Abstract
<p class="zhengwen">Soil magnetic properties measurements are relatively fast and inexpensive but have been proved to be sufficient for preliminary investigations in diverse socio-developmental issues. This paper presents results of a reconnaissance study of soil colour and magnetic susceptibility (c) in eastern Botswana, where ~80% of the population resides. The work is a first step to creating a database of rock and soil magnetic properties and to document spatial variations in magnetic properties in the country. These measurements are important as constraints for interpretation of available aeromagnetic data and can also be exploited for environmental soil research (pollution) and land-use planning (agriculture). The soils sampled include derivatives of varying types and provenance such as Archean gneissic granitoids, metamorphosed rocks (granulites), volcano-sedimentary assemblages, Karoo basalts, and alluvial sediments. A soil colour chart was used since soil colours and magnetic properties are diagnostic of its parent rock sources and weathering profiles. Soil magnetic susceptibilities were measured at both low frequency (0.46 MHz, clf) and high frequency (4.6 MHz, chf), thus allowing calculation of frequency-dependent susceptibility (cfd, cfd%) for detecting ultra-fine ferromagnetic minerals.</p>It was found that soils with Hues ranging from 7.5YR to 10YR have appreciable amount of magnetic materials and soils with Hues of 2.5YR are generally nonmagnetic. The results of soil magnetic susceptibility profiles show spatial variation closely related to the variation in basement rocks, which provides excellent evidence that the magnetic susceptibility variation reflects basement rocks or bedrock composition (soil parent material). In relation to the Botswana physiographic units, soils from the hardveld (Precambrian) show the highest susceptibilities, followed by those from the sandveld, with the lowest values being from the alluvial. The frequency dependent magnetic susceptibilities indicate the presence of ultra-fine super-paramagnetic minerals such as magnetite/maghemite. It is suggested that a systematic and continuous programme of rock and soil magnetic measurements would benefit various socio-economic and development priority sectors of Botswana. This also applies to many developing countries in Africa where soil physics and measurement of soil susceptibility in particular, is generally still at an embryonic stage.
Highlights
Economies in many southern African countries, including Botswana, largely rely on mining and agriculture, and several programmes have been instituted to enhance and promote these sectors
This work forms part of a multidisciplinary long term project to study various rock and soil environmental characteristics to provide a basic understanding of soil physical properties and processes, and to discuss methods for solving practical problems related to environmental, hydrological, land use and agricultural issues (e.g., Gwosdz et al, 1982; King & Ranganai, 2001; Moidaki, 2001; Zhai et al, 2003)
The study has revealed important soil magnetic signatures from soils derived from different rocks that span significant geologic time periods
Summary
Economies in many southern African countries, including Botswana, largely rely on mining and agriculture, and several programmes have been instituted to enhance and promote these sectors. Aeromagnetic surveys have proved to be the most cost-effective tool in mapping complex geology and sporadic outcrops (e.g., Gunn et al, 1997), as the data are relatively unaffected by the presence of surficial material such as weathering or overburden. This is the case for Botswana where exposure of pre-Tertiary formations is restricted to about 25% of the country This would not give information on the grain sizes of the minerals
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