dolomites with light beds highlights thesignificance of anoxic and dysaerobic environmentscycles during early Carnian deposition.

0 842 6

2000

0 250

Lagenella martiniTuvalian

Enzonalasporites vigens

Cor

devo

lian

- Ju

lian

Cor

devo

lian

- Ju

lian

APIGamma Ray TOC (%)

Base of organic-richness event

Top of organic-richness event

MFS

Ladi

nian

5200

5300

Kodiat El Alfa

(KEA-5 Well)

Jebel Rheouis

(Oued El Bhima section)

Cap Bon

(CB-101 Well)

EXPLANATION

Cor

devo

lian

- Ju

lian

0 180APIGamma Ray

MFS

Top of organic-richness event

Base of organic-richness event

5100

5000

Ear

ly C

arni

an

Bla

ck L

imes

tone

Mid

dle

Gyp

sum

Organic-rich dolomitic limestone alternating with darrk grey to black shale levels

Tuva

lian

Tuva

lian

Lower Gypsum (Tr1)

Lower Siltstone (Tr2)

Black Limestone (Tr3)

Middle Gypsum (Tr4)

Yellow Dolomites (Tr5)

Dolimite

Anhydrite, gypsum

Salt, gypsum, anhydrite

Limestone

Alternation dolomiteand shale

Siltstone

Black limestone, dolomites and shales

Bla

ck

lim

es

ton

es

600

20

700

800

40

60

80

100

Bla

ck

do

lom

ite

s

Patinasporites densus, Partitisporites novimundanus, P. maljawkinae, Pseudoenzo-nalasporites summus, Lagenella martini, Camero-sporites secatus

P. densus, P. summus,Vallasporites ignacii

Julia

n -

Cor

devo

lian

1400

1450

1500

1550

1600

0 150

Gamma RayAPI

Ear

ly C

arni

an

Bla

ck li

mes

tone

/dol

omite

Mid

dle

Gyp

sum

Tran

sgre

ssiv

e sy

stem

s tr

act (

TS

T)

Hig

hsta

nd s

yste

ms

trac

t (H

ST

)

TS

mfs

?

Gulf of Tunis

(ROD-1 Well)

MFS

MFS

MFS

Figure 8. Distribution of organic-rich early Carnian strata in four Tunisian sections in relation with available palynologic data andcharacteristic TOC as well as Gamma ray (GR) peaks (compiled from Couture and Marti 1965; Khessibi and Pacaud 1968; Soussi et al.1998; Soua 2007; Mehdi et al. 2009).

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The low TOC values characterizing some earlyCarnian sections/wells (Figure 2) could be explained bythe absence of the basal part of the early Carnian interval(typically organic-rich in the Oued El Bhima and Oued ElForka of the Rhéouis-type section) where the silty level(Tr2 of Burollet 1956) that directly underlies the organic-rich level is reportedly absent in several sections through-out the North–South Axis region and exceptionally withinthe western and eastern flanks of Jebel Rhéouis (Burolletand Dumestre 1952; Salaj and Stranik 1970; Ben Ferjaniet al. 1990; Chandoul et al. 1993; Soussi et al. 1996;Kamoun et al. 1998) and in northcentral as well as north-eastern Tunisia (Bonnefous 1967; Crampon 1971; Salajand Bajanik 1972; Biely and Rakus 1972; Burollet 1973;Boufares and Mekki 1984; Touati et al. 1995; Smaoui1996; Salaj and Mzoughi 1997). This explains to someextent the direct deposition of the organic-rich black stro-matolitic dolomites interbedded with black shales and few

gypsum levels on the lower gypsum unit. In addition, thevalues of TOC of less than 1% could be linked to theorganically leaner location of the logged sections/wells orprobably they were deposited beneath the OMZ (Figure 7;Barrett 1998; Soua and Tribovillard 2007). Figure 2 andTable 1 show good TOC potential for the Carnianrecorded in the Gulf of Tunis (Figure 9; Soua 2007). Infact, the Triassic section has been encountered only in theROD-1 well with a thickness of 2616 m. It has beendivided into three units (Boufares and Mekki 1984) ofLadinian to late Carnian age (Soua 2007, and referencestherein). The first unit (basal unit) is 1781 m of grey toblack sublithographic limestone alternating with greyanhydrite, black indurated shales, and indurated grey todark dolostones. The second unit is composed of blackdolostone interbedded with few anhydritic and blacksandy bioclastic limestone levels and black shales, withthe presence of siltstone levels. Finally, the third unit is an

300

600

900

1200

1500

1800

997

1924

Eq.

