PROCEDINGS OF THE 10TH TUNISIAN PETROLEUM EXPLORATION & PRODUCTION CONFERENCETUNIS, OCTOBER 13th 18rd,2008

Abstract

High-resolution chemostratigraphic records for the Late Cenomanian-Early TuronianBahloul Formation in five spaced localities (Tunisia) provide new insight into thepalaeoceanographic evolution of the southern Tethyan margin. Based on geochemicalproxies (V/(V+Ni), Si, Ti, U, D*, Ba, Cr), the Bahloul is interpreted as deposited within a highproductivity and warm and humid climatic conditions. Enhanced humidity triggered probablyriverine influxes, resulted in a sluggish water circulation and consequent anoxic/euxinicconditions favouring the preservation of organic matter at the bottom.

Introduction

During the Late Cenomanian-Turonian times, Tunisia situated at the southern Tethyanmargin was characterised by a general slope almost dipping to the north, promoting variablefacies. The general facies trend was related to a complex tectonic and halokinetic palaeorelief(Bishop, 1988; Perthuisot, 1981). Organic-rich C/T sediments occur in three main provinces,northern, central and central/southern Tunisia (Fig. 1) and are grouped into the BahloulFormation originally defined by (Burollet et al., 1952; Burollet, 1956). Five spaced sectionswere sampled, the Jerissa [JR] and Guern Halfaya [GH], Jebel Kherij [JK] (in the NorthernProvince); Bargou [BR] (Central provence) and Oued Hessi [OH] (southern province) (Figure1;Fig. 2).

Structural setting

The Cenomanian-Turonian organic-rich strata in onshore Tunisia/Algeria and offshoreTunisia are mainly found in the first province and the distribution of the organic contentis strongly controlled by halfgraben systems (Fig. 1) and in some areas by Triassic salt diapiricmovements during the Cretaceous (Perthuisot, 1981; Soua and Tribovillard, 2007; Soua etal., 2009).

The rift grabens formation is related to the opening of the Neotethys to the north andCentral Atlantic to the southwest.

Notably, this rifting in onshore and on the Shelf of Tunisia/Algeria did not begin beforeLate Jurassic-Early Cretaceous. However, late Triassic-Early Jurassic rifting is in fact associatedwith the separation of the eastern part of the Apulian micro-plate block related at thattime to the northern African Plate.

RIVERINE INFLUXES, WARM AND HUMID CLIMATIC CONDITIONS DURING THE LATEST CENOMANIAN-EARLY TURONIAN BAHLOUL DEPOSITION

SOUA Mohamed1, TURKI ZAGHBIB Dalila2, TRIBOVILLARD Nicolas3

1 Entreprise Tunisienne d'Activités Pétrolières, Charguia II, Tunisia2 Faculté des Sciences de Tunis, Département de Géologie3 UMR 8110 et FR1818, Sciences de la Terre, université Lille-1bâtiment SN5, 59655 Villeneuve-d'Ascq cedex, France

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So, two main Mesozoic rifting events occurred in this northern African margin (i.e. Algeriaand Tunisia), namely (1) the Late Jurassic to Early Aptian, associated with E-W-trendinghalfgrabens and related volcanism e.g. in the Pelagian block, and (2) from the post-Aptianto Early Cenomanian, associated with NW-SE trending halfgrabens (Soua and Smaoui, 2008;Soua et al., 2009).

These two recorded extensional phases were interrupted by a regional top-Aptiancompressional phase (the so-called Austrian phase) which formed the typical unconformitydetected in seismic profiles and in outcrops. The general graben trends are locally complicatedby other faults (the Cenomanian NNE-SSW trend faults, and Turonian ENE-WSW trendingfaults) which have partly dissected the grabens into smaller segments.

Commonly, around C/T times a general change from extension to compression occurredin North Africa, which was related to the closing of the Neotethys and the onset of NorthAtlantic rifting. This resulted in the inversion of the former rift grabens from the precedingextensional phase. However, some authors assume that the C/T period was characterizedby extension activities associated with a transtension and pullapart movement.

Extensional Infracretaceous tectonics was used to explain Cenomanian-Turonianhalfgraben tectonics, resulting in fragmentation into small tectonic blocks.

Fig. 1: Distribution and properties of Cenomanian-Turonian organic-rich Bahloul in Tunisia and NE Algeria.Data compiled from Askri et al. (1995), Delsol (1995), Busson and Cornée, (1996), Busson et al. (1999),Bishop (1988), Ben Ferjani et al. (1990), Soua and Tribovillard (2007). GH: Garn Halfaya, JR; Jerissa, BR:Bargou; OH: Oued Hessi (Gafsa)

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Some authors assume that the rift movements were somehow blocked or were restrainedduring the anoxic Cenomanian-Turonian deposition.

In northwestern and central Tunisia as well as on the Pelagian Province, Cretaceousdiapiric movements of Triassic salt locally also played a role in controlling C/T deposition(Perthuisot, 1981, Camoin, 1991, Bédir et al., 2001). They are characterized by a markedthickness reduction and partly by development of a detrital level (sandstones, conglomerates).The diapiric rise, starting from Aptian (and even before) to approximately middle Eocenewas probably continuous, but it increased during periods of tectonic instability. Thus earlierdiapiric movements and rise-up are super-imposed to the extensional features favouringdepocenters individualisations in the central parts of rim-synclines. The peridiapiric seriesare characterized by a marked thickness reduction (Perthuisot, 1981).

