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Lullemmeden Basin
The Iullemmeden Basin with a tectono-epeirogenic origin is one of the three important sub-Saharan inland basins. It extends for about 1 000 km from north to south and 800 km from east to west. The basin consists of a broad syncline with a maximum depth of about 1000 m, except within two major depressions which are structurally controlled. Three principal fault-trends predominate. These are NNE-SSW, NW-SE and WSW-ENE. The most important is the NNE-SSW-trend which is responsible for the existence of the two depressions within the basin. The NW-SE trending faults mark the western limit of the basin and display a downthrow of about 500 m. The WSW-ENE faults are concentrated in the north-eastern part of the basin on the western margins of the Air mountains. The epeirogenic movements within the basin are evident from the beginning of the Palaeozoic and continued until the late Cretaceous when the opening of the Goa trench took place. These movements were responsible for the south-western progradation of sediments within the basin. Sedimentation resumed during the Palaeozoic and continued throughout the Mesozoic and Cainozoic with occasional breaks. The Cambro-Ordovician formations rest directly on the basement in the northeastern portion of the basin along the margins of the Air mountains where basement outcrops have been intensively peneplained. Further towards the centre of the basin, Devonian and Carboniferous formations outcrop and are punctuated by inliers of basement rocks. Permian and Lower Cretaceous deposits outcrop progressively further south and the sedimentary sequence becomes increasingly younger south-westwards. Upper Cretaceous and Tertiary deposits outcrop around the centre of the basin forming an undulating plain with occasional hills with an elevation of about 100 m above the plains. The geological history of the basin displays a close relationship between tectonics and sedimentation.
Stratigraphy and tectonics
The general stratigraphic sequence of the basin is now fairly well established thanks to the efforts of many pioneer French geologists including Joulia (1959, 1963, and 1989), Radier (1959), Greigert (1966), and Faure (1966). Geologists of the French Atomic Energy Agency including Bigotte, Obellianne, and Valsardieu worked extensively in Niger. Jones (1948) and Kogbe (1979) published monographs on the Nigerian sector of the basin and Alidou (1983) also published a monograph on the sector of the basin in northern Benin. Table 1 summarises the stratigraphic sequence within the Iullemmeden Basin.
Palaeozoic
The Palaeozoic formations outcrop in the north-eastern portion of the basin, locally known as the Tamesna Basin. They form a band around the southern portion of the Tassilli highlands within the Central Sahara. Alidou (1983) described Palaeozoic trace-fossils from the south-western portion of the basin in northern Benin Republic thus confirming the presence of Palaeozoic outcrops in the area.
The basement horst of 'In Guezzain' divides the Tamesna basin into two sections; the Tin Seririne or Tim Mersoi section to the east and the Tassilli Oua N' Ahaggar section to the west. The Tin Seririne section is folded into a large syncline oriented roughly north-south while the Tassilli Oua N' Ahaggar trends east-west but is folded into several small synclines with north-south orientation. The lithostratigraphy of the Palaeozoic sequence in the lullemmeden Basin is shown in Table 1.
The Palaeozoic sediments in the northern portion of the lullemmeden Basin are subdivided into two sections by a major fault (see Figure 1). The lower section is unconformable on the basement complex and consists of Cambrian and Ordovician sandstones and shales overlain by Silurian calcareous-silty clays and sandstones with graptolites. One erosional unconformity exists at the base of the Cambrian and a more important one occurs at the top of the In-Azaoua sandstones of Middle Ordovician age.
The upper section is unconformable on the lower one described above, and comprises lowennost conglomerates succeeded by Lower Devonian sandstones (Grcs d'ldekel).
The Middle Devonian consists of sandstones with ferruginous oolites, and calcareous shales with a rich brachiopod fauna. The Upper Devonian is represented by alternating coarse-grained ferruginous sandstones and fossiliferous gypsiferous shales. The Carboniferous consists of three sedimentary sequences: the Terada Series; the Lower and Upper Tagora Series (Table 2).
Table 1 Idealized lithostratigraphic section of the N-E sector of the Iullemmeden Basin (after Valsardieu, 1971 )
The Terada Series consists of basal conglomerates and f1uvio-glacial sandstones described by Valsardieu (1971). The Lower and Upper Tagora Series consist of fluviodeltaic sandstones. Each of the series is overlain by marine shales or an alternation of marine shales and continental sandstones. After the deposition of the marine Carboniferous deposits, there was a long period of erosion and continental sedimentation which ended with the Upper Cretaceous transgression over the basin.
