Quantative Analysis and Paleoecology of Middle to Upper Eocene Ostracods

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Revue de micropaléontologie 59 (2016) 409–424

Original article

Quantative analysis and paleoecology of Middle to Upper Eocene Ostracods

from Jebel Jebil, central Tunisia

Analyse quantitative et paléoécologie des Ostracodes de l’Éocène moyen et supérieur de Jebel Jébil,

Tunisie centrale

a,∗ b a a

Aïda Amami-Hamdi , Ferid Dhahri , Dhouha Jomaa-Salmouna , Kmar Ben Ismail-Lattrache ,

Najeh Ben Chaabane

Department of Geology, El Manar II, University of Tunis El Manar, Faculty of Sciences of Tunis, 2092 Tunis, Tunisia

Department of Earth Sciences, Sidi Ahmed Zarroug, University of Gafsa, Faculty of Sciences of Gafsa, 2112 Gafsa, Tunisia

Abstract

One hundred and seventy collected samples from Jebil section have been carefully studied for their ostracod content and referred to 41 species

belonging to 20 genera. Their vertical distribution allowed to distinguish ﬁve successive associations of ostracod assemblages; two of which

are correlated with the Early Lutetian, one with the Late Lutetian, another association with the Bartonian and the last one with the Priabonian.

Community structure of the collected ostracod fauna has been studied; three indices have been calculated for each sample: Shannon (diversity),

Margalef (richness) and Equitability indexes. In the lower and the middle part of the Formation, they indicate a stable environment supporting

high diversity ostracod communities; whereas in the upper portion the environmental conditions were unstable characterized by low diversity.

The results of a multivariate statistical method, using the cluster analysis and the Detrended Correspondence Analysis of the 41 ostracod species

and the 170 samples, have led to conclude that the most effective environmental factor in the study area is the paleodepth and of less importance

oxygenation and salinity. Thus, it allowed to distinguish four palaeoenvironmental intervals within the Cherahil Formation: the ﬁrst one represented

by taxa that are known from the shallower parts of the shelf; the second interval includes the majority of the encountered species of inner neritic

shelf with normal salinity; the third one, corresponding to an outer neritic domain; and the last interval refers to a circalittoral environment, is

comprised mainly of Cytherella angulata and of Soudanella laciniosa triangulata.

Keywords: Ostracods; Middle to Late Eocene; Central Tunisia; Community structure; Multivariate statistical analysis; Palaeoenvironmental intervals

Résumé

Cent soixante-dix échantillons prélevés de la section Jebil ont été soigneusement étudiés pour leur contenu en ostracodes ; la faune est composée

de 41 espèces appartenant à 20 genres. La distribution verticale de ces espèces a permis de distinguer cinq associations successives d’ostracodes,

dont deux sont corrélées avec le Lutétien inférieur, une avec le Lutétien supérieur, une autre avec le Bartonien et la dernière avec le Priabonien.

La structure communautaire de la faune à ostracodes recueillie a été étudiée, trois indices ont été calculés pour chaque échantillon : indice de

Shannon Waever (diversité), de Margalef (richesse) et d’Équitabilité. Dans la partie inférieure de la Formation, ils indiquent un environnement stable

soutenant des communautés d’ostracodes à valeurs élevées de diversité ; tandis que dans la partie supérieure, les conditions environnementales

ont été instables caractérisées par une faible diversité. Les résultats de l’analyse de cluster et de l’analyse détendancée de correspondances des

41 espèces d’ostracodes et des 170 échantillons ont permis de conclure que, dans la zone d’étude, le facteur environnemental déterminant est la paléo-

profondeur et de moindre importance l’oxygénation et la salinité. Ainsi, cette étude a permis de distinguer quatre intervalles paléoenvironnementaux

le long de la Formation Chérahil : le premier est souligné par des taxons connus dans les parties très peu profondes de la plateforme interne ; le

∗

Corresponding author.

