DIEP RIVER: Topography, geology and soils

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Map of the DIEP River

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TOPOGRAPHY
The Diep River rises in the Riebeek Kasteel mountains 65 km north-east of Cape Town.  The Diep River catchment covers the Malmesbury district and the area west of Paarl, including Durbanville. (Ref 2)
The Department of Water Affairs gives the catchment area above gauging station at Visserhoek as 1 360 km2.  (Ref 2)

The length of the river is approximately 60km (Beaumont and Heydenrych 1980).

The Diep River drains south-west from the Riebeek-Kasteel Mountains in the north-east of the sub-area, through the wheat producing areas of the Swartland, to enter the sea at Milnerton, via the Rietvlei wetland. The Mosselbank River drains the southeastern portion of the region, and is the main tributary of the Diep River. (Ref 1)
The profile of the Diep River (from the origin near Malmesbury to the mouth at Milnerton)  is illustrated below:
    GEOLOGY                                    
The predominant geological formation in this area is the Malmesbury Formation but Cape Granite and what are thought to be Klipheuwel Beds are also present, though to a lesser extent.  The Malmesbury rocks are dark, medium grained, sub-greywackes with interbedded blue and  sometimes purplish shales.  Graded bedding is not uncommon in the greywackes and may be suggestive of a turbidate origin.  There is evidence of multiple folding and late shearing at Blaauwbergstrand a few kilometres north of the lagoon.  In the Gran Sasso Quarry, jointing and folding apear in the working face.  The Malmesbury sediments have been dated at 595 million years (Alsopp and Kolbe, 1965).  (Ref 2)
The Cape Granite is characteristically light grey, porphyritic and intrudes into the sediments of the Malmesbury  Formation and has been dated at 553 million years.  Large microcline microperthite phenocrysts are plentiful and form a cospicous feature of the granite. 

The Klipheuwel Beds outcrop at the village of Klipheuwel and the contact with the Cape Granite is visible in a large disused quarry.  At this intensive contact the feldspar of the granite is almost completely altered to kaolinite.  However, due to the small areas of outcrop of the Cape Granite and Klipheuwel Beds their contribution to the lagoon sediments is small in comparison to that of the Malmesbury Formation.  (Ref 2)

The Malmesbury Formation rocks are poorly exposed, consisting of low rolling hills, and have been subject to low grade regional metamorphism never proceeding beyond the biotite stage.  The sediments of the Malmesbury Formation consist of a variety of shales, greywackes, quartzites and grits, with occasional bands of conglomerate, limestone, dolomite, chert, basic lavas and tuffs.  In the catchment of the Diep River, arenaceous rocks are being quarried for building material. (Ref 2)
    SOILS                                    
The dominant source of clay minerals in Milnerton Lagoon sediments is most likely to be the Malmesbury Formation rocks which crop out or are present under the soil of most of the catchment area of the Diep River.  Some clay minerals could be derived from the small areas of Cape Granite and Klipheuwel Formation.  (Ref 2)
  References                                      

Ref 1:  Department of Water Affairs and Forestry, South Africa. 2004. Berg Water Management Area: Internal Strategic Perspective. Prepared by Ninham Shand (Pty) Ltd in association with Jakoet and Associates, Umvoto Africa and Tlou and Matji, on behalf of the Directorate: National Water Resource Planning. DWAF Report No P WMA19/000/00/0304.

Ref 2:  Estuaries of the Cape, Report No. 28, Rietvlei(CW 24)and Diep(CW25)CSIR RESEARCH REPORT 427

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