The Palaeozoic sequences thin towards the centre of the basin, each overlapping the previous one from north to south (see Figure 1). Sedimentation is obviously structurally controlled.
The Palaeozoic geological history of the Iullemmeden Basin is characterised by the presence of erosional unconformities separating the mega-sedimentary sequences. Two minor structural unconformities exist at the end of the Silurian and the middle of the Carboniferous. They may be traces of Caledonian and Hercynian orogenic events.
The Continental Intercalaire (Permian to Lower Cenomanian)
Killian (1931) defined the Continental Intercalaire as continental sediments above the marine Carboniferous and below the Upper Cenomanian transgression. They therefore range from Permian to Lower Cenomanian in age. In the Iullemmeden Basin, the Continental Intercalaire has been subdivided by 10ulia (1963) into several distinct major units separated by mappable unconformities. They consist of the following units (see Figure 2):
The Tegama Group at the top
The Dabla Series / Irhazer Group
The Agades Sandstone
The Izegouandane Formation at the base
Figure 1 Cross-section of the Tin Seririne Basin. 1 = Timesugar sandstone; 2 = Graptolite • Schales'; 3 = Idekel and Touaret sandstones; 4 = Akara 'Schales'; 5 = Amesguer sandstone; 6 = Farazgkat sandstones (or Teragh arkose); 7 = Talach clays; 8 = Guezouman sandstones; 9 = Tchinezogue clays; 10 = Tarat sandstones; 11 = Madaougla clays; 12 = Izggouandane Series; 13 = Irhazer clays; 14 = Tegama sandstones.
The Izegouandane Formation
This formation outcrops between Tim Mersoi in the northwest and north of Agades above the 1 r 30' N latitude. It consists of arkoses, shales, sandstones and sandy shales of detritic fluvio-Iacustrine origin deposited under a semi-arid climate. The thickness decreases southwards from a maximum of about 300 m. Fossil wood fragments of Dadoxylon rolli have been collected from the basal arkosic sediments.
They are common in the German Permo-Triassic and thus confirm the Permian age of the formation. Fossil vertebrates including the reptile Moradisaurus grandis Taquet (1967, 1969) were collected from the top of the formation. This reptile, with close affinity to species described from Texas and Russia, dates the top of the formation as Upper Permian (Taquet, 1972, 1976). The Izcgouandane Formation is undoubtedly Permian in age.
According to Valsardieu (1971) and Cazoulat (1985) the Izegouandane Formation was deposited during a tectonically active period. At the flank of the basin around Air, acid volcanic intrusions were emplaced, probably during the early stages of sedimentation. The intrusions became prominent towards the end of the Permian when the volcano-sedimentary series of Moradi were deposited.
Agades Sandstone Group
This group outcrops at the western flank of the Air massif (Figure 2). It is separated from the underlying Izegouandane Formation by an unconformity. The Agades Sandstone Group is made up of several mappable units including at its base, the Teloua sandstones which may be sub-divided into two members of fine- to coarse-grained sandstones with cross-stratification. 'Pieces of magmatic rocks' within the basal conglomerates might have originated from ring complexes at Damagaram and N!ounio which were emplaced around 300 2: 20 M.a. (Karche & Vachette, 1978). They may also have originated from the volcanic activity of Carboniferous or Permian age discussed earlier. This conglomerate thins out and later disappears northwards. The Teloua sandstones are overlain by conglomerates, coarse-grained sandstones of Tehirezrine-l and analcimolites of Abinky or Karafou.
Figure 2. Schematic structural section of the Iullemmeden Basin west of the Air Mountains. North-south section, on 6° longitude line.
The Agades sandstone diminishes in thickness from north-west (250 m) to the south-west. It consists of detrital, poorly-sorted sediments. Only the base of the Teloua sandstones is dated, by reptilian trace fossils of Chirotherioum, a primitive form described from the Lower Triassic (Taquet, 1976). The stratigraphic position of the Agades Sandstone Group between the Permian Izegouandanne Formation and the Irhazer shales of Late Jurassic age suggests a Triassic to a Lower-Middle Jurassic age for the Agades Sandstone. More detailed dating of the different formations within the Agades Sandstone Group has not been possible to date due to scarcity of fossils.
The Dabla Series or Irhazer Group
This series consists of two formations; the Tchirezrine-2 sandstones and the Irhazer shales. They outcrop in the northeastern portion of the Iullemmeden Basin where they attain a maximum thickness of 600 m (Valsardieu, 1971).
