Subsidence history in basins of northern Algeria

Sedimentary Geology 156 (2003) 213 – 239 www.elsevier.com/locate/sedgeo

Subsidence history in basins of northern Algeria Rabah Bracene a,*, Martin Patriat b, Nadine Ellouz b, Jean-Michel Gaulier b a

Sonatrach, Institut Alge´rien du Pe´trole Bat. C, BP 68, Boumerdes, Algeria b Institut Francßais du Pe´trole, 92852 Rueil-Malmaison cedex, France

Received 2 July 1999; received in revised form 2 July 2001; accepted 19 July 2002

Abstract The Tellian foreland in Algeria represents a part of the southern Tethyan margin during the Mesozoic. Its tectonic evolution includes a rifting stage during the Triassic and Liassic times characterised by tilted blocks and early diapiric events during the Liassic, a post-rift regime from Middle Jurassic up to the Late Cretaceous and basin inversion during the Tertiary related to the African and European plates convergence. The subsidence modelling supported by surface and subsurface data integration emphasises different subsidence phases.

(1) Liassic subsidence phase under Tethyan and Atlantic control related to crustal thinning. (2) Late Jurassic, Cretaceous and Tertiary subsidence phases, where the first one is linked to diapiric events and the second in eastern Algeria to the effects of the rifting event developed more easterly in the Gulf of Gabe`s and Sirte. The last one is flexural and occurs in foreland basins during the Tertiary. Taking into account the subsidence record of each structural domain, a correlation between tectonic events and the subsidence phases can be established. D 2002 Published by Elsevier Science B.V. Keywords: Northern Algeria; Subsidence; Rifting stage; Mesozoic; Tilted blocks

1. Introduction Petroleum research in northern Algeria has been developed in the last few years, deep wells have been drilled (Fig. 1a) and seismic profiles have been acquired by the Algerian Petroleum Company (Sonatrach). In the present paper, we provide an overview

* Corresponding author. Fax: +213-24-81-90-81. E-mail address: [email protected] (R. Bracene).

of the subsidence history of this area, integrating petroleum data and surface data. This study deals with Algerian basins located southerly of the Tellian front (Fig. 1b). Several geological studies have been carried out (structural geology, stratigraphy, sedimentology and others) in these areas but most of them do not include subsurface data (deep well, seismic profiles and geochemistry analysis). Geodynamic evolution was considered in several publications (Guiraud, 1973; Kazi Tani, 1986; Guiraud and Bosworth, 1997; Bracene et al., 1998; Saint Bezar, 1999; Frizon de Lamotte et al., 2000; Bracene and Frizon de Lamotte, in press).

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Fig. 1. (a) Location of the wells analysed in northern Algeria. (b) Geological framework of the studied area.

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The purpose of this paper is: (1) to give an overview of the subsidence evolution of this part of Algeria corresponding to the southern Tethyan margin; (2) to correlate the subsidence phases with tectonic events and (3) to compare the geological domain behaviours presented by the studied areas. Following a discussion of the overall geological setting of northern Algeria and a stratigraphic overview, the geodynamic context will be presented together with an assessment of the different subsidence phases. This work constitutes our contribution to the Peri-Tethys Programme.

2. General geological context This section reviews the main geological domains of northern Algeria to locate the studied areas. 2.1. Northern Algeria structural framework From the north to the south in northern Algeria, along the Great Kabylia meridian (Fig. 1b), i.e. from the Mediterranean Sea to the Saharan Platform, the following structural domains can be distinguished: – The internal zones called ‘‘Kabylide domain’’ at the North, which corresponds to the Tethyan northern margin during the Mesozoic times. This domain represents the Maghrebian orogen hinterland domain. Its structure displays imbricate tectonic units constituted by metamorphic and sedimentary rocks (Durand Delga and Fontobe´, 1980; Wildi, 1983; Bouillin, 1986; Bouillin and Olivier, 1992; Saadallah, 1992). – The flysch zones are currently located both south and north of the Kabylian Massifs. In their initial organisation Mauretanian and Massylian flyschs are deposited southerly of Kabylia ridge. This palaeogeographic position has been subject to discussion for a long time (Bouillin 1977; Durand Delga and Fontobe´, 1980; Wildi, 1983; Bouillin and Olivier 1992; Saadallah, 1992). The Massylian flyschs are considered as an intermediate domain (oceanic?) located between the northern and the southern Tethyan margin. The present structural position results from partitioning of the deformation in plastic basinal units in front of the Kabylian basement backstop. – The Tell external zones representing the Tethyan southern margin (Tellian domain) before

