Bar for scale = 10 mm

Chikhi-Aouimeur, F., Abdallah, H., Pons, J.M., and Vicens, E.

African margin. It occurs together with porifers, ostreids, gastropods, benthic foraminifers and ostracods, indicatinga platform margin setting preceding the wide Late Cenomanian transgression.Durania arnaudi and D. blayaci are well represented in the Bahloul fine-stratified limestones, below the LowerTuronian levels with ammonites, as Thomasites rollandi or Pseudaspidoceras flexuosum. These radiolitids correspondthen to the Upper Cenomanian/basal Turonian transition. Vaccinites cf. rousseli has been found in the upper partof the Bahloul limestones at Jebel Chemsi.Hippuritids may be locally abundant in the Gattar dolomites, as in Jebel Zitouna where Hippurites resectus occurin disseminated monospecific assemblages or together with radiolitids. The uppermost part of Gattar is referred tothe lower part of the Mammites nodosoides Zone (uppermost Lower Turonian). At Jebel Ben Younès its upper partbecomes rich in anhydrite (Beïda Anhydrite Member) and rudists are found in the uppermost beds (Eoradiolitesliratus and Durania arnaudi). Mammites nodosoides and Choffaticeras luciae indicating the uppermost LowerTuronian have been found in the lateral equivalent of these anhydrites.Overlying the Gattar, the Annaba Clays correspond to a transgression phase characterised by the sedimentationof clays with benthic and planktonic foraminifers. These last are composed of whiteinellids, heterohelicids andhedbergellids. Some scarce small Bournonia africana have been recovered from Jebel Ben Younès.Rudists are best represented in the limestones or the clayey limestones and evaporites of the Bireno Member.Radiolitid fauna is highly diversified: Radiolites praesauvagesi, R. trigeri, Praeradiolites ponsianus, and alsoEoradiolites liratus generally quoted in the Cenomanian, are abundant. Biradiolites angulosus is scarce and Bournoniaafricana (synonym of B. lecoqui and B. fourtaui) occurs in dense assemblages at some beds. Some specimens ofDistefanella bassani, Apricardia sp. and Hippurites libanus were also found. These assemblages are particularlywell represented at Jebel Ben Younès, Jebel Chemsi and Jebel Zitouna. At Jebel Sidi Bouhlel, the tidal-flat faciesof the upper part of Bireno change laterally to small bio-constructions of peculiar species. A level with a denseassemblage of small monopleurids, another with bouquets or lenticular assemblages of Radiolites lefebvrei, or alast one with large up-standing Vaccinites praegiganteus have been recorded.Unfortunately, the two last members (Annaba and Bireno) do not yield ammonite fauna. Biostratigraphy is basedon benthic foraminifers (Nezzazatinella picardi, Cuneolina pavonia,...), planktonics (Heterohelix reussi, Whiteinellaparadubia, Hedbergella delrioensis,…) and ostracods (Brachycythere dumoni, Cythereis mdaouerensis, Cytherellagambiensis). According to their regional distribution these rudists can be attributed to the Mid Turonian.The species found have been mentioned from the Turonian-Coniacian platforms of the western Tethys. Nevertheless,their assemblages seem to have been affected by environmental conditions probably more difficult due to a warmclimate on assemblages living in very shallow water. Emersion structures (bird eyes, mud-cracks) and saltaccumulations are the omnipresent expression of this aridity that should explain the poor diversity within a singlelevel in spite of their abundance as well as the smaller size of individuals when compared with European faunas.

Jebel ZitounaThis locality corresponds to an intermediatearea where rudists developed during theHST. They show very high diversity andabundance. The assemblages aredominated by Bournonia or by Radiolites.

CONCLUSIONS

The region of Gafsa results to be of great interest for the study of theCenomanian-Turonian rudists.

The Upper Cenomanian yields Praeradiolites biskraensis. As thisspecies is widespread and always found below Neolobites vibrayeanusalong the southern margin of the Tethys, it is a good marker fromMorocco to the Middle East.

Durania is frequent in the deeper facies of the Upper Cenomanian-basal Turonian; such as in the basin deposits of the Bahloul and ofthe Gattar Equivalent occurring together with ammonites and planktonicforaminifers.

At Mid Turonian, deposits are less differentiated and composed ofshallow platform facies. Rudists are very abundant, particularly theradiolitids (Eoradiolites, Radiolites, Biradiolites, Bournonia,Distefanella,…). They are found in the highstand deposits and theydisappear after the generalised emersion of the considered region.

A better calibration of the Cenomanian-Turonian rudist fauna richnessand distribution in south-central Tunisia would be possible by comparingthis rudist fauna with that of the Kasserine region.

References

1- Abdallah H., Ben Youssef M. and Saïd-Benzarti R., 2003- Biostratigraphy of the Annaba Clays and the Lower Bireno succession, Middle Turonian, Gafsa area, northern southernTunisia. Revue de Paléobiologie, Genève (juin 2003), 22, pp. 439-456.

2- Abdallah H., Sassi S.,Meister C and Souissi R., 2000- Stratigraphie séquentielle et paléogéographie à la limite Cénomanien-Turonien dans la région de Gafsa- Chotts (Tunisiecentrale).Cretaceous Research, 21, pp. 35-106.

3- Abdallah H., Sassi S., Souissi F., Meister C., Néraudeau D., Saadi A. and Saïd-Benzarti R., 2003- Sequence Stratigraphy at the Cenomanian-Turonian Boundary in the NorthernChotts Range, Southern Tunisia. In The Geology of Northwest Libya, Sedimentary Basins of Libya- Second Symposium, eds M.J. Salem and K.M. Oun, volume 1, pp. 99-138.

