The soils in this region are highly weathered. It is generally accepted th a t their pedogenesis corresponds to th a t of soils developed under climatic conditions more hum id th an those prevailing (Gavaud, 1966). Information on th e m any episodes of climatic change th a t have taken place in the region is incomplete. O u attara (1990) provides a sum m ary from m any prim ary sources based on dated cores from Lake Chad and terrace stratigraphie correlations:
100.000 - 50,000 YBP At least one period of extreme aridity w hen sand dunes (‘ancient erg*) formed down to the 12th parallel.
40.000 - 30,000 YBP Evidence of a wet period w ith deposition of fluvial and lake sediments and aeolian reworking.
30.000 YBP Arid phase.
29.000 - 20,000 YBP Humid climate throughout subtropical Africa w ith a decrease in tem perature in the la tte r p a rt of the period probably due to an incursion of polar fronts.
20.000 - 13,000 YBP Arid phase with a maximum aridity a t 17,000 YBP and the associated deposition of the most recent dunes ("recent erg") associated w ith much more vigorous winds th an present (Talbot, 1980).
12.000 YBP - present Many oscillations w ith dry periods a t 10,000 and 7,500 YBP and wet periods a t 11,000, 9,000 - 8,000, 6,000 and
3,000 - 3,500 YBP. Rainfall was less intense and more evenly distributed th an a t present w ith vegetation under a dense and perm anent vegetation cover.
Most workers accept th a t the climate has been tropical in the region during th e late Q uaternary as suggested by the deep w eathering and the form ation of ferricrete terraces.
3.3. Geology.
South w estern Niger comprises part of the lullem eden sedim entary basin which extends to p arts of Benin, Mali and Nigeria. Covering m ost of th e basin are the Miocene Continental Terminal sediments (Gavaud, 1966; Greigert, 1966). G reigert (1966) distinguished three m ain series in the formation: th e upperm ost clayey sandstone (Moyen Niger), the lignitic clayey sandstone, and the siderolitic series at the base. The study area is located on the youngest Moyen Niger series, comprising a soft, thick (400 m), sedim entary rock with massive, medium to fine-grained sandstone clays and clayey sediments of quartz, kaolinite and goethite mineralogy (Greigert, 1966; Gavaud, 1977; Sombroek and Zonneveld, 1971; O uattara, 1990; Legger, 1993).
The sedim ent is incised by ancient river valleys comprising late Cretaceous to early T ertiary sedim entary rocks. At the time of the formation of these sedim ents the land surface was stable resulting in pediplanation and the formation of now fossil valleys (Dallols). The eastern p art of the region is dominated by the Dallol Bosso w ith ancient dune deposits a t the surface. During the late Tertiary, or early Q uaternary, several layers of ferricrete (laterite) formed w ithin the Continental Term inal on horizontal surfaces. They were probably formed during interruptions to the sedim entary process (Greigert, 1966). At the end of the T ertiary the Continental Term inal was uplifted (Greigert, 1966; Gavaud, 1977) and considerable erosion resulted in a landscape of very flat, ironstone capped, low plateaux separated by broad, low gradient, sand valley systems, sloping to dry stream beds (Wilding and Daniels, 1989; O uattara, 1990). Several sequences of ferricrete have been found, the m ain sequence is at 220 m, 240 m and 260 m above the current sea level (Wilding and Daniels, 1989). The valley systems are buried in an aeolian sand m antle a few m etres thick, associated with the ‘ancient erg’. The subcircular dunes do not show preferential orientation (Gavaud, 1966; Grove and W arren, 1968). The ancient erg is found in all neighbouring West Afiican countries to a latitude of 12®N (Grove,
1958).
F erricrete is defined as a duricrust cemented w ith iron and / or alum inium (Oilier and Galloway, 1990). It is commonly found in w eathered profiles in repeated phases
and in formations of ranging ages (Oilier and Galloway, 1990). Ferricrete has a v ariety of forms including pisolithic and verm icular (McFarlane, 1983) and may range betw een a few centim etres to tens of m etres thick. The sapolite usually b eneath the ferricrete, leached or unleached of Fe, varies from a few m illim etres to over 100 m etres (McFarlane, 1983). The ferricrete and leached sap(blite underlying Niamey is shown in Plate 3.3.1.
P late 3.3.1. Leached sapAlite of the ferricrete terrace underlying Niamey.
In ferricrete formations the degradation of prim ary m inerals, such as feldspars is accompanied by the formation of new secondary minerals, which typically belong to clay m ineral groups. During initial and interm ediate stages of w eathering, complex clay m inerals accum ulate and together w ith prim ary m inerals are the major
constituents in surface soils and sediments. As chemical w eathering proceeds the proportions, in surface soils and sediments, of Na, K and Ca decrease and th a t of A1 increases and these changes are reflected in the accumulation of kaolin group m inerals (Kronberg and Melfi, 1987). The most abundant and im portant constituents of this group of m inerals are the iron oxides goethite (a-FeOOH), which give yellow to brown colours, and haem atite (a-FegOg), which gives red colours.
The landscape in the vicinity of the study area comprises four geomorphic elem ents (Wilding and Daniels, 1989): broad gently sloping plateaux w ith discontinuous sand cover; sandy valley systems th a t slope gently from the ironstone-capped plateaux tow ard dry ephem eral stream s; broad terrace-like sand plains; and the dallols.