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inversion, is composed by Mesozoic and Cainozoic deposits. It corresponds to an allochthonous made of several units forming a pile of nappes (Caire, 1957; Mattauer, 1958; Kieken, 1974; Vila, 1980; Wildi, 1983). The record is mainly Mesozoic and Cainozoic in age. – The Tellian foredeep developed southerly of the previous domain is filled by Miocene deposits and is overlapped by the previous Tellian external tectonic units (Caire, 1957; Mattauer, 1958; Kieken, 1974; Vila, 1980; Wildi, 1983). The Tellian foredeep is mainly trending west to east and rests unconformably on the foreland. – The foreland zones gather the Pre-Atlas, the Mesetian block and the Atlasic domain. Outcrops are essentially represented by the Mesozoic deposits in the western and central Saharan Atlas (Elmi, 1978; Kazi Tani, 1986; Aı¨t Ouali, 1991; Elmi et al., 1998). The Palaeogene deposits are lacking in these areas but they are only present easterly in the Hodna Massif (Guiraud, 1973; Kieken, 1974), and in the Aure`s Mountains (Laffitte, 1939; Vila, 1980; Wildi, 1983) (Fig. 1b). – The Saharan Platform northern border (Benoud, Melkhier) corresponds to the Atlasic Tertiary foreland basins. This domain is weakly deformed by the Tertiary compressive phases (Boudjemaa, 1987; Frizon de Lamotte et al., 2000; Bracene and Frizon de Lamotte, in press). Following this structural framework, our studied areas correspond to the geological domains located southerly of the Tellian front (Fig. 1b) i.e. the Tellian foredeep, the foreland zones and the Saharan Platform northern border. The general geometry is illustrated by the NW – SE parallel cross-sections (Fig. 2) built from seismic profiles (Bracene and Frizon de Lamotte, in press). From the west to the east, we distinguish the Telagh trough, the Pre-Atlas, the Hodna and the Aure`s basins (Fig. 1b). Westerly, a high called ‘‘Mesetian block’’ is located between the Telagh trough and the Atlasic domain (Figs. 1b and 2a), and disappears to the east (Figs. 1b and 2b,c). 2.2. Stratigraphy Below, we review the Mesozoic and Cainozoic lithostratigraphy in the studied areas.

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Fig. 2. Seismic cross-section (TWT second) through the studied areas (from Bracene and Frizon de Lamotte, in press. Location, see Fig. 1b).

The Mesozoic and the Cainozoic facies become more marine from the south (Saharan Platform) towards the north (Tell) and from the southeast (Ksours Mounts) to the northeast (Aure`s Mountains). In the Ksours Mounts at the southwest (Fig. 3) the total thickness is about 5000 m (Delfaud, 1986) and reaches 10 000 m in the Aure`s Mountains (Vila, 1980; Bureau, 1986; Kazi Tani, 1986).

2.2.1. Triassic The Triassic sequence drilled in the Bch borehole (Fig. 1a) begins by conglomerate; followed by sandstones interlayered by clays then evaporites at the top. These evaporites are lacking in some areas as in the Bourlier well (Fig. 4) drilled in the Mesetian block representing a high during the Mesozoic and the Cainozoic times.

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In other areas, the Triassic evaporites constitute the major material of the diapirs (mixture of gypsum and clay). In that case, blocks of dolomite, sandstone, mineralised Palaeozoic metamorphic and alkaline igneous rocks associate these evaporites (Augier, 1967; Caratini, 1968; Guiraud, 1973).

Fig. 3. Synthetic stratigraphic column of the Ksours Mounts (from Delfaud, 1986).

2.2.2. Jurassic The Jurassic succession outcrops widely in the western Atlas and has been studied by Bassoulet (1973), Elmi (1978), Benest (1982, 1984), Kazi Tani (1986), Aı¨t Ouali (1991) and Elmi et al. (1998 and references therein). In the western Saharan Atlas, the Liassic succession is composed of four sequences called L1, L2, L3 and L4 by Aı¨t Ouali (1991). The sequences L1 and L2 are Hettangian – Sinumerian in age. They correspond to dolomites and limestones of inner platform. The sequences L3 and L4 are Domerian and Toarcian in age and they are made of marls interbedded by limestones and characterise the slope deposits and a fast deepening environment (Aı¨t Ouali, 1991; Elmi et al., 1998). The thickness of the L3 and L4 sequences decrease and the facies pass laterally to limestones. These variations can be linked to the tilted block geometry and the early to diapir growth up (Figs. 5a,b and 6a,b). The carbonate facies follow during the Early Dogger up to the Callovian where a detrital spreading prevails in the western and central Saharan Atlas (Bassoulet, 1973; Kazi Tani, 1986; Elmi et al., 1998). The detrital input disappears towards the east, namely the Bourlier area (Bo) (Fig. 4), DoghmaneAı¨n Ouessera high, Ouarsenis Ridge (Mattauer, 1958) and Hodna Massif (Bertraneu, 1955; Guiraud, 1973). The Late Jurassic follows by sandstones and clays in the western Atlas (Bassoulet, 1973; Delfaud, 1986; Kazi Tani, 1986). This detrital material diminishes towards the north and disappears completely in the Hodna where the facies pass to limestones and marls (Bertraneu, 1955; Guiraud, 1973; Aissaoui, 1979). Several hard grounds, top laps and reef facies characterise the Late Jurassic succession (Caratini, 1968; Aissaoui, 1979; Herkat, 1982; Nouiouat, 1994). The reef facies are linked to diapir growth up during the Late Jurassic in the central Saharan Atlas (Herkat, 1982; Nouiouat, 1994). These facies are also described

