Introduction
(The Cape Granite Suite: A short introduction by Arnaud Villaros, 2006)
The deposition, deformation and metamorphism of the Malmesbury group occurred between 1.2 Ga and 510 Ma. From the oldest until the most recent the Malmesbury Group is divided in 3 groups: The Swartland, Malmesbury, and Klipheuwel:
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The Swarland Group is the oldest, and is composed by three distinct formations (Berg River, Bridgetown and Mooresburg). All of them present a greenschist facies metamorphism (chlorite schist and biotite-feldspar-quartz schist.). The Bridgetown formation is a particularity in the Malmesbury Supergroup. Lithology of this formation corresponds with mainly dark green metavolcanic rock, associated with dolomites and cherts. This formation is commonly interpreted as tectonic sliver of oceanic crust within the sequence. Globally the deformation of the Swartland Group is characterised by intrafolial, isoclinal folding, thrusting and imbrication and upright open to tight folding.
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The Malmesbury Group is a clearly detrical sequence, composed of three formations (Piketberg, Tygerberg and Porterville). It includes conglomerates derived from the Berg River formation (in the Piketberg formation), arenites, greywackes, sandstones and shales. This group has not been metamorphosed. Structurally, the Malmesbury Group is characterized by upright open to tight folding and axial planar cleavage.
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Finally the Klipheuwel Group ends the Malmesbury Supergroup. As the Malmesbury Group, it’s a mostly detrical sequence. Sediments derive from both the lower Malmesbury Supergroup and the CGS; the Klipheuwel group mostly occurs in fault-bounded basins on the top of either the rest of the Malmesbury Supergroup, or granites from the CGS. Three formations are described: the Franshoek formation is made of conglomerates, grits and shales. The Magrug formation consists of coarse sandstone, while the Populierbos formation is characterized by finer sediments (mudstone, shale). Metamorphism and deformation structures haven’t been described in the Klipheuwel.
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About a dozen of Panafrican plutons intrude the Malmesbury Supergroup. On the base of petrological and geochemical characteristics, three types of granites are identified (S, I and A), each with several subtypes. Each type is also found only in specific parts of the Saldanhan belt. Same characteristics of these granites are:
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Cape granite plutons occur in clusters (also called TMG Basement).
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Consist of granites, gabbro, diorites and felsic igneous melts.
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Geohydrology
Malmesburg Supergroup
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Aquifers generally yield less than 2 l/s
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Aquifers are highly anisotropic and aquifer parameters vary significantly
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Cape Granites
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Foundation structures are classified as secondary aquifers.
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Mostly granites and gneiss.
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Older granites have better water potential, due to higher erosion of structure.
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If erosion is deeper than water table, likelihood of water very high.
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Cape granites at bottom of TMG
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