The biostratigraphy of carbonate deposits of the middle and upper eocene in northeastern off-shore Tunisia

Palaeogeography, Palaeoclimatology, Palaeoecology, 36 (1981): 191--211 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

191

THE BIOSTRATIGRAPHY OF CARBONATE DEPOSITS OF THE MIDDLE AND UPPER EOCENE IN NORTHEASTERN OFF-SHORE TUNISIA

HECTOR BISMUTH and JEAN BONNEFOUS Socidtd de Recherches et d'Exploitation des Pdtroles en Tunisie, El Manar II, Route X, P.O. Box 145, Tunis (Tunisia) SociOt~ Nationale Elf-Aquitaine (Production), Centre Micoulau, Avenue du President Angot, 64018 Pau C~dex (France) (Received April 8, 1981)

ABSTRACT Bismuth, H. and Bonnefous, J., 1981. The biostratigraphy of carbonate deposits of the Middle and Upper Eocene in northeastern off-shore Tunisia. Palaeogeogr., Palaeoclimatol., Palaeoecol., 36: 191--211. Several holes drilled recently off the coast of northeastern Tunisia (Gulf of Hammamet and the Pelagian Sea) have cut into a thick formation of entirely carbonate rocks of Priabonian and Upper Lutetian age. These deposits with a "Liburnian" facies have no equivalent on-shore in the outcrops of the Tunisian Eocene. They are characterised by an unusual brackish-marine faunal association and by the total absence of argillaceous layers and detrital siliceous material. This formation, well dated by its microfauna and by the age of the layers which surround it, can be divided into three distinct units, from top to bottom: (1) Limestones with Miliolidae, Peneroplidae, Pseudochrysalidina, Lituonella and Discorinopsis find with dasycladacean algae; at their top, layers rich in Microcodium can be observed. (2) Marine platform limestones with Nummulites and Discocyclina. (3) Crystalline and vacuolar dolomites which are probably secondary. Identical facies to those of the upper unit can be found in outcrops on the islet of Lampione in the Pelagian Sea. Therefore, and for the first time, we can establish that they are of Late Eocene age. Moreover, a comparison has been made between the facies on Lampione and those facies which have been previously described in the southern Apennines, characterising the "Spirolina cenozone". The possibility of a younger age for this cenozone (Late Lutetian and Priabonian rather than Paleocene) is envisaged. INTRODUCTION T h e d i f f e r e n t t y p e s o f facies a n d f a u n a w h i c h c h a r a c t e r i s e t h e series o f t h e M i d d l e a n d U p p e r E o c e n e i n T u n i s i a are r e l a t i v e l y w e l l k n o w n . I n p a r t i c u l a r , t h i s is d u e t o t h e w o r k s o f P e r v i n q u i ~ r e ( 1 9 0 3 ) , S o l i g n a c ( 1 9 2 7 ) , C a s t a n y ( 1 9 5 1 ) a n d B u r o l l e t (1956). Burollet, the o r i g i n a t o r of a lithostratigraphic nomenclature for central Tunisia, distinguishes in this geologic interval the 0031-0182/81/0000---0000/$02.75 © 1981 Elsevier Scientific Publishing Company

192

evaporite "Djebs" Formation, defined west of Sfax, in the region of Mezzouna, and the Souar clays Formation, identified west of Enfidaville, close to the Gulf of Hammamet. Burollet has also described nodular and shelly limestones, rich in Ostraceae, which are present at Djebel Cherahil in central Tunisia. It is precisely the latter which Comte and Dufaure (1973) proposed to name the formation, or rather facies, of the shelly limestones of Cherahil. As is to be expected, the Cherahil shelly limestones occupy geographically an intermediate position between the zone of evaporite deposits of the Djebs Formation and that of the fine, argillaceous sediments of the Souar Formation with their essentially pelagic microfauna. Oil wells drilled during the last ten years off the coast of northeastern Tunisia (Gulf of H a m m a m e t and the Pelagian Sea) have occasionally encountered the Middle and Upper Eocene with a para-reefal carbonate facies. This is completely different from all the facies known on mainland Tunisia; therefore, none of the names of previously defined formations is suitable. On the other hand, there are striking similarities between the Eocene limestones of these off-shore wells and that which constitute the islet of Lampione, an Italian possession to the west of Lampedusa. It was found necessary to reconsider the biostratigraphy of the outcrops on Lampione which successive authors had attributed to the Jurassic (Segre, 1960) and then to the Maastrichtian (Borsetti and Colantoni, 1973). The limestones on Lampione are in fact Upper Eocene and correspond to the uppermost parts of a new formation, the "Halk el Menzel carbonates", which we propose for the deposits made up entirely of limestones and dolomites of Late to Middle Eocene age, encountered in several holes in the Gulf of Hammamet. The definition and the description of this new formation lead us to place it in relation to other formations of the same period, and to specify the microfaunal c o n t e n t and the area of geographic distribution. We will also have to discuss the paleogeographic implications associated with the presence of this carbonate platform and with the stratigraphic hiatus, which normally affects its top. DISTRIBUTION OF FACIES OF THE MIDDLE AND UPPER EOCENE IN TUNISIA (Figs. 1 and 2)