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Fig. 4. Synthetic correlation N – S from the Saharan Platform to the Mesetian block, showing Meso – Cainozoic thickness and facies variations (from Frizon de Lamotte et al., 2000. Location, see Fig. 1b).

by Caratini (1968) on the Doghmane-Aı¨n Ouessera high and are in agreement with the Triassic outcrops (diapirs) observed in Chellala area. Thickness decreasing towards this high are pointed out by seismic sections and some of them show Triassic salt mobilisation during Late Jurassic as in the central Saharan

Atlas. This salt activity also agrees with reworked elements of the Triassic observed in the Lower Cretaceous by Herkat (1992). A discontinuity tops the Jurassic (Figs. 6a and 7) and was interpreted as indicative of a compressional event (Kazi Tani, 1986).

Fig. 5. (a) Seismic section (TWT second) SW – NE oriented strike to a salt ridge in the Akerma area. The Upper Liassic thickness decreases towards the SW (salt ridge) where the salt of the Triassic becomes thicker. The Triassic (salt) thickness decreases towards the NE. The Upper Liassic deposition is contemporaneous with the salt mobility. (b) Salt pillow in the Akerma area. The salt displacement occurs during Late Liassic times (from Yelles-Chaouche et al., 2001. Location, see Fig. 1b).

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Fig. 7. Seismic section (TWT, second) W – E oriented showing the Jurassic thickness change towards the Mesetian block Jurassic and tilted blocks during the Liassic times (rifting) and unconformities of the Miocene/Cretaceous and the Cretaceous/Jurassic (location, see Fig. 1b).

2.2.3. Cretaceous The Lower Cretaceous is made of detrital deposits of Neocomian, Barremian and Albian age. The facies

during the Barremian are continental in the western and central Atlas and become argillaceous to marly and thicker towards the Tellian margin (Figs. 6 and 8)

Fig. 6. (a) Seismic section (TWT, second) N – S oriented showing the Liassic thickness variation linked both to the normal active fault and the salt growth up during the Liassic. This seismic section also shows the Upper Jurassic top laps and the thickness variation. Note that the Upper Jurassic thickness increase is not affected by the salt growth up. (b) Northern part of the section (a) showing a structure due to the mobility of the Triassic salt during Late Liassic – Dogger? On this section, we can also observe the Cretaceous thickness change (location, see Fig. 1a and b).

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Fig. 8. Seismic section (TWT, second) oriented N – S crossing the south Tellian front, the foredeep until the foreland. The Cretaceous thickness increases towards the north (location, see Fig. 1b).

and the Aure`s Mountains (Laffitte, 1939; Vila, 1980; Wildi, 1983; Bureau, 1986; Kazi Tani, 1986). The Aptian made of limestones ‘‘calcaires a` Orbitolines’’ overlies the previous detrital input and becomes clay-rich to the north (Tell) and the east (Aure`s Mountains). Some reefs facies surrounding diapirs are linked to salt mobility during the Aptian (Sn Repal, 1970). Cross-bedded sandstones of Albian age overlie the Aptian then clays alternating with marls; then limestones at the top. The sandy facies diminish towards the Tell and the Aure`s Mountains where they pass to facies rich in clay. The Cenomanian follows by marls interbedded by gypsum in the western and central Saharan Atlas and southern of the Hodna Massif. The facies and the thickness change towards the Tell where they become more marine and thicker (Guiraud, 1973; Kieken, 1974; Bureau, 1986; Kazi Tani, 1986). In eastern Algeria (South Atlasic Front), the Upper Cenomanian corresponds to oolitic limestones producing oil. Anoxic facies of Turonian cover the Cenomanian, then oolitic limestones alternating with levels of