The Tchirezrine-2 sandstones overlie the Tchirezrine-l sandstones of the Agades Group unconformably. They are probably Late Jurassic in age. The Irhazer Formation consists of sandy clays or carbonates deposited under fluviatile or lacustrine conditions. The depositional environment was calm and sub-tropical. The very large thickness of this formation as well as the lithologic characteristics suggest a slow but continuous depression of the basin and the absence of any major tectonic movement.
The Tegama Group (Le Groupe du Tegama)
This group includes all continental sediments outcropping along a vast plateau between the Iullemmeden and the Chad Basins. The group extends through the north and the centre of the basin as far south as the 15° N latitude where it has been penetrated by a borehole. The Tegama Group lies directly on the Irhazer clays, unconformably. It is characterised by sediments that are dominantly sandy with clayey or calcareous intercalations (Bellion, 1989). These fluviatile and lacustrine deposits occurred within broad valleys and marshland under humid tropical and semi-arid conditions. Previous workers (Faure 1966, Greigert 1966, Joulia, 1959a), have sub-divided the Tegama Group into two series. These consist of the Tegama Series, overlain by the Farak Series. The Tegama Series was subdivided into 3 formations (Tazole or Tiguedi at the base, overlain by the Eirhaz and Echkar, respectively, see Table 1).
The Tegama Series consists of sandstones, siltstones and clays deposited during two major sedimentary cycles. The basal Tazole Formation and the Echkar Formation at the top consist of coarse feldspathic sandstones, with crossstratification. The intermediate Eirhaz Formation is more argillaceous or sandy-calcareous. It is rich in fossil bones of dinosaurs and crocodiles. South of AYr the Eirhaz Formation thins out and the Tegama Group consists predominantly of sandstones (Faure, 1966).
The Farak Series is a transitional series between the lower continental deposits of Lower Cretaceous age and the upper marine deposits of Middle and Late Cretaceous age. The thickness is around 200 m. The series is characterised by finely bedded variegated massive clays and fine sandstones and siltstones, locally micaceous. At the top the Farak Series contains several layers of lacustrine limestones with fish, crocodile and dinosaurian remains (Greigert et ai., 1954; Taquet, 1976).
The Continental Intercalaire in the Iullemmeden basin terminates with fine sediments marking the end of a major detritic stage. According to Faure (1966), the Farak Series is a lateral equivalent of the Alanlara Formation of marine origin in eastern Niger. This formation is dated as Early to Late Cenomanian. This therefore implies that the top of the Continental Intercalaire is diachronous. It is older than Late Cenomanian in the Damergou and slightly younger in Tefidet. The maximum thickness of the Tegama Group is about 800 m north of Demergou. It attains its minimum thickness of about 200 m at In-Ababgarit further south. To the west at Tamesna the thickness varies between 500 and 600 m (Lapparent, 1953). Facies variation is frequent.
The Gundumi and Illo Formations of north-western Nigeria
In north-west Nigeria, the Gundumi and Illo Formations along the south-eastern flank of the Iullemmeden Basin are lateral equivalents of the Tegama Group (Figures 3a & b) (Kogbe, 1981). The Gundumi Formation lies unconformably on the basement and consists of basal conglomerates and gravels with sand and variegated clays increasing upwards. The maximum thickness is about 350 m. The Illo Formation includes interbedded clays and grits with an intermediate pisolitic and nodular clay member which attains over 240 m in thickness.
Figure 3 (a) Schematic lithostratigraphic section through the south-eastern part of the Iullemmeden Basin. W= Wurno, Du= Dukamaje; T = Talika; G-J = Gundumi and Illo.
Figure 3 (b) Lithostratigraphy of the Iullemeden Basin in northwest Nigeria.
The Upper Cenomanian and Lower Turonian
The Later Cretaceous resumed with a major marine transgression during which the Tethys Ocean extended inland as far south as the Iullemmeden Basin, where it was probably in contact with a south Atlantic marine transgression through the Benue Trough (Kogbe, 1979, 1981; Figure 4).
Figure 4. Palaeogeographic map of West Africa during the Cenomanian and Early Turonian. Hatched area denotes marine inundation and arrows direction of transgressions.
The Cenomanian transgression extended into the Lower Turonian, putting an end to the development of the Continental Intercalaire. The Cenomanian - Turonian deposits consist predominantly of calcareous sandstones, fine sandstones, shales with gypsum, glauconite and fossils. They show rapid changes in facies and typify a detritic continental platform style of sedimentation, following a general transgression interrupted by a brief regressive episode. In the Iguellala mountains north-east of Fennert, Greigert (1966) described a section with 25 m of slightly calcareous sandstones, sometimes glauconitic and phosphatic, with slightly sandy shales with gypsum.
