The Hammamet, Gabès and Chotts basins (Tunisia): a review of the subsidence history

The Hammamet, Gabes and Chotts basins (Tunisia):

a review of the subsidence history

Martin Patriata, Nadine Ellouza,*, Zied Deyb,Jean-Michel Gauliera, Hatem Ben Kilanib

a Institut Franc�ais du Petrole, 1–4 Avenue de Bois Preau, 92852 Rueil-Malmaison cedex, FrancebEntreprise Tunisienne d’Activites Petrolieres, BP 367, 1002 Tunis, Tunisia

Received 20 October 1999; received in revised form 4 July 2001; accepted 19 July 2002

Abstract

We modeled the subsidence of three Mesozoic basins in Tunisia by using well data, seismic lines and field data. The results

are presented here with an attempt to relate the succession of events deduced from the subsidence evolution analysis to the

geodynamic evolution of the northern African margin.

The Mesozoic and Cenozoic geological history of Tunisia has been broken down into two major periods separated by a

transition period. The first period from Trias to Aptian was characterized by a constant subsidence of variable intensity spread

over most of Tunisia, and over all the studied basins. This period was dominated by extension accompanying the Tethyan rifting

during Trias–Lias. The transition period during the upper Cretaceous was characterized by extension and by the weak effects of

far-seated compressional events. It is coeval with the first appearance of salt mobilization. The last period, in Tertiary, marked

the appearance of the major collisional events, mostly during Neogene. During this last period, basins were characterized by

very different behaviour, depending on their location: East or West of the North–South axis and close or not to the Tellian

nappes.

Among the three basins we studied, we paid special attention to the southernmost Gabes basin located south of the present

eastern margin of Tunisia. The first-order evolution of its subsidence is similar to that we infer for most of the Tunisian basins.

We emphasize the role of the structuration of the Gabes basin in the partition of the subsidence through time.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Tunisia; Maghreb; Atlas; Subsidence modeling; Tectonic controls; Paleostress field

1. Introduction

The present-day margin of North Africa repre-

sents the southern margin of the Miocene western

Mediterranean basin. The original southern margin of

the Tethyan Ocean, Triassic to Lower Jurassic in age,

has been highly tectonized during Cenozoic. At

present, the paleomargin is represented in the Atlas

regions of the Maghreb.

This paper aims to bring out elements about the

geological history of the Tethyan margin by a study

of its subsidence through time. It focuses on the

evolution of Tunisian basins located on the ancient

Tethyan margin. We used industrial wells, seismic

0037-0738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0037 -0738 (02 )00290 -7

* Corresponding author. Fax: +33-1-47-52-70-67.

E-mail address: [email protected] (N. Ellouz).

www.elsevier.com/locate/sedgeo

Sedimentary Geology 156 (2003) 241–262

lines and field data from the literature to reconstruct

the Meso- and Cenozoic subsidence of these basins.

The evolution of subsidence rates of three Tuni-

sian basins (Fig. 1) will be described on the Hamma-

met, Gabes and Chotts basins. We correlate the

temporal variations of the subsidence rate to the

margin evolution from rifting to tectonic inversion.

The subsidence of the Atlas basins began in Triassic

time or even before (Permian), and extensional pro-

cesses continued through Jurassic time. Thus, a short,

well identified, rifting phase is difficult to isolate.

Unfortunately, most of the wells are not reaching

these levels. The Permo-Triassic rifting phase, well

documented at the surface, will not be discussed here

at a basin scale because of the lack of information in

well data. Only the evolution since Lias is illustrated

in this paper.

We subsequently study in detail the Gabes basin by

an analysis of the evolution of subsidence rates along

two cross-sections. One is parallel and one is perpen-

dicular to the NW–SE faults which constitute the

structural scheme in this basin.

Fig. 1. Schematic structural map of Tunisia modified after Ben Ferjani et al. (1990). CB: Cap Bon; Gt: Gafsa trough; Kd: Kairouan depression;

Ki: Kasserine Island; Kk: Kerkennah; Me: Medenine; SAf: South Atlas fault.

M. Patriat et al. / Sedimentary Geology 156 (2003) 241–262242

2. Geological setting

2.1. Stratigraphy (Fig. 2)

2.1.1. Trias

The major paleogeographic zonation of Trias took

place during the Late Triassic. The Late Triassic series

show a lateral facies change along a NW–SE line

joining Cap Bon toward the Garian mole (Libya).

To the west, the salt Triassic domain belonging to

the Germanic type is linked to the large evaporitic

Saharan basin. To the east of this line, the Alpine type

carbonate facies prevail, similar to that observed in

southern Tunisia (J. Rebach), in Libya and in Sicily.