E-mail addresses: [email protected] (A. Amami-Hamdi), [email protected] (F. Dhahri), [email protected] (D. Jomaa-Salmouna),

[email protected] (K. Ben Ismail-Lattrache), [email protected] (N. Ben Chaabane).

http://dx.doi.org/10.1016/j.revmic.2016.10.001

410 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

second comprend la majorité des espèces rencontrées aux environnements à salinité normale de la plateforme interne ; le troisième correspond au

domaine de plateforme externe et le dernier intervalle traduit un environnement circalittoral, composé principalement de Cytherella angulata et de

Soudanella laciniosa triangulata.

Mots clés : Ostracodes ; Éocène moyen à supérieur ; Tunisie centrale ; Structure communautaire ; Analyse statistique multivariée ; Intervalles paléoenvironnementaux

1. Introduction intercalations of bioclastic limestone (oyster rich) and marls rich

in foraminifers and ostracods. These deposits correspond to the

Several micropalaeontological (Oertli, 1976; Bismuth et al., Cherahil Formation (Comte and Dufaure, 1973), which is sub-

1978; Said-Benzarti, 1978; Bismuth, 1981; Mechmeche, 1981; divided into three units, which are as follows from base to top:

Ben Ismail-Lattrache and Bobier, 1996; Ben Ismail-Lattrache, the Lower Cherahil, the Siouf and the Upper Cherahil Members

2000; Trabelsi et al., 2015) and petroleum studies (Said-Benzarti (Fig. 2).

and Kharbachi, 1995) have been realized on ostracods from

Tunisia. 2.1. Lower Cherahil Member

However, rare are the studies that attempted a paleoecolog-

ical analysis of Eocene ostracod assemblages from Tunisia. As This member can be divided into three main units:

to other countries from North and West Africa, there exist sev-

•

eral studies related to the paleoecology and paleogeography of the lower unit (J1–J48) is 46 m thick and comprised of lam-

Eocene ostracods (Elewa et al., 1999; Elewa, 2002; Elewa, 2004; inated green clays with crystallized calcite interbedded with

Elewa, 2005; Shahin, 2005; Elewa, 2007; Shahin, 2008; Amami- thin argillaceous limestones and a lumachellic limestone bed;

•

Hamdi and Ben Ismail-Lattrache, 2013; Amami-Hamdi et al., the middle unit (J48–J88) is 40 m thick and comprised of

2014). In Libya, El Waer (1992) used the ostracod morphology yellowish sandy clays, sometimes gypsiferous and slightly

and carapace nature of Eocene ostracods for paleoenvironmental phosphatic with ferruginous concretions and ﬂint nodules.

reconstructions. In West Africa, Sarr (1995 and 1999) studied These levels are interbedded with centimetric dolomite beds

the migration of ostracods during the Middle Eocene and their rich in nummulites;

•

relation with the paleoenvironmental evolution of the western the upper unit (J88–J122) is 35 m thick and composed of

senegalian deposits; the same author (2012) illustrated a link laminated gray-green clays intercalated with fossiliferous

between changes in ostracod faunas and the paleogeographic argillaceous limestones containing ferruginous concretions.

evolution of the Senegalese basin. Elewa et al. (2001) focused

his study on the reconstruction and interpretation of the paleoen- 2.2. Siouf Member

vironmental conditions that prevailed during the deposition of

the Middle Eocene succession of Northern Somalia by means of It is 5 m thick and composed of lumachellic gray sandy

ostracod assemblages. In Egypt, Elewa (2004) realized a pale- limestones. This level is rich in large benthic foraminifera: Num-

oecological study of the Eocene series in the region of Cairo, mulites gizehensis, Discocyclina roberti, Discocyclina sella,

based on the quantitative analysis of ostracod assemblages. Operculina sp. and Alveolina sp.