marls. This succession is rich in organic matter and constitutes the main source rock in the eastern part of Algeria where the Upper Cenomanian and the Turonian constitute the main petroleum system. The Senonian succession follows by limestones and marls (Kieken, 1974; Bureau, 1986; Kazi Tani, 1986). Reefs facies surround some diapirs in the Aure`s Mountains (Perthuisot et al., 1990), they are coeval with reworked elements of the Triassic described in western Algeria (Ciszak, 1993; Vila and Charrie`re, 1993), the central Saharan Atlas (Herkat, 1992) or conglomerates in the Ourasenis Mounts (Mattauer, 1958) and the Hodna Massif (Bertraneu, 1955; Guiraud, 1973; Kieken, 1974). 2.2.4. Palaeocene The Cainozoic succession in Algeria is well described in the Hodna Massif southern border (Kieken, 1974). The Palaeocene begins by marls interbedded by coquinoid limestones. Its thickness changes laterally from 245 m in Drw well to zero in Id.2 and Deg wells (Fig. 9a,b). In the western Atlas, the Palaeocene is lacking (Bracene and Kireche, 1986;

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Fig. 9. (a) Location of the correlation (b) in the Hodna basin. (b) Correlation from the east to the west showing the Palaeocene and the Miocene unconformities in the Hodna basin. The Cretaceous thickness increases to the west (location, see Fig. 1b).

Kazi Tani, 1986) and rests unconformably on the Maastrichtian in the Hodna Massif (Kieken, 1974). It does not seal structures resulting from compressional events (Bracene and Frizon de Lamotte, in press). 2.2.5. Eocene The Eocene is made of bituminous marls, cherty limestones, and phosphates and fish teeth bearing limestones. The Lutetian follows by dolomites, marls

and gypsum (Sn Repal, 1970; Guiraud, 1973; Kieken, 1974). 2.2.6. Oligocene Oligocene facies are mainly sandy, they are made of reddish sandstones, clays, siltstones and gypsum. In the Hodna Massif, the Oligocene deposits lie unconformably on the Cretaceous and the Palaeogene rocks (Kieken, 1974) and seal compressive structures in the Hodna Massif (Kieken, 1974;

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Courme-Rault, 1985; Bracene and Frizon de Lamotte, in press). 2.2.7. Miocene The Miocene facies and thickness vary quickly from one area to another. The facies include conglomerates, sandstones alternating with clay and carbonate levels. In Ogs Well, for instance, the Miocene begins by limestones, then marls alternating with sandstones and limestones. To the South of the Hodna Massif, three cycles called M1, M2, M3 have been defined (Kieken, 1974; Courme-Rault, 1985). The oldest sediments are of Late Burdigalian age and the most recent levels are Pontian in age (Courme-Rault, 1985). Everywhere in the studied area, the Miocene deposits lie unconformably on the previous strata (Fig. 9b).

3. Tectonic setting The main events discussed in this paper (Table 1) are. – A syn-rift episode during the Triassic and Liassic times (Elmi, 1978; Biju-Duval and Dercourt, 1980; Elmi et al., 1982; Aı¨t Ouali, 1991; YellesChaouche et al., 2001). This episode is characterised by tilted blocks geometry (Figs. 6, 7, 8 and 10a,b), synsedimentary tectonic structures, emersion and erosion of the apex blocks, thickness variations (Bourezg, 1984; Aı¨t Ouali, 1991; Elmi et al., 1998) and mobility of the Triassic salt (Figs. 5a,b and 6a,b). During the Liassic, trending SW– NE normal faults (Fig. 11a) and salt ridges emphasised by seismic sections are in agreement with a NW – SE extension direction (Aı¨t Ouali, 1991; Bracene and Frizon de Lamotte, in press). – A post rift episode follows during the Middle Jurassic up to the Late Cretaceous (Bracene and Frizon de Lamotte, in press). The Middle Jurassic deposits seal the previous tilted bocks (Fig. 10a,b) and display a typical ‘‘steer’s head’’ geometry. During this period, the facies and the thickness vary towards the north (Tell) and to the Aure`s Mountains where they become rich in clay and thicker. Previous works developed in the western Algeria and in Morocco describe a transcurrent regime during the Dogger and the Malm (Mattauer et al., 1977; Laville and Petit, 1984; Laville,