2.1.2. Jurassic

In central and eastern Tunisia, the Jurassic is

composed of platform facies limestones and dolo-

mites. To the North, the series were deposited in more

open marine conditions. An elongated high zone

called the N–S axis is identified, comparable to the

northeastern part of Tunisia where reduction or hiatus

is observed. Two main subsiding areas are distin-

guished, the subsiding Tunisian trough characterized

by deeper facies (Ammonitico Rosso), and the

‘‘Chotts’’ region characterized by platform carbo-

nates.

2.1.3. Cretaceous

Until Central Tunisia, the Early Cretaceous is

composed of complex deltaic material coming from

northern Sahara, and by shallow marine facies

towards the North.

The Barremo–Aptian in central and meridional

Tunisia is characterized by carbonate and detritic

deposits. Toward the North, they are represented by

reef carbonate facies, and finally by open marine

marls. Facies and thickness repartition is controlled

by structural factors among which the halokinetic

movements seem to play a major role.

At early Albian stage, a large part of Tunisia

(Saharan platform, Jeffara, central and eastern Tuni-

sia) emerged. Only the northern part of Tunisia and

the Gafsa basin have a continuous sedimentary record

during the Albian period.

The Cenomanian is transgressive all over Tunisia.

In the littoral Saharan platform, it is represented by

thin dolomites and evaporites. North of the Saharan

platform, the Southern Sahel (Sfax basin) and the gulf

of Gabes are grouped in a unique subsiding zone

called the Gabes basin, showing a thick platform of

marls and limestones. In the septentrional part of

Tunisia and on both sides of the N–S axis, the

deposits are pelagic and continuous with the Early

Cretaceous basin.

During Turonian–Maastrichtian, a continuous

transgressive phase occurred characterized by the

progression of the marine domain towards the South.

The facies repartition is close to the Cenomanian one.

Since Early Cretaceous sedimentation was very con-

trasted on both sides of the N–S axis.

To the West, the basins are NE–SW oriented and

received up to 600 m of Maastrichtian limestones.

Eastern Tunisia and the Pelagian basins correspond to

platforms with 110j–120jN sedimentation axis.

2.1.4. Tertiary

The Paleocene–Eocene series overlie Maastrich-

tian limestones. Towards the South, these series start-

ing with marls show an epineritic and laguno-littoral

facies rich in phosphate. In the north, in the Tunisian

trough, pelagic shales thickness can reach 2500 m,

whereas it is absent in central Tunisia in the zone

called the Kasserine Island and in the southern Tunisia.

From Oligocene to Early Miocene, the sedimenta-

tion is characterized by thick continental deposits, the

Fortuna sandstone, passing laterally towards the East

to carbonates.

Then, in all of Tunisia, uplift occurred responsible

for the large hiatus in the central and southern Tunisia.

A Middle Miocene transgressive phase is marked

by the shell limestones of the Ain Grab formation

(Langhian) well developed in the Hammamet basin.

Subsequently, the sedimentation becomes shal-

lower as expressed by the continental, epineritic and

fluvio-lacustrine deposits of the Cap Bon and the

Oum Douil groups dated Middle to Late Miocene.

The Miocene to Pliocene transition is characterized

by the Segui fluvio-lacustrine formation.

Finally, fine continental detritus represent the Plio-

Quaternary facies.

2.2. Structural zonation of Tunisia

General reviews and summaries of the structural

geology of Tunisia have suggested regional subdivi-

M. Patriat et al. / Sedimentary Geology 156 (2003) 241–262 243

sions into various tectonic zones or provinces (e.g.

Castany, 1951; Kanes et al., 1979; Vila, 1980). The

main boundary elements are the nappe front, the

North–South axis and the south Atlas fault (Fig. 1).

The Burollet and Rouvier (1971) structural zona-

tion is generally accepted and is described from north

to south across Tunisia (Fig. 1).

2.2.1. Northern Tunisia

The northern Tunisia area constitutes the north-

eastern termination of the North Africa ‘‘Atlas’’ oro-

genic belt, which corresponds to the southern segment

of the ‘‘Alpine arc’’, extending westwards through

Algeria and Morocco and eastwards through the

Mediterranean sea and Sicily. From NW to SE, the

Northern Tunisia onshore area can be subdivided into

two major domains: the Tellian fold and thrust belt

whose major nappes setting is dated Middle Miocene,

bordered to the SE, by the foreland domain (Fig. 1).

At the front of the Tellian fold and thrust belt lies

the zone of diapirs. It is characterized by the surface

exposure of SW–NE salt diapirs, which coincide with

anticlines fold and thrust emergence. Despite the

presence of up to 1000 m of Triassic–Lower Jurassic

evaporites, the salt diapirs are not exposed at a similar

scale elsewhere in Tunisia.

