Geology of the Limpopo River Basin

The Influence of Geology on the Landscape

There is a close, fundamental relationship between soils and the geology of the rocks that lie beneath them. Most soils are weathered and modified sediments from underlying bedrock, or have been transported to their present location by some form of mass transit.

Another relationship that it is important to recognise is the close link between geology and geohydrology/groundwater. The lithology and geological structure of a region has a direct result on the storage, quality, availability and recharge potential of groundwater resources.

While the majority of aquifers in southern Africa are associated with secondary porosity - secondary aquifers - the Limpopo River basin features some of the few alluvial aquifers in the region, with subsurface flow of the Limpopo River and some of the tributaries providing groundwater to towns and mines along the main stem river during periods of low flow.

Geology of the Limpopo River Basin

The rocks of the southern African region represent the entire cross section of geological history (Schlüter 2006).

The Geology of the Limpopo River basin is characterised by a series of significant geological features:

The Kalahari Craton;
The Limpopo Belt;
The Archaean Craton;
The Karoo System; and
The Bushveld Igneous Complex

A map of the generalised geology of the Limpopo River basin is presented below, followed by individual descriptions of these features.

The Kalahari Craton

The term craton refers to the ancient, stable foundation of the continental Earth's crust (Pidwirny 2008). The Kalahari craton comprises the Kaapvaal craton, Zimbabwe craton and the Limpopo Belt.

The Limpopo Belt

The Limpopo Belt, or Limpopo Mobile Belt, as it is sometimes know is a Precambrian formation (Schlüter 2006) that joins the Kaapvaal craton to the Zimbabwe craton, running East to West (Chinoda et al. 2009) and covering a large portion of the Limpopo River basin (Chinoda et al. 2009). The Limpopo Mobile Belt has experienced significant mineralisation and follows very closely the Limpopo River valley (Ashton et al. 2001).

The exposed parts of the Limpopo Belt are distributed throughout the South African, Botswanan and Zimbabwean portions of the basin (Chinoda et al. 2009). The Limpopo Mobile Belt, as it is sometimes known, is divided into three zones:

The Northern Marginal Zone;
The Central Zone; and
The Southern Marginal Zone.

The Archaean Craton

Archaean craton are the oldest remaining fragments of the original continents of the earth, predominately comprising crystalline granitic and gneissic rocks, with greenstone belts and dolerite dykes and sills (Ashton et al. 2001)

The Karoo System

The Karoo System is the origin of the major carbonaceous shale and coal deposits (Council for Geoscience 2010) that have supported the South African coal mining industry, providing fuel for domestic use and export for the past century and a half. This system is associated with younger sedimentary and crystalline rocks, overlying large areas of the southern portion of the basin (Ashton et al. 2001).

The coal deposits have been preserved in four downfaulted basins in the Limpopo Province, known as the Waterberg, in the vicinity of Lephalale (Ellisras), Soutpansberg (along the northern flank of the Soutpansberg mountains), Limpopo (along the southern bank of the Limpopo river, west of Musina) and Springbok Flats coalfields. These coalfields contain large reserves of coal, with the Waterberg coalfield reckoned to hold more than 40% of South Africa's in situ mineable coal reserves. These vast resources are presently being mined at the large Grootegeluk coal mine. The small Tshikondeni mine is the only currently operating coal mine in the smaller Soutpansberg coalfield and yields high-grade coking coal for Iscor's steel mills. (Council for Geoscience 2010)

The Bushveld Igneous Complex

Containing a large portion of the region's mineral wealth (Ashton et al. 2001), the Bushveld Igneous Complex comprises largely basic mafic and ultramafic rocks and intermediate intrusive rocks (SARDC 2002). The Bushveld Igneous Complex is covers the southern, South African, portion of the basin, overlain in places by the Waterberg Group of sandstones (CGIAR 2003).

Geology of the Lower Limpopo River Basin

The geology of the lower portion of the Limpopo River consists largely of consolidated and unconsolidated sedimentary rocks (Ashton et al. 2001), including argillites, fluvial sandstones and mudstones (Chinoda et al. 2009). These sediments form a region of shallow sloping plains interrupted occasionally by exposed granitic intrusions (SARDC 2002). The sediments of this region are largely alluvial in origin, including Ferrigenous arid sands (Chinoda 2009). The coastal zone is lined with interior dunes, including some consolidated dunes, and coastal dunes (Chinoda et al. 2009).

This region has very little in the way of significant geological structure, with the exception of two large sets of differing ages (Chinoda et al. 2009).

Significant Geological Features of the Limpopo River Basin

There are a number of geological features worth noting, many of them are attractions for regional and international tourists.

Tswaing Crater

The Tswaing Crater is approximately 1 kilometre in diameter, with rim approximately 60 metres high. Tswaing, which means Place of Salt in Tswana, is located approximately 40 km north of South Africa's capital city, Pretoria. It is thought that the crater was created by a meteor 60 metres across, travelling at 4 000 km/hr some 220 000 years ago. The crater is a significant tourist attraction, receiving thousands of visitors every year. (South African Tourism 2010).

Pilanesberg

Pilanesberg is located mid-way between Pretoria and Gaborone, in South Africa, approximately 40 km north of Rustenburg. Pilanesberg is the remains of an extinct volcano, comprising three concentric rings of hills that rise from a relatively flat landscape. The hills of this remarkable geological feature form a natural enclosure for animals of a game reserve that attracts many thousands of visitors every year. Pilanesberg exhibits evidence of human civilisations dating back as far as the Stone and Iron age. (Pilanesberg 2010).

Erosion Cycles in the Limpopo River Basin

Erosion cycles during the early Tertiary period formed the African denudational surface at high or medium plateau level, such as the highveld in South Africa. Its major occurrence is southwest of the southern divide of the Limpopo River Basin, the high-level plateau zone in Zimbabwe, the elevated areas near Polokwane (Pietersburg) in South Africa, and the flat-topped hills in eastern Botswana and Limpopo Province in South Africa.

Further erosion in the late Tertiary period formed the Post-African denudational surface. Various phases of this surface are dominant in the Limpopo River Basin. The most recent erosion was active during the Quaternary period, primarily downstream of the main rivers and tributaries in the basin area.

Most of the land within the Limpopo River Basin in Mozambique was formed by aggradational surfaces during the Quaternary and Tertiary periods, except for a band of Cretaceous rocks occurring north of the Save River to the border with Zimbabwe. Extensive, well-developed alluvial formations occur in the middle and lower reaches of the Limpopo River, and in the watercourses of the non-perennial rivers entering the Limpopo River. The oldest formations (Palaeocene and Eocene rocks) are of marine facies and correspond to the calcareous sandstones and conglomerates, which are disconformably overlain and border the effusive Karoo formations on the western border with South Africa.

The main unit of the Quaternary cover is a thick, homogenous mantle of yellowish-brown, saline, sodic, calcareous, sandy clay loam extending over the vast interior of Gaza Province west of the Limpopo River. It builds large, slightly sloping plateaus called Mananga developed over sedimentary, coarse and siliceous rocks. Near the incised valleys, the basal gravels have been exposed after erosion of the Mananga cover. Different cycles of weathering and landscape lowering reworked the resistant gravels into basal gravel floors. The highest gravels correlate to the red sandstones and conglomerates of the late Tertiary. The higher, lower and young gravels are associated with the respective Mananga platforms.

Source: FAO 2004