The sediments are richly fossiliferous and contain lamellibranchs, gastropods, brachiopods and ammonites including Neolobites vibrayeanus, a typical Upper Cenomanian ammonite which was found at the base of the section. Vascoceras, Paravascoceras, Acanthoceras and Nigericeras were found at the top, thus confinning a Lower Turonian age for the top of the section. North-westwards, the sequence continues undisturbed as far as Mount TazerzaH Kebir after which it thins and becomes more sandy and fossiliferous. West of the 3° 40' meridian in the republic of Mali, the deposits become more continental with prominent cross-bedded sandstones separating the upper level with Nigericeras from the lower level with Neolobites.
The maximum thickness of the Cenomanian-Turonian deposits occurs between the Iquellala mountains and the TazerzaYt Kebir mountains which suggests that the Upper Cenomanian and Lower Turonian transgression of the Tethys Ocean invaded the Iullemmeden Basin from the north-east. This view is supported by the enhanced marine nature of the sediments in the same region. It is therefore unlikely that the transgression was from the west of the Adrar des Iforas as suggested by some authors (petters, 1978).
The Upper Turonian and Lower Senonian
The Lower Turonian calcareous limestones with Nigericeras are directly overlain by an alterating sequence of limestones and shales or clays. These sediments can be sub-divided into two portions. The lower portion is poorly fossiliferous while the upper portion is more calcareous and fossiliferous. It is referred to as the 'Serie des calcaires blancs' by French workers.
This whitish limestone outcrops along the 3° Meridian north of Tenekert in Mali as far as north-east of Adar Doutchi at Tan Haidara in Niger (Figure 5). The limestone contains remains of gastropods, echinoids, and lamellibranchs which are difficult to identify with any precision. They are overlain by shales and limestones of Lower Senonian age.
Figure 5 The Cenomanian-Turonian and Lower Maastrichtian transgressions in the Iullemmeden Basin. AD = Adar Doutchi; TK = Monts Tazersait Kebir; I-M = Monts Iguellala and Monts Maya. Modified after Greigert (1966).
The rather considerable thickness of the 'Serie des calcaires blancs' attaining 250 - 300 m around Tama"ia, suggests slight subsidence of the basin during the upper Turonian.
The Upper Senonian (Maastrichtian)
The Maastrichtian is characterised by alternating fine sandstones, siltstones and clays rich in lignite and fossil vertebrates. This paralic sequence is intercalated with marine shales with limestone, deposited during a brief episode of marine incursion (Kogbe, 1972, 1976, 1979, 1981). The Maastrichtian deposits of the Iullemmeden Basin in north-western Nigeria constitute the Rima Group of sediments, comprising the Taloka, the Dukamaje and the Wurno Fonnations (Table 3). These sediments outcrop extensively along the south-eastern flank of the basin particularly in the sector commonly referred to as the Sokoto Basin. Their type sections have been described by Kogbe (1981). The Rima Group lies unconformably on the pre-Maastrichtian beds. This erosional unconformity is well exposed in Wurno.
The Taloka Fonnation, with a maximum thickness of about 100m, consists essentially of white, friable fine sandstones and carbonaceous mudstones and shales.
The Dukamaje Formation consists predominantly of shales with some limestones and mudstones. The formation varies from about 12 m at the type locality to less than 0,5 m, 50 km further south. The fauna includes the Upper Maastrichtian ammonite Lybicoceras sp., found at Dukamaje and Gilbedi. The presence of two horizons in Gilbedi is significant and supports the view that the bone bed is due to the action of winnowing forces along the strand-line of a transgressive sea during the Maastrichtian (Kogbe, 1974).
The Wurno Formation is quite similar to the Taloka Formation. The sediments consist of pale, friable fine sandstones, siltstones and intercalated mudstones (see Figure 7a). In boreholes, the sediments are dark-coloured due to the presence of carbonaceous material and finely disseminated iron sulphides. Small-scale 10ad-casL~, bioturbation structures and flaser bedding are common in the Wurno, as is the case with the TaJoka Formation.
The base of the Wurno Formation is only observed around Wurno; south of Rabat, the intermediate Dukamaje Formation thins out and the Wurno and Taloka Formations can no longer be differentiated. They thin south-westwards and outcrop very rarely. The Wurno Formation is over 50 m thick. According to Bellion (1989), the top of the Wurno Formation grades into very fossiliferous limestones towards the west of the basin. The fossils confirm an Upper Maastrichtian age.