Por

to F

arin

a

Mid

dle/

Upp

er P

lioce

ne

328

Age Fm Lithology(m) (m)

10161089

145914951535

Ain

Gra

bE

l Har

ia

Tri

assi

cL

angh

ian

Lat

e P

aleo

cene

Y

pres

ian

Early Carnian Black limestone/dolomite

Rhéouis Formation

Middle Gypsum

Yellow dolomites

Julia

n -

Cor

devo

lian

1400

1450

1500

1550

1600

0 150

Gamma Ray(API)

Ear

ly C

arni

an

Rhé

ouis

- B

lack

lim

esto

ne -

M

iddl

e G

ypsu

m -

Y

ello

w d

olom

ites

Raoued -1 well

Figure 9. Lithology of the Raoued-1 well section and the organic-rich and calcareous early Carnian succession plotted against relatedGamma ray signature (after Soua 2007).

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anhydrite, dolomite, and shale sequence. Black dolostonesfrom 1555 m revealed TOC of up to 2.86% (Figure 2;Table 1). The early Carnian is revealed to be more carbo-naceous in Gulf of Tunis when compared with the NOSAsections and could be similar to the section described bySmaoui (1996) in the UTQ-1 well. The little difference canbe considered in the organic richness of the early Carnian(Table 1).

4.2. Biostratigraphy and sequence stratigraphy

The organic-rich ‘Black Limestones–Dolostones’ levelof the Rhéouis Formation was deposited during theEarly Carnian Anoxic Event (Aonoides to Austriacumzones), but this Black Limestone facies could in someregions continue up to the late Carnian (e.g. in ROD-1,Soua 2007). This reflects probably local upwelling con-ditions during black shale deposition. Burollet (1956)originally attributed the black limestones and shales inthe Jebel Rhéouis area (section Rhs) to the ‘Muschelkalk’based on collected oysters and venerids bivalves(reported also in Salaj 1978); however, this stratigraphicinterpretation was later re-evaluated by Soussi et al.

(2001) who by means of palynology and geochemistrydemonstrated that this level is characterized by apalynomorph assemblage formed by Patinasporitesdensus, Partitisporites novimundanus, P. maljawkinae,Pseudoenzonalasporites summus, Lagenella martini, andCamerosporites secatus, which indicates early Carnian age(Julian). Alternatively, Kamoun et al. (2001) assigned tothis organic-rich level a Ladinian to Carnian age on thebasis of benthic formaminiferal assemblage (F3 a/b) con-stituted essentially of (1) F3a, Triadodiscus eomesozoicus,Lamelliconus sp., Endotrianella wirzi, and Acicularia sp.assemblage (Mekraneb dolomite, southern Tunisia), (2)F3b, Aulotortus ex. gr. Praegaschei, T. eomesozoicus, andAcicularia sp. assemblage (black limestones, centralTunisia) (Figure 10).

Similarly, Castany and Degalier (1956) attributed theTriassic series outcropping in Koudiat El Alfa (KEA, seeFigure 2 for location) to the ‘Keuper’; then Kamoun et al.(1998), by means of benthic foraminiferal assemblages,assigned a Carnian age to the section. The neighbouringwell (KEA-5) has been subject to a palynological investi-gation by Khessibi and Pacaud (1968), who assigned aMiddle to Late Triassic age to the succession and noted

Carn

ian

Julian

Tuvalian

Aonoides

Austriacum

Dilleri

Subbullatus

Anatropites

AonAonides

Triadicum

"Oedipus"

Dilleri

Crasseplicatus

Subbullatus

Plinii

Italicus

F3

F4

Duplicisporites continuus

Aul

ispo

rites

-A

ratr

ispo

rites

Acm

e Z

one

Aulisporites astigmosus

Aul

ispo

rites

-Ara

tris

porit

es A

cme

Zon

e

Dup

licis

porit

es c

ontin

uus

asse

mbl

age

(Lat

e Ju

lian)

(

Aus

trot

rach

ycer

as a

ustr

iacu

s S

ubzo

ne)