2.2.2. The Tunisian Atlas

The Tunisian Atlas is a region of variable tectonic

styles in central Tunisia. It is essentially consisting of

NE–SW trending folds as in the Algerian Saharan

Atlas (Burollet and Rouvier, 1971). Several authors

considered the Tunisian Atlas as an example of thin-

skin tectonics mainly detached on Triassic and Creta-

ceous decollement levels mainly during Miocene to

Quaternary time (Frizon de Lamotte et al., 1998;

Outtani et al., 1995). It is bounded to the East by

the North–South axis and to the South by the South

Atlas fault, a possible location of basement reactiva-

tion (Pique et al., 1998).

The paleofeature known as ‘‘Kasserine Island’’

formed a stable block made of Late Cretaceous and

Tertiary shallow water deposits (Burollet and Rouvier,

1971). The Cretaceous carbonate facies of this zone

generally pass from a subcontinental domain in the

south to a marine domain in the north (Marie et al.,

1984). The Kerkennah high, located on the eastern

part of Tunisia, presents a similar evolution. The

anticlines developed in the northern part of the zone

(e.g. Selloum, Sidi Aich, Zaouia) are elongated in a

NE–SW direction, whereas their trend becomes east–

west to the south (e.g. Kebar, Meloussi, Orbata).

Large synclinal basins, generally filled up by Mio-

Pliocene deposits, separate these anticlines.

2.2.3. The eastern platform

The eastern platform onshore, which includes the

Cap Bon and Sahel areas, extends eastwards from the

North–South axis to the Pelagian Sea.

In contrast with the Atlas domain, the surface

expression of folding is very subdued east of the

North–South axis, consisting mainly of broad low-

amplitude arches along NE–SW axes or elongated

domes (Touati, 1985). In the northern part of the

Kairouan depression, close to the Gulf of Hammamet,

N–S trending folds developed, in which a complete

sequence from Lower Cretaceous is expressed at the

surface.

Offshore, this domain is represented by the Ham-

mamet and Gabes basins separated by a high area

centered on the Kerkennah islands (Fig. 1).

2.2.4. The Saharan platform

The South Atlas Fault marks the limit between the

Tunisian Atlas and the Saharan Platform. The area is

underlain by strongly deformed Precambrian base-

ment, but has been dominantly influenced by minor

rather than real compressional movements throughout

the Paleozoic to Cenozoic. A significant exception

occurs in the vicinity of Medenine where Lower

Cretaceous folding on axes orientated NW–SE and

E–W may reflect the pre-Turonian left lateral regime

of the Mediterranean region (Bouaziz et al., 1998).

2.3. Geodynamic evolution and timing of deformation

The rifting in Tunisia associated to the propagation

of the Tethys toward the West, began in Permian time

(Robertson Research International, 1983; Barrier et

al., 1993; Ben Ferjani et al., 1990). The Early Mes-

ozoic is characterized by extensional paleostress of

N–S orientation. After Jurassic time, the transition to

compression took place through a complex succession

of extensional phases during the whole Cretaceous

(Letouzey and Tremolieres, 1980; Barrier et al., 1993;

Bouaziz et al., 1994, 1996, 1998).


Fig. 2. Schematic stratigraphy of Tunisia after Anderson (1996); time scale is from Odin (1994).


The first clearly identified folding phase in the

‘‘Atlas domain’’ took place during Eocene times. The

second main tectonic phase is Tortonian in age (Bur-

ollet and Rouvier 1971) and was followed by minor

phases until the last folding in the Plio-Pleistocene

(Villafranchian). Significant folding probably oc-

curred in the Late Cretaceous, which has been des-

cribed by Letouzey and Tremolieres (1980), and is

well known and documented on the seismics in

Algeria (Guiraud, 1990).

The Alpine belt corresponds to a fold and thrust

belt developed during the Alpine tectonic cycle,

composed of four principal compressional pulses

(Ben Ayed, 1986; ETAP, 1999). Evaporite layers,

mostly Triassic in age, play as a deep decollement

level. Additionally, the Triassic salt is remobilized in

diapirs (Fig. 2). They started to grow up during the

Aptian (Snoke et al., 1988) or even earlier (Bedir,

1995). Since that time, salt tectonics is considered

as dominant especially along and East of the

North–South axis explaining the individual evolu-

tion recorded by small isolated basins.

3. Methodology

3.1. Data

We had access to three types of data: industry wells

data, seismic reflection and field data from published

sources.

The wells, localized in Fig. 1, represent the major

contribution of this work. Thirty wells are localized in

the Gabes basin, 18 wells in the Hammamet and 6 in

the Chotts. All these 54 wells are located in eastern

and southern Tunisia.

Field data have been used to constrain datings,

facies, possible hiatus and tectonic phases, in addition

to wells and provide us a basis for this discussion.

The seismic lines used cover the offshore Gabes

and Hammamet basins.