4- Meister C. and Abdallah H., 2005- Précision sur les successions d’ammonites du Cénomanien- Turonien dans la région de Gafsa, Tunisie du centre-sud. Revue de Paléobiologie,Genève (juin 2005), 24 (1), pp. 111-199.

Jebel Sidi BouhlelAs the western part of the NorthernChotts Range (NCR) is tiltedwestwards, the lower part of theseries is buried in this locality. Theupper Cenomanian and thelowermost Turonian correspondto deep anoxic facies. During midTuronian, sedimentation becameuniformly shallow through the NCRand allowed rudists installation.Three monospecific assemblagesare found and are restricted to thislocality.

Jebel ChemsiIs a part of a high blocktilted westwards andseparating two basins(Zammour and SidiMansour).Isolated Durania arep r e s e n t i n d e e penvironment but rudistsbecome abundant inthe shallow water of them i d T u r o n i a n .Assemb lages a red i v e r s i f i e d b u tRadiolites is generallythe dominant genus.

Jebel Ben YounèsThis structure is limited in its southern part bythe Gafsa Fault, it belonged to the Mejel BelabesBasin where evaporites deposited during theearly-mid Turonian are very thick (AnhydritesBeida Member). The evaporites concentrationis very high inside the basin while on the marginrudists can grow in small thickets.

(1) Université H. Boumediènne, Bab Ezzouar - BP 32 El Alia, Algiers 16000, Algeria; fet_aouimeur@yahoo.fr(2) Laboratoire de Géoressources, CERTE, Technopôle de Borj Cédria, BP 95, 2050 Hammam-Lif, Tunisia; Hassen.Abdallah@inrst.rnrt.tn

(3, 4) Universitat Autònoma de Barcelona, Departament de Geologia, Facultat de Ciències, Edifici Cs. 08193 Bellaterra, Spain.

During Late Cenomanian and Turonian the Gafsa region presented a framework of basins and highs where rudists became well represented when localconditions were favourable. Recent studies in the area on ammonites and microfossils (1, 4) have refined the biostratigraphy, and others on sedimentologythe depositional history (2, 3), allowing a fairly precise biostratigraphical and environmental setting of the rudist formations. Nevertheless, this faunaalthough frequent and well preserved has not been adequately described. Sampling carried on the northern, southern, and eastern margins of the Gafsa-Metlaoui Basin allowed us to describe its composition and distribution.Praeradiolites biskarensis is frequent in a bed 30 cm-thick in the alternating clays and carbonates of the uppermost Zebbag Formation s.s. (Jebel Zitouna,Jebel Ben Younès). This bed is located few meters below the chalky limestones with Neolobites vibrayeanus, as in many other localities of the Tethyan

The Cenomanian – Turonian rudists of the Gafsa region, Tunisia

1, 2, 3 Radiolites trigeri (Coquand, 1859)4, 5 Bournonia africana Douvillé, 1910

6 Eoradiolites liratus (Conrad, 1852)7 Hippurites resectus Defrance, 18218 Praeradiolites biskarensis (Coquand, 1862)

1 2 3

5 6 7

8

4

Gattar

Anhydrites Beida

1 Biradiolites angulosus (d’Orbigny, 1850)2 Hippurites libanus Douvillé, 1913

3, 4 Praeradiolites ponsianus (d’Archiac, 1835)5 Durania arnaudi Choffat, 1891

6, 7 Durania blayaci (Toucas, 1903)

1

2

3

6

5

4

7

1 Vaccinites praegiganteus Toucas, 19042, 3, 4 Radiolites lefebvrei Bayle in Rolland, 1881

5 Monopleura sp.

1

2

3

5

4

1, 6, 7, 8 Bournonia africana Douvillé, 19102, 3 Distefanella bassanii Parona, 19014, 5 Radiolites praesauvagesi Douvillé, 1908

9 Hippurites resectus Defrance, 1821

1

2 3

54

6 7 8

9Praeradiolites biskraensis

Durania arnaudiDurania blayaci

Apricardia sp.Monopleura sp.

Eoradiolites liratus

Radiolites praesauvagesiRadiolites lefebvrei

Praeradiolites ponsianus

Bournonia africanaBiradiolites angulosus

Radiolites trigeri

Distefanella bassanii

Hippurites libanus

Vaccinites rousseli

Hippurites resectusVaccinites praegiganteus

Pseudaspidoceras flexuosum

S. (Jeanrogericeras) reveliereanus

Mammites nodosoides

Neolobites vibrayeanus

Thomasites rollandi rollandi

Neoptychites cephalotus

Choffaticeras luciae

Heterohelix reussi

Whiteinella paradubia

Thomasinella punica

Heterohelix globulosa

Nezzezatinella picardi

Cuneolina pavonia

Paracypris mdaouerensis

Brachycythere dumoni

Spinoleberis yotvataensis

Cytherella gambiensis

Pterygocythere pennata

Annaba Clays Lower BirenoGattar /Eq. Gattar

BeidaAnhydritesBahloul /

Eq. BahloulUpper Zebbag s.s.

RUDISTS

AMMONITES

FORAMINIFERS

OSTRACODS

UpperCenomanian

UppermostCenomanian -Lower Turonian

LowerTuronian

Lower - MiddleTuronian

MiddleTuronian

Lithostratigraphy