3000

2750

2321

2511

Uni

ts 4

to 5

Uni

ts 1

to 7

Uni

ts 2

to 4

DJM-1 DE-1

MFS

5200

5300

Jebel Rheouis CB-101

EXPLANATION

Cor

devo

lian

- Ju

lian

MFS

5100

5000

Ear

ly C

arni

an

Bla

ck L

imes

tone

Mid

dle

Gyp

sum

Tuva

lian

Dolimite

Anhydrite, gypsum

Salt, gypsum, anhydrite

Limestone

Alternation dolomiteand shale

Siltstone

Black limestone, dolomites and shales

20

40

60

80

100

Bla

ck

do

lom

ite

s

Julia

n -

Cor

devo

lian

1400

1450

1500

1550

1600

Ear

ly C

arni

an

Bla

ck li

mes

tone

/dol

omite

Mid

dle

Gyp

sum

mfs

Lagenella martiniTuvalian

Enzonalasporites vigens

Cor

devo

lian

- Ju

lian

Ladi

nian

KEA-5

600

700

800

P. densus, P. summus,Vallasporites ignacii

?

ROD-1

Cor

devo

lian

- Ju

lian

Tuva

lian

Lower Gypsum (Tr1)

Lower Siltstone (Tr2)

Black Limestone (Tr3)

Middle Gypsum (Tr4)

Yellow Dolomites (Tr5)

Patinasporites densus, Partitisporites novimundanus, P. maljawkinae, Pseudoenzo-nalasporites summus, Lagenella martini, Camero-sporites secatus

Aul

ispo

rites

-Ara

tris

porit

es A

cme

Zon

e

Triassic system Ammonites BF Palynoflora

Buratti et al. (2012) Mzoughi et al. (1994) Soussi et al. (1998) Mehdi et al. (2009) Soua (2007)

Figure 10. Biostratigraphic calibration of the organic-rich strata on the basis of palynology and benthic foraminifera. BF: benthicforaminifera (zones of Peybernes et al. 1993; Kamoun et al. 1998, 2001); palynoflora (zones of Mehdi et al. 2009; Buratti et al. 2012).

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that typical palynomorphs occur in the lower part of theseries. Later, Mehdi et al. (2009) noted that the lowersuccession is characterized by the occurrence of lateLadinian–Carnian sporomorphs (Praecirculina granifer,C. secatus, Enzonalasporites vigens, Duplicisporites gran-ulates, and P. novimundanus) associated with typicalCarnian palynomoprphs such as P. densus, P. summus,and Vallasporites ignacii, L. martini, and Aulisporitesastigmosus, confirming the palynologic investigation ofKhessibi and Pacaud (1968). This age calibration is con-strained by the fact that L. martinii is restricted to theJulian–early Tuvalian interval (Litwin et al. 1991;Hardenbol et al. 1998; Roghi 2004) and A. astigmosusgenerally belongs to early Carnian assemblages (Hochuli1998).

In addition, the early Carnian of the ‘Dorsale’ area ismarked by alternating marls, limestones, and laminatedblack shales containing bioclastic debris, radiolarians,ostracoda, and scarce benthic foraminifera (Kamounet al. 2001). Biely and Rakus (1972) and Salaj (1978)described several unconformities marked by angular dis-cordances within the Triassic of the area.

In Jebel Baouala–Lansarine (Bl, Ln sections), Kamounet al. (2001) described very thin (from 2 to 8 m) massiveblack dolomites and attributed early Carnian age on thebasis of Tunethyris punica. So the age of these BlackLimestones–Dolostones is now well documented as earlyCarnian. However, earlier, Adil (1993) as well as Adil andAlouani (1996) considered these black dolomites in JebelBaouala as being lower Liassic without providing anybiostratigraphical argument, although they are overlainunconformably by an Upper Triassic sandstone sequence.The fossiliferous pelecypod-bearing dolomites have beendocumented also by Calzada et al. (1994). The distributionand extension of these black carbonates revealed more thandescribed earlier by Burollet (1973) since they have beencharacterized in the west of the Dorsale (Kamoun et al.2001) and in north-central (e.g. Ch, Bl, Ln sections) as wellas in northeastern Tunisia (e.g. ROD-1 well) (Perthuisot1981; Kamoun et al. 2001; Soua 2007) (see Table 1 forsection and well codes). Along the North–South Axis, theseblack limestones are very well documented, especially inthe type section Jebel Rhéouis (Rhs section). They arereported as early Carnian black shales (Soussi et al. 2001;Kamoun et al. 2001; Mehdi et al. 2009).

