Influence of Altitude on the Petrological Features of a Soil Climosequence in the Humid Tropical Zone of Cameroon
Although lateritic soils are well documented in the humid tropical zone, work on adiabatic effect, which imposes local climatic variations with increasing altitude, has received very little attention in this zone. The aim of this work was to study and specify a soil climosequence in the humid tropical zone of Cameroon and to show the influence of altitude on the geochemical functioning of tropical soils. The work was done in the field and completed by a battery of laboratory analyses. The results enabled the definition of a soil climosequence for the humid tropical zone in Cameroon. This climosequence is marked by soils whose evolution is conditioned by the geochemistry of iron, silicon and aluminium. In the plateau ecosystems (500-800 m altitude), iron is expressed either as hardened materials (e.g. ferruginous nodules or duricrust) or light patches on the soils, while silicon and aluminium are combined as kaolinite. In the high plateaus (800-2000 m altitude), soil evolution is dominated by aluminium geochemistry, where aluminium forms gibbsite (either indurated or loose) often associated with small amounts of goethite and hematite. Finally, in the mountainous massifs (> 2000 m altitude), soil evolution is controlled by silica geochemistry, where silica is combined with amorphous aluminium to form allophane. In this soil, the presence of a dark and thick humiferous andosolic horizon in the upper part of the profile is very remarkable. Overall, this soil climosequence reflects geochemical functioning processes which grade with altitude in the humid tropical zone of Cameroon, similar to that commonly observed under the temperate climate. This general scheme could be extrapolated to other humid tropical regions with similar climatic and relief conditions as those observed in Cameroon. However, this general tendency often includes site-specific modifications caused by human activity and local variations of environmental conditions.
Essential differences between the bauxite deposits along the southern and northern edges of the Guiana Shield, South America
Comparison of the buried bauxite deposits of the Onverdacht-Lelydorp area in Suriname (northern edge of the Guiana Shield) with those of the Trombetas area in Amazonas-Para, Brazil (southern edge of the Guiana Shield) reveals essential differences in: (1) the age and duration of the bauxitization process (Eocene-Oligocene at about 20 m.y. and Pleistocene at about 1 m.y.); (2) the mineralogy of the iron component (exclusively hematite and a mixture of hematite and aluminum-bearing goethite); (3) textures (mainly concretionary and cellular textures compared with granular and massive textures); and (4) the character of the covering layer (Miocene and younger unconsolidated sediments against Pliocene residual soil).The geographic position, climatic history, and parent rock in both areas are as similar as ever can be expected in geology; thus, the differences between the bauxite deposits in both areas can probably be attributed to the differences in age and duration of the bauxitization and to the presence or absence of a postbauxite sedimentary cover.
Formation of Red Beds in Modern and Ancient Deserts
New evidence supports the theory that the hematite pigment in many red beds, particularly those associated with evaporites and aeolian sandstones, formed after deposition in hot arid or semiarid climates. The evidence includes field, petro-graphic, and chemical data collected from studies of two stratigraphic sequences that contain red beds: (1) Recent, Pleistocene, and Pliocene deposits in the Sonoran desert of northeastern Baja California, Mexico, and (2) late Paleozoic deposits in Colorado. The sequence in the Sonoran desert contains examples of red beds forming today in a hot dry climate. They are associated with bedded evaporites and occur where regional faunal, floral, and pedological evidence indicates rainfall has been low throughout the postdepositional history of the sediments. The facies associations reflect deposition in fluvial and fluvial-marine transitional environments. Red arkose fanglomerates occur on the flanks of the highland source areas, and red muds, probably of intertidal and shallow subtidal origin, occur in the transition sediments. Both of these red-stained facies show progressive stages of in situ alteration of nonred sediments to hematite-stained red beds, and in each, the iron in the stain is derived from intra-stratal alteration of iron-bearing detrital grains, particularly iron silicates such as hornblende and bio-tite. The late Paleozoic red beds of Colorado contain rock types and facies associations similar to those of the Sonoran desert; they are interpreted as ancient counterparts of those red beds. Numerous lines of field and petrographic evidence indicate that the hematite pigment in the late Paleozoic red beds formed in place after the sediments were deposited in desert basins.