Mixed methods integrating quantitative and qualitative data

collection and analysis were explored in recent studies (Gliozzi 2.3. Upper Cherahil Member

and Grossi, 2004; Mazzini, 2004, 2005; Elewa, 2004, 2005;

Guasti, 2005; Van Itterbeeck, 2007, Gliozzi and Grossi, 2008; This member can be divided into two units:

Grossi and Gennari, 2008; Sarr, 2012; Amami-Hamdi and Ben

•

Ismail-Lattrache, 2013 and Amami-Hamdi et al., 2014). They the lower unit (J129–J149) is 20 m thick and represented by

were applied to Middle and Upper Eocene deposits, which are green laminated claystones, rich in iron oxides and gypsum,

rich in pelagic microfauna. and intercalated with centimetric dolomite beds;

•

We have sampled and studied in detail the Middle and Upper the upper unit (J149–J184) is 35 m thick, comprised of ﬁne

Eocene succession of the Jebil outcrop in order to obtain a greenish sandy clays and interbedded with metric lumachellic

detailed report of its ostracod assemblages. limestones, rich in Ostrea lamellosa. These limestones are

represented by biomicritic grainstone texture with bryozoans,

2. Stratigraphy bivalves and algae.

The Jebil section is located in central Tunisia, near the Khit 3. Materials and methods

◦ ◦

El-Oued village (18 10 N and 3 71 E); the area is covered by

the geological map of Haffouz (1:50,000 scale) (Fig. 1). Many authors have analyzed different aspects of the Jebil

The Middle and Upper Eocene marine deposits are section. Ben Ismail-Lattrache (2000) has been the ﬁrst to

200 m thick and well exposed in Jebel Jebil. They present study the planktic and benthic foraminifera, as well as the

A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424 411

Fig. 1. Geological map of the study area (after Dhahri and Boukadi, 2010).

412 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

Fig. 2. Lithostratigraphy and description of microfacies observed in the Jebil section.

A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424 413

ostracod fauna. Subsequent studies have covered structural Apostolescu and Magné (1956) is attributed to the same zone

aspects (Rigane et al., 1994; Rabhi, 1999; Dhahri and Boukadi, of this interval. In Egypt, Shama and Helal (1993) assigned

2010). the Costa praetricostata praetricostata zone to the Early

One hundred and seventy samples were collected for this Lutetian.

study; they were washed after treatment with diluted H2O2. The Loculicytheretta semirugosa interval zone: this zone com-

dried residues were divided into three grain-size fractions (after prises the interval between two last occurrence (LO) events, the

they were collected with sieves of 250, 100 and 63 ␮m, in mesh). LO of L. semipunctata (Apostolescu and Magné, 1956) at the

◦

Then they were dried in an oven at 50 C. The picked ostra- base and the LO of L. semirugosa at the top. It occurs within

cods and planktic foraminifers were identiﬁed with the help of the middle part of the Lower Cherahil member. In Tunisia,

a stereoscopic binocular. Bismuth et al. (1978) assigned a late Early Lower Lutetian age

The Middle and Upper Eocene deposits have delivered to this zone, characterized by abundant ostracods, characteris-

a rich microfauna characterized by abundant ostracods and tic of the Early Lutetian: Loculicytheretta prima Bismuth and

foraminifers. The preservation of the planktic foraminifers and Oertli, 1978; Soudanella tarabulusensis El Waer, 1992; Para-

ostracods is in general good. Taxonomic analysis and species cypris eskeri Bassiouni and Morsi (2000); Paracypris buisae

identiﬁcation of ostracods was completed with reference to Morsi, 2003 and Leguminocythereis sadeki Bassiouni, 1969.

previous works (Bassiouni 1969, 1971; Oertli, 1976; Bismuth, This interval zone is attributed to the Lutetian in Algeria (Apos-

1981; Mechmeche, 1981 and El Waer, 1992). The species abun- tolescu and Magné, 1956) and to the Middle Lutetian (El Waer,

dance and richness were calculated and normalized to 10 g 1992) in Libya.

of each dried and sieved sample. An analysis of the commu- Loculicytheretta minuta interval zone: this zone is cha-

nity structure (i.e., species richness, Shannon, Margalef and racterized by the extinction of both L. semirugosa at its base

