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2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition Algiers, 10 – 13 April 2005

The two studied outcrops of the Triassic deposits are located about 2 Km westward

the Slouguia village where are separated by the main road GP leading to Testour (NW Tunisia)(Fig1). They were first investigated by Solignac [10] who considered them as normal stratigraphic successions in lower contact with Triassic material. This latter was regarded as an allochtonous unit overlying the encasing strata. The same interpretation was held by Daly [3] and Truillet and Delteil [11]. In the context of cartographic works made by the Tunisian Geological Survey (O.N.M.),Bajanik and Biely [1] referred to the same successions as reversed in an abnormal contact with the saliferous complex. El Ouardi [4] confirmed the same hypotheses and interpreted the Triassic materiel as a salt dome.

This paper deals with a detailed cartographic revision (1/25 000) of these outcrops based on tight sampling and accurate observation of the contact Triassic and related material/encasing units.

The Triassic saliferous complex was found on a normal succession of the layers, via clear sedimentary contacts, different units. That does not exclude posterior tectonic activity. At the J.Tellet Mabrouka area, this complex overlies a marly level of an albian age as attested by a pelagic foraminiferal association among which Ticinella primula. At the J. Sene ed Jemel, the overlain strata consist of Uppermost Albien (Vraconian) limestone/marl alternations as indicated by the association of Rotalipora and the index microfossil Planomalina buxtorfi. The same Triassic deposits are overlain,through an erosion surface, by Cenomanian and/or Campanian units as confirmed by the association: Rotalipora greeborvensis and Rotalipora Cushmani.. / G. elevata, G. stuartiformis, G. buloides, G.arca and Rosita fornicata.

These updated biostratigraphic data confirm the idea of an interstratified Triassic

material as previously announced by Vila et al.[12&13]. The evidence of Paleocene exiguous scarps as well as the unconformity of Eocene deposits on Lower Cretaceous strata attests the bottom instability during deposition. The lower erosion contact Triassic/Albian and the roll shapes was found in the albian formation indicate that the lateral flozing of the Triassic material took place during the Albian in a shallow marine environment on a gentil slope. The Eocene unconformity: (Fig.2)

Above the Paleocene marly levels, a conglomeratic bed underlies a sandstone bar which is rich in Glauconite and Phosphates. This bar is overlain by the Lowermost Ypresian carbonates that are elongated in a NW-SE band forming the major relieves of the area. The succession is followed by massive Nummulites-rich limestone beds alternating with sandstone

P24The Medjerda Valley (N. Tunisia) Triassic complex: updated biostratigraphy

and Geodynamics revisited

BEN SLAMA Mohamed Montassar1, MASROUHI Amara1, GHANMI Mohamed1, BEN YOUSSEF Mohamed2& ZARGOUNI Fouad1

1 Département de géologie, Faculté des Sciences de Tunis, 1060 El Manar II, Tunis, Tunisie 2 INRST, Hammam Lif, Tunisie. Key word: Medjerda valley, north Tunisia, Triassic, Albian, composite salt glacier.

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levels. These sandstone levels are of a metric thickness with the intercalation of shaly to marly joints bearing Echinid fragments, small Nummulites specimens, Globigerina, Lepidocyclina, Globorotalia and rare Algae.

Eastward J. Chitana, the following cross section can be followed from oldest on: metric pink limestone beds yielding an Upper Cretaceous microfaunal association that characterizes the Abiod Formation (Campanian-Maatrichtian).A polygenic conglomeratic bed separates this Formation from the adjacent carbonate unit rich in Nummulites. To the right side of a Wadi bou Kralfa ravine (Fig.3) we observed a 20 m-thick limestone succession with disseminated chert nodules of an Albian age as attested by the beared Hedbergella. After a 1m-thick conglomeratic bed we found the Lower Eocene limestone unit. Thus, it appears that Lower Eocene units overly unconformably various cretaceous strata.This unconformity was observed at the O. bou Kralfa, J Tellet Mabroka and J. Chitana outcrops. The most spectacular one is that of O. bou Kralfa where a gentil angle with the adjacent Albian units is observed (Fig.4). This unconformity fossilizes the beginning of the Lower Ypresian folding followed by a regressive-transgressive cycle. The regional data confirm these observations [2;13;15&16]. The Oligocene unconformity:

From East to West : The lumachellic sandstone beds of Oligocene age always covering with a weak angle unconformity [7;9&15] the Nummulites-rich Eocene units at the eastern edge of the O. Blidha anticline and the dolomitic Triassic units to the East of J. Sene ed Jemel. Conclusion:

The relationship between the studied saliferous material outcrops and the encasing layers characterize an interstratified Triassic material in Albian strata. The Oligocene unconformity on the Triassic saliferous complex does not correspond to erosion activity that has reached the Triassic in its original position ; furthermore the Mesozoic series ante-Aptian units which does not outcrop.The flattening of these Oligocene units does not allow to find « an intrusive Triassic structure ». Furthermore, the unconformity of Eocene deposits on subjacent series excludes the alloctonous position of the Triassic complex.

