SELATI RIVER: Topography, geology and soils

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Map of the SELATI River

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TOPOGRAPHY
"The upper reaches of the Selati sub-catchment consist of steep-sided valleys with sharply defined cliff faces on the eastern side of the escarpment.  The local relief changes dramatically as the escarpment drops sharply on its eastern flank to the flat or undulating terrain of the dry Lowveld region.  Here, the topography is characterized by gently sloping valleys and isolated outcrops of granitic rocks."  (Ref 2)
    GEOLOGY                                    
"In its upper reaches, the sub-catchment is underlain by rocks of the Transvaal Sequence, which form the major portion of the Drakensberg Mountain range.  The lithology is dominated by quartzites, chert, hornfels, basic lava and dolomite.  The dolomite outcrops, in particular, are important sources of good quality water in the upper catchment and contribute water throughout the year."  (Ref 2)
"Further to the east and south, rocks of the Gravelotte Group (part of the Murchison Sequence) and Rooiwater Complex outcrop in the vicinity of the town of Gravelotte .  Quartzite, schists, basic lava and granitic rocks dominate the Gravelotte Group lithology.  These greenstone formations contain important deposits of antimony and gold, with minor deposits of mercury and zinc.  An extensive deposit of heavy mineral sands (illmenite, rutile and zirconium) is located near the town of Gravelotte ."(Ref 2)
"The felsites and gabbros of the Rooiwater Complex are easily eroded and contribute increased levels of sediments in the valley depressions and river channels.  Further to the east, the rocks of the crystalline Basement Complex outcrop at various points across the Lowveld.  These rocks consist mainly of potassium-rich granites and gneisses with feldspar and mica outcrops.  The exposed weathered granites are easily eroded, particular where the vegetation cover is sparse or denuded.  Accelerated rates of sediment production occur in this region, with large amounts of coarse and fine sediments accumulating in river channels.  The slightly harder rocks of the Mashimale Suite are intrusive into the rocks of the Basement Complex and are visible as stack-like outcrops of fine-grained, light-coloured granites."  (Ref 2)

"The Phalaborwa area is underlain by granite and gneiss rocks of the Basement Complex.  These were intruded by a cyclical series of alkaline-rich magmas that have given rise to the highly mineralised carbonatite and pyroxenite rocks of the Phalaborwa Complex.  Plugs and dykes of fine-grained syenite that were intruded during the alkaline phase of the Phalaborwa Complex are resistant to weathering and remain as conspicuous stack-like hills and ridges.

North-east to south-west trending dykes and sills of undifferentiated mafic intrusives (mainly Karoo age dolerite) occur over much of the Lowveld Basement Complex, with particularly well-developed examples of dyke swarms in and around the Phalaborwa area.  These rocks are less resistant to weathering than their granite and gneiss host rocks and give rise to negatively weathered linear features in the landscape, often forming or delineating drainage lines." (Ref 2)

    SOILS                                    
"Soils in the Selati sub-catchment can be divided into four broad groups or regions:
  • Small areas of shallow, sandy or gravely soils the upper reaches;
  • Areas of sandy colluvial soils at the foot of the escarpment, grading into red apedal soils in the western part of the sub-catchment
  • Shallow, brownish to greyish-brown sandy soils overlying coarsely weathered rock in the eastern portion of the sub-catchment; and
  • Transported alluvial deposits of coarse to fine-grained sands and silts located along drainage lines."    (Ref 2)
  References                                      

Ref 2: www.mining.wits.ac.za/FinalReport-Section5.doc
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