Equitability indexes; Dodd and Stanton, 1990) and multivari- and the markers species at its top. It occurs within the upper

ate analysis (i.e., cluster analysis and detrended correspondence part of the Lower Cherahil marls of Late Lutetian age. This

analysis), were performed on our ostracod dataset with the help assemblage zone is characterized by the presence of abundant

of the PAST software-PAleontology STatistics: 1.52 v (Hammer ostracods (Loxoconcha tarabulusensis El Waer, 1992; C. angu-

et al., 2001). lata El Waer, 1992; Bairdia samdunae El Waer, 1992 and

Soudanella laciniosa triangulata Apostolescu, 1961) associated

4. Results with larger benthic foraminifers such as Nummulites gizehensis

(Forskal, 1775) (A and B forms) and N. discorbinus Schlotheim

4.1. Biostratigraphy (1820).

In central Tunisia, Mechmeche (1981) described this zone in

Collected from the Upper and Middle Eocene deposits of the the Middle Eocene deposits. In Libya, El Waer (1992) assigned

Jebel Jebil, the ostracod assemblages comprise more than 41 a Late Lutetian to this zone.

species belonging to 20 genera. Loculicytheretta cavernosa interval zone: this zone of Bar-

The zonation of these ostracods, used in this paper (Oertli, tonian age, is restricted between two LO respectively those

1976; Bismuth et al., 1978; Mechmeche, 1981; El Waer, of L. minuta at the base and L. cavernosa (Oertli, 1976)

1992), is calibrated with the coeval local planktic foraminifera at the top. It occurs within the lower part of the Upper

biozonal scheme (Ben Ismail-Lattrache, 2000). A correlation Cherahil marls. It is correlated with the Middle Eocene Hep-

between these zones with their regional and worldwide equiv- taloculites cavernosa zone recognized by El Waer, 1992 in

alents has been realized. From base to top, we distinguish Libya. In Egypt, this zone is correlated with the Lutetian Dig-

(Fig. 3): mocythere ismaili–Uromuellerina saidi zone (Shahin et al.,

Loculicytheretta semipunctata interval zone: it constitutes 2008).

the range zone of the marker species L. semipunctata, corre- Loculicytheretta aff. gortanii interval zone: this zone is

lated with the Turborotalia frontosa planktonic foraminiferal characterized by the LO of both L. cavernosa at its base

zone Berggren and Pearson (2005) of earliest Lutetian age. and the marker species at its top. The co-occurrence of the

This biozone is recognized in Tunisia by Oertli (1976), nominate taxon with the Priabonian taxon Loculicytheretta tune-

Bismuth et al. (1978), Mechmeche (1981), Said-Benzarti and tana (Oertli, 1978), Propontocypris tarabulusensis El Waer,

Kharbachi (1995), Ben Ismail-Lattrache (2000), Amami-Hamdi 1992 and Hermanites libyaensis El Waer, 1992 and with the

and Ben Ismail-Lattrache (2013) and Amami-Hamdi et al. planktic foraminiferal marker species Turborotalia cerroazulen-

(2014) within the base of the Lower Cherahil marls. It is sis Cole, 1928 (Wade and Pearson, 2008) allows to correlate

characterized by the abundance of Loculicytheretta harshae, this interval with the Priabonian. In Tunisia, Oertli (1976),

Buntonia ramosa Bassiouni, 1969; Argilloecia ghalilae El Waer, Bismuth et al. (1978), Mechmeche (1981), Said-Benzarti and

1992; Costa libyaensis El Waer, 1992; Acanthocythereis salahii Kharbachi (1995), Ben Ismail-Lattrache (2000) and Amami-

Bassiouni, 1969; Acanthocythereis tarabulusensis El Waer, Hamdi and Ben Ismail-Lattrache (2013) described this zone

1992; Paleocosta mokattamensis Bassiouni, 1969. In Libya and in the Priabonian deposits. In Libya, this zone is correlated

according to El Waer (1992), this biozone is homologous to the with the Priabonian Heptaloculites aff. gortanii zone (El Waer,

Heptaloculites harshae zone of Early Lutetian age. In Algeria, 1992).

