The geological setting
in western Niger
Frieder Graef
Department of Soil Science and Land Evaluation, University of Hohenheim, 70593 Stuttgart, Germany, e-mail: graef@uni-hohenheim.de
Introduction
Geological surveys in West Niger were carried out e.g. by Greigert (1961, 1963), Greigert and Pougnet (1967), Machens (1966, 1967, 1973) and Ferier (1981). The geological map of W-Niger provided in this work is essentially based on the mapping of Greigert (1961).
W-Niger is situated on various formations of the Precambrian West African shield (Liptako-Gourma region in the west, Aïr in the north-east) and on Palaeozoic, Mesozoic and Tertiary formations of the Iullemeden basin (Figure 1: Geological setting in the Iullemeden Basin, Greigert 1963, modified) , which extends from Algeria over Mali to Benin and northern Nigeria.
In the Palaeozoic a gulf open to the north
filled the southern limits of the Aïr and Adrar with deposits overlapping
towards the south (Greigert 1963). During the Mesozoic and Tertiary the
West African shield receded and was periodically invaded by marine transgressions
diminishing in thickness to the south and passing laterally into continental
series. Uplift movements beginning in the Middle Eocene gave the basin
its present aspect. It was subsequently filled inland delta-like with continental
fluvial and lacustrine oolithic iron-containing series (Greigert and Pougnet
1967). Further uplift movements caused these sediments to form an extended
land surface undergoing ferralitic weathering and residual iron-oxide accumulation
in a humid tropical climate (Leprun 1979). Subsequent dry periods caused
an induration and sealing of the iron-rich surface, so the present ironstone-capped
plateaux and terraces ("cuirasses" Fr.) were formed. These plateaux were
carved during the Pleistocene humid periods by fluvial systems originating
locally and from the Aïr (Gavaud 1966). Silicate karst phenomenons
are also attributed to this period (Bergoeing and Dorthe-Monachon 1997)
or to earlier humid phases (Sponholz 1989). More recent tectonic activities
were found by Vicat et al. (1994). At least two dune transgressions (Erg
I and Erg II) during Pleistocene aridification phases were identified (Ousseini
1986, Völkel 1989).
The Precambrian basement shield occurs in the west (Liptako), where its geological boundary is also traced by the river Niger, and in the north (Aïr), where it is partly concealed by dunes and smooth cover sands. In the eastern and central part the sediments of the Iullemeden basin prevail, which are also partly concealed by cover sands.
Detailed maps of the Liptako basement area (So) were established by Machens (1967) and Ferier (1981), who divided the basement area into Prebirrimian, Birrimian and dolerites of younger age. The Prebirrimian, composed of migmatites, few granulites and amphibolites, is found only as local enclosures within the batholiths of the more recent Birrimian. The Birrimian covers most of the region. It consists on one hand of large batholiths, which are composed of granites, granodiorites and quartz bearing diorites and on the other of thick layers of argillic schists, greywackes and sericitoschist series with a rather weak metamorphism. Schists with high metamorphism and intercalations of magmatic rocks occur locally between these sedimentary rocks. Younger dolerites injected during a phase of fracturing of the basement shield and formed longitudinal dikes (Machens 1973). The Precambrian basement area of the Aïr is composed of metamorphic sedimentary and volcanic deposits of many kilometres thickness and with many granite intrusions (Kilian 1932). In the geological map the various basement substrates are not differentiated.
The sedimentary cover of the Iullemeden Basin can be divided into a) series of Infracambrian age, b) the Perm to Cretaceous "Continental intercalaire", c) the Upper Cretaceous to lower Eocene "Continental hamadien", d) the Eocene to Pliocene "Continental terminal", and e) Quaternary alluvial deposits, Regs, dunes and sand deposits:
- The Infracambrian series e.g. the Kirtachi and Niamey quartzite and the Dounga-Goungou series (Machens 1967) horizontally overlying the basement occur only to a small extent near the Niger river and are not shown in the map.
- The "Continental intercalaire", which is located at the western edge of the Aïr can be roughly divided into 4 groups: The Perm to Triassic arkose of Izegouandane (jP), the Jurassic sandstone of Agadez (ci1), the lower Cretaceous claystone of Irhazer (ci2) and the vast lower Cretaceous group of Tégama (ci3), which is composed of sandy, clayey or calcareous fluvial indurated sediments (Greigert 1963).
- The "Continental hamadien" sediments, which were deposited during the upper Cretaceous to Lower Eocene, show in an almost circular pattern tracing the Iullemeden basin. They are divided into a) the Upper Cenomanien and Lower Turonien consolidated sandy-clayey gypsum and carbonates (Cr6a-5) at the edge of the Tegama group, b) the Upper Turonian white limestone and marls (Cr6b), c) the Lower and Middle Sénonien limestone and claystone banks (Cr7), d) the Upper Sénonian sandstones, mudstones and shales (Cr9-8), e) the Lower Eocene marine carbonates and claystones (eIII-VI) and f) the indifferent Continental hamadien deposits, which are composed of claystone, sandstone, limestone and conglomerates (Csc).
- The "Continental terminal" deposits cover the central part of the Iullemeden Basin. At the edge of the Birrimian they have only few decimetres’ depth but they attain 450 m depth in the centre of the basin. The base was assigned by Greigert and Pougnet (1967) to the Middle Eocene, whereas Machens (1973) indicated an approximative Mio-Pliocene age for the middle to upper strata. Four series were distinguished. The siderolithic series of Adar Doutchi with kaolinitic iron-rich claystone (ct1) and a maximum depth of 80 m. The clayey-sandy series with lignites (ct2). The indifferent lower Continental terminal (ct1-2). The upper clayey sandstone series (ct3) with a maximum thickness of 50-100 m (Greigert and Pougnet 1967) and consisting of variously textured and hardened mixture of quartz, iron-oxides and kaolinite.
- Several types of alluvial deposits can be distinguished: Recent deposits of various composition are found along the Niger river and its tributaries (A) and various terrace generations can be distinguished along the Niger river (Thevoz et al. 1994, Bergoeing and Gilliard 1997) and at ancient Niger tributaries (Bergoeing and Dorthe-Monachon 1997). The Dallols (qa²) with alluvial sand and clay deposits represent ancient Pleistocene river valleys originating from the Aïr, that were later covered with small dunes (Bui 1986). Lastly alluvial plains of Pleistocene age (qa1) with quartz gravels are found in the region south of Maradi.
- In the north the vast Quaternary regs of the Ténéré du Tamesna (qR) occur.
- Moving dunes (qd2), fixed dunes, and other cover sands (qd1) show an increase with latitude. They were formed in arid Pleistocene and Holocene phases and now cover wide areas of the Iullemeden Basin and the basement (Ousseini 1986, Wilding and Daniels 1989).
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Figure 1: Geological setting in the Iullemeden Basin, Greigert 1963, modified