Evaluating weathering of palaeosols in Cameroon (Central Africa) as a tool for paleoenvironmental reconstruction

Abstract

Palaeosols formed from different igneous rocks have been identified and characterized in the humid savannas (1400 mm mean annual rainfall, 22 °C mean annual temperature) of the Adamaoua region in north-central Cameroon by means of macro- and micromorphology, geochemical analyses and bulk mineralogy. The studied sequence at Mangoli is about 11 m deep and is divided into three sections, in terms of weathering products: (1) Precambrian granite, located in the base; (2) vesicular porphyritic basalt, in the middle section; (3) aphanitic basalt in the uppermost layer. The products of weathering from the Precambrian granite consist of the following horizons, from bottom to top: a saprolite with granite structure, a yellowish to whitish fine saprolite, and Btg and Bt horizons. The saprolite horizon is dominated by quartz, feldspars (plagioclases and microcline), and muscovite. Primary minerals, except quartz, show weathering, while clay fills the pores and fractures and kaolinite dominates the clay mineralogy. The granite weathering products are slightly acidic (pH ranging from 5.9 to 6.3) and display low values of Fe extracted with dithionite-soluble Fe (Fed) and high Chemical Index of Alteration (CIA) and Mineral Index of Alteration (MIA) values. The middle section of the sequence consists of different areas of the basalt’s spheroidal weathering zone, from the more advanced to the less weathered phase. The clay mineralogy is dominated by kaolinite. The zones with the most alteration are the fractures, where thick, reddish clay coatings are observable. In these areas, the minerals are well crystallized. The kaolinite crystals share areas with Fe oxides. The upper section of the sequence constitutes the modern soil from the aphanitic basalt and consists of a spheroidal weathering zone divided into three horizons. The lowest is a C horizon, overlaid by Bt horizons. The C horizon shows a high degree of weathering in some areas, but it is still possible to identify the primary minerals. Their products of weathering are slightly acidic (pH = 6) and have high Fed, CIA and MIA values, which are indicative of intense weathering. The results suggest warm and humid climates during the formation of all of the palaeosols, similar to modern environmental conditions. We conclude that Palaeosol 1, formed from the granite, represents a soil formed during the Mid-Miocene Climatic Optimum, when the temperatures are assumed to have been 3–6 °C warmer than today. Features discovered during field investigations reveal that the former valley at the surface of the landscape has been filled by lava flows and today occupies the top of the plateau. Meanwhile, the former top of the plateau is located on the slopes and lower altitudes; the burial of soils by volcanic materials at Mangoli increased the altitudes of the former plateau and resulted in landscape inversion (slopes). These palaeosols and their top layers that induced the slope inversion have not yet been described and appear as a unique example worldwide.