414 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

Fig. 3. Stratigraphic distribution of ostracofauna in the Middle and Upper Eocene lithostratigraphical units of the Jebel Jebil.

A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424 415

Fig. 4. Diagrams of the community structure indices of 41 ostracod taxa samples recorded from the Middle and Upper Eocene intervals of the Jebil section.

4.2. Paleobioecology diversity communities dominated by the taxa Loculicytheretta

aff. gortanii.

4.2.1. Community structure analyses- diversity indices

Community structure analyses have been performed on the 4.2.2. Multivariate analysis

ostracod assemblages collected from the Cherahil Formation, 4.2.2.1. Cluster analysis. In order to depict the detailed pale-

in the Jebil section. Diversity takes into account not only the oenvironmental evolution of the Middle to Upper Eocene

number of species, but also the distribution of individuals in succession at the Jebil section, a multivariate statistical approach

these species. Three assemblage structure indexes were calcu- was carried out based on the analysis of ostracod assemblages.

lated for each sample: Shannon (diversity), Margalef (richness) Cluster analysis based on the similarity coefﬁcient (Jaccard coef-

and Equitability indexes (Fig. 4). ﬁcient) using the paired-group method was applied to the data

In the lower part of the Cherahil Formation, samples display matrix. Hence, four distinct biofacies can be discerned (Fig. 5),

a rather medium diversity and Equitability (which may reach which are often characteristic of a speciﬁc paleoenvironment:

the value of 1), coupled with high richness up to a maximum Biofacies A: is represented by A. salahii, P. mokattamensis,

value of 4 (Samples J2, J10, J56 and J59), indicating a stable C. libyaensis and Isobuntonia pseudotuberata. This ostraco-

environment able to support a mature assemblage. fauna was encountered at the base of the Lower Cherahil

Within the middle part of the Cherahil Formation (J112 to Member. This biofacies has a low P/B ratio (8%).

J152), samples display a high diversity and richness with maxi- Biofacies B: which includes species that are increasingly

mum values of Shannon (1.6) and Margalef indexes (up to 5 in abundant in the L. semipunctata and L. semirugosa zones. The

sample J129), coupled with a peak of Equitability around 1.6. majority of these species are recognized in the Lower Cherahil

These parameters reﬂect a relatively stable environment, leading Member. This biofacies has a low P/B ratio (5%).

to the establishment of several mature communities of ostracods. Biofacies C is the most visible cluster in the dendrogram and

The top of the Upper Cherahil Member (J152 to J184), dis- regroups species that are more common in the upper part of the

plays very low values for the Shannon and Equitability indexes, Lower Cherahil Member and the lower part of the Upper Cher-

coupled with low species richness (with a Margalef index ahil Member, and are dominant in the L. minuta and L. cavernosa

value below 1), indicating an unstable environment with low zones. The P/B ratio increases to reach a maximum at 20%.

416 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

Fig. 5. R mode clustering (taxa) based on Jaccard coefﬁcient applied to 41 ostracod species. The Pelagic index is also provided.

Biofacies D is represented by species which dominate the Group I: same as Biofacies D, is represented mainly by

ostracod record in the Loculicytheretta aff. gortanii zone. This opportunistic species of high frequency (Loculicytheretta aff.

biofacies is mainly present in the top of the Upper Cherahil gortanii); Group II: similar as Biofacies C includes the species

Member. The Pelagic index is relatively high (42%). As shown of ostracods L. minuta, L. tunetana, L. cavernosa, C. angulata,

in Fig. 5, biofacies D is characterized by several reticulated L. tarabulusensis, Soudanella laciniosa triangulata, B. sam-

ostracod fauna. dunae, Asymmetricythere youseﬁ and Brachycythere omarai;

It is worth noting the coincidence of community analyses Group III: the same as Biofacies B, are characterized by

results and cluster analysis results. the presence of species L. semipunctata, L. semirugosa, Lox-

oconcha vetustopunctatella, Reticulina proteros, P. eskeri,

L. sadeki, Costa aff. bassiouni, Costa aff. saidi and A. ghalilae;

Group IV: similar as Biofacies A includes C. libyaen-

4.2.2.2. Detrended Correspondence Analysis (DCA). In the

sis, I. pseudotuberata, P. mokattamensis and A. salahii

studied section, detrended correspondence analysis performed

species.