According to Scallon (1980) and Gueinn (1980), euxi-nic conditions in the Ben Kheir area (see Figure 2 forlocation) were diachronous at both the base and the top.They distinguished a typical facies of Carnian black shaleand shaly limestone based on the palynomorph assemblageconstituted of Classopolis torosus, Duplicisporites cf. gran-ulates, Alisporites sp. Classopollis spp., ?Spheripollenitessubgranulatus, Triadispora sp., and ?Concavisporites sp. inthe interval between 3532 and 3812 m. The sedimentationis represented by an organic-rich and condensed euxinic

section, suggesting the development of restricted circulationwithin ‘mini-basins’ (Bedir 1995). This is confirmed byseismic studies over the same basin by Bedir (1995), whodefined laminated and well-stratified Carnian carbonatestermed T3 (Figure 5). Generally speaking, the earlyCarnian black shales contain dwarfed and impoverishedbenthonic fauna as well as pelycepods, lamellibrachs, gas-tropods, brachiopods, and ammonites (Burollet 1956;Calzada et al. 1994; Soussi et al. 1996; Kamoun et al.,1998). However, the dysaerobic conditions in the Gulf ofGabes (DE-1, DJM-1 wells, e.g. Mzoughi et al. 1994; Ha2009; Buratti et al. 2012) appear to have begun at thesame time (Mzoughi et al. 1994) but seem to have persistedmuch longer through the Carnian here than in centralTunisia (Buratti et al. 2012). Especially within this area(Figure 2, for location), the oxygen depletion is assumedto have prevailed during almost all of the Carnian, whereasit was limited to the early Carnian in Central Tunisia only afew hundred kilometres away. This is inferred from thebiostratigraphic investigation on the DE-1 well made byMzoughi et al. (1994), who defined Units 4 and 5 (Figure5). These units are characterized by the distribution oftypical Carnian species represented by Camerosporitessecatus, Triadispora sp., Voltziaceaesporites heteromorpha,Aratrisporitespalettae, and Calamospora tener withinshale, black dolomite, and shaly limestone succession.

The minimum duration of the organic-rich facies in theinvestigated Triassic sections (e.g. Rhs, DE-1, DJM-1,KEA-5, BK-1 ROD-1) includes the whole Aulisporitesastigmosus assemblage and the Lagenella martini assem-blage of Roghi et al. (2010) (Figure 10). Soussi et al.(2001) denoted the first occurrence of Vallasporites igna-cii, Granuloperculatipollis rudis, and Patinasporites den-sus in the basal part of the organic-rich Carnian sedimentsfrom the Rhs section at 227.40 Ma (Hardenbol et al.1998). Alternatively toward the northeast, in the KodiatEl Alfa (KAE-5 well), situated a few kilometres from theRhs section, Mehdi et al. (2009) denoted the lastappearance of Gliscopollis Meyeriana (at 226.07 Ma),Aulisporites astigmosus, and Lagenella martini (at224.74 Ma) in the upper part of the black limestone-levelequivalent. The same palynological scheme has beenpresented in the Gulf of Gabes (DE-1 and DJM-1 wells)by Mzoughi et al. (1994) and Buratti et al. (2012).This reasoning implies an estimated duration of ~2 millionyears. Toward the northeast, in the Gulf of Tunis, theCarnian section from the ROD-1 well yielded benthicforaminiferal assemblage composed of Glomospira sinen-sis, Glomospirella vulgaris, Triadodiscus eomesozoicus,Endotriadella wirzi, Ammodiscus parapriscus, Aulototuspraegaschei, and ostracods (Simeonella sp.; Figure 10).The whole Triassic section in the ROD-1 has beenassigned to the Ladinian–Carnian stage.

The onset of deposition of the black limestone level ofthe Rhéouis Formation generally coincides with the base of

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the early Carnian Transgressive Systems Tract (TST) (Haqet al. 1987; Vail 1988; Peybernes et al. 1993; Chandoulet al. 1993; Hardenbol et al. 1998; Soussi et al. 2001;Kamoun et al. 2001) of a major third-order Tethyan eustaticsea level (Haq et al. 1987; Vail 1987; Hardenbol et al.1998). A similar sequence stratigraphic interpretation waspresented by Soussi et al. (1996). Particularly in the Rhssection, the sequence boundary (SB) is confused with thetransgressive surface (TS), probably due to an underlyingdiscontinuity (Kamoun et al. 1998).

