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Chapter 75. Marine ostracods of the Upper Miocene of the well Ashtart 1 (Gulf of Gabès, southeastern Tunisia)

Chapter 75. Marine ostracods of the Upper Miocene of the well Ashtart 1 (Gulf of Gabès, southeastern Tunisia)

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1088 G. BONADUCE

et al.



INTRODUCTION

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

Parakrithe.

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.



GENERAL

DATA

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.

1. LV(x59).



PLATEl-Fig.



Marine Upper Miocene Ostracods of Well Ashtart I , Gurfof Gab& 1091



x



TEXT-FIG.

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).



PLATE2-Fig.



1092



TABLE

~-LITHO-AND

CHRONOSTRATIGRAPHYOF THE W E L L ASHTART 1.



Appearance marine microfauna

Top frequent planktonic microfauna

Diagraphic boundary

Top of gypsum

Appearance marine microfauna

Diagraphic boundary

Appearance Borelis



Depth

feet

m

0’- 0

254‘- 77

404‘- 123

813’- 248

1125’- 343

1420’- 433

1519’- 463

1540’- 469

1780’- 542

1935’- 590

1980’- 603



Diagraphic boundary



2765’- 843



Diagraphic boundary



3604’-1099



256



Diagraphic boundary

Top frequent planktonic microfauna

Diagraphic boundary



4240’-1292

4340’-1323

4773’-1455



356



Litho-biostratigraphy

Depth datum level rotary table

Sea bottom

1st sample collected



Thickness(m)



46

125

95

90

30

127

253



6



Diagraphic boundary



4795’-1461



Diagraphic boundary



5186’-1581



Appearance Amphistegina



5220-1591



Appearance Heterostegina



5240’-1597



Appearance Miogypsina



5500’-1676



Appear. Eulepidina & Spiroclypeus



5580’-1701



Appear. Miogypsinoides cf. complanata



5730’-1746



Appear. Pararotalia viennoti Imexicana



6000’-1829



Diagraphic boundary



6345’-1934



Appearance Nummulites

Diagraphic boundary

Appear. Loculicytheretta cavernosa



6360’-1938

6368’-1941

6900’-2103



162



Diagraphic boundary



7548’-2300



241



Appear. Loculicytheretta minuta



7690’-2344



Diagraphic boundary



7771’-2368



Diagraphic boundary



9126‘-2781



Top oil-field with Nummulites

Base oil-field

Diagraphic boundary

Diagraphic boundary

Final depth reached



9448’-2880

9704‘-2958

9777’-2980

9787’-2983

9900’-3017



120



360



Age



Quatern.

(266 m)

Pliocene

(120m)



Formations



Holocene

Not observed

Villafranchian

Piacentian

Tabianian Raf-Raf

Upper

Oued be1 Khedim

Messinian



Upper

Miocene

(636m)



Lower

Melqart

Messinian

Tortonian Somag

Middle

Serravallian

Oum Douil

Miocene

to

(462m)

Langhian

Mahmoud

Ain Grab

Middle

Langhian

Salammb6

Miocene basal

(basal)

Burdigal.

to

Aqui t an.

Lower

Miocene

Ketatna

(240m)

Basal

AquitaniG

Miocene Chattian

and

Upper Stampian

Oligocene

(233 m)

Lower

Oligocene Lower

(7 m)

Stampian

“vascus”

Priabonian

Eocene



Upper

Lutetian



Cherahil

“B”

Reidche



Lower

Lutetian



536

(1076,50)

78



Cherahil

“A”

El Guerria



Ypresian

59,50



Chouabine



BUROLLET 1956



BIELY et AL. 1972



6



I



WIMAN 1976



COLLEUIL 1976



~~



HAMMAMET

SANDS



J V



S l D l BARKA

CLAYS

NABEUL

SANDS

POTlERS

CLAYS



OUED EL BIR



HAMMAMET

SANDS



f ';1

z

0



5



-



BISMUTH 1984



w

0



2



SlDl



BARKA

CLAYS



'



0 - 1

P



2



NABEUL

SANDS



W



z

W



0



0

-J

a



POTIERS

CLAYS



. g



MESSINIAN OUED EL BIR



UPPER

IESSINIA

LOWER



z



-z



a



3FF-SHORE . DRlLLlNGS

FOURNIE 78 BISMUTH 84



Raf -Rof



Raf-Rat



0. Be1 KheLm



0. Eel Khedm



Melqart



Melqart



u)



In

w



I



I



I



I



1



z

z

0

c



4



a



0



tv)



TORTONIAN

Bdqlio



I



Ain Grob



#fi

I



I



iERRAVAI



LANGHl4

BURDIGAL.



BURDIGA



I



~exr-plo.2-Miocene lithostratigraphical units of East Tunisia (Bismuth, 1984).



I



Ain Grob



Ain Grob



I

I



Ain Grab



I



I



1094 G.BONADUCE

et al.



LITHO-AND

BIOSTRATIGRAPHY

In Table 1 the bio-, litho-, and chronostratigraphical frame of the complete Well Ashtart 1 is

presented.

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

activity.

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.



THEOSTRACOD

ASSEMBLAGES

AND THEIR

SIGNIFICANCE

Text-figure 3 shows the stratigraphical and quantitative distribution of 5 1 selected ostracod

species.

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.

sp. 4.

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.



1096



TEXT-FIG.

3-Stratigraphical distribution and abundance of some ostracod species in the Well Ashtart-1 sequence.



1097



- -



1098 G. BONADUCE

et a].



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

extinct.

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.



ACKNOWLEDGEMENTS

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



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