1985; Kazi Tani, 1986; Aı¨t Ouali, 1991; Jacobshagen, 1992; Jossen and Filali-Moutei, 1992; Pique´ et al., 1998). This transcurrent regime is not evidenced on our subsurface data. However, the Upper Jurassic shows top laps and is topped by a discontinuity which has been observed in the central Saharan Atlas by Kazi Tani (1986). Kazi Tani links this discontinuity to a compressional event but Jurassic compressive structures sealed by Early Cretaceous have not been observed anywhere in the field or on the subsurface data. Furthermore, during the same period in the Tellian domain (Babors), Early Cretaceous deposits include conglomerate facies (Leikeine, 1971; Kireche, 1993). According to Kireche (1993) these conglomerates mark apex blocks erosion. Furthermore, during the Early Cretaceous, westerly of the Hodna Massif, extensional events occur (Fig. 11b), known in the Aure`s Mountains (Bureau, 1986) and in Tunisia (Burollet and Ellouz, 1986). The discontinuity observed on the seismic sections represents a sequence boundary (offlap), which appear to be more linked to sea level change (Bracene and Frizon de Lamotte, in press) than to compressional events (Kazi Tani, 1986) or to break up unconformity. During the Cenomanian, synsedimentary NW –SE normal faults were observed westerly of the Hodna Massif (Fig. 11c) and they emphasise an extensional event at this time. The sedimentation made of marls and limestones continuous until the Late Lutetian. Conglomerate occurrences during the Santonian are known in the Hodna and also in the Tell (see Leikeine, 1971 and Obert, 1981). According to Kireche (1993) in the Tell (Babors) but also in the Hodna, these facies do not seal compressive structures, they rather underline apex blocks erosion during extension. The major sedimentation change occurs mainly during the Oligocene with sandy deposits. – A compressional regime followed, linked to Europe/Africa plate convergence (Durand Delga and Fontobe´, 1980; Wildi, 1983; Frizon de Lamotte et al., 2000). The main compressive phases in Algeria are: – Late Lutetian (Kieken, 1974; Frizon de Lamotte et al., 1990; Bracene and Frizon de Lamotte, in press) or Middle Eocene as pointed out by Guiraud and Bosworth (1997). The structure axes resulting from this event are trending NE – SW.

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Table 1

– Miocene (Tortonian) (Kieken, 1974; Courme-Rault, 1985). This phase is coeval with the allochthonous emplacement. – Pliocene ‘‘Villafranchian phase’’ (Boudjemaa, 1987; Frizon de Lamotte et al., 1990; Ghandriche, 1991). The Villafranchian structure axes are E – W trending in eastern Algeria (Biskra to Mandra). The shortening is NW –SE directed with a shortening ratio of about 20% (Vially et al., 1994; Frizon de Lamotte et al., 2000) in western Algeria (Ksours

Mounts), but easterly in Aure`s Mountains, shortening ratio is not evaluated.

4. Analysis (1-D) of the subsidence curves 4.1. Data The wells used for the subsidence modelling in this study were selected according to their structural position, the thickness encountered and the precision of the stratigraphic limits.

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Fig. 10. (a) Seismic section (TWT, second) W – E oriented showing Liassic tilted block sealed by the Middle Jurassic deposits. (b) Seismic section (TWT, second) N – S oriented showing Liassic tilted blocks (rifting) sealed by the Middle Jurassic deposits, and the Middle and Upper Jurassic thickness variations (location, see Fig. 1b).

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Fig. 11. (a) Main Liassic active faults in the studied area (from Frizon de Lamotte et al., 2000). (b) Main active faults in the southern border of the Tell during the lower Cretaceous. (c) During the Middle Cretaceous (location, see Fig. 1b).

To cover all the structural domains corresponding to the studied area and to make an account of the subsidence evolution, 20 wells were used (Fig. 1a). Among these wells, 15 were real ones and 5 were synthetic. These last ones were composed by using seismic sections interpretations combined with field data. In this study, we have avoided wells showing strong thickness induced by tectonic complexity as in the Tellian domain or along the South Atlasic Front. For the subsidence modelling, we use the Genex software developed by the French Petroleum Institute

(IFP) and the stratigraphic scale from Odin (1994). The data include deep wells (well logging), geochemistry analysis, and seismic sections from Sonatrach petroleum company. For each well, the available data are the age and the thickness of the sequences drilled, the eroded thickness (evaluated, see below), the geothermal gradient (from Drillstem test ‘‘DST’’ and corrected Bottom Hole Temperature ‘‘BHT’’), the geochemistry parameters (Maximum Temperature, ‘‘Tmax ’’, Vitrinite Reflectance ‘‘PRV’’ and Thermal Alteration Index ‘‘IAT’’), the estimated palaeobathymetry and the porosity or the lithology component.