During the Middle and Late Eocene in Tunisia deposits were arranged according to isofacial lines roughly NW--SE orientated. In the north of the Saharan platform and around the mole of Kasserine, both devoid of Eocene sediments, gypsum and dolomites of the Djebs Formation are to be found. These are particularly thick in the Mezzouna region. Beyond this fringe of lagoonal deposits, towards the northeast, one can observe, over a fairly wide area, thick deposits with marked neritic Fig.1. Geographical distribution of the different facies in Tunisia (on-shore and off-shore) during the Middle and Late Eocene.

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194 characteristics, in which the a m o u n t of argillaceous layers increases towards the northeast. In this way one can successively distinguish: (a) The deposits in which limestones predominate, generally fossiliferous, coquinoid or bioclastic; such deposits are to be found at Djebel Cherahil (the type locality for the formation of the same name), in the Nasr Allah 1 well (NA. 1) close to the " n o r t h - - s o u t h axis", Chaker 1 (CH. 1), in the Sahel of Sfax, and at Gabes 1 (GA. 1) to the northeast of the island of Djerba. (b) Then the deposits which are basically marly and with an exclusively benthic microfauna, including more or less thick and more or less frequent intercalations of bioclastic limestones and oyster and barnacle coquina. The Djebel Djebil in central Tunisia provides a good example of this facies which, already present in the Constantinois in Algeria, runs obliquely into Tunisia and is found in a large part of the Gulf of Gabbs (Ashtart oilfield), it then continues very likely b e y o n d there into off-shore Libya. Due to the importance of the argillaceous sediments in these deposits, t h e y could be attributed to the Souar clays Formation. However, their association with the Cherahil Formation appears more appropriate to us, owing to the fact that once again it concerns a sedimentation in a shallow marine environment with an occasional t e n d e n c y to confinement. The biostratigraphy of these rocks is essentially established with the help of the rich population of ostracodes which t h e y contain; by taking the single genus Loculicytheretta into account, we are able to achieve a fine biozonation (Oertli, 1976; Bismuth et al., 1978). A fairly narrow strip of land, marked out by the Kairouan 1 (KRN. 1), Ketatna 1 (KE. 1) and Kerkennah 1 (KK. 1) wells and terminating at the Miskar 1 (MKR. 1) well in the Gulf of Gab~s, shows in the Middle and Upper Eocene more open marine facies than the preceding ones. They show mixed features with the co-existence, or the alternation, of benthic and planktonic micro faunas. Continuing with this progression towards the northeast, we enter the vast area in which argillaceous deposits with pelagic microfauna have a clear predominance. This is the Souar Formation, identified to the west of Enfidaville, in which we find the classic succession of planktonic biozones of the Tethys (Salaj, 1975, 1980). Therefore, it seems natural for us to link this evolution of facies of the Middle and Upper Eocene to a regular paleobathymetric increase from the southwest towards the northeast. THE GULF OF HAMMAMET AND THE HALK EL MENZEL FORMATION (Figs.l, 2 and 3) The general t e n d e n c y towards an increasingly argillaceous and pelagic sedim e n t a t i o n in the northeastern direction does not persist in the Cap Bon Peninsula, however. In the northern part of the Djebel Abderrahmane the plankton persists, but, in the Upper Lutetian and the Priabonian, numerous

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196 On the island of Zembra, off the northwest tip of Cap Bon, the only Eocene deposits which we know of are exclusively calcareous and contain the same associations of algae and large foraminifera as those which characterise the Rein6che limestone and the overlying beds (Bismuth et al., 1972). The t e n d e n c y towards a deepening of the sedimentary floor in a SW--NE direction is refuted in the Gulf of Hammamet, where the geology begins to be fairly well known, due to a number of holes drilled by various oil companies during the last ten years. In this way the argillaceous and pelagic deposits of the Souar Formation, present as far as the H a m m a m e t W 1 (HW. 1) drill hole, are fairly abruptly substituted by a carbonate platform devoid of argillaceous levels and of detrital siliceous material. This limestone and dolomite formation is particularly noticeable in the oil-bearing structure of Halk el Menzel; we therefore propose to call it the "Halk el Menzel carbonate F o r m a t i o n " .