To correlate the TST we used the gamma ray (GR) andthe organic carbon potential (TOC) patterns of the KEA-5,CB-101, and ROD-1 wells and the Rhs section, respec-tively (Figure 8).

The third-order Highstand Systems Tract (HST) over-lying the organic-rich early Carnian TST along the North–South Axis is generally represented by shallower deposits,such as gypsum, thin ostracod-bearing white limestonebeds, dark grey marls associated with lignitic debris, andfine-grained sandstones (e.g. Burollet 1956; Chandoulet al. 1993; Soussi et al. 1998; Kamoun et al. 2001). Inthe Tellian domain, thick stromatolitic limestonesinterbedded with calcareous tempestites of the BechateurFormation belonging to the middle Carnian HSTwere deposited in the area (Bechateur, Ras El Koran,Figure 5) and probably in Ichkeul and Hairech meta-morphic structures in northern Tunisia (e.g. Bolze 1954;Ghorabi and Henry 1992; Kamoun et al. 2001), implyinga pronounced diachroneity of this lithostratigraphic unit.However, toward southeastern Tunisia, in the Jeffara andsouth Dahar area, the Black Limestones–Dolostones arereplaced locally by yellow bioturbated dolomite, termedthe Mekraneb Formation by Bouaziz (1986) or Dolomitesinférieures Formation by Busson (1967), which exhibits afossiliferous and glauconitic level at the top, marking theflooding surface (mfs). In the Sidi Toui area (Figure 2),Arfaoui and Montacer (2004) described early Carnianorganic-rich platy dolomitic and limestone levels thatmark the TST. This interval is overlain by greenish ammo-noid-bearing shales marking the HST, which could corre-late with the Middle Gypsum unit in northcentral Tunisiaand to the stromatolitic limestones in the Bechateur, RasEl Korane, Ichkeul, and Hairech localities (Figure 5).

Unfortunately, no data are available from the offshoreGulf of Hammamet or the Mediterranean. Figure 6 is aN–S seismic line and interpreted section in the Gulf ofTunis, near the Dorsale and the Northern Tunisian trough.Early Carnian black shales may also be present within thisbasin on the basis of biostratigraphy from the ROD-1 well.

4.3. Molecular geochemistry and environment ofdeposition

Detailed organic geochemical description of the earlyCarnian black limestones of the Rhéouis Formation and

available early Carnian source rock data from outcropsand wells have been provided by Scallon (1980), Touatiet al. (1995), Smaoui (1996), Soussi et al. (1996), Arfaouiand Montacer (2004), and Soua (2007). Organic-richCarnian black shales are characterized by mixed-typeII/III kerogen, i.e. planktonic marine type II and ligneousand hemicellulosic continental type III kerogen (HI ran-ging between 50 and 250 mg HC/g TOC; OI between 30and 70gCO2/gTOC) with TOC concentrations of up to7.5%, indicating excellent source rock qualities for oiland gas. A sufficient maturity level of these laminatedblack shales is confirmed by Tmax values between 429°Cand 446°C (Figure 11). Organic petrological and palyno-logical studies on these sediments indicate kerogen domi-nated by dark brown to black colour fluorescent organicmatter associated with amorphous organic matter (AOM)(hemicellulosic continental origin). The higher molecularweight n-alkanes reveal a wide range of normal alkanes.The moderate molecular weight n-alkanes and acyclicisoprenoids such as pristine and phytane occur in lowconcentrations in the early Carnian black shales. In addi-tion, they display a smooth homologous series within thehigher molecular weight n-alkane, suggesting a significantinput of higher land plant organic matter into these sedi-ments. The presence of pristine and phytane in lowamounts within these sediments, inferred from the Pr/Phratios, is indicative of a reducing environment (anoxiccondition) and reflect significant input from the chloro-phyll of higher plants, algae, and photosynthetic bacteriaand from archaeobacteria (Cooper 1990). This low Pr/Phratio in the early Carnian organic-rich black shales sug-gests also an aquatic depositional environment under redu-cing bottom conditions (Burgan and Ali 2009). Theinvestigation of triterpanes and steranes within organic-rich Carnian samples taken from Jebel Rhéouis and JebelSidi Toui was performed by Arfaoui and Montacer (2004).Biomarker maturity parameters revealed that this organic-rich series has been subject to deposition within reducingconditions in both areas.