The studied area, as for the neighbouring region [14&16] has undergone a moderate ante-Oligocene compression. It is the initiation of the Ypresian or Pyrenean phase. This study shows that the Chitana-J. Sene Ed Jemel area is a submarine composite salt « glacier » installed during Lower Albian and then laterally expanded rapidly till the Uppermost Albian (Vraconian). BIBLIOGRAPHY : [1] BAJANIK S. ET BIELY J., 1973 : Carte géol.Tunisie au 1/50 000, feuille n°26, Oued Zargua. Publ. Serv. Géol. Département de géologie de l’Office National des Mines (ONM) Tunisia. [2] BUROLLET, P.F. 1956 : Contribution à l’étude stratigraphique de la Tunisie centrale. Ann.Min. et Géol., Tunis18,350p. [3] DALY K. ,1981 : Étude tectonique tangentielle dans les monts de Mejez El Bab. Actes du premier congrès National des sciences de la Terre (Tunis, Septembre 1981). [4] EL OUARDI H., 1996 : Halocinèse et rôle des décrochements dans l’évolution géodynamique de la partie médiane de la zone des dômes (Tunisie septentrionale). Thèse.Univ.Tunis II, Fac.Sc, Tunis.p242. [5] FLETCHER, R. C., M. R. HUDEC, AND I. A. WATSON, 1995 : Salt glacier and composite sediment-salt glacier models for the emplacement and early burial of allochthonous salt sheets, in M. P A, Jackson, D. G. Roberts, and S. Snelson, eds., Salt tectorics: a global perspective: AAPG Memoir 65, p. 77-108 [6] GHANMI.M, 2003 : Géodynamique de la plate forme saharienne et de sa marge septentrionale au Crétacé. Son évolution vers l’Atlas septentrional, interférences du rifting et de l’halocinèse. L’Habilitation universitaire spécialité: Géologie. Univ. Tunis El Manar II, Fac.Sc.Tunis.347p. [7] PERTUISOT V., 1978 : Dynamique et pétrogenèse des extrusions triasiques en Tunisie septentrionale 1978, p312. [8] RABHI M., GHALI A. HAMMAMI M. , EL OUARDI H., BEN AYED N. ET TURKI M.M.1999: Carte géol.Tunisie au 1/50 000, feuille n°27 de Mejez el Bab. Publ. Serv.géol. département de géologie de l’Office National des Mines. [9] SAINFELD P. 1952 : Les gîtes plombo-zincifères de Tunisie. Ann. Mines Géol., Tunis, 9, 285p. [10] SOLIGNAC M., 1925 : Les recherches de pétrole en Tunisie. R. Pétrolifère, Paris, broch. 59pp. [11] TRUILLET R. ET DELTEIL J., 1982 : Allochtonie alpine de la " zone des diapirs " de Tunisie septentrionale et du NE Algerie.C.R.Acad.Sc.Paris, t. 294 série II, 1982,.1143-1146. [12] VILA J.M., BENKHEROUF et CHARRIERE A. 1994: Interprétation du matériel triasique de la région de l’Ouenza (confins algéro-tunisiens) : un vaste « glacier de sel » sous marin albien, à l’image des structures offshore d’Aquitaine, C.R.Acad. Sci.Paris,t.318, serie II, p.109 à 116, 1994. [13] VILA J.M. 1995 : Première étude de surface d’un grand « glacier de sel » sous marin : l’est de la structure Ouenza-Ladjebel-Méridef (confins algéro-tunisiens). Proposition d’un scénario de mise en place et comparaisons, Soc.géol.France, 1995,t.166,n°2,pp.149-167. [14] VILA J.M, 1997: Définition des structures halocinétiques du 3ème type : les dômes de sel-plis du Sud-Est Constantinois (anticlinaux à cœur salifère et à amincissements synsédimentaires antérieurs) Réun.sp. Soc.géol.France « les Marges Téthysiennes d’Afrique du Nord », Paris, 16-17 décembre, res. 2 p., p.93, livre en dépôt à la Soc.géol. France. [15] VILA J.M., GHANMI M., BEN YOUSSEF M. ET JOUIROU M., 2002 : Les « glaciers de sel » sous-marins des marges continentales passives du nord-est du Maghreb (Algérie-Tunisie) et de la Gulf coast (USA) : comparaisons, nouveau regard sur les « glaciers de sel » composites, illustré par celui de Fedj el Adoum (Nord-Ouest tunisien) et revue globale. Eclogae geol.Helv. (2002) pp 347-380 . [16] ZARGOUNI F. 1977 : Étude des mouvements ascensionnels du complexe triasique dans la chaîne de Lansarine (Atlas Tunisien-Zone des “Diapirs”. Note du service géologique de Tunisie n° 43, 1977.

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2nd North African/Mediterranean Petroleum & Geosciences Conference & Exhibition Algiers, 10 – 13 April 2005

Fig.1 Geological map of the study area

Fig.2 Lateral variation of the Ypresian deposits in the investigated area

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Fig.3 Geological cross section in the W. bou Kralfa

Eocene limestone Albian limestone

Unconformity plane

Conglomerates

Fig.4 Ypresian / Albian unconformity