Localisation of the typical profile of the formation: It has been established in the Halk el Menzel 1 (HEL. 1) bore hole, drilled in the Gulf of H a m m a m e t in 1977, 83 km east-northeast of Sousse; the geographic coordinates of this well are: 11°33'02"45 E, 35°55'50"48 N. Thickness: Approximately 260 m in HEL 1. Principal lithologic and faunal characteristics: One can schematically subdivide the Halk el Menzel carbonate Formation into three distinct lithologic members which are, from top to bottom: (a) Marine and dismicrite limestones, rich in Miliolidae, Peneroplidae and Valvulinidae, probably from a confined environment and which correspond to lagoonal or back-reef deposits; a particular abundance of Microcodium can be observed at their upper part. (b) Marine limestones, often sparite, characterised by an abundance of large foraminiferas, such as Discocyclina and Nummulites. This leads us to assume an environment of shallow deposits with free access to the open sea. (c) Finally the dolosparites in which there is no faunal element and which are probably secondary. Age of the Formation: Middle and Late Eocene. The reasons for this particular age attribution are explained in the following paragraphs. THE LIMESTONES OF LAMPIONE: THEIR RELATIONSHIP WITH THE HALK EL MENZEL CARBONATES (Figs.4, 5 and 6) The islet of Lampione is the only example in the Pelagian Sea of an outcrop of the Upper Eocene limestones encountered by certain holes in the Gulf of H a m m a m e t ; hence our interest in the limestones of Lampione. As far as we know, there have been very few publications concerning this outcrop. In a work on the Pelagian islands Segre (1960) attributed a Jurassic age to these limestones. This was based on a simple comparison of the facies with the Sicilian Jurassic (Egadi islands and the region of Trapani). Borsetti and Colantoni (1973) believed them to belong to the Maastrichtian on the

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evidence, a m o n g o t h e r things, o f certain d a s y c l a d a c e a n algae (Salopekiella inopinata GuStS); nevertheless, t h e y n o t e d t h e " L i b u r n i a n " c h a r a c t e r o f the m i c r o f a c i e s present. In 1965, a geological section was studied on t h e islet o f L a m p i o n e b y Rivier and B o n n e f o u s and a n u m b e r o f samples were t a k e n . T h e section, w h i c h has a thickness o f m o r e t h a n 35 m , shows a succession o f massive sub-

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A micropaleontological study of these samples has been carried out by Bonnefous and Bismuth (1965). This same material was the object of a reexamination (Bonnefous and Artru, 1968). On this occasion, several thin plates were referred to Professor Cuvillier who contributed a substantial a m o u n t of detailed information. The microfauna observed is fairly rich and varied, and is composed of numerous benthic foraminifera, the most significant of which are (see Plates I--IV): Lituonella robertiSchlumberger, Orbitolites complanatus DouvillS, Discorinopsis (Arenagula) cf. kerfornei Allix, Praerhapydionina cf. huberi

200 Henson, Pseudochrysalidina sp., Miliolidae, Spirolina sp., Dendritina sp., Discorbis sp.; associated with this microfauna are very rare corals, gastropods and algae, among which we can not e Distichoplax biserialis Dietrich. Certain levels show f r eq ue nt sections o f Dasycladaceae; these are t he same ones illustrated by Borsetti and Colantoni (1975) which t hey nam ed Salopekiella inopinata Gu~id; in our opinion, these Dasycladaceae of L a m p i o n e are very close to Praturlonella salernitana Barattolo 1. The association which we just have described allows us to attribute a Late Lutetian--Priabonian age to the limestone of Lampione, with a facies close to the one which characterises the Liburnian described by Bignot (1972) in Istria or on the Dalmatian coast (Bignot, 1975). It also allows a comparison with the Upper Eocene Limestones found in the Medoc region of southwest France (Poignant, 1960, 1961).