5. Discussion and outlook

5.1. General remarks

The Ladinian–lower Carnian drastic lithological changefrom carbonates (including local organic-rich levels) togypsum, clastic, and terrigenous sedimentation has beenwidely characterized in western Tethyan margins (Golonka2007) and grouped under the Reingraben event (Schlagerand Schöllnberge 1974), which characterizes the well-known CPE (Simms and Ruffell 1989).

In Jebel Rhéouis (Oued El Bhima, Oued El Forka, andKsar Rechaiech sections, Soussi et al. 1996; Kamoun et al.1998), the organic-rich shales and carbonates of the earlyCarnian are represented essentially by fosiliferous beds rich

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in (1) bioclasts of lammellibrachs, gastropods, and crinoids,(2) microfaunal assemblages represented by radiolariansand benthic foraminifera, and (3) palynomorphs representedgenerally by circumpolles species and monosaccate forms.Black dolomites, limestones, and organic-rich shales arerich in framboidal pyrites (Soussi et al. 1996). Thissequence overlies bioclastic carbonates and is overlain inturn by a gypsum, shale, and fine-grained sandstone succes-sion witnessing the demise of a supposed carbonate plat-form characterizing the western Tethyan margin (Figure 1).However, remnant carbonate levels in the overlying Carnianseries are documented. These black shales and limestonesrich in framboidal pyrites are documented also in the Gulf

of Tunis within the ROD-1 well (Boufares and Mekki1984), in the CO2-bearing Carnian level of the UTQ-1well (Smaoui 1996), as well as in the Cap Bon (CB-101well, Touati et al. 1995).

In general, pyrite in modem anoxic environments isalmost exclusively represented by framboidal form (e.g.Vallentyne 1963; Schallreuter 1984; Oenema 1990).Ancient anoxic sediments are also documented as rich inframboid pyrites such as in the early Silurian hot shales,Frasnian Kellwasser level, and Cenomanian–TuronianOAE-2 successions (e.g. Riquier et al. 2006; Lüninget al. 2000; Soua and Tribovillard 2007; Soua and Chihi2014; Soua 2014).

Figure 11. Early Carnian maturation zones (Tmax). References for maturation are listed in Table 1.

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In the Tunisian early Carnian case study, the demise ofthe carbonate platform, which is recognized by the onsetof the generalized anoxic conditions inferred from thedeposition of black limestones, dolomites, and shales,does not seem to have occurred suddenly as documentedin the western Tethyan margin (Figure 12; Hornung et al.2007). For the western Tethys, terrigenous input has beensuggested as the factor primarily responsible for thedemise of carbonate platforms and reefal constructions inassociation with the increased salinity related to the con-tinuing break-up of Pangea, extensional movements insome parts, as well as to the increased CO2 emissionrelated to the intense volcanic activity as inferred fromthe northwestern African margin (Tunisia, Morocco,Guinea, Mali, Mauritania) (Weigand and Ragland 1970;Biju-Duval et al. 1977; Bertrand and Coffrant 1977;Laubscher and Bernoulli 1977; Manspeizer et al. 1978;Bertrand et al. 1982; Kurtz 1983).

A monsoonal circulation system has been adopted byParrish (1999) to explain the climatic model of the Carnian inthe Tethyan realm associatedwith a rapid tectonic subsidencerate due to isostatic equilibrium (Garzanti 1999). Theseeffects may be related to the general Tethyan palaeogeo-graphic conditions characterized by the accelerated Pangeabreak-up and the northward separation of the Cimmerianplates from northern African margins (Figure 1).

The Tunisian shelf, which is characterized by thedeposition of a widespread carbonate platform andorganic-rich black limestone and shales in the earlyCarnian with high organic carbon potential up to 7.5%of TOC (in the Rhs section, Soussi et al. 1998) (Figure 2),has been subject to well-stratified ocean water. This