Biostratigraphy

The Bahloul Formation was deposited starting from the latest Cenomanian within theuppermost Rotalipora cushmani foraminiferal Zone to earliest Turonian, base or near theHelvetoglobotruncana helvetica Zone. Especially the upper limit of the unit is known to bediachronous and its age ranges from the Whiteinella archaeocretacea archaeocretacea Zoneto the lower H. helvetica Zone (Maamouri et al., 1994; Soua, 2004; Soua, 2005; Soua andTribovillard, 2007). Two different radiolarian assemblages were pointed out in BR section,one late Cenomanian assemblages dominated by multi-segmented nassellarians, and aTuronian one characterized by higher S/N values (Soua et al., 2006). Between these two

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Fig. 2: Distribution of values of geochemical proxies throughout four Bahloul sections superposed to thebiostratigraphic data; light grey band is the significant identified spikes

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assemblages, some species become pseudo-extinct due to diagenetic process and/orpreservation.

Trace and major elements chemostratigraphy

In order to compensate for organic carbon and carbonate dilution, element concentrationswere normalized to Al.

4.1. Detrital flux variations

In order to evaluate the detrital inputs into the sedimentary basin, Machhour et al.(1994) have determined the D* geochemical relationship, that is expressed by Al/(Al + Fe+ Mn), as a proxy. In the northern province (GH section) (Figure1), the mean D* value is0.63 in the lower part of the Bahloul limestones (Fig. 2) and increases through the upperBahloul exceeding 0.71 against 0.55 in typical terrigenous shales Wedepohl (1978). In thesouthern province (OH section) increased D* in the organic-rich interval (a mean value ofabout 1.94) indicates an enhanced rate of terrigenous supply to the sediment during theBahloul deposition. However, Ti/Al ratios show a clear decline through the five Bahloulsections (Figure 2). This different behaviour can be explained by the fact that D*, is generallyrelated to fluvial contributions, while Ti/Al indicates aeolian inputs (Wehausen and Brumsack,2000).

4.2. Bottom-water redox conditions

The V/(V + Ni) ratios and Cr could indicate redox conditions during the Bahloul deposition.The V/(V + Ni) curve data, show range of 0.80-0.36 in the northern province; 0.37-0.93 inthe central province; 0.33-0.70 in the southern province (Fig. 2). However, Hatch andLeventhal (1992) assume that 0.54-0.80 interval, indicate anoxic conditions. Some highervalues, (> 0.80) are suggestive of euxinic conditions, while values situated beneath 0.54,are interpreted as oxic conditions at the bottom. The enrichment of the Cr/Al average shaleindicate an algal source of the organic matter, which concentrates Cr, of higher redoxpotentials and consequent availability of reduced Cr3+ to react with organic matter (Hatchand Leventhal, 1992).

4.3. Primary productivity-sensitive elements

Barium can be indicative of surface biological productivity fluctuation. The Bahloulshows moderate to high Ba/Al ratios with an average value of 30 in GH, JR and BR sections(Fig. 2) against a mean of 196 measured for OH section situated in the southern province.Deposition of the Bahloul sediments within the oxygen minimum zone (OMZ) (Soua andTribovillard, 2007), which is consistent with results from redox-sensitive elements, impliedmoderate to high degree of barite saturation. Si/Al ratio is also considered as a potentialproxy for primary productivity, which across the Bahloul in BR section assumes particularlyhigh values, up to 37 and a mean value of about 9 against 3.39 in the average shale (Grometet al., 1984), and 3 in the remaining sections. This is probably due to specially abundantradiolarians in cherts, cherty black shales (BR section) (Soua et al., 2006).

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4.4. Packages

Based on the chemostratigraphic data, the Bahloul Formation has been subdivided into three/four main Packages, each with a number of Units and Subunits based on thecorrelation of key elemental ratios. The methodology follows that described by Pearce etal. (2005). An example of the chemical stratigraphic zonation for the Bahloul sections isshown in Figure 3, where three/four of the Packages and their units are highlighted. Thechemostratigraphic data were generated at sample intervals of 30 cm. Correlations wererelatively consistent between sections and were used to complement semi-regionalcorrelations based on biostratigraphic data and well-log stacking patterns (Fig. 2, Fig. 3).

Gamma ray was reproduced from Th, K and U profiles of each section.

Conclusion

Depth profiles of selected geochemical indicators for the Bahloul Formation (Figure 2;Figure 3) are presented and interpreted in terms of changes in detrital delivery to thesediment, primary productivity and basin-bottom redox conditions during the time of theBahloul deposition.

Higher D* values concurrent with lower Ti/Al ratios are interpreted as caused by enhancedriverine material contribution, due to more humid climate during the OAE2.

As a consequence of the efficient circulation mode, recycling of nutrients from deeperwaters could have been more marked and coinciding with enhanced siliceous, carbonateand organic matter production as testified by relatively higher Si/Al.

Associated lower V/(V + Ni) ratios and Cr concentrations could testify of an increase inthe oxygen supply to deep waters resulting in enhanced sinking organic matter to beoxidized.

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Fig. 3: Correlation of geochemical elements and Gamma ray throughout three selected Bahloul sections. Notefour main packages were generated for Jerissa and Garn Halfaya sactions, the lower Bahloul is characterizedby high gamma ray values. GH: Garn Halfaya section, GAF: Gafsa section

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