Environment of the deposit: Geochemical analyses carried out by A rt ru ( B o n n e f o u s and Artru, 1968) on these same samples from L am pi one and n o t ab ly the b o r o n c o n t e n t t ha t exceed 330 ppm , indicate a high degree of saltiness which leads one t o expect regressive tendencies and/ or a back-reef or lagoonal t y p e o f envi r onm ent for the deposit, which agrees with the interp r e t a t i o n o f the microfacies. GEOGRAPHIC EXTENT OF THE HALK EL MENZEL FORMATION In addition to the wells drilled in the Halk el Menzel oil-bearing structure, this f o r m a t i o n is also to be f ound in the o u t c r o p of the islet of Lampione, where only its u pper m e m b e r is represented. In the Gulf of H a m m a m e t this f o r m a t i o n has also been f ound in wells drilled by Shell, in Byrsa 2bis and Oudna 1, and also by Total's Ksar 1 (XSR. 1) well. In the A k o u d a 1 (AKD. 1) bore hole the identified Eocene deposits, with their abundance of Nummulites and Discocyclina, can only be ascribed t o the middle part of the Halk el Menzel F o r m a t i o n . One can say the same for the E ocene limestones on the island of Zembra, of which the affinities with the Rein~che limestone of Cap Bon we have indicated above. The Riccio 1 well of Agip, in Italian t e r r i t o r y and in the immediate proximity o f Lampione, has p e n e t r a t e d a great thickness of t he Halk el Menzel F o r matio n . Much f u r t h e r to the east, facies virtually identical to those of this formation have been e n c o u n t e r e d in the Aqualta 1 bore hole drilled by Aquitaine Malta in the vicinity of the island of Malta. It is possible that these E o c e n e facies may also be represented in the lowest layers visible in outcrops on the island o f Malta. T h e r e f o r e , t h e microfacies shown at the e x t r e m e base of t he " l o w e r reefal l i m e s t o n e " (Felix, 1973) and which he believes to belong t o 1This similarity to Praturlonella salernitana has been confirmed by R. Deloffre to whom our material from Lampione has been submitted (pers. comm.).

201

the Oligocene, could well correspond to the terminal strata of the Halk el Menzel Formation, due to their Peneroplidae fauna. The extent of the Halk el Menzel Formation may spread beyond the area we have just defined. Indeed, a striking analogy exists between the limestones of Lampione (upper unit of the Halk el Menzel Formation) and those which, in southern Italy, have been attributed to the Paleocene and ascribed to the "Spirolina c e n o z o n e " (Sartoni and Crescenti, 1962). In the southern Apennines, where this biozone has been identified, the Paleocene dating is based essentially upon the rare Alveolina specified by Hottinger. Chiocchini and Mancinelli (1977) have debated the age of this formation, and they do not exclude the possibility of a younger age than Paleocene. As on Lampione, Miliolidae, Peneroplidae (among others Spirolina) and dasycladacean algae can be observed. Praturlonella salernitana, an alga discovered in these levels of southern Italy by Barattolo (1978), strongly resembles the form present on Lampione. The only difference (but is it a determining one?) between the facies of these two regions is as follows: the presence of Coskinolina and the absence of Lituonella in southern Italy, as opposed to the presence of Lituonella and the absence of Coskinolina in the outcrop of Lampione. The limestones attributed to the Spirolina biozone are limited by two unconformities, as are thelimestones of the Halk el Menzel Formation in holes in the Gulf of Hammamet. Indeed, they are directly overlain by the Miocene (with a complete absence of Oligocene rocks), while lying themselves, with an obvious stratigraphic hiatus, on limestones rich in rudists of Late Cretaceous age. Is this however a mere accidental analogy and a simple convergence of facies? Maybe it is now time to reconsider the Paleocene age attributed to the Spirolina cenozone. But of course, it is up to the Italian geologists, and more particularly the stratigraphers, to clarify this problem. In order to do this, it will no d o u b t be necessary to return to the Alveolina material observed in the Spirolina cenozone limestones of the southern Apennines. It is essentially because of this Alveolina material that the Spirolina cenozone has been ascribed to the Paleocene. Moreover, extending the work of Bignot (1973) and Schroeder (1974), it will be necessary to continue with the basic research on the genus Coskinolina and Lituonella in order to specify their stratigraphic significance in greater detail. PALEOGEOGRAPHIC IMPLICATIONS (Fig.l)

It is still rather premature to a t t e m p t to outline the precise contours of the Eocene carbonate platform evident in the Pelagian Sea, but from now on one can give a rough idea of the places where the Halk el Menzel Formation has been encountered; this leads us to believe that these deposits extend over a fairly wide area. The existence of this carbonate platform, with its parareefal character, adjacent to regions in which deposits of the same age are pelagic and argillaceous, implies an abrupt raising of the sedimentary floor, so abrupt in fact,