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Fig. 12. Shale transit time versus depth of Deg well and eroded thickness estimation. In this well, the eroded thickness is about 800 m.

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Fig. 13. Curves of total subsidence and subsidence rates for wells: (a) BchTtaDrw, (b) Bch, (c) Tta, (d) Drw.

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Fig. 14. Curves of total subsidence and subsidence rates for wells: (a) Rjb, (b) Drwfic, (c) Tebfic, (d) Dogfic.

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4.2. Eroded section estimates For the modelling by Genex software, two unknown quantities must be correctly appraised. The first one is the eroded thickness and the second is the heat flow. In this order, the eroded thickness can be deduced from regional geological data and can be appraised also by plotting shale transit time versus depth within the real wells. Using this method (Serra, 1979, 1985; Magara, 1986) for example in Deg well (Fig. 12), the eroded thickness is 800 m corresponding to the Maastrichtian and partially to the Campanian. By this method, the eroded thickness was evaluated in the real wells. The eroded thickness in Ogs well (Fig. 1a) reaches 2500 m. The Deg cross-plot outlines an under-compaction of the Campanian shale. This means that the Upper Cretaceous was not buried during the Palaeogene and is consistent with the Palaeogene gap in Draa Ghozlane area (Deg) or the western Saharan Atlas (Bracene and Kireche, 1986; Kazi Tani, 1986). Moreover, this cross-plot illustrates the Miocene angular unconformity pointed out by the seismic sections (Figs. 6, 7 and 10) and known at the regional scale. This eroded thickness estimation reduces the unknown parameters to the heat flow. Therefore, taking into account the tectonic context of northern Algeria during the subsidence modelling, we have to calibrate the heat flow values with the thermal geochemistry data (PRV and Tmax). For this, several runs and hypotheses concerning the heat flow are considered (constant, variable, rifting). 4.3. Subsidence history since the Mesozoic up to the present The continuous and thick Mesozoic and Cainozoic sequences (Triassic to Miocene) have not been drilled in northern Algeria up to now. Such a section likely can be found in the Hodna area or in the Aure`s Mountains. To summarise the subsidence history in the Tellian foreland in Algeria, we have constructed a composite curve (Fig. 13a) by combining subsidence features of the most subsided areas. It includes data of Bch well (Fig. 13b) for the Jurassic, Tta well (Fig. 13c) for the Cretaceous and Drw well (Fig. 13d) for the Cainozoic.

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The aim of this curve is to give first assessment of the subsidence evolution. The inferred thickness from this curve is about 9000 m and is close to the Aure`s basin estimate (10 000 m). According to the synthetic subsidence curve BchTtaDrw (reference to each real well), strong subsidence phases occur during Jurassic and Cretaceous times. The Liassic starts with a mean rate of 50 m/Ma until the Toarcian (185 Ma). During the Toarcian, the distribution of the subsidence shows a spatial heterogeneity. At this time, erosion is expressed in the Bch well, while synchronously, subsidence accelerates laterally in other areas. The subsidence heterogeneity implies that strong subsidence occurs in the basin besides mild subsidence or even uplift on the top of the blocks or salt domes. This is in agreement with the field data (Elmi et al., 1998) and can been deduced from seismic sections (Figs. 6, 7 and 10). During the Late Jurassic times (Malm), the subsidence increases and reached a rate of 200 m/Ma for Bch and Tta wells, drilled close to diapiric areas (Fig. 1a,b). The subsidence rate fell during the Cretaceous to 40 m/Ma. This rate represents a mean value for the Cretaceous because during the Cenomanian the rate was about 250 m/Ma and during other periods of the Late Cretaceous, it did not exceed 50 m/Ma. This phase is followed by a slow subsidence and even a tendency to uplift in the western Saharan Atlas during the Late Cretaceous. The Palaeocene is characterised by uplift and by an erosion rate of about 50 m/Ma in the western Sahara Atlas (Fig. 14a). However, a moderate subsidence in the Hodna basin continues until the Lutetian (Figs. 13d and 14b) and the Aure`s Mountains (Fig. 14c). The Early Oligocene corresponds to an erosive phase followed by subsidence during the Late Oligocene. During the Miocene, erosion occurs in the thrust belt and the forebulge and a coeval weak subsidence in the foreland basins such as M’sila Basin (Fig. 2c) or the Tellian foredeep (Fig. 8).