The major discontinuity underlying the Rima group (Kogbe, 1981) is both erosional and tectonic in origin. The important tectonic episode which occurred in the Benue valley during the late Santonian (Wright, 1976) must have affected parts of the lullemmeden Basin before the deposition of the Rima Group of sediments.
Lower Tertiary
Palaeocene
The Palaeocene has been subdivided into three formations (Monod, 1939; Cornet, 1943, 1948; Radier, 1959; Kogbe, 1979, 1981, 1986). In north-west Nigeria, the Palaeocene sediments constitute the Sokoto Group, consisting of the D:mge, Kabmbaina and Gamba Formations. Kogbe (1981) introduced a fourth subdivision which is made up exclusively of ferruginous oolites (Figure 6a).
Figure 6 (a) Idealized section through the Mesozoic-Cenozoic sequence of the southern flank of the Iullemmeden Basin (northwest Nigeria). 1 = conglomeratic fluviatile pebbles; 2 = fossil wood; 3 = siltstone; 4 = lignite; 5 = fossil bone; 6 bioturbation structures; 7 = gypsum; 8 = laminated shale; 9 = ferruginous oolites; 10 = lateritic crust.
The oldest Palaeocene unit is the Dange Formation which consists of slightly indurated bluish-grey shale, interbedded with thin layers of yellowish-brown limestone. In outcrops, the maximum thickness of the formation is about 22 m near Sokoto, but in subsurface weBs, it attains a thickness of over 45 m. The Dange Formation is overlain by the Kalambaina Formation which consists of a marine white, clayey limestone and shale. The thickness varies between 5 m in the north to over 18 m further south. The formation is rich in invertebrate fossils, mainly echinoids, corals, nautiloids, lamellibranchs and gastropods. Figure 6a is a section at the cement quarry near Sokoto showing the Kalambaina and Gamba Formations and the ferruginous oolite horizon.
The Gamba Formation overlies the Kalambaina and consists of grey laminated shale with 'false folding' due to the removal by solution of parts of the underlying limestone of the Kalambaina Formation and subsequent slumping of the overlying beds. The Gamba Formation is characterised by a distinct horizon with abundant phosphatic pellets and coproliths.
The Oolitic Ironstone overlies the Gamba Formation. The oolites are primary. According to Du bois & Lang (1981), the Gamba Formation and the Oolitic Ironstone are of Ypresian (Middle Eocene) age. It is, however, more probable that the Oolitic Ironstones are of Lower Eocene age, since the Gamba Formation has been precisely dated as Upper Palaeocene by diagnostic microfauna (Kogbe et ai., 1972).
The Cretaceous - Tertiary boundary is marked in the south-eastern sector of the Iullemmeden Basin by an unconformity which is both structural and erosional.
The Continental Terminal
The Continental Terminal is made up of all the continental deposits overlying the marine Eocene and underlying the Quaternary sediments. The term was introduced by Killian (1931) for Tertiary non-marine sediments in Sub-Saharan Africa. The lower and upper boundaries of the Continental Terminal are of geodynamic significance. In the Iullemmeden Basin, the Continental Terminal was subdivided into three major units defined by Greigert (1966) as the 'Serie Siderolithique de L'Adrar Doutchi' (Ct 1) at the base, 'la serie argilo-sableuse A lignites' (Ct 2) in the middle and 'Les gres argileux du Moyen Niger' (Ct 3) at the top. According to Lang et al. (1986) Ct 1 is of Lower to Middle Eocene age and marks the end of the marine sedimentary cycle in the Iullemmeden basin. Kogbe & Sowunmi (1975) dated the Continental Terminal in northwest Nigeria (Gwandu Formation of the Sokoto basin) as Miocene - Pliocene with a noticeable absence of the Oligocene which probably represents a period of nondeposition. Figure 6b attempts to correlate the Continental sequences in the Iullemmeden Basin with those of other basins in Africa.
Figure 6(b) Possible correlation of some major continental sequences in Africa. U = Upper; L = Lower; M = Middle.
Quaternary
The end of the Cenozoic in the Iullemmeden Basin is marked by uplift movements and erosion, resulting in the formation of terraces capped by ferruginous crusts or laterites.
Data source: Stratigraphy and tectonic history of the Iullemmeden Basin in West Africa. C.A. Kogbe, 1991
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