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Chapter 75. Marine ostracods of the Upper Miocene of the well Ashtart 1 (Gulf of Gabès, southeastern Tunisia)
1088 G. BONADUCE
The Miocene ostracod assemblages of the southern Mediterranean Basin and surrounding areas
are poorly known. As far as we know, the contributions dealing with this subject are the monographic works of Bossio et al. (1976), Ducasse and Cirac (1981) for Morocco, Sissingh (1972),
Guardia et al., (1974), Guernet et al. (1984) for Algeria, and Van Hinte et al. (1980) for Libya.
Very interesting information on the Messinian ostracod faunas are given in the thesis by Carbonel
(1971) for Morocco.
In Tunisia, Bismuth (1984b) could mention only five papers making rather short references to
Upper Miocene ostracods. They are from Benson (1976a, 1976b), Bizon et al. (1980) and Demarcq
et al. (1976). The indication of marine species is only found in Fournib (1978); this author, describing the new “Melqart carbonates” formation from the Upper Miocene of MELQART 1, another
borehole drilled in the Gulf of Gabts, cites the presence of Hemicythere deformis and of the genus
The scarce information on Upper Miocene Tunisian ostracods is certainly not related to their
paucity in the sediments, at least in most of the offshore drillings of Eastern Tunisia. The abundance
and diversity of the ostracod assemblages was previously pointed out (Bismuth, 1976) in the Upper
Miocene of the Gulf of Gab&, associated with abundant bryozoans, corals and exclusively benthic
foraminifers (including the remarkable presence of Borelis).
On the basis of previous studies, we believe there is an urgent need for detailed work on the
systematics of these rich ostracod populations and on their stratigraphical, palaeoecologic and
palaeobiogeographical significance. The present paper deals with only a part of this project
and with a selected number of 51 species, most of which are partially illustrated (pls. 1 and 2).
All the systematics will be the object of a subsequent monographicpaper.
The offshore Well Ashtart 1 was drilled in 1971 by SEREPT for the Aquitaine Tunisie-ETAP
association. It is located on the Tunisian shelf about 80 km SE of Sfax (Text-fig. 1). The water
depth at the drilling site was 217 feet (66 m). The rotary table, which is the basis of all depth
measurements was 37 feet (11 m) above sea level and, consequently 254 feet (77 m) above the
sea bottom. The drilling was conducted vertically.
The section studied is comprised, gross0 modo, between 800 and 2900 feet (245 to 870 m) and
includes all the terminal Miocene (Messinian) and part of the Pliocene and Tortonian sediments
which embrace it.
1. Cytheridea n. sp. 1. LV of complete carapace (x71). Fig. 2. Cytheridea n. sp. 2. LV (X68).
Fig. 3. Peteraurila n. sp. 1. LV ( x 59). Fig. 4. Paijenborchella n. sp. 1 LV of complete carapace ( X 84). Fig.
5. Pauenborcheflalaskarevi Krstic and Pietrzeniuk. LV ( x 90). Fig. 6. Graptocythere n. sp. 1. LV of complete
carapace ( x 68). Fig. 7. Hemicytherid n. sp. 4. LV ( x 60). Fig. 8. Hemicytherid n. sp. 1. LV ( X 74). Fig. 9.
Hemicytherid n. sp. 3. LV ( x 68). Fig. 10. Chrysocythere paradisus Doruk-LV ( x 65). Fig. 11. Chrysocythere
n. sp. 1. LV (x68). Fig. 12. Chrysocythere n. sp. 2. LV of complete carapace (x65). Fig. 13. Hemicytherid
n. sp. 2. LV ( x 81). Fig. 14. Trachyleberid n. sp. 1. LV of complete carapace ( x 62). Fig. 15. Cytherelloidea
petrosu Doruk. LV of complete carapace (x68). Fig. 16. Occlusacytheris n. sp. 1. LV (X84). Fig. 17.
Pterygocythere sp. LV ( x 54). Fig. 18. CythereZla n. sp. 1. LV of complete carapace ( X 71). Fig. 19. Loxoconcha
n. sp. 1. LV ( x 96). Fig. 20. Loxoconchagibbosofoveo~ata(Seguema) LV( ~ 8 1 ) .Fig. 21. Truchyleberisn. sp.
Marine Upper Miocene Ostracods of Well Ashtart I , Gurfof Gab& 1091
1-Location of the Well Ashtart-1 in the Gulf of Gab&.