5. Relation between subsidence and tectonics The Tethyan margin evolution during the Mesozoic can be summarised by periods of strong and

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weak subsidence followed by coeval erosion and subsidence during the Tertiary. These subsidence phases agree with the previous tectonic context. They express Atlantic and Tethyan opening and the Tethyan margin inversion (formation of the Maghrebian orogen). 5.1. Liassic During the Early Jurassic, the strong subsidence may be directly related to crustal thinning. The h factor is about 1.13 in the Saharan Atlas (Aı¨t Ouali, 1991). The structural framework is dominated by tilted blocks (half grabens) consistent with the southern Tethyan margin crustal thinning. During the Liassic, normal faults affect this margin. It underwent dislocation, individualising subsidence zones with variable rates (Aı¨t Ouali, 1991; Elmi et al., 1998). During the Late Liassic, subsidence was weak or even transformed into uplift on the apex blocks but continuous laterally in the depocentre. Dogfic well (Fig. 14d) and seismic sections (Figs. 6, 7 and 10) point out well this subsidence heterogeneity observed also on the field in the western Saharan Atlas (Aı¨t Ouali, 1991) and the Telagh trough (Elmi et al., 1998).

5.3. Cretaceous During Early Cretaceous times, the subsidence rate fell to 40 m/Ma. It corresponds to the thermal subsidence during the post rift regime. An increase of subsidence rate characterises the Cenomanian (Fig. 15a,b) that exceeds 250 m/Ma in the central Saharan Atlas (Tta), the Pre-Atlas or in the Aure`s Mountains. Easterly in Tunisia and Lybia (Gulf of Gabe`s and Sirte), at the same time, a similar increase of the subsidence is described (Patriat et al., 2003). This subsidence phase is linked to eustatic rising and major changes in the African/Eurasian kinematics. During the Cenomanian and the Turonian, the accretion rate of the central Atlantic is very high (3.8 cm/year) and at the Coniacian a rapid change in the relative movements of the African/Eurasian plates occurs (Olivet et al., 1984). In the Hodna Massif, the first steps of the deformation described are Late Cretaceous in age (Bertraneu, 1955; Guiraud, 1973; Guiraud and Bosworth, 1997), but the Upper Cretaceous does not seal compressive structure. Evidence for this phase comes argued mostly by the conglomerate facies which can translate more erosion of apex blocks or cap rocks of diapirs than a compressive phase.

5.2. Middle – Late Jurassic 5.4. Tertiary The post-rift episode is from the Middle Jurassic up to the Late Cretaceous. After a period of weak subsidence during the Dogger, the Upper Jurassic subsidence rate increases and reaches 200 m/Ma (Fig. 13). This peak constitutes an anomaly occurring during post rift regime (thermal subsidence). However, this phase concerns wells (Bch and Tta), specially those drilled close to outcrops of Triassic salts (Fig. 1b) and never in areas where the diapiric events are not evidenced (Mesetian block or Saharan Platform). Therefore, it appears that diapiric events have strongly affected subsidence during the Late Jurassic. During the Late Jurassic, salt growth up is demonstrated and likely recurrent normal faults can be observed. This subsidence anomaly could constitute a response to the evaporites volume displaced laterally. The mobility of the evaporites induces space (rim syncline) where thick deposits can take place.

The Palaeogene begins with an erosive phase in the entire western part of Atlasic domain. The Ksours Mounts, Djebel Amour and Oulad Naı¨l have been uplifted during the Late Cretaceous– Early Tertiary (Bracene and Kireche, 1986; Kazi Tani, 1986) and towards the east (Aure`s Mountains) the emerged zones become narrow. This emersion predates the main inversion, which occurs at the Late Lutetian. The African and the European plate convergence starts during the Late Lutetian and ends at the Pliocene initiation (Frizon de Lamotte et al., 2000). After this compressional phase (Late Lutetian) followed by an erosion phase during the Early Oligocene, the subsidence goes on moderately with an average rate of 50 m/Ma (Figs. 13d and 14b). This subsidence occurs in foreland basins coeval with deformations during the Miocene (Tortonian) (Fig. 16).

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Fig. 15. Curves of total subsidence and subsidence rates for wells: (a) Deg, (b) Tya, (c) Tlgfic.

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Fig. 16. Seismic section (TWT, second) showing the Palaeogene and Miocene unconformities, Cretaceous thickness increases from Drw well to Drwfic (location, see Fig. 1b).

Since the end of the Miocene up to the present time, no subsidence has been recorded in northern Algeria foreland basins.