202 that is was not able to p r o m o t e the establishment of coquinoid limestones. The result was that between the Pelagian clays of the Souar Formation and the Halk el Menzel limestones no fringe, however narrow, of ostracean shelly deposits comparable to those of the Cherahil Formation, has been observed. In the Eocene, the paleobathymetric profile which can be traced between the Kasserine mole and the region around the Pelagian islands would therefore be affected by a marked dissymetry: from the southwest towards the northeast it would first indicate a fairly regular increase in depth, and then a rapid raising of the sedimentary floor. All this suggests that the miliolid and peneroplid facies of Lampione and of the upper part of the Halk el Menzel Formation [the same type as those which Henson (1950) believed to be associated with back-reef deposits] had been able to settle in the shelter of high zones {reefs or emerged terrain). The regions totally devoid of Eocene deposits, indicated by certain holes in Cap Bon, such as Kelibia 1 and 2 (KA. 1 and KA. 2), Korba 1 {KO. 1) or in the Gulf of H a m m a m e t Hergla Nord 1 (HGN. 1), display the same features in this area of shoals. In this context, one is able to give a better explanation for the existence of the fore-reef facies of Zembra and for the facies which can be observed near to the village of Rein~che in the northern part of Cap Bon. The deposits of the Halk el Menzel Formation (whether they constitute a true carbonate platform of a particular spatial dimension, or are merely a group of separate patch reefs), are based on complex substrata, consisting of juxtaposed variable blocks which attained their high position at different geological periods. The Halk el Menzel 1 bore-hole (HEL. 1) {where the formation of the same name has been defined) has crossed, immediately beneath the dolomites which form the base, biostromal limestones with abundant rudist debris. These have been attributed to the Cenomanian--Turonian by an analogy of facies. Therefore, an important stratigraphic gap characterises the surface which, on this site, separates the Halk el Menzel Formation from its substrata. A virtually identical superposition, with the same sedimentary gap, is to be found in the island of Zembra. Here Bismuth et al. {1972) have discovered coral and rudist reefal limestones, including levels with Cuneolina and Accordiella conica Farinacci. These reefal limestones, probably having a Turonian--Coniacian age, support limestones with large foraminifers of the Upper Lutetian. Similar facies, of Middle Eocene age, are present at A k o u d a 1 (AKD. 1) but, in this bore hole, nearly complete deposits of Early Eocene, Paleocene and Senonian age separate them from the biostromal rudist limestone. In the zone of the Korba 1 (KO. 1) and Cap Bon 1 and 3 (CB. 1, CB. 3) on-shore holes, Cretaceous deposits of Cenomanian to Coniacian age have also shown rudist biostromes of high energy. However, in CB. 1 and CB. 3, and in the outcrops of the Djebel Abderrahmane which surround these wells, the overlying deposits, including Upper Eocene, show a fine sedimentation in a pelagic environment. After a fairly short more marine interlude, during the course of which a Campanian limestone typical of the pelagic facies {Abiod Formation) was