on the 170 samples was used to analyze the ostracod assem-

Species Leguminocythereis cirtaensis and Legu-

blages. The resulting DCA species plot shows two axes (with

minocythereis africana, occupying a position at the same

positive values greater than 1) that account respectively 51 and

distance of Groups II and III, with low frequencies, are present

16% (Fig. 6). The maximum information is provided by Axis 1

in almost all the analyzed samples.

(51%); the ﬁrst two axes represent 67% of the total information

In general, the positions of the samples in the diagram appear

about the correlations between variables. The graphic tests with

to be strongly inﬂuenced by the microfaunal frequency and

axis 3 to 7 do not provide additional information, giving a less

diversity. Nevertheless for both Groups I and IV, they appear

clear picture of the distribution of variables. However, the graph

to be only characterized by low frequency of ostracods rather

relating the Axis 1 and Axis 2 was used to conﬁrm the ostracod

than taxonomic composition.

clusters distinguished by the dendrogram.

There are several palaeoecological data on the Eocene ostra-

The distribution of the ostracod species of Jebil section on

cods of North and West Africa (Babinot, 1973; Bismuth et al.,

the ﬁrst two axis allows to distinguish four clusters according to

1978; Colin and Carbonel, 1982; Donze et al., 1982; Peypouquet

Axis 1 (the ﬁrst one represents negative values: < −0.5 and the

et al., 1983; Carbonnel and Johnson, 1989; Bassiouni and Luger,

three other ones represent positive values: > 0.25, that match the

1990; El Waer, 1992; Keen et al., 1994; Elewa and Ishizaki,

four clusters previously recognized:

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Fig. 6. Scatter plot of 41 ostracod species recovered from 170 samples coming from the Middle and Upper Eocene intervals of the Jebil section according to Detrended

Correspondence Analysis taken along two Axes.

1994; Elewa et al., 1998; Elewa, 1999; Elewa et al., 1999; Thus, the Axis 1 which accounts 51% of the total variance

Elewa, 2004; Elewa and Morsi, 2004; Elewa, 2005; Elewa, 2007; could represent the ecological parameter “depth”. The interpre-

Sarr, 2012; Amami-Hamdi and Ben Ismail-Lattrache, 2013 and tation of ostracod cluster distribution along this axis reﬂects

Amami-Hamdi et al., 2014), which afford a possible synecolog- signiﬁcant sensitivity to paleodepth ﬂuctuations. However, the

ical characterization of ostracod assemblages. Axis 2 which accounts 16% of the total variance might represent

Group II dominated by rather deep taxa like C. angu- the ecological parameters salinity or oxygenation.

lata, B. samdunae and Soudanella laciniosa triangulata, which

reﬂect an outer platform domain (Bassiouni and Luger, 1990).

While, the ostracode fauna recorded in Group III are domi- 5. Paleoecological implications

nated by taxa (Reticulina proteros and P. eskeri) known from

the deeper parts of the shelf: outer shelf (Bassiouni and Luger, The result of quantitative analyzes (community analyses,

1990) and reﬂecting reduced dissolved oxygen level (Babinot, cluster analysis and DCA) of 41 species of ostracods and qual-

1973; Colin and Carbonel, 1982; Whatley and Coles, 1991). itative analysis of the associated planktic foraminifers allowed

The ostracod assemblage of Group IV speciﬁcally proliferates to give important information that distinguish several palaeoen-

in an inner environment (Peypouquet et al., 1983). In fact, vironmental intervals within the Cherahil Formation (Fig. 7).

at the presence of sighted species with eye tubercle such as Interval 1- Biofacies A–B and Group III–IV (samples

Acanthocythereis typical of infralittoral environment of 0–150 m J1–J88); ostracod association reveals a rather medium diversity

depth (Keen et al., 1994) where minimum oxygen zone is and a high richness with the occurrence of signiﬁcant species

low and the deep ocean circulation is sufﬁcient (Donze et al., of middle to outer platform environment with the abundance of

