Groundwater in Namibia - an Explanation to the ... - BGR

Unit 8, “Non-porous sandstone, conglomerate, quartzite”
forms a unit which was mainly deposited during Damaran
times. It comprises the basal Nosib Group of the Damara
Sequence which was laid down in, or marginal to, intra -
continental rifts, and consists of quartzite, arkose, conglo -
merate, phyllite, calc-silicate and subordinate limestone,
as well as the deep water sediments of the Auas Formation
deposited on the edge of a narrow developing ocean. This
unit further contains the flat-lying basal and upper sandstones
of the Nama Group of southern Namibia.
A sequence of the 180 million year old basalts of the
Kalkrand Formation, that are about 360 metres thick, occur
in the Mariental area. Volcanic rocks of the Etendeka
Formation in north-western Namibia have ages of about
135 million years. Both formations are flat-lying. Extensive
dolerite sills and dyke swarms are related to the volcanic
rocks which have formed in connection with the continental
break up of Gondwanaland and form unit 9 “Volcanic rocks
(Karoo and younger)”.
Many different, highly folded rock types of Mokolian
and Namibian ages are included in unit 10 “Metamorphic
rocks, including quartzite and marble bands”. These
extend from the Gobabis area to the Gamsberg region and
then southwards to Helmeringhausen.
The 1800 million year old Reho both Sequence is thought
to have formed in the back-arc basin of a magmatic arc
and comprises schist, phyllite, amphibolite and quartzite.
Rocks of the Sinclair Sequence accumulated within an intra -
con ti nental rift. Deposi tion
of quartzites took place in
narrow fault-bounded
troughs in today’s Helme -
ringhausen-Solitaire area
after a cycle of magmatic
activity. Damaran rocks present
in this unit include
schists of the basal Nosib
Group, marbles of the Ugab
and Kudis Subgroups, schist,
phyllite and amphibolite of
the Chuos Formation and
marble, schists and amphi-
Etendeka Formation at Grootberg
bolites of the Karibib and Kuiseb Formations, including the
Matchless Amphibolite Belt.
Damaran meta-sediments have been intruded by granites
in a broad zone between Otjiwarongo and Okahandja and
the coast. The intrusion accompanied the mountain building
process between 650 and 450 million years ago. Where
individual granite bodies are too small to be shown on
the Map, they were included in unit 11, “Metamorphic
rocks, including quartzite and marble bands; with granitic
intrusions”.
Unit 12, “Granite, gneiss, old volcanic rocks” finally
includes a multitude of lithologies almost covering the entire
geological history of Namibia. Vaalian rocks comprise the
gneisses of the Epupa Complex and the intrusives of the
Kunene Complex. Early Mokolian rocks include gneisses
and meta-volcanics of the old metamorphic complexes, metavolcanics
of the Orange River Group, granites of the Vioolsdrif
Suite, gneisses of the Elim Formation and meta-volcanics
and gneisses of the Khoabendus Group. Middle to late Mokolian
rocks of this unit are the meta-volcanics of the Rehoboth
Sequence, the gneisses of the Namaqualand Metamorphic
Complex, the meta-volcanics of the Sinclair Sequence, the
granites of the Fransfontein Suite, as well as the younger granites,
for example the Gamsberg Granite. The unit also com -
prises a range of syn- to post- tectonic granites which intruded
Damaran sediments during the course of the Damaran
Orogeny. Complex intrusions with ages of about 135 million
years occur in a zone extending from the coast north
of Swakopmund in a northeasterly
direction. Some of
them are extremely complex
layered intrusions and contain
rhyolite, grano phyre,
granite, syenite, foyaite, gabbro,
dunite, pyroxenite and
carbonatite. They are not
related to any orogeny and
interpretated as a result of
a hot mantle plume, and
have also been included in
this unit.
Kevin Roberts
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