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| Mineralogical characteristics and genesis of black cotton soils in Nairobi Valley of Kenya |
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Received:October 04, 2024
Revised:January 03, 2025
Accepted:January 07, 2025
Published Online:August 05, 2025
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| DOI： |
| KeyWord:Nairobi Valley; Black cotton soil; Mineralogical characteristics; Geological genesis |
| Author | Institution |
| Shiding Miao |
Jilin University |
| Zhaopu Shen |
China Road and Bridge Corp. |
| Xuelian Wang |
Chengdu Comprehensive Rock and Mineral Testing Center of Sichuan Geological and Mineral Exploration and Development Bureau |
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| Abstract: |
| This article focuses on compositional and mineralogical characteristics of Black Cotton Soil (BCS) in the Nairobi Valley of East African Rift Valley in Kenya. Based on previous researches, e.g., structural geology, sedimentary petrology, paleogeographic climate, hydrology, the Pleistocene archaeology related to BCS geological genesis was analyzed, and formation mechanism of BCS rich in alkaline water system or volcanic rocks river lake sediments under semi-closed conditions was proposed. The main mineral of modern BCS in Nairobi South Ring Road was calcareous montmorillonite, which contains illite mixed layer minerals and a small amount of contact metamorphic plagioclase, quartz and zeolite. Compared to ancient clays, the Mg content of modern BCS has significantly decreased, and contents of Al and Ca were increased. At geological age of 0.7-0.9 Ma, the organic content of BCS was relatively high, and the Fe2O3 content was slightly lower, the negative charge carried by silicate plates decreases, and the unit structure changes from trioctahedron to dioctahedron. The sedimentary rocks of BCS in Nairobi Valley were derived from coarse-grained basalt and pyroclastic rock. The development of such sedimentary rocks began from 3.3-0.8 Ma, which was validated in the Late Cretaceous (Turonian Maastrichtian stage), spanning from basic olivine green basalt, Limuru and Bishoftu coarse-grained rocks to the Quaternary volcanic glassy clastic sediments. The development process follows path of volcanic glass → high silica zeolite → montmorillonite, and the sedimentary water environment follows a sequence of acid at the initial stage and then alkali at the final stage. The intermediates rich in Si were excreted with the semi-closed environment. Results of paleogeographic climate and archaeological research show that the East African region has evolved from humid to semi-arid, and climatic factors have also promoted the development of BCS sedimentation. |
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