OVERVIEW OF THE IMPACT OF MINING ON THE ... - IIED pubs

5.1.1 Geology, topography and soils
The Olifants basin is located over the eastern lobe of the Kalahari Craton, forming the largest sub-basin of the
Limpopo basin. The Archaean Craton rocks comprise predominantly crystalline granitic and gneissic rocks,
intruded by various Greenstone belts as well as dolerite dykes and sills, and silicified sedimentary formations.
Karoo System rocks overlie large areas of the southwestern portion of the basin and these are also associated
with younger sedimentary and crystalline rocks consisting predominantly of sandstones, carbon-rich mudstones,
conglomerates and shales. Recent sedimentary deposits line most of the river valleys and provide important
farming areas (Du Toit, 1960).
In the western (South African) portion of the Olifants basin, the Bushveld Igneous Complex forms an extremely
important feature and contains a very large proportion of the region’s mineral wealth. The geological features of
this area consist mostly of basic mafic and ultramafic intrusive rocks, accompanied by extensive areas of acidic
and intermediate intrusive rocks. At the southern and eastern periphery of this area, large dolomite and
limestone formations occur, accompanied by extensive mineralization along their contact zones. Several areas
of the northern portion of the Olifants basin consist of various deposits of consolidated and unconsolidated
sedimentary rocks, with important belts of intrusive Greenstone rocks that are heavily mineralised.
The north-south trending rhyolites and lavas of the Lebombo Mountains mark the eastern border between South
Africa and Mozambique, and separate the South African and Mozambican portions of the basin. The eastern
(Mozambique) portion of the Olifants basin consists largely of unconsolidated and consolidated sedimentary
rocks with granitic intrusions exposed as erosional remnants in the landscape
In the southern portion of the basin, the extensive, carbon-rich sedimentary rocks of the Karoo System contain
enormous economic reserves of coal and are the site of intensive coal mining activities. Elsewhere, and
particularly prominent in the northern and eastern parts of the basin, harder, silicified sandstones and cherts, as
well as syenitic and granitic outcrops, form stack-like erosional remnants that protrude above the generally
undulating terrain (Du Toit, 1960).
The topography of the Olifants basin is extremely varied, ranging from approximately 150 metres above sea
level where it joins the Limpopo River in Mozambique, to over 2,000 metres in the mountainous region marking
the transverse position of the northern extension of the Drakensberg Mountains. Most of the basin consists of
relatively undulating terrain separated by ranges of steep-sided hills and mountains. The northeastward flowing
Olifants River and its major tributaries have incised deep gorges through the hills and mountain ranges that form
spectacular landscape units. Generally, the river valleys tend to be broad and flat-bottomed, with river channels
that are slightly or moderately incised into the surrounding parent material.
Several small sections of the northeastern parts of the Olifants basin (especially in the Shingwedzi and Letaba
sub-catchments) have very little or poor drainage, and are usually considered to be endorheic (internally
draining). These areas are often marked by the formation of clay-bottomed pans where rainfall collects and
evaporates to leave small deposits of salts. The drier northern and eastern portions of the Olifants basin are
generally subjected to mechanical (physical) weathering processes, in contrast to the predominance of chemical
weathering processes in the wetter headwater regions of most tributaries.
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