Groundwater in Namibia - an Explanation to the ... - BGR
Groundwater in Namibia - an Explanation to the ... - BGR
Groundwater in Namibia - an Explanation to the ... - BGR
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Unit 8, “Non-porous s<strong>an</strong>ds<strong>to</strong>ne, conglomerate, quartzite”<br />
forms a unit which was ma<strong>in</strong>ly deposited dur<strong>in</strong>g Damar<strong>an</strong><br />
times. It comprises <strong>the</strong> basal Nosib Group of <strong>the</strong> Damara<br />
Sequence which was laid down <strong>in</strong>, or marg<strong>in</strong>al <strong>to</strong>, <strong>in</strong>tra -<br />
cont<strong>in</strong>ental rifts, <strong>an</strong>d consists of quartzite, arkose, conglo -<br />
merate, phyllite, calc-silicate <strong>an</strong>d subord<strong>in</strong>ate limes<strong>to</strong>ne,<br />
as well as <strong>the</strong> deep water sediments of <strong>the</strong> Auas Formation<br />
deposited on <strong>the</strong> edge of a narrow develop<strong>in</strong>g oce<strong>an</strong>. This<br />
unit fur<strong>the</strong>r conta<strong>in</strong>s <strong>the</strong> flat-ly<strong>in</strong>g basal <strong>an</strong>d upper s<strong>an</strong>ds<strong>to</strong>nes<br />
of <strong>the</strong> Nama Group of sou<strong>the</strong>rn <strong>Namibia</strong>.<br />
A sequence of <strong>the</strong> 180 million year old basalts of <strong>the</strong><br />
Kalkr<strong>an</strong>d Formation, that are about 360 metres thick, occur<br />
<strong>in</strong> <strong>the</strong> Mariental area. Volc<strong>an</strong>ic rocks of <strong>the</strong> Etendeka<br />
Formation <strong>in</strong> north-western <strong>Namibia</strong> have ages of about<br />
135 million years. Both formations are flat-ly<strong>in</strong>g. Extensive<br />
dolerite sills <strong>an</strong>d dyke swarms are related <strong>to</strong> <strong>the</strong> volc<strong>an</strong>ic<br />
rocks which have formed <strong>in</strong> connection with <strong>the</strong> cont<strong>in</strong>ental<br />
break up of Gondw<strong>an</strong>al<strong>an</strong>d <strong>an</strong>d form unit 9 “Volc<strong>an</strong>ic rocks<br />
(Karoo <strong>an</strong>d younger)”.<br />
M<strong>an</strong>y different, highly folded rock types of Mokoli<strong>an</strong><br />
<strong>an</strong>d <strong>Namibia</strong>n ages are <strong>in</strong>cluded <strong>in</strong> unit 10 “Metamorphic<br />
rocks, <strong>in</strong>clud<strong>in</strong>g quartzite <strong>an</strong>d marble b<strong>an</strong>ds”. These<br />
extend from <strong>the</strong> Gobabis area <strong>to</strong> <strong>the</strong> Gamsberg region <strong>an</strong>d<br />
<strong>the</strong>n southwards <strong>to</strong> Helmer<strong>in</strong>ghausen.<br />
The 1800 million year old Reho both Sequence is thought<br />
<strong>to</strong> have formed <strong>in</strong> <strong>the</strong> back-arc bas<strong>in</strong> of a magmatic arc<br />
<strong>an</strong>d comprises schist, phyllite, amphibolite <strong>an</strong>d quartzite.<br />
Rocks of <strong>the</strong> S<strong>in</strong>clair Sequence accumulated with<strong>in</strong> <strong>an</strong> <strong>in</strong>tra -<br />
con ti nental rift. Deposi tion<br />
of quartzites <strong>to</strong>ok place <strong>in</strong><br />
narrow fault-bounded<br />
troughs <strong>in</strong> <strong>to</strong>day’s Helme -<br />
r<strong>in</strong>ghausen-Solitaire area<br />
after a cycle of magmatic<br />
activity. Damar<strong>an</strong> rocks present<br />
<strong>in</strong> this unit <strong>in</strong>clude<br />
schists of <strong>the</strong> basal Nosib<br />
Group, marbles of <strong>the</strong> Ugab<br />
<strong>an</strong>d Kudis Subgroups, schist,<br />
phyllite <strong>an</strong>d amphibolite of<br />
<strong>the</strong> Chuos Formation <strong>an</strong>d<br />
marble, schists <strong>an</strong>d amphi-<br />
Etendeka Formation at Grootberg<br />