Because of the drilling technique, the vertical distribution of the taxa extends artificially
downward. As a result, it is almost impossible to establish with certainty the first appearance of
a certain taxon, whereas we are able to define precisely its extinction.
1. Aurila n. sp. 3. LV ( x 48). Fig. 2. Aurila n. sp. 4. LV ( x 52). Fig. 3. Aurila n. sp. 5. LV ( X 68).
Fig. 4. Aurilafieudenthali Sissingh-LV of complete carapace ( x 59). Fig. 5. Aurila n. sp. 6. LV of complete
carpace ( ~ 6 5 ) .Fig. 6. Aurila n. sp. 7. LV of complete carapace ( ~ 6 2 ) .Fig. 7. Aurila n. sp. 1. L v (X62).
Fig. 8. Aurila bradleyuna Ruggieri-LV ( x 59). Fig. 9. Hemicytherid n. sp. 5. LV of complete carapace ( X 57).
Fig. 10. Cimbaurila n. sp. 2. LV ( x 74). Fig. 11. Aurila n. sp. 8. LV of complete carapace ( X 74). Fig. 12.
Hemicytherid n. sp. 6. LV of complete carapace (x57). Fig. 13. Cimbaurila n. sp. 1. LV (X84). Fig. 14.
Cimbaurila diecii (Sissingh). LV of complete carapace ( x 71). Fig. 15. Cimbaurila cimbaeformis (Seguema).
LV ( x 81). Fig. 16. Hemicytherid n. sp. 7. LV of complete carapace ( x 84). Fig. 17. Hemicytherid n. sp. 8.
LV (X68). Fig. 18. Hemicytherid n. sp. 9. LV of complete carapace ( x 81). Fig. 19. Ruggieria n. sp. 2.
LV of complete carapace (x77). Fig. 20. Ruggieria n. sp. 1. LV of complete carapace ( ~ 8 1 ) .Fig. 21.
Actinocythereis n. sp. 1. LV of complete carapace ( x 84).
CHRONOSTRATIGRAPHYOF THE W E L L ASHTART 1.
Appearance marine microfauna
Top frequent planktonic microfauna
Top of gypsum
Appearance marine microfauna
Top frequent planktonic microfauna
Depth datum level rotary table
1st sample collected
Appear. Eulepidina & Spiroclypeus
Appear. Miogypsinoides cf. complanata
Appear. Pararotalia viennoti Imexicana
Appear. Loculicytheretta cavernosa
Appear. Loculicytheretta minuta
Top oil-field with Nummulites
Final depth reached
Oued be1 Khedim
Aqui t an.
BIELY et AL. 1972
S l D l BARKA
OUED EL BIR
0 - 1
MESSINIAN OUED EL BIR
3FF-SHORE . DRlLLlNGS
FOURNIE 78 BISMUTH 84
0. Be1 KheLm
0. Eel Khedm
~exr-plo.2-Miocene lithostratigraphical units of East Tunisia (Bismuth, 1984).
In Table 1 the bio-, litho-, and chronostratigraphical frame of the complete Well Ashtart 1 is
The Upper Miocene of Ashtart 1, which this paper is particularly concerned with, is 636 m
thick and can be subdivided into three formations. Text-figure. 2 shows the correlations between
the Miocene lithostratigraphical units of eastern Tunisia used by different authors. From the
bottom upward, the three formations identified in Ashtart 1 are, successively:
The Somla Sands Formation
Encountered between 2765 feet (843 m) and 3604 feet (1099 m) this unit essentially consists of
detrital argillaceous and siliceousmaterial the continental origin of which is obvious. This mollassic
series contains only reworked microfossils from older strata (Globotruncana, Heterohelix, etc.).
The geological position and lithological characters of this unit fully agree with the definition
given by Colleuil (1976) of the Somla Sands formation which he attributed to the Tortonian.
Only the upper levels of this formation have been studied at Ashtart 1, about thirty metres
below its top, just to check that there were no occurrences of taxa which had not been seen before
in the overlying beds. Indeed, the assemblages found in the samples of that interval were very
similar to those observed higher up and, because of the clastic lithology, are considered to be a
result of caving.
The Melqart Carbonates Formation
This unit, mainly composed of bioclastic and sometimes oolitic limestones (often vacuolar
due to the dissolution of the clasts and oolites) interbedded with very fossiliferous clays, also
includes strata of nearly reef environment such as biosparitic boundstones built by corals (Porites).