Finally, synthetic wells have also been created,

based on seismic interpretation, and analysed.

3.2. Calculation of tectonic subsidence

The tectonic subsidence of the basins has been

calculated from the back-stripping method developed

by Steckler and Watts (1978), using GENEX software

(IFP). Results are presented with a water isostatic

balance and, for data homogeneity in the Peri-Tethys

consortium, we used Odin’s chronostratigraphic scale

(Odin, 1994).

The embedded data of the subsidence modeling are

age, thickness and lithology of selected sequences on

the studied columns. When direct or indirect data are

not available on wells (cores or diagraphies), the

porosity/depth curves are based on type laws estab-

lished in GENEX for each lithology.

The most sensitive sources of errors remain the

dating of formations as well as the evaluations of

eroded thickness and paleobathymetries.

All the results of the modeling are presented in this

paper as curves displaying the tectonic subsidence

according to time and tectonic subsidence rates (in

meters by millions of years).

Original GENEX files provide subsidence rates

during time intervals corresponding to the sampled

units. In order to be free from a local stratigraphic or

tectonic effect, which may vary from one well to

another, we optimized the subsidence rate sampling,

in order to obtain subsidence rates expressed million

of years by million of years. We are thus able to

compare one well to another.

4. Analysis of subsidence data

4.1. Behaviour of the different studied basins

4.1.1. Chotts area

The Chotts basin is located south of the Tunisian

Atlas, in western Tunisia, close to the Atlas deforma-

tion front.

Only a small number of wells were drilled in the

Chotts basin and its surroundings (seven wells), and

these wells are not all localized in the basin center. As

Fig. 3. Cumulated tectonic subsidence (curves) and tectonic subsidence rates (columns) for the three studied Tunisian basins. (a) The

Hammamet basin. Dotted picks correspond to tectonic subsidence rates greater than the scale extremes. Their numerical value is indicated.

Dating errors account for these nonrealistically large values. (b) The Gabes basin. (c) The fictitious* Chotts basin. *The Chotts basin tectonic

subsidence curve is constructed after the Nf1 well from Lias to Santonian, and the Gnt1 well from Campanian to present.


Fig. 4. Cumulated tectonic subsidence (curve) and tectonic subsidence rates (columns) for three real wells representative of the three studied

Tunisian basins. (a) Bfc1 well, Hammamet basin. Dotted picks correspond to tectonic subsidence rates greater than the scale extremes. Their

numerical value is indicated. Dating errors account for these nonrealistically large values. (b) Ely2 well, Gabes basin. (c) Nf1 well, Chotts basin.


a result, the representativity of the subsidence curve of

the basin is sometimes questionable. A composite

well has been constructed based on Cf1 (until the

Albian) and Gnt1 (from the Cenomanian) in order to

better represent the Chotts area evolution (Fig. 3c).

Gnt1 has been chosen because it is the only well

showing subsidence during Tertiary. Cf1 has been

chosen because it is located on the eastern side of

the Chotts basin paleodepocenter. Unfortunately, it did

not reach the Triassic. Based on the Cf2 well, we

extrapolated an additional Liassic thickness of 800 m

between the Cf1 well terminal depth (TD) and the

Triassic top.

During the whole Jurassic, subsidence was impor-

tant and constantly around 50 m/Ma (Figs. 3c and 4c).

However, datings are not reliable enough during

Jurassic to distinguish any particular phase.

In Cenomanian, after a period marked by a slowing

down of the subsidence rates (and locally by erosion),

the basin evolution was stopped (Fig. 4c). The tec-

tonic evolution during the Late Cretaceous and Cen-

ozoic is difficult to quantify due to the gap of

sedimentation (erosion or nondeposition?). Gnt1 is

the only well showing a continuous sedimentary

record in the Chotts region since the Cenomanian. It

is located north of the Chotts basin, in the Gafsa

trough. A very marked rise in subsidence rates char-

acterized the Late Cretaceous (Fig. 3c). This accel-

eration corresponds to the one observed on the

Saharan platform in western Atlas Tunisia and in the

Algerian Aures (Bracene et al., this volume).

We cannot consider the dating of Tertiary levels as

very precise in Gnt1 well. Thus, durations, localiza-

tions in time and therefore subsidence rates during

Fig. 5. Seismic section in the Hammamet basin showing the Late Eocene erosion phase and the unconformity of the Early Oligocene.


Tertiary cannot be determined with great accuracy.

Nevertheless, subsidence variations show that the

Chotts area has been influenced by the Alpine colli-

sion episodes, especially during the Pliocene phase.

4.1.2. The Hammamet basin

The offshore Hammamet basin, northernmost

studied basin, is located on the present margin of

the Pelagian Sea in eastern Tunisia (Fig. 1).

