Relationships between pedology, geomorphology and stratigraphy in the Dallol Bosso of Niger, West Africa

Abstract

Relationships between pedology, geomorphology and stratigraphy were investigated in a dry valley of Niger, the Dallol Bosso. The sediments of the Dallol are mostly fluvial sands which have been reworked by eolian processes. Two sections of the valley were studied, each representing a different ecologic zone; one was in the pastoral zone (350 mm rain/year), the other in the rainfed agriculture (millet) zone (550-600 mm rain/year). In both zones, the same five morphologic units are found: plateau capped by ferruginous sandstone, scarp exposing unconsolidated Miocene sandstones, alluvial fan surface, dunes and sand-plugged ancient channels. The dunal unit and ancient channels were the focus of the study in the two areas. Dunes cover about 80% of the valley while ancient channels occupy about 20%. Throughout the Dallol Bosso sandy dunal soils generally show only incipient pedogenic development (organic matter accumulation and color Bw horizon). Most are classified as Psamments. Their fertility is low: percent organic C is less than 1% and CEC less than 3 meq/100 g soil. Their pH is acid, ranging from 5.2 to 6.5 in the surface horizon. Exchangeable Al comprises up to 50% of exchangeable cations. Base saturation is variable but is generally higher than 35% in the top 50 cm of the soil. Their soil moisture regime is ustic to weakly aridic and they are droughty. The high sand content and sparse vegetation cover of these soils make them highly susceptible to wind erosion. Field evidence of eolian activity includes blowouts at the base of trees, barren active dunes and buried A-horizons. Factor analysis of quartz grain-size and shape distributions suggest that sorting by wind on the basis of both size and shape is occurring. The medium to fine and more angular shaped sand fractions appear most susceptible to wind transport. Soils developed in old channel deposits tend to be variable in texture, ranging from sand to sandy clay. They tend to be alkaline where the water-table is shallow, due to concentration of Na2CO3 by evaporative pumping. Due to the high pH, silica is mobilized and recombines with bases to form smectite. Greenish gley sub-surface horizons and segregation of Fe-oxide nodules are also associated with a shallow water-table. Low-lying areas are often strewn with Fe-oxide concretions and sheets of ironstone (groundwater laterite) a few square meters in area are sometimes found along the edges of former ponds.