This undoubtedly marine formation is 253 m thick here and extends from 1935 feet (590 m) down
to 2765 feet (843 m). Its top more or less coincides with the last occurrence of Borelis melo. In this
formation, planktonic foraminifers do not occur, but only benthonic forms such as Borelis,
Dendritina, Miliolids, Ammonia gr. beccarii, A. tepida, Elphidium crispum, E. spp., Cellanthus cf.
craticatulus, Elphidiella sp., Glabratella spp., Schakoinella sp., Pararotalia aff. audouini and
Discorbis sp. FourniC (1978), who was the first to describe the Melqart formation, thought it was
of Tortonian age and a lateral equivalent of the Oum Douil formation well known in the outcrops
of northeastern Tunisia, Recently, Bismuth (1984a) moved the Melqart formation to a higher
stratigraphical level, proposing for it a Lower Messinian age, because of the great similarity
between its facies and faunas and those also attributed to the Lower Messinian in outcrops in
Sicily and the Italian peninsula, as well as those in Morocco and western Algeria.
The “Oued be1 Khedim” Formation,
The upper part of the last Upper Miocene series, the “Oued be1 Khedim” formation, 127
metres thick and extending from 1519 feet (463 m) down to 1935feet (590 m), is mostly composed of
brackish or evaporitic sediments with gypsiferous marls, gypsum layers and soft, fine gypsifcrous
sandstones. From its lower part and upwards, one can observe the successive disappearance of all
the marine species inherited from the underlying Melqart limestones and the colonisation by an
association of benthonic foraminifers and ostracods much more tolerant in respect to the wide
variations in environmental salinity and, following this temporary adaptation to high stress
conditions, the extinction of all kinds of life. The last 70 metres of the formation seem to be azoic.
Marine Upper Miocene Ostracods of Well Ashtart I . Gulf of Gab& 1095
As in the whole Gulf of Gab&, the Messinian sediments in Well Ashtart 1, represented by
the Melqart and Oued be1 Khedim formations, appear as the last megasequence of sedimentation
in the Miocene. It is regressive as it undoubtedly begins with marine facies and ends with nearly
emersive thick evaporitic layers. It is also a post-tectonic sedimentary sequence as testified by
the underlying ‘‘SomSia sands” formation which is mollassic facies contingent on intense orogenic
The Messinian sediments of Ashtart 1 perfectly registered the fast detcrioration of the palaeoenvironment that happened in the Mediterranean Sea during the Upper Miocene “salinity crisis”.
The “Raf-Raf” Clays Formation
At a depth of 1519 feet (463 m) a most important unconformity underlines the boundary
between the last evaporitic upper Miocene sediments and those open marine sediments beginning
the Pliocene sedimentary cycle. Indeed, the clays of the Raf-Raf formation, which here are Tabianian
(Lower Pliocene) in age due to the presence of very scarce Globorotalia margaritae, which contrast
sharply in their lithology as well in their planktonic foraminifera1 content with the azoic gypsum
layers of the Oued be1 Khedim formation. They testify to the restoration of normal marine
environmental conditions, perhaps for a shallow water connxtion to the deep sea.
Text-figure 3 shows the stratigraphical and quantitative distribution of 5 1 selected ostracod
On the basis of our data on the vertical evolution of the ostracod assemblages, we have been
able to reach the following preliminary conclusions:
1) The number of species and specimens was generally very high during the Lower Messinian,
with three abundance maxima corresponding to the following levels: A) just above the Tortonian-Lower Messinian boundary (& sample 2700 feet); B) at about the median part of the
Lower Messinian (isample 2390 feet); and C ) just before the Lower-Upper Messinian
boundary (& 2040 feet).
The Tortonian-Messinian boundary did not show any apparent important biostratigraphical
event for the species taken into consideration. In fact, the assemblages found in the Tortonian
detrital sediments are all considered as contaminations from the above Messinian.
2) During the Lower Messinian, mainly corresponding to peak B, we noted the last occurrence
of the following 12 species which seemed to characterise the sediments of the older part of the
Lower Messinian : Aurila n. sp. 3, Cimbaurila n. sp. 1, Cimbaurila n. sp. 2, Cytherella n. sp. 1,
Cytherelloidea petrosa Doruk, Cytherelloidea n. sp. 1, Graptocythere n. sp. 1, Occlusacythereis
n. sp. 1, Ruggieria n. sp. 1, Hemicytherid n. sp. 1, Hemicytherid n. sp. 2 and Hemicytherid n.