Despite the few wells reaching older series than

Aptian, we considered the subsidence as constant and

very homogeneous during the Jurassic and Early

Cretaceous until the Aptian (Fig. 3a), though pre-Late

Cretaceous variations of the subsidence curve are

based on only few deep wells and are not necessarily

representative of the whole basin.

The Cretaceous period is characterized in the

Hammamet basin by a period of intense subsidence

identified during Aptian and by a period of moderate

subsidence during the Cenomano-Turonian (Fig. 3a).

The change due to the collision is prominent in the

Hammamet basin. At the end of Cretaceous to Paleo-

cene, the sedimentary signal showed a pronounced

oscillation. The amplitude of this oscillation is strongly

dependent on eroded thickness evaluations. The ampli-

tude of erosion has been estimated from seismic inter-

pretations. Even though it can be overestimated, an

episode of erosion and uplift, is clearly identified and

localized at the beginning of Paleogene (Figs. 5–6).

In Langhian, the trend of moderate subsidence or

local erosion (Figs. 3a and 4a) that was active since

Paleogene completely changed. The Hammamet basin

developed extensively with subsidence rates reaching

150 m/Ma. This very strong subsidence, synchronous

to the nappes emplacement to the North (Rouvier,

1977), could correspond to the formation of a flexural

depression in the front of these nappes (Burollet and

Ellouz, 1985).

During the Tortonian subsidence rates decreased to

reach values of 30–40 m/Ma, which characterized the

subsidence of the basin up to the Miocene and were

coeval to the main compressive event. During Plio-

Quaternary, the rates are similar in amplitude on

validation points. This quite fast subsidence corre-

sponds to the transtensive reactivation of the Pelagian

rift between Sicily and the present margin of Tunisia

Fig. 6. Seismic section showing a Plio-Quaternary graben in the Hammamet basin.


(Figs. 5–6), to the east of the North–South axis

(Boccaletti et al., 1987, 1990).

4.1.3. The Gabes basin

The wells located onshore, east of the North–

South axis, in the Sahel, have also been analysed

and integrated in the so-called Gabes basin.

Located in the south-easternmost part of Tunisia,

the Gabes basin is influenced by the same tectonic

evolution as that of the Sirt basin (WNW–ESE

direction between Tunisia and Libya, Fig. 1).

In Jurassic time, corresponding to the extensional

processes, subsidence (Figs. 3b and 4b) was sus-

tained, around 25 m/Ma, until Malm. Subsidence

slowed down at the end of Malm, earlier than in the

Hammamet and Chotts basin. At the end of this

distension phase, in Barremian, rates increased and

reached values superior to 50 m/Ma. This acceler-

Fig. 7. Schematic structural map of the Gabes basin. Major (thick lines) and secondary normal faults are indicated. The location of the two

reference structural cross-sections (aaV and bbV) deduced from seismic lines (Kilani, in ETAP, 1999) and illustrated on top of Figs. 8 and 9 is

shown as thin gray lines. The location of the two cross-sections (AAV and BBV) illustrated in Figs. 8 and 9 and discussed in the text is shown as

thick gray lines.


Fig. 8. Tectonic subsidence rate along a NW–SE cross-section. The uppermost cross-section is deduced from seismic lines (Kilani, in ETAP,

1999) and is shown as a structural reference. The eight following cross-sections are tectonic subsidence rates correlations between seven wells

located on Section BBV (Fig. 7) for eight different time intervals from Turonian to Plio-Quaternary.


Fig. 9. Tectonic subsidence rate along a SW–NE cross-section. The uppermost cross-section is deduced from seismic lines (Kilani, in ETAP,

1999) and is shown as a structural reference. The eight following cross-sections are tectonic subsidence rates correlations between five wells

located on Section AAV (Fig. 7) for eight different time intervals from Turonian to Plio-Quaternary.


Fig. 10. (a) Bouguer anomaly along the EGT’85 profile after Midassi (1982) and Klingele et al. (1992). (b) Heat flow along the EGT’85 profile after Ben Dhia (1987) and Lucazeau

and Ben Dhia (1989) and Cermak et al., (1992). (c) EGT’85 Velocity Profile after the Research Group for Lithospheric Structure in Tunisia (1992) and Bobier et al., (1991). E, F1, F2

and G are the shot points localized on Fig. 1.

M.Patria

tet

al./Sedimentary

Geology156(2003)241–262

254

ation is synchronous to similar events observed in

eastern Tunisia and in Algeria but is not expressed in

the Hammamet basin. At a large scale, it is synchro-

nous to the opening of the central African rift and to

the inception of rifting in the Sirt basin (Guiraud and

Maurin, 1992; Guiraud, 1990). This event is coeval

to the deposit of a thick flysch series on distal

margin in the Tellian zone. We have to be cautious

about the Jurassic history. All the wells analysed in

the Gabes basin are located on apex of blocks and

probably do not reflect the real subsidence of the

Gabes basin itself.

