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

3.1.1 Geology, topography and soils
The Zambezi basin is located between the three Archaean Cratons: the Central African Craton to the northwest,
the Tanzanian Craton to the northeast and the Zimbabwe lobe of the Kalahari Craton to the south. The
Archaean Craton rocks comprise mainly granitic terrain, intruded by various Greenstone belts and dolerite
dykes and sills. Karoo System rocks overlie large areas of the central portion of the basin and these are
intruded by younger (Cretaceous) crystalline rocks, consisting predominantly of shales, sandstones and
conglomerates. Recent sedimentary deposits line most of the river valleys (SADC-MSCU, 1998; Chenje, 2000).
In the east, the southern extension of the east African Rift Valley, containing Lake Malawi, forms the eastern rim
of the basin. A second rifting structure, the Luangwa and Lunsemfwa Rift Valleys in Zambia lie almost parallel
to the Lake Malawi basin. The zones between the three Archaean Cratons are marked by several mobile belts
where considerable seismic activity has taken place and mineralization has occurred along fault and shear
planes (SADC-MSCU, 1998).
The crystalline rocks underlying the central region of the Zambezi basin have been formed through
metamorphosis of sedimentary deposits. These rocks have complex structures and contain most of the
mineralization recorded in the basin.
The topography of the Zambezi basin is very varied. The elevation of the catchment rim ranges from some
1,000 metres in the west (in Angola), to over 3,000 metres in the east at points along the eastern rim of the Lake
Malawi Rift Valley (e.g. the Zomba and Mulanje Mountains in Malawi), to sea level at the Zambezi mouth at
Chinde in Mozambique.
The terrain separating the Lake Malawi and Luangwa River rifts is also marked by several points of high
elevation, notably the Nyika and Viphya plateaux marking the border between Zambia and Malawi, as well as
the Livingstone Mountains along the northern border with Tanzania (Chenje, 2000). On its western flank, the
Luangwa Rift Valley is marked by the high Muchinga escarpment. The central portion of the Zambezi basin is
marked by undulating terrain with low hills and escarpments formed by outcrops of quartzites, granites, schists
and gneisses. In the north, the region of the Zambian Copperbelt, quartzitic sandstones outcrop at surface.
Many of these rocks are stained green by oxides and carbonates of copper (Mendelsohn, 1961).
In its upper reaches, the Zambezi River flows along a relatively broad, moderate-gradient valley, until reaching
the Victoria Falls. At this point, the river has incised through some 60 metres of Karoo Supergroup basalts and
flows in a deep, narrow gorge until reaching Lake Kariba. The river gradient in these middle reaches is
relatively low, and the river flows within a deep valley flanked on the Zambian and Zimbabwean banks by steep
terrain. At Cahora Bassa, the river gradient steepens slightly, flowing through a second set of narrow gorges
before emerging into the lower reaches within a broad shallow valley. Downstream of the town of Tete in
Mozambique, the Zambezi River meanders along a broad, flat-bottomed valley and is joined by the Shire River
from the north before reaching the delta at the coast. Features of the Zambezi delta (increased salinity, tidal
movements) are recognizable some 120 kilometres inland from the coast, and the riverbanks are marked by
noticeable flood levees along most of its length (Chenje, 2000).
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