6. Conclusion and discussion The previous sections emphasise that the framework of the southern Tethyan margin in Algeria is characterised by a polyphased geological history. During the Mesozoic, the southern Tethyan margin was organised as troughs and ridges mostly trending SW –NE. They have undergone, through time and space, strong and weak subsidence and heterogeneity behaviours. 6.1. Subsiding areas They are characterised by strong subsidence rates during the Mesozoic. The total subsidence in the Telagh trough (Fig. 15c) and the western Atlasic domain is 6000 m (Fig. 17). The thickness increases towards the Aure`s Mountains where subsidence accel-

eration occurs during the Barremian up to the Middle Cretaceous. In the Aure`s Mountains where the Mesozoic is about 10 000 m thick (Vila, 1980; Bureau, 1986; Kazi Tani, 1986), the Jurassic sequences are not reached by wells and the seismic sections do not clearly show horizons below the Cretaceous sequences. We consider it likely that during the Jurassic, the Aure`s Mountains underwent a similar central and western Saharan Atlas evolution. From the Cretaceous, a regional tilting eastwards but also northwards induces a fundamental change in palaeogeography and sedimentary deposition. The Palaeozoic (basement) becomes deeper from the SW to the NE. This tilting of the Mesetian block explains the Palaeozoic deepening from the west (wells H33, H36, Fig. 2a) to the east (Bo well). The eastward Palaeozoic deepening can be also observed on the seismic section (Fig. 7). This eastwards tilting continues during the Palaeogene and the western and central Saharan Atlas undergo emersion, it thus prevents to follow the evolution after the Late Cretaceous. However, the evolution can be traced for the Tertiary in the Hodna basin and the Aure`s Mountains

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where the Tertiary sequences are present and the subsidence is continuous. The sedimentation rate is moderate, particularly in the Hodna basin (Fig. 14b,c). These subsiding areas during the Miocene (Sersou, Fig. 1b, see also Fig. 8; M’sila Basin, Fig. 9a, see also Fig. 2c) correspond to foredeeps and the uplifted areas constitute either the forebulge (Atlasic system) or the thrust belt (Tellian domain or the Hodna Massif). 6.2. The ridges Ridges are also trending SW –NE and characterised by weak subsidence rates (Fig. 17). On the Mesetian block, the Bo well (Fig. 18a) shows that the subsidence is about 1000 m and consistent with seismic data (Fig. 7). The Mesozoic is very thin and this feature can be found in the northern Saharan Platform border (Benoud) where the total subsidence is about 2000 m (Fig. 18b,c). On these resistant blocks, indications of very slow subsidence amplitudes are evidenced. The Upper Jurassic subsidence phase (known in the subsiding areas, Fig. 13a,b) does not appear because of the absence of diapir manifestations (salt mobility).

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During Cretaceous, the Saharan Platform is affected by subsidence (Fig. 18b,c), which can be correlated to opening of the Tellian trough (Kazi Tani, 1986), the extensional in the eastern regions (Bureau, 1986), or the Central African rift (Janssen et al., 1995; Guiraud, 1998). In summary, the southern Tethyan margin in Algeria presents not only NW –SE tilted blocks (Figs. 6a and 8), but also a NW – SE tilting (Fig. 7). This geological framework leads to migration of the subsidence over time and space. During the Triassic after the sandy deposition corresponding to the syn-rift phase, evaporites seal the Triassic tilted blocks. These facies have not been deposited on the Mesetian block. During Liassic normal faults affect this part of the southern Tethyan margin and the subsidence is reactivated. From the Dogger up to the Late Cretaceous a post-rift regime prevails mostly in the western areas (Ksours Mounts, Telagh trough and central Saharan Atlas). The Hodna Massif and the Aure`s Mountains evolution seems to be different during the Cretaceous where the Mesozoic and Cainozoic sequences are more complete, more marine and thicker. Extensional events are

Fig. 17. Mean total subsidence curves for the seven zones distinguished in this paper. These curves are made with the mean of the subsidence rates calculated from the modelled wells.

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Fig. 18. Curves of total subsidence and subsidence rates for wells: (a) Bo, (b) Hmk, (c) Mzr.

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recorded, they translate effects of rifting occurring easterly in the Gulf of Gabe`s and Sirte. During the Tertiary, compressional regime dominates inducing hiatuses, erosion on forebulges and thrust belts. The subsidence is observed only in some areas corresponding to foredeeps.

Acknowledgements The authors wish to thank M.D. Takherist, Head of the Sonatrach Exploration Division, and M.D. Bekkouche coordinator of the Peri-Tethys Programme at Sonatrach, who gave access to well data and seismic profiles and authorised publication. Special thanks are given to Pr. D. Frizon de Lamotte, Dr. R. He´bert and Dr. B. Saint Bezar for their constructive criticisms. We are indebted to Dr. B. Ce´le´rier, Pr. S. Elmi, Dr. A. Mauffret and Dr. F. Beekman for constructive remarks that helped to improve the original manuscript.

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