The Cretaceous period is characterized by fast and

strong variations in subsidence during Aptian and

Albian, partly linked to the halokinetic movements,

followed by the large Cenomano-Turonian subsidence

characteristic of all Tunisian basins.

Except for two brief episodes of erosion, the

Cenozoic is characterized in the Gabes basin by

sustained subsidence, particularly in the central part

of the basin during the Middle Eocene (Ellouz, 1984).

This Paleogene evolution is similar to that known in

the Sirt basin (van Der Meer and Cloetingh, 1993).

Finally, the Plio-Quaternary subsidence corre-

sponds, like in the Hammamet basin, though

smoother, to the recent opening of the Pelagian Sea.

4.2. Structuration of the Gabes basin

For the Gabes basin, we worked out a finer

analysis of its structuration. Thanks to the paleoba-

thymetries well constrained in the Gabes basin and of

moderate amplitude, the analysis of the Gabes basin

subsidence can be detailed. It is consequently possible

to evaluate the importance of local structural control

on subsidence. The permanent role of salt tectonics in

the behaviour of the different blocks should be real-

ized. Halokinesis has been demonstrated to be active

since Aptian time in Tunisia (Perthuisot, 1981; Snoke

et al., 1988), but could be much older (Bedir, 1995) as

in Algeria (Bracene et al., 2003; Bracene and Frizon

de Lamotte, in press). In order to bypass the effects of

salt tectonic or local heterogeneities, synthetic wells

have been constructed and analysed.

The Gabes basin is structured mainly by faults of

NW–SE direction (Fig. 7) (Burollet and Ellouz, 1985;

Ben Ferjani et al., 1990). The behaviour of the basin is

illustrated from 11 wells situated on two cross-sec-

tions: one (Fig. 8) NW–SE parallel to structures

passing through seven wells, and another (Fig. 9)

SW–NE, perpendicular, passing through five wells.

Along the two cross-sections, mean subsidence rates

have been calculated for representative periods.

Fig. 11. Averaged tectonic subsidence curves of the three studied basins. The Hammamet basin curve integrates 18 wells, the Gabes basin curve

integrates 30 wells and the Chotts basin curve 6 wells.


Unfortunately, only a few wells reach deeper and

older levels than the Turonian. Thus, calculation was

done from Turonian, the representative period for the

whole Cenomano-Turonian large subsidence (Ellouz,

1984).

The analysis of the two profiles confirms the NW–

SE orientation prevailing in the Gabes basin, guided

by the geometry of the fault network. Over time, from

Late Cretaceous, the deformation center and the

location of the central graben axis migrated to their

present position, close to Ashe1 and to the present

basin center. The two highest periods of activity in the

Sirt basin, the Paleocene and Middle Eocene (van Der

Meer and Cloetingh, 1993), are synchronous, in the

Fig. 12. Maps of tectonic subsidence rates calculated for eight selected time intervals. (A) From Barremian to Paleocene. (B) From Lutetian to

Plio-Quaternary. The background of these figures corresponds to the topography and to the major tectonic structures of Tunisia. Paleostress

directions (extensional in blue; compressional in red) are compiled after Letouzey and Tremolieres (1980), Barrier et al. (1993) and Bouaziz et

al. (1994, 1998).


Gabes basin, to the two periods of development of a

narrow NW–SE graben of restricted extension cen-

tered around Ashe1 basin. This common evolution

confirms the close connection between the basins of

Sirt and Gabes.

The distribution of subsidence rates along the

NW–SE profile and the contrasting evolution are

only partly explained by the unidirectional structura-

tion commonly accepted for the Gabes basin. The

evolution of the subsidence maxima position, and

especially the total inversion of the trend observed

around the Miocene, suggest a deep structural control

of the subsidence, and cannot be explained by faults

parallel to the NW–SE profile.

4.3. Crustal cross-section and the EGT’85 profile

According to the EGT’85 seismic refraction profile

(Research Group for Lithospheric Structure in Tuni-

sia, 1992; Bobier et al., 1991; Figs. 1 and 10), the

Fig. 12 (continued ).


thinnest part of the Tunisian crust is located in the

northern part of the profile, in the Tellian zone.

Toward the south, the crust thickens progressively

until the Saharan platform. Locally, in the Chotts

basin, a moderate thinning of the crust bordering the

Saharan platform reflects the strong individualization

of this basin, even at crustal scale. On this crustal

architecture, the sedimentary basins developed with a

maximum thickness on the Kasserine Island (con-

firmed by the negative Bouguer anomaly). On the

northern part of the profile, the heterogeneity of the

crust is marked by the positive anomaly above the

present Mediterranean margin, in the area of allochth-

onous stacked units at the surface.