1982), conﬁrms this attribution. Else, the abundance of a high inner neritic taxa with normal salinity (Whatley, 1983). How-

ornamented form (P. mokattamensis and Costa spp.), which ever, the low P/B ratio indicates a shallow water depth. The

tolerate a high pH and Mg++/Ca++ ratio (Peypouquet et al., relationship between environmental variables and species sug-

1980 in Carbonel, 1988), may indicate deposition at inner shelf gests a preference of these species to oxygenated conditions and depth. relatively diversiﬁed ecosystem.

418 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

Fig. 7. Paleoecological and palaeoenvironmental evolution (Paleodepth) of the Cherahil Formation in the Jebil core.

A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424 419

Fig. 8. Distribution of the Middle and Upper Eocene Ostracoda associations in Jebil section from the coastal line to the abyssal domain.

Interval 2- Biofacies C and Group II (samples J96–J149); the Interval 3- Biofacies D and Group I (samples J152–J184);

subsequent dominance of the taxa Soudanella laciniosa triangu- show a very low diversity and richness indexes coupled with

lata, C. angulata and B. samdunae that seem to have inhabited a very high faunal dominance characterizing an association of

the shallow inner shelf as well as the deep outer shelf depths opportunistic species: Loculicytheretta aff. gortanii, which tol-

(Bassiouni and Luger, 1990; Whatley and Coles, 1991), together erate spontaneous variations of depth and dissolved oxygen in

with high values of the diversity indexes indicates deepening the upper part of the section. The higher pelagic index suggests

trend during deposition of interval 2 from inner to outer neritic an increase of water depth from an outer neritic to a circalittoral

depths, and marks of environmental stability. These data cou- environment, that was conﬁrmed by the global sea trend marked

pled with a relative increase of pelagic index suggest an outer in the Priabonian by Miller et al. (2005). It reﬂects semi pelagic

neritic domain with normal salinity. within a circalittoral environment.

420 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

Plate 1. Examples of typical ostracod species collected from the studied section: 1, Acanthocythereis salahii Bassiouni, 1969 × 100, lateral view of right valve, Lower

Lutetian (sample J10); 2: Loculicytheretta semipunctata (Apostolescu and Magné, 1956) × 150, carapace in ventral view, Lower Lutetian (sample J41); 3: Loculi-

cytheretta semirugosa (Apostolescu and Magné, 1956) × 100, lateral view of right valve, Upper part of Lower Lutetian (sample J14); 4: Loculicytheretta minuta

(Oertli, 1978) × 200, lateral view of right valve, Upper Lutetian (sample J122); 5, 6: Reticulina proteros Bassiouni, 1969 × 150, 5- carapace in dorsal view; 6- lateral

view of left valve, Lower Lutetian (sample J58); 7: Loculicytheretta tunetana (Oertli, 1978) × 150, carapace in ventral view × 150, Bartonian (sample J145); 8:

Loculicytheretta cavernosa (Apostolescu and Magné, 1956) × 150, carapace in ventral view, Upper Lutetian-Bartonian (sample J122); 9: Loculicytheretta aff. gor-

tanii (Ruggieri, 1963) × 150, carapace in ventral view, Priabonian (sample J155); 10: Loculicytheretta harshae El Waer, 1992 × 100, carapace in ventral view,

Lower Lutetian (sample J14); 11: Xestoleberis tarabulusensis El Waer, 1992 × 200; lateral view of right valve, Bartonian (sample J140); 12, 13: Ruggiera aff.

glabella Bassiouni, 1969 × 100, 12- lateral view of right valve, 13- carapace in ventral view, Lower Lutetian (sample J1); 14: Brachycythere (Digmocythere) ismaili

Bassiouni 1971 × 100, carapace right side, Lower Lutetian (sample J14).