203 deposited, one notes at Korba 1 (KO. 1) a total absence of deposits for the entire duration of the Paleocene--Eocene. All these facts point to a certain instability in the sea bed which has affected this region. In the Aqualta 1 bore hole, off the island of Malta, interposed between the fine crystalline dolomite which constitutes the base of the Eocene deposits and the dolomitic limestones with Dicyclina attributed to the Upper Cretaceous, are "basaltic deposits (with abundant augite and olivine) probably showing three principal subaerial coulees ( . . . ) and a volcanic~sedimentary complex with basalt and interstratified red clays ( . . . ) " (Boulouard et al., 1972). This volcanic manifestation is probably the result of intense tectonic activity of which this part of the Mediterranean was the centre towards the end of the Cretaceous period. The youngest substratum which has been observed below the Halk el Menzel F o r m a t i o n was encountered by the Ksar 1 (KSR. 1) bore hole, drilled by Total, and with its marly and pelagic facies it corresponds to the high levels of the lower Lutetian. This is a valuable piece of information which confirms the post-Early Lutetian age attributed to the Halk el Menzel Formation in this region. Indeed, it also supports the view of a fairly contemporary raising of the sedimentary floor with the Rein~che limestone deposit. The positive movements, which almost everywhere have provoked sedim e n t a r y gaps and which have also promoted the establishment of the Halk el Menzel carbonates, continued after this formation was deposited. Moreover, numerous corroborative observations lead one to believe that, towards the end of the Eocene or towards the beginning of the Oligocene, this carbonate platform experienced a more or less prolonged period of emersion. Indeed, the top of the Eocene limestones, in the Halk el Menzel wells as well as in the Aqualta well, shows abundant Microcodium spp. At the present time (Bodergat, 1974) it is presumed that these structures are associated with the activity of micro-organisms in a subaerial environment, hence constituting a form of decay of the emerged surface layers protected by a nowdisappeared paleosol. In addition, none of the holes in the Gulf of H a m m a m e t which have cut into this formation have shown any trace of Oligocene deposits. Deposited above the Upper Eocene limestones, and therefore after an important sedimentary hiatus, are limestones more or less made up of corallinacean algae, Bryozoa and Amphistegina of the Lower to Middle Miocene. The sedimentary hiatus is perhaps less important in Malta if, like Felix (1973), one attributes a Late to Middle Oligocene dating to the Lepidocyclina limestone of t h e "lower reefal limestone" extracted from Aqualta 1, above the peneroplid limestone of the Halk el Menzel Formation. An additional indication of the emersion of this carbonate platform is provided by certain wells drilled by Shell, such as Oudna 1, which show the existence of continental or lacustrine episodes; in the reddish clays intercalated between the Halk el Menzel Formation and the Miocene, non-marine ostracodes and charophyte oogonia have been observed. These same levels have shown traces of volcanism.

204 PLATE I (bar: 0.2 ram) (p. 205) Ataxophragmiidae 1. Valvulininae: ? Valvulina sp., Lampione Isl., JBS-65. 2. Verneuilininae: Lampione Isl., JBS-65 113. 3. Valvulininae: ? Valvulina sp., Lampione Isl., JBS-65 109. 4. Valvulininae: ? Valvulina sp., Lampione Isl., JBS-65 109. 5. Ataxophragmiinae: Pseudochrysalidina cf. floridana Cole, HEL. 1, 1145 m. 6. Ophthalmidium sp., Lampione Isl.; JBS-65 109. 7. Miliolidae, Lampione Isl., JBS-65. Peneroplinae 8. Dendritina sp., equatorial section, Lampione Isl., JBS-65 112. 9. Dendritina spp., equatorial and axial sections, Lampione Isl., JBS-65 112. PLATE II (bar: 0.2 mm) (p. 206) Peneroplinae 1. Dendritina sp., axial section, Lampione Isl., JBS-65 112. 2. Dendritina sp., or Hottingerina sp., axial section, Lampione Isl., JBS-65. 3. Spirolina sp., Lampione Isl., JBS-65 113. 4. Spirolina cf. cylindracea Lamarck, Lampione Isl., JBS-65 112. 5. Spirolina sp. aft. Praerhapydionina huberi Henson, Lampione Isl., JBS-65 112. 6. Discorinopsis sp., Lampione I s l , JBS-65 113. 7 , 8 . Discorinopsis sp. cf. D. kerfornei (Allix), Lampione Isl., JBS 112. 9. Lituonella roberti Schlumberger, Lampione Isl., JBS-65. PLATE III (bar scale in mm) (p. 207) 1. 2. 3. 4. 5. 6. 7.

Lituonella cf. roberti Schlumberger, Lampione Isl., JBS-65. Praerhapydionina cf. delicata Henson, Lampione Isl., JBS-65. ? Taberina sp. or Orbitolites sp., Lampione Isl., JBS-65. Discorbidae, axial section, Lampione Isl., JBS-65 104. Discorbidae, section parallel to spiral side, Lampione Isl., JBS-65 109. Chapmanina cf. gassinensis (Silvestri), subaxial section, HEL. 1, 1050 m. Lituonella roberti Schlumberger, axial section in a dismicritic limestone, Lampione Isl., JBS-65 115. 8. Alveolina, subaxial section in a biosparitic limestone, HEL. 1, 1195 m. 9. Alveolina sp., equatorial section, XSR. 1 , 6 0 6 m.

PLATE IV (bar scale in ram) (p. 208) 1. 2. 3. 4. 5. 6. 7,8 9.