From the three basins studied in this paper, only the

Chotts basin is located across the EGT profile. Accu-

mulated thickness of Meso- and Cenozoic sediments

are inversely proportional to those of Paleozoic sedi-

ments. The two Meso-Cenozoic subsident troughs

situated along the profile, the Tunisian trough, and

the Chotts basin are separated by the Kasserine Island,

a Meso-Cenozoic high whose position corresponds to

the Paleozoic depocenter.

The structure visible on the profile EGT’85 assimi-

lated to the Hairech ridge corresponds precisely to the

zone of diapirs where they pierce the surface. The

seismic arrivals have presumably been strongly influ-

enced by diapirs. We therefore will not draw any

conclusion from the shape and position of this ridge

(that does not correspond to any prominent feature in

the gravimetric or heat flow data).

4.4. Geodynamics and subsidence

Tunisia is framed by the Tethyan Ocean to the

north with its Pelagian branch to the east, and by the

Gulf of Sirt to the southeast. This hinge situation is

responsible for the development of fracture families of

various directions. This could be the origin to the

permanent segmentation and atypical subsidence

demonstrated in this study that affected Tunisia during

the whole Mesozoic (Fig. 11). This segmentation

continued more irregularly during the Cenozoic, east

of the N–S axis. It is therefore difficult to imply a

simple geodynamical concept to explain the geolog-

ical evolution of Tunisian basins. Because of this

strong heterogeneity in time and orientation, the geo-

logical stages cannot be described in terms of a single

succession of syn-rift and post-rift periods (Hlaiem et

al., 1997). Unfortunately, a quantitative approach of

the Triassic subsidence is not possible because of the

lack of data.

Due to the evolution of these subsidence rates and

to the geodynamic setting of Tunisia, we decomposed

the Meso-Cenozoic geological evolution of Tunisia in

three key periods. These periods are slightly different

to that commonly adopted for the single-phase evo-

lution of classical margins.

4.4.1. Main rifting period from Lias to Neocomian

(205–113 Ma; Fig. 12a)

In continuation with Triassic extension, this period

is characterized by extensional brittle deformation

(Bouaziz et al., 1994, 1996), subsidence rates during

the Jurassic and Cretaceous were not very high

compared to what is known in Algeria (Kuhnt and

Obert, 1991; Bracene et al., 2003). In Tunisia, the

tectonic subsidence is spread out over a very long

period of time when compared to the duration of the

Tethyan rifting.

4.4.2. Transition period, Aptian to Upper Cretaceous

(113–65 Ma; Fig. 12b–c)

This period is characterized by a strong variability of

subsidence owing to different geological reasons. Sev-

eral extensional pulses are coeval with (1) the first

halokinetic movements, (2) a compression phase prom-

inent in the Tellian zone in Algeria (Obert, 1974;Wildi,

1983), that is not clearly identified in Tunisia. Varia-

tions in subsidence rates described during Aptian are

followed by a discrete unconformity between Albian

and Aptian in central and southern Tunisia (Ben Ferjani

et al., 1990; Ouahchi et al., 1998).

During Cenomano-Turonian, an acceleration of the

subsidence rates is obvious like in the whole Magh-

reb. Part of the observed subsidence is influenced by

several parameters: (1) eustasy which reached a max-

imum amplitude of 200 m, but is only responsible for

a minor part of the subsidence acceleration; (2)

variations in the central Atlantic accretion rate, 3.8

cm/year during Cenomanian, whereas it decreased

during Coniacian linked to a drastic change in direc-

tion of the Africa/Eurasia relative movement (Olivet

et al., 1984).

In the Maghreb, this plates kinematic evolution

induced a change from a regime of sinistral trans-


tension between Africa and Eurasia (Iberia) to a regime

of dextral transtension between Africa and the Italo-

Dinaride block (Ziegler, 1988; Zouari et al., 1999).

During this transition period, the Triassic ductile salt

layer played very likely a major role in the behaviour of

some basins showing atypical subsidence rates.

4.4.3. Convergence phase (65–0 Ma; Fig. 12d–g)

The Cenozoic behaviour of Tunisian basins is

dominated by the inception of Europe/Africa collision.

Burollet and Ellouz (1985) outlined that the con-

vergence and associated compressional structure can

be expressed in a basin as a combination of uplifted

zones and subsiding areas (see also Cloetingh et al.,

1985). The passage to Tertiary is expressed as a

smooth surrection of the borders of the Gabes basin

and on the contrary as a brief but strong subsidence

acceleration in the Hammamet basin. This is probably

linked to the first collision phase in the northeast of

Tunisia. This phase is not well known in the field in

Tunisia (Letouzey and Tremolieres 1980), but some of

its effects are known west in the Maghreb, in Algeria

(Guiraud, 1990) and in the north of Africa in general

(van Der Meer and Cloetingh, 1993; Guiraud, 1998).