3) The part of the Lower Messinian between samples at 2380 feet and 2080 fiizt (a thickness of
about 100 m) showed an abrupt decrease in the abundance of specimens, whereas the number
of species (in spite of the disapperarance of the previously cited 12 species) remains constant.
This interval was followed in the upper part of the Lower Messinian by a new abundance
maximum for the following selected species found in the levels 1970 feet to 2070 feet : Aurila
n. sp. 4, Aurila n. sp. 8, Trachyleberid n. sp. 3, Cytheridea n. sp. 1, and Pterygocythere sp.
We also noted that some species such as Aurila n. sp. 6, Aurila n. sp. 7 , and Actinocythereis
n. sp. 1 reached their maximum abundance at the end of the Lower Messinian, being represented
by only a few specimens in the older sediments.
3-Stratigraphical distribution and abundance of some ostracod species in the Well Ashtart-1 sequence.
1098 G. BONADUCE
4) The end of the Lower Messinian and the subsequent Upper Messinian, samples 1960 feet
to 1530 feet characterised by gypsiferous clays, showed the abrupt extinction of most of the
marine species and the survival of euryhaline species such as Cytheridea? acuminata Bosquet,
Cytheridea n. sp. 2, and an increase in the brackish species Cyprideis gr. torosa (Jones).
5 ) The Upper Messinian, the top part of which is completely azoic, corresponds to the well
known “salinity crisis” and was followed by Pliocene sediments which showed the re-establishment
of normal marine conditions, even if the ostracod fauna taken into consideration appeared
rather scarce and poorly diversified. It was represented by Aurila bradleyana Ruggieri, Aurila
n. sp. 5, Cimbaurila cimbaeformis (Seguenza) and Cytheridea n. sp. 2.
6) The bathymetry of the studied sequence after the Tortonian up to the base of the Upper
Messinian was clearly included within the depth of the continental shelf. The presence of numerous
species of Aurila, Cimbaurila, Peteraurila, Loxoconcha, Hemicytherids, Xestoleberis, Cytheretta,
Callistocythere, Tenedocythere, Urocythereis and Sagmatocythere seems to indicate a general
bathymetry not exceeding 70 m. But in addition to these genera, we frequently noted the
presence of species of Chrysocythere, Trachyleberis, Acanthocythereis, Ruggieria, Costa,
Pterygocythereis, Semicytherura, Krithe and Polycope, the known habitat of which is at depths
exceeding 80 m. Moreover, we observed the constant occurrence of very near-shore and euryhaline
genera such as Cytheridea, Paijenborchella, Miocyprideis and Caudites. The evaluation of the
those assemblages suggests that the palaeobathymetry during the Lower Messinian was about
80 to loom, in the proximity of a coral-reef environment and with strong influence from a
shallower environment. It also suggests the periodic inlet of seasonal brackish-water elements,
possibly from sebkhas bordering the coast.
During the Lower Messinian, we observed the disappearence of some species characteristic
of the deeper environment and belonging to the genera Krithe, Ruggieria, Trachyleberis and
Occlusacythereis, suggesting a regression phenomenon.
Just before the end of the Lower Messinian, at the time underlined by Peak C, normal marine
conditions were temporarily re-established, but only some of the species taken into consideration were able to recolonise the environment and reach their previous quantitative levels.
7) The Upper Messinian was characterised by the catastrophic event known as the “Mediterranean salinity crisis” which caused the complete destruction of the marine ostracod fauna and
the development of a brackish environment as demonstrated by the colonisation by Cyprideis
torosa and the survival of some euryhaline species of Cytheridea before all kinds of life became
8) Above the sample at 1530 feet, and corresponding to the first Lower Pliocene sediments,
the marine environment is re-established. The ostracod fauna however, is very reduced in
number of species, among them ; Aurila bradleyana, Aurila n. sp. 5, Cimbaurila cimbaeformis,
Cytheridea? acuminata, Cytheridea n. sp. 2 and Trachyleberis n. sp. 1. Some of these show a
certain quantitative abundance (Aurila bradleyana and Cytheridea n. sp. 2).
9) As previously mentioned, the Messinian sedimentary sequence is 380 m thick. On the basis
of the previous palaeobathymetrical interpretation, it appears evident that during the marine
Lower Messinian (250 m thick) the bathymetry was more or less constant. This means that
during this interval a subsidence of at least 250 rn occurred.
We are deeply indebted to the management of the oil companies ETAP (Enterprise Tunisienne
d’ActivitCs Pktrolitres) and ELF-AQUITAINE TUNISIE for their kind authorization to publish