Nummulites, equatorial section, XSR. 1 , 8 6 4 m. Nummulites sp. and Discocyclina sp., axial sections in a biomicritic limestone, HEL. 2, core No. 2, 1 7 9 8 . 4 0 m. Nummulites cf. gizehensis, equatorial section, XSR. 1 , 8 5 6 m. Discocyclina sp., axial section, in a biomicritic limestone, HEL. 2, core 2, 1798. 40m. Discocyclina sp. and N u m m u l i t e s sp., axial sections, Birsa 2 bis, 1366 m. In the same level, Nummulites fabiani, N. bouillei and N. incrassatus have been determined by Blondeau (pers. comm.). Gymnocodiacea, Lampione Isl., JBS-65 114. Dasycladacean algae thought to be Praturlonella salernitana, Barattolo, Lampione Isl., JBS-65. Microcodium, HEL. 1, 1060 m.

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209 CONCLUSION T he lithostratigraphic n o m e n c l a t u r e of Tunisia is already fairly much e n c u m b e r e d ; so, the establishment of a new form at i on, t hat of the Halk el Menzel carbonates would n o t be necessary even to d e n o m i n a t e a particular Middle and Upper E o c e n e facies, which was u n k n o w n in onshore Tunisia. Indeed, our approach has been justified by the recognition of a true sequence o f sedimentation with a parareefal character and a marked regressive t e n d e n c y . This sedimentary sequence, which in the locations examined is not necessarily isochronous at its base, is established on a vast mole embracing the Pelagian islands and at various periods f u n c t i o n e d as a high zone. Each time t hat the Halk el Menzel F o r m a t i o n has been e n c o u n t e r e d it has been enclosed by two major sedimentary gaps, occasionally associated with periods o f volcanic outflow. Th e islet o f L a m p i o n e is the only o u t c r o p of this f o r m a t i o n to be identified in the Pelagian Sea. It could be possible t hat in the southern Apennines t h e c e n o z o n e of Spirolina limestones shows the same sequence of sedimentation: t h e y show evidence of microfacies remarkably similar t o those on L a m p i o n e and are also enclosed by two sedimentary gaps. T he platform of the Halk el Menzel limestones can be regarded as a prefiguration of the platf o r m which was t o e x t e n d over a m uc h larger area in t he Early Miocene. Evidence o f this is shown by limestone deposits containing algae (Rhodolites) which can be observed at n u m e r o u s places in the Mediterranean.

ACKNOWLEDGEMENTS We wo u ld like to t ha nk t he following companies who have authorised t he publication o f this work and who furnished data from a n u m b e r of drillings: SEREPT, SNEA(P), SH E L L , OEMV, T O T A L and AGIP. We also t h a n k Mr. Bignot and Mr. Poignant of t he University of Paris VI, and Messrs. Oertli, Deloffre and Ham aoui [SNEA(P)] whose advice has been of invaluable help. The actual p r o d u c t i o n of this publication would not have been possible witho u t the collaboration of Messrs. Poirel and Ghelila for the p h o t o g r a p h y and o f Messrs. Meaouia and Gliguem for the figures. T he translation into English has been carried out by Miss A. T racy of Bradford University. REFERENCES

Barattolo, F., 1978. Su di una nuova dasicladacea (alghe verdi) nel Paleocene dell' Appennino meridionale. Boll. Soc. Nat. Napoli, 87:1--76.