In Eocene, the first important manifestation of

uplift appeared in the north of Tunisia. It corresponds

to the first phases of the Alpine collision and is

illustrated by the seismic sections in the distal Ham-

mamet basin (Figs. 5 and 12).

To the Middle Miocene, the observed changes are

associated with the setting up of Tellian nappes to the

north of Tunisia, which have been emplaced during

Burdigalian–Langhian time (Rouvier, 1977). The

consequences of this event, and especially regional

isostatic equilibrium, are expressed differently accord-

ing to the geographical location of basins, and espe-

cially to their distance with regard to the nappe front.

5. Conclusions

Due to the peculiar location of Tunisia, at the

junction of western Mediterranean and eastern Medi-

terranean sea, the geodynamic evolution gave rise to

complex rifting processes which (1) was longer (cumu-

lating several pulses) than in the rest of the Maghreb,

and (2) registered different extension phase orienta-

tions inducing a strong segmentation of the basins.

By studying the subsidence of Tunisian basins, we

can emphasize the particular tectonic trends. From our

study, the Meso-Cenozoic geological history can be

broken down into three very different periods.

Triassic–Liassic rifting initiated a general episode

of tectonic subsidence of variable amplitude but

synchronous all over Tunisia. The first period (Liassic

to Neocomian) is dominated by extension. The sinis-

tral strike slip at Maghreb scale between Africa and

Eurasia was effective during this period and could

help to maintain a subsidence that spread over a

particularly long period in Tunisia but with low

subsidence rates.

Post-rift thermal subsidence started at the end of

Neocomian and was interrupted by several pulses

linked with new extensional events during the Creta-

ceous. From Aptian to Maastrichtian, Tunisia under-

went a change corresponding in time to that observed

in the global Maghreb kinematics from a sinistral

transtensive regime to a dextral regime. The very

brief accelerations in the subsidence could suggest

the existence of collision phases at the edges of the

studied region. The inception of diapirism is thought

to begin around Aptian time. It could thus explain

some of the features observed during this period.

Finally, the Tertiary was characterized by the

effects, uplift or subsidence, of the different stages

of the Alpine collision.

Most probably due to inherited structures and

depending on their position in Tunisia, the different

basins studied do not all have the same behaviour:

� West of the N–S axis: In the Chotts basin, the

subsidence stopped as early as in Campanian after

a period of strong subsidence during Jurassic time.

During Late Cretaceous, the singular evolution of

Gnt1 well, with much more subsidence than the

Chotts basin itself, has to be compared to that of

the Algerian Aures basin located very close to this

well (Bracene et al., 2003). A similar and amplified

evolution is known in the Tunisian Atlas (Ellouz,

1984). The post-Early Cretaceous subsidence was

greater in Gnt1 and the Aures basin, which are

supposed to be in the same structural position.� East of the N–S axis: During Jurassic time, as in the

Chotts region, the Gabes and Hammamet basins

showed strong subsidence. Yet, in this area, unlike

in the Chotts region, the subsidence evolution did


not stop after the Jurassic episode, but remained

important during Cretaceous time. The Gabes basin

had an almost continuous evolution during this

period, more or less similar to the neighbouring Sirt

basin. From the Gabes and Hammamet basins, only

the latter (the closest to the Tellian nappes) shows an

evolution clearly influenced by the successive

collisions during the Cenozoic.� Mostly during Cenozoic, the effects of the different

collision phases depend mainly on the position with

respect to the Tellian nappe’s front. This is

corroborated by the structural and sedimentary

trends visible in Fig. 12: in the Hammamet basin,

the orientation of the basin axis is parallel to the

Tellian (SW–NE) front. This is particularly prom-

inent since the Neogene. On the opposite, the Gabes

basin exhibits a perpendicular NW–SE axis.

Subsidence evolution seems to be largely deter-

mined by the compression during the Cenozoic, but

this has to be locally modulated with the effects of salt

tectonics.

In the Gabes basin, we found evidence of a sub-

sidence segmentation that cannot be totally explained

by the structure of this basin. Among other things, the

translation of depocenters during the Miocene could be

more easily explained by the existence of deep-rooted

structural trends oblique to those known in the basin.

Acknowledgements

We are grateful to Mr. A. Gaaya, Mr. R. Ghenima

and Mr. H. El Heuchi from ETAP for their contribution

in this project and the permission to publish some

internal results and sections. We thank them for the

long talks and warm contacts which existed between

the two teams during the preparation of this paper. The

constructive criticisms of two anonymous reviewers

are acknowledged. Thanks also to M.-F. Brunet and S.

Cloetingh for their editorial work and help.

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The Hammamet, Gabès and Chotts basins (Tunisia): a review of the subsidence history

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