stratification is the primary cause of separating oxygenatedsurface water from anoxic and maybe euxinic deeperwater (e.g. Brumsack 2006; Soua and Tribovillard 2007;Soua 2011). Humid climatic conditions for this time per-iod have been inferred from DJM-1 well by Buratti et al.(2012), who admitted that the early Carnian sporomorphvegetation was composed essentially of bennettitales,cycadales, lycopodiophyta, and equisetophyta. This hygro-phytic assemblage is believed to have been in need ofhumid conditions for reproduction (Visscher et al. 1994;Vakhrameev 2010). This is in concordance with the pre-vailing regional humid and monsoonal climate conditiondefined by Simms and Ruffell (1989) as the CPE and welldocumented in the Tethyan realm (Hochuli and Frank2000; Hornung and Brandner 2005; Hornung et al. 2007;Kozur and Weems 2007; Rigo et al. 2007; Kozur andBachmann 2010). A less-diversified assemblage has beenreported from DE-1 well (Mzoughi et al. 1994), fromJebel Rhéouis (Soussi et al. 2001), and from KEA-5core-drill (Mehdi et al. 2009), which could confirm theextension of the CPE to central Tunisia, albeit with lessstrength than recorded in DJM-1 well (Buratti et al. 2012).

This humid climate in association with the monsoonalactivity (Parrish 1999) would therefore have intensifiedthe water stratification and caused oxygen-depleted(O2-depleted) conditions due to the increased run-off andincoming nutrient.

In addition, and referring to Roghi et al. (2010), thisCarnian anoxic event has been widely characterized espe-cially in the northwestern Tethyan margin and at highlatitudes (Figure 12), which could be a key of correlationto the North African margin.

Tunisia

Algeria

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Libya Egypt

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Figure 12. Present geographic distribution of the main studied sections around the early Carnian (Julian/Tuvalian boundary) showingthe generalized anoxic event. (Redrawn after Roghi et al. 2010).

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5.2. Depositional model

With the proposed depositional model (Figure 7), the sedi-mentation of the Carnian carbonate platform in the Tunisianshelf setting seems to have been complex. It has beendocumented in early studies that this carbonate platformmay continue up to the middle Carnian in Bechateur(Béchateur Formation, Kamoun et al. 2001), the equivalentof the Middle Gypsum unit of Burollet (1956), as well as inUtique (UTQ-1), Gulf of Tunis (ROD-1), and Cap Bon(CB-101), where organic-rich sedimentation continuedthrough the middle Carnian (Boufares and Mekki 1984;Touati et al. 1995; Smaoui 1996; Soua 2007).

The proposed depositional model implies a regionalstructural framework with strongly distensional trendsinduced by the northward separation and movement of theCimmerian plates from the north African margin producinghalf-grabens. This model is associated with the regionalpluvial event (Simms and Ruffell 1989), which triggeredand increased riverine influx due to the enhanced run-offstimulating high productivity in surface waters. Theincreased humidity may be the consequence also of theenhanced CO2 emission associated with the tholeiitic volca-nic activity such as that recorded in northern and northcentralTunisia in the Early to Late Triassic (e.g. Kurtz 1983).

The deposition of the organic-rich black shales hasoften been associated with the intensification and expan-sion of an OMZ along the southern Tethyan marginmainly for the Cenomanian–Turonian OAE-2 model(Barrett 1998; Lüning et al. 2004; Brumsack 2006;Turgeon and Brumsack 2006; Soua and Tribovillard2007; Soua 2011). In most cases, the deposition of theseorganic-rich black shales is accompanied by initial trans-gression prior to and/or within short-lived severe anoxicevents. Taking into account this depositional model, dur-ing early Carnian OMZ development periods, black shaleaccumulated throughout the northern margin of Africa.Oil/gas-prone organic-rich strata have been deposited inthe initial marine sediments of narrow rifts and in parts ofthe subsiding margin (Figure 7).

It is noteworthy that the organic-rich early Carnianlimestones, dolostones, and shales are the principal hydro-carbon source in the Cap Bon peninsula (Touati et al. 1995)and the principal source of CO2 in UTQ-1 (Smaoui 1996).In addition, good hydrocarbon potential has been recentlyoutlined in the Gulf of Tunis in ROD-1 with 2.86% of TOC(Soua 2007). The area underlying the OMZ is characterizedby yellow to white carbonate accumulation, which is thecase of the early Carnian carbonate platform of Bechateurand Ras El Korane (Figures 5 and 7).

Following these observations and the depositionalmodel presented in this paper, we propose to separate theblack limestone member (black limestone, dolostone,shale, or mixed carbonate) from the Rhéouis Formationfor the following reasons:

● The Rhéouis Formation has been defined byBurollet (1956) based on its evaporitic Triassic suc-cession (Burollet 1973; Salaj 1978; Chandoul et al.1993).