A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424 421

Plate 2. Examples of typical ostracod species collected from the studied section: 1: Isobuntonia pseudotuberata (Apostolescu and Magné, 1956) × 150, lateral

view of left valve, Lower Lutetian (sample J10); 2: Loxoconcha vetustopunctatella Bassiouni, Bou Khary, Shama and Blondeau, 1984 × 150, lateral view of

right valve, Lutetian (sample J46); 3: Soudanella laciniosa triangulata Apostolscu, 1961 × 150, lateral view of right valve, Upper Lutetian (sample J122); 4:

Acanthocythereis tarabulusensis El Waer, 1992 × 100, lateral view of left valve, Lower Lutetian (sample J2); 5, 6: Asymmetricythere youseﬁ Bassiouni, 1971 × 100,

5- lateral view of left valve, 6- carapace in dorsal view, Middle Lutetian (sample J129); 7: Soudanella tarabulusensis El Waer, 1992 × 150, lateral view of

right valve, Lower Lutetian (sample J66); 8: Costa aff. bassiounii Cronin and Khalifa, 1979 × 100, lateral view of left valve, Lower Lutetian (sample J16); 9:

Loxoconcha tarabulusensis El Waer, 1992 × 150, lateral view of right valve, Priabonian (sample J155); 10: Bairdia samdunae El Waer, 1992 × 75, lateral view

of right valve, Upper Lutetian (sample J100); 11: Parcypris eskeri Bassiouni and Morsi, 2000 × 150, lateral view of right valve, Lower Lutetian (sample J52); 12:

Argilloecia ghalilae El Waer, 1992 × 100, lateral view of right valve, Lower Lutetian (sample J41); 13: Cytherella angulata El Waer, 1992 × 150, lateral view of

right valve, Upper Lutetian-Bartonian (sample J118).

422 A. Amami-Hamdi et al. / Revue de micropaléontologie 59 (2016) 409–424

Paleoecological model paleoenvironmental evolution in space and in time. The sec-

The paleoecology of ostracods, typical of Southern Tethys, ond one is based on statistical approach (cluster and detrended

studied in Algeria (Faid, 1999), Libya (El Waer, 1992; Whatley correspondence analysis), considering different environmental

and Arias, 1993) and Egypt (Elewa and Ishizaki, 1994; Elewa, variables such as paleodepth, paleosalinity and oxygenation. It

1997; Elewa et al., 1998; Elewa, 1999; Elewa et al., 1999; Elewa, has permitted to determine the possible relationship between the

2002; Elewa, 2004; Elewa and Morsi, 2004; Elewa, 2005 and distribution of the Middle and Upper Eocene ostracods and the

Elewa, 2007) is adopted for the reconstruction of the paleoeco- oceanographic characteristics of central Tunisia. These analyses

logical model of Tunisian ostracods. revealed that the ostracod associations provided by the studied

The variations of ostracod associations obtained from the material are characteristic of a platform environment from inner

studied section and the resultant statistical analysis (Hierarchical shelf to circalittoral.

and Correspondence detrended analysis) led to the reconstruc-

tion of a model showing the marine ostracod distribution

Disclosure of interest

depending on the depth, salinity and oxygen.

In conclusion, this paleoecological model has permitted to

The authors declare that they have no competing interest.

distinguish different paleoenvironments according to the studied

ostracod associations (Fig. 8):

Association A- represented by Loculicytheretta aff. gortanii Acknowledgments

and H. libyaensis that are known from the shallower parts of the

shelf: inner shelf.

The authors are grateful to Taniel DANELIAN (Revue

Association B- includes the taxa A. salahii, A. tarabulusensis,

de Micropaléontologie Editor-in-Chief) and to the reviewers

Costa aff. bassiouni and P. mokattamensis, speciﬁcally prolifer-

Ashraf M.T. ELEWA (Minia University, Al Minya,¯ Egypt)

ates in an inner neritic environment.

and Raphaël SARR (Cheikh Anta Diop University, Dakar) for

Association C- comprising of Reticulina proteros and

thoughtful comments that improved the manuscript.

P. eskeri that are known from the deeper parts of the shelf: outer

shelf.

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