210 Bignot, G., 1972. Recherches stratigraphiques sur les calcaires du Cr6tac~ sup4rieur et de l'Eoc~ne d'Istrie et des r~gions voisines. Essai de r~vision du Liburnien. Travaux du Laboratoire de Micropal4ontologie, No. 2, Universit~ de Paris VI, Paris, 353 pp. Bignot, G., 1973. Pr~cisions sur Coskinolina liburnica Stache. Rev. Micropal~ontol., 15(4): 195--206. Bignot G., 1975. La transgression Eocene et les terrains liburniens en Dalmatie septentrionale (Yougoslavie). Rev. Micropal~ontol., 18(1): 15--22. Bismuth, H., Lazaar, A., Lorenz, C. and Rakus, M., 1972. Reconnaissance g6ologique de l'fle de Zembra (Golfe de Tunis, Tunisie). C. R. Acad. Sci. Paris, S~r. D., 275: 2807-2810. Bismuth, H., Keij, A.J., Oertli, H.J. and Szcechura, J., 1978. The genus Loculicytheretta (Ostracoda). Bull. Centre Rech. Explor.-Prod. Elf-Aquitaine, 2(2): 227--263. Bodergat, A. M., 1974. Les Microcodiums. Milieux et modes de d~veloppement. Doc. Lab. G~ol. Fac. Sci. Lyon, No. 62: 137--235. Bonnefous, J. and Artru, P., 1968. Compl~ments ~ l'~tude des ~chantillons des ties P61agie (Lamp~duse et Lampione). Italie. R a p p o r t interne SNPA, Centre de Recherches de Pau, note R/ST No. 21611, 15 pp. Bonnefous, J. and Bismuth, H., 1965. Etude des ~chantiilons de la coupe de Lampione. R a p p o r t interne SEREPT, Tunis, D6partement Exploration, EX 11-04, 6 pp. Borsetti, A. M. and Colantoni, P., 1973. Stratigrafia dell' isola di Lampione (canale di Sicilia). G. Geol., (2), 39(1): 313--320. Boulouard, C., Calandra, F., Deres, F. and Fourni~, D., 1972. Etude stratigraphique et s~dimentologique des s~ries s~dimentaires travers~es par le sondage d'Aqualta 1, Malte. R a p p o r t interne SNPA, Centre de Recherches de Pau, note R/GEO No. 48/72, 8 pp. Burollet, P. F., 1956. Contribution ~ l'6tude stratigraphique de la Tunisie centrale. Ann. Mines G~ol. Tunis, 1 8 : 3 5 0 pp. Castany, G., 1951. Etude g~ologique de l'Atlas tunisien oriental. Ann. Mines G6ol. Tunis, 8 : 6 3 2 pp. Chiocchini, M. and Mancinelli, A., 1977. Microbiostratigrafia del Mesozoico in facies di piattaforma carbonatica dei monti Aurunci (Lazio meridionale). Stud. Geol. Camerti, III. 109--152. Comte, D. and Dufaure, Ph., 1973. Quelques pr~cisions sur la stratigraphie et la pal~ogdographie Tertiaires en Tunisie centrale et centro-orientale du Cap Bon ~ Mezzouna. Livre Jubilaire M. Solignac, Ann. Mines G~ol. Tunis, 26: 241--256. Felix, R., 1973. Oligo-Miocene stratigraphy of Malta and Gozo. Meded. Landbouwhogeschool Wageningen, 73-20: 1--103. Fourni~, D., 1972. Les facies calcaires de l'fle de Lampione et leurs homologues probables dans le sondage d'Aqualta 1, Malte. R a p p o r t interne SNPA, Centre de Recherches de Pau, R/GEO No. 51172, 4 pp. Henson, F. R. S., 1950. Middle Eastern Tertiary Peneroplidae (Foraminifera), with Remarks on the Phylogeny and T a x o n o m y of the Family. West Yorkshire Print Co., Wakefield, 70 pp. Oertli, H., 1976. The evolution of Loculicytheretta in the Eocene. Abh. Verh. Naturwiss. Ver. Hamburg, NF, 18/19 (Suppl.): 153--160. Pervinqui~re, L., 1903. Etude g~ologique de la Tunisie centrale. Dir. Tray. Publ. carte g~ol. Tunisie. De Rudeval, Paris, 359 pp. Poignant, A., 1960. Aper~u sur la microfaune de l'Eoc~ne sup~rieur du M~doc. Rev. Micropal$ontol., 3( 1 ) : 31--36. Poignant, A., 1961. Quelques pr~cisions sur les Lituonelles de l'Eoc~ne du M6doc. Rev. Micropal~ontol., 4(1): 66---67. Salaj, J., 1975. Contribution ~ la microbiostratigraphie du M~sozofque et du Tertiaire de la Tunisie septentrionale. In: V~ Coll. Afr. Micropal~ontol., Addis Ababa, 1972. Rev. Esp. Micropaleontol., pp. 703--783.

211 Salaj, J., 1980. Microbiostratigraphie du Cr6tac6 et du Pal6og~ne de la Tunisie septentrionale et orientale (hypostratotypes tunisiens). Institut G~ologique de Dion:~z ~tflr, Bratislava, 238 pp. Sartoni, S. and Crescenti, U., 1962. Richerche biostratigrafiche nel Mesozoico dell' Appennino meridionale. G. Geol., 2, 29: 161--304. Schroeder, R., 1974. Der Typus der Foraminiferen-Gattung Coskinolina Stache 1875. N. Jahrb. Geol. Pal~/ontol., 11: 702--706. Segre, A.G., 1960. Geologia. In: Zavatteri et al., Biogeografia delle isole Pelagie. Acad. Naz. dei XL, IV--XI: 115--162. Solignac, M., 1927. Etude g~ologique de la Tunisie septentrionale. Dir. G~n. Trav. Publ. (Serv. Mines), Tunis, 756 pp.