● The Triassic system in northern and central Tunisiahas long been divided by geologists on the basis offacies (Keuper, Muschalkak, Alpin, Germanic, etc.)and not on the basis of age constraints, althoughsome stratigraphic progress has been made since theworks of Soussi et al. (2001) on palynomorphs aswell as Kamoun et al. (2001) on benthic foraminifera.

● The Carnian in northern and central Tunisia hasexceptionally recorded the Tethyan palaeogeo-graphic events when compared with neighbouringsouthern Tethyan margin localities in Algeria andMorocco. This sequence is revealed to be similar toseveral sections logged in the northwestern Tethyanmargin such as in Germanic, southern Alps, south-ern Apennines, and Po basins where a thick carbo-nate platform formed including early Carnianorganic-rich shales proven as good source rocks(Stefani and Burchell 1993). A similar sequencehas been described by Touati et al. (1995) in theCap Bon, where the Carnian source rock potentialhas been confirmed using oil/oil correlation and inthe Utique area (Smaoui 1996) where the Carnianblack carbonate flowed over 96% of CO2. Thissequence seems to extend into the Gulf of Tunisbasin with good source rock potential (2.86% ofTOC, Soua 2007).

● The depositional sequence of the Carnian in the dif-ferent studied sections and wells (Table 1), such as inJebel Rhéouis, Koudiat El Alfa, CB-101, ROD-1, andJebel Baouala-Lansarine, is very different from theoverlying series (consisting generally of gypsum, sili-ciclastics, paralic, and continental series).

These arguments seem to be sufficient to revise theTriassic series and give new nomenclature to the differentintervals instead of Rhéouis Formation, which could begraded up to a group rank. We propose the term RaouedFormation for the organic-rich Black Limestones–Dolostones level inferred from the ROD-1 (Raoued-1)well, because it shows a representative section with expli-cit GR signature (Figures 8 and 9).

6. Conclusions

The review of the early Carnian black shales in northern,central (Raoued Formation) as well as in southern(Mekraneb Formation in Jebel Sidi Toui) Tunisia centreson probable highest quality Triassic source rock of thecountry. The deposition of the organic-rich black shaleshas often been associated with intensification and expansion

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of an OMZ along the southern Tethyan margin. In mostcases, the deposition of these organic-rich black shales wasaccompanied by initial transgression resulting from differ-ent mechanisms (accelerated separation of the Cimmerianfragments, distensive regime, rapid tectonic subsidence,climatic pluvial event, monsoonal circulation system, CO2

emission related to tholeiitic volcanism, mixing mechan-isms, etc.) prior to and/or within a short-lived severe anoxicevent. Although this anoxic event is recorded within theRaoued and locally in Mekraneb Formations and detectedby means of geochemistry (TOC) and wireline logs (mainlyGR), unfortunately, no large-scale carbon-isotopic investi-gation or high-resolution biostratigraphic analysis has beenconducted, as required to correlate these worldwide events.The biostratigraphy based on benthic foraminifera and paly-nological investigations has been the subject for the wholeTriassic successions and the early Carnian section did nottake the whole chance of high-resolution biostratigraphicscheme. Dysoxic to anoxic conditions have been interpretedusing molecular geochemical ratios where the presence ofpristine and phytane in low amounts, inferred from thePr/Ph ratios, is indicative of a reducing environment andreflect significant input from the chlorophyll of higherplants, algae, and photosynthetic bacteria. Although pri-mary productivity also could be interpreted from lithologi-cal and faunal content, carbon isotopic investigation has tobe undertaken and coupled with chemostratigraphic analy-sis (major and trace elements) to allow a worldwide corre-lation between the southern and northern Tethyan margins.

AcknowledgementsI would like to thank Professor Richard T.J. Moody (KingstonUniversity London) who provided a thorough review and editedthe English language of the text and an anonymous reviewer forproviding numerous suggestions, which improved the text. Theuseful constructive reviews and suggestions as well as the editor-ial work by the chief editor Robert Stern is much appreciated.The author is also grateful to the Faculty of Sciences at TunisUniversity staff that facilitated the field trip to Jebel Rhéouis andsurrounding area in March 2013 and the fruitful discussions withWalid Ben Ali. Thanks are also given to ETAP (Tunisia), whichfinanced the field trip to Jebel Sidi Toui and the whole Dahar-Jeffara basin in April 2010. Fruitful discussions with OussamaEchihi and Sofien Haddad (ETAP) are also much appreciated.

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