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(b) Secondary aquifers
(i) Intrusive rocks
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A summary of the more important geological formations and the rock types thereof, which
as secondary (hard rock) aquifers, comprise by far the greater portion of NatallKwaZulu
is given in Table Kl. Intrusions of Karoo dolerite occur extensively in the province,
especially in the interior. These commonly take the form of sub-horizontal or slightly
inclined sheets or sills, which may vary in thickness from about a metre to more than
100 m. Due to their resistance to erosion, the thicker dolerite sheets in the interior of
Natal/KwaZulu result in high-rising prominent flat-topped topographic features, which
characterize much of the interior of the province. With ascent in the sediments of the
Karoo Supergroup, the incidence and magnitude of the dolerite sheet intrusions increases
markedly. In rocks below the Karoo Supergroup, except in some places in the sandstones
of the Natal Group, intrusions of Karoo dolerite are relatively rare. Vertical or inclined dyke
intrusions of Karoo dolerite, which seldom exceed about 6 m in width, are also generally
rare in the province, by comparison with the incidence of sheet intrusions. Dykes tend to
be somewhat more common in the upper portions of the Karoo Supergroup. Adjacent to
the dolerite sheets, especially the thinner sheets, and also in the case of the dykes, the
country rocks are often disturbed for a distance of a few metres. The Karoo dolerite
sheets (and where they occur), the dykes of the same material, are of major significance
for groundwater in Natal/KwaZulu.
• Geological structure
The coast and coastal hinterland areas generally for a distance of up to some 80 km from
the sea are strongly faulted, which is indicative of the rifted margin of the sub-continent
at that point. The distribution of the faults reflects a conjugate shear fracture pattern
resulting from tensional conditions prevailing during the disruption of Gondwana, which
initiated the coastline in the late Jurassic (160 million years ago). The faults are all of the
tensional or gravity type, the fault surfaces dipping towards the downthrow side at steep
angles of about 80 0 • The structure of this part of Natal/KwaZulu is mainly one of tilted