bolites of <strong>the</strong> Karibib <strong>an</strong>d Kuiseb Formations, <strong>in</strong>clud<strong>in</strong>g <strong>the</strong><br />
Matchless Amphibolite Belt.<br />
Damar<strong>an</strong> meta-sediments have been <strong>in</strong>truded by gr<strong>an</strong>ites<br />
<strong>in</strong> a broad zone between Otjiwarongo <strong>an</strong>d Okah<strong>an</strong>dja <strong>an</strong>d<br />
<strong>the</strong> coast. The <strong>in</strong>trusion accomp<strong>an</strong>ied <strong>the</strong> mounta<strong>in</strong> build<strong>in</strong>g<br />
process between 650 <strong>an</strong>d 450 million years ago. Where<br />
<strong>in</strong>dividual gr<strong>an</strong>ite bodies are <strong>to</strong>o small <strong>to</strong> be shown on<br />
<strong>the</strong> Map, <strong>the</strong>y were <strong>in</strong>cluded <strong>in</strong> unit 11, “Metamorphic<br />
rocks, <strong>in</strong>clud<strong>in</strong>g quartzite <strong>an</strong>d marble b<strong>an</strong>ds; with gr<strong>an</strong>itic<br />
<strong>in</strong>trusions”.<br />
Unit 12, “Gr<strong>an</strong>ite, gneiss, old volc<strong>an</strong>ic rocks” f<strong>in</strong>ally<br />
<strong>in</strong>cludes a multitude of lithologies almost cover<strong>in</strong>g <strong>the</strong> entire<br />
geological his<strong>to</strong>ry of <strong>Namibia</strong>. Vaali<strong>an</strong> rocks comprise <strong>the</strong><br />
gneisses of <strong>the</strong> Epupa Complex <strong>an</strong>d <strong>the</strong> <strong>in</strong>trusives of <strong>the</strong><br />
Kunene Complex. Early Mokoli<strong>an</strong> rocks <strong>in</strong>clude gneisses<br />
<strong>an</strong>d meta-volc<strong>an</strong>ics of <strong>the</strong> old metamorphic complexes, metavolc<strong>an</strong>ics<br />
of <strong>the</strong> Or<strong>an</strong>ge River Group, gr<strong>an</strong>ites of <strong>the</strong> Vioolsdrif<br />
Suite, gneisses of <strong>the</strong> Elim Formation <strong>an</strong>d meta-volc<strong>an</strong>ics<br />
<strong>an</strong>d gneisses of <strong>the</strong> Khoabendus Group. Middle <strong>to</strong> late Mokoli<strong>an</strong><br />
rocks of this unit are <strong>the</strong> meta-volc<strong>an</strong>ics of <strong>the</strong> Rehoboth<br />
Sequence, <strong>the</strong> gneisses of <strong>the</strong> Namaqual<strong>an</strong>d Metamorphic<br />
Complex, <strong>the</strong> meta-volc<strong>an</strong>ics of <strong>the</strong> S<strong>in</strong>clair Sequence, <strong>the</strong><br />
gr<strong>an</strong>ites of <strong>the</strong> Fr<strong>an</strong>sfonte<strong>in</strong> Suite, as well as <strong>the</strong> younger gr<strong>an</strong>ites,<br />
for example <strong>the</strong> Gamsberg Gr<strong>an</strong>ite. The unit also com -<br />
prises a r<strong>an</strong>ge of syn- <strong>to</strong> post- tec<strong>to</strong>nic gr<strong>an</strong>ites which <strong>in</strong>truded<br />
Damar<strong>an</strong> sediments dur<strong>in</strong>g <strong>the</strong> course of <strong>the</strong> Damar<strong>an</strong><br />
Orogeny. Complex <strong>in</strong>trusions with ages of about 135 million<br />
years occur <strong>in</strong> a zone extend<strong>in</strong>g from <strong>the</strong> coast north<br />
of Swakopmund <strong>in</strong> a nor<strong>the</strong>asterly<br />
direction. Some of<br />
<strong>the</strong>m are extremely complex<br />
layered <strong>in</strong>trusions <strong>an</strong>d conta<strong>in</strong><br />
rhyolite, gr<strong>an</strong>o phyre,<br />
gr<strong>an</strong>ite, syenite, foyaite, gabbro,<br />
dunite, pyroxenite <strong>an</strong>d<br />
carbonatite. They are not<br />
related <strong>to</strong> <strong>an</strong>y orogeny <strong>an</strong>d<br />
<strong>in</strong>terpretated as a result of<br />
a hot m<strong>an</strong>tle plume, <strong>an</strong>d<br />
have also been <strong>in</strong>cluded <strong>in</strong><br />
this unit.<br />
Kev<strong>in</strong> Roberts<br />
19