|4.3 THE NAMIBIAN AND CAMBRIAN AGES
The rocks and their weathered derivatives of the Namibian age are from strata aged some 900 to 450 Ma. The larger part of Namibia is underlain by rocks of these precambrian ages which are grouped into a number of litho-stratigraphic units . The most important systems of this period are as far as road building materials are concerned, the Damara Sequence and the Nama Group which are belonging to the same period than the less important Gariep Complex, all with syn- to post-tectonic granitic intrusions. All these units can be found in different areas of the country. The Damara Sequence is restricted to the northern and central areas between Windhoek and Tsumeb while the Nama Group and Gariep Complex occur in the southern respective south-western parts of Namibia. All these rocks and derived materials are exposed to an arid climatic environment which favours mechanical disintegration as major form of weathering.
The basal arenitic succession with local evaporitic rocks and alkine ignimbrites of the Nosib Group is the oldest group in the Damara Sequence. Widespread carbonate deposition follows ( Swakop Group, the second oldest group in the Damara Sequence) and interbedded mica and graphitic schist, quartzite as well as massflow deposits point to a variable, unstable condition south of a stable platform where mainly carbonates occur ( Otavi Group, the northern equivalent to the Swakop Group). Near the southern margin of the orogen deep water-fans, facies equivalents of the northern carbonates were deposited on either side of an ocean forming the Auas Formation and the Tinkas Member. Thick schistose metagreywacke and metapelite ( Kuiseb Formation) overlie the above rocks and contain a narrow 350 km-long zone of interbedded oceanic greenstones ( Matchless Member).
The succession, predominantly consisting of quartzites, schists, limestones, dolomites and marbles, is quite variable as far as its lithology is concerned. These varieties resulted in the subdivision of the Damara rocks into three major zones :
4.3.1 DAMARA SEQUENCE: NORTHERN ZONE
The Namibian Damara Sequence: Northern zone can be divided in three groups with further subgroups nd formations:
At the base of the Damara Sequence the Nosib Group is prevailing  and contains mainly arenaceous rocks and conglomerates. The overlying Otavi Group is composed of dolomite and limestone with intercalations of quartzite and shale. At the top the Mulden Group consists mainly of quartzite and conglomerate but also contains substantial intercalations of shale, marl and phyllite.
The Nosib Complex can be encountered in the Chuos Mountains south-east of Usakos and in the eastern parts of the Kaokoveld. This formation consists of quartzite, sandstone, limestone and arenaceous rocks. The dolomites of the Otavi Group are showing typical karst features such as the forming of caverns in the rock and the occurrence of dolines and sinkholes below the surface. The Otavi Group dolomite is also quarried at Tsumeb, Abenab, on the farm Gabus 52 north of Otavi, at Outjo and between Kamanjab and Opuwa (Codes 36, 37, 38, 44, 45, 46, 47, 48, 53, 54, 57, 58, 59, 60, 62, 63, 64 and 68 in table 10). The physical properties of commercially quarried dolomite of the Otavi Group are shown in table 13 and those of weathered Mulden phyllite in table 14 :
TABLE 13 PHYSICAL PROPERTIES OF QUARRIED DOLOMITE: OTAVI GROUP
| Gabus (Otavi) | 2,85-2,95 | 22 - 23 | 20 - 25 |
| Abenab (Grootfontein) | 2,85-2,95 | 15 - 18 | 25 - 30 |
| Tsumeb (Mine) | 2,85-2,95 | 15 | 15 - 30 |
| Outjo | 2,85-2,95 | 17 - 23 | 20 - 25 |
Flakiness Index: TMH 1 : Method B3: Flakiness Index of a coarse aggregate is determined by gauging screened-out
fractions with appropriate slots which are specified.
TABLE 14 ROAD-BUILDING PROPERTIES OF WEATHERED MULDEN PHYLLITE
4.3.2 DAMARA SEQUENCE: CENTRAL ZONE
The central and southern zones of the Damara Sequence have the same groups, subgroups and formations:
The largest area of the central zone of the Damara Sequence is underlain by many varieties of schist, by marble and some quartzite. Outcrops of rocks of the Nosib Group can be found in the south western parts of the area and those of the Mulden Group only in the northern parts. Volumetrically the central zone is composed of marble, schist and granite in approximately equal proportions.
The engineering properties of the schist vary considerably with the metamorphic grade . Different schists can be established from low-grade to medium-grade biotite schist mainly in the north-east to medium-grade cordierite schist grading into gneiss at places in the south-west of the area. The properties regarding strength, permeability and weathering characteristics are a function of their foliation. The higher-grade schists have a more homogeneous to massive appearance whose engineering properties can approach those of gneiss, even of quartzite. The deposits of calcareous and dolomitic marbles of Karibib and the Rössing formations in the region of Karibib, Usakos and Swakopmund are used extensively for concrete aggregates and road building materials (Codes 28, 30, 31 in table 10). The three main groups of the Cambrian granites which are intruding all formations of the Damara Sequence are Salem Granite, the Sorris-Sorris Granite and the Donkerhuk Granite. The granites are susceptible to weathering. The road building properties of Salem granite are listed in table 15 :
TABLE 15 ROAD BUILDING PROPERTIES OF WEATHERED SALEM GRANITE
Quartzites are concentrated in the Nosib Group at the base of the sequence but are also found in layers higher up. In comparison with the quartzites of the Nosib Group the quartzites of the Swakop Group often grade into quartz schist and quartz mica schist which accounts for the lower strength of these rocks.
4.3.3 DAMARA SEQUENCE: SOUTHERN ZONE
The southern zone of the Damara Sequence consists mainly of schist and quartzite which makes up 60% and 30% respectively. Other rocks in this succession are granite, marble, amphibolite and other rock types.
The quartzite is largely restricted to the Nosib Group at the base of the sequence although layers of quartzite are also found in the Auas Formation . Thinner quartzite layers which often grade into quartz mica schist can also be encountered in the other formations of the Swakop Group. East of Witvley virtually all Damara outcrops are belonging to the Kamtsas Formation which consists of mainly quartzite and arkose. The schist in this group is generally biotite bearing and has undergone low-grade to medium-grade metamorphism and is strongly foliated with the resulting variable physical properties such as permeability and strength in the different directions of the rock. The quartz content varies considerably vertically and laterally. The quartzitic component is more resistant to erosion than the mica schist component with the resulting typical ridges in this material. In spite of the high quartzitic content in it the rock seldom reaches the strength of a true quartzite. The quartzites of the Auas Formation are generally quite pure and are in spite of their high mica content extensively used as building material in the Windhoek area (Code 70 in table 10).
The biotite schist of the Kuiseb Formation is found all over the Windhoek area. It can be seen in many of the road excavations around Windhoek. These excavations are usually stable except for some minor wedge failures due to the low rainfall in this area. The biotite schist is known for its rapid weathering property. Due to the absence of other suitable road building materials biotite mica schist of the Khomas Subgroup has been used extensively as road construction material in the Windhoek area: see table 16. But this material must be met with caution. The Namibian Department of Transport has learnt some quite expensive lessons in using mica schist as road building material. For instance, during the construction of main road 52 between Windhoek and the Matchless Mine in 1971, a slightly plastic, rapid-weathering schist containing about 30% of biotite and muscovite, was used for fill construction in a large number of fills from between six and twenty metres in height in this mountainous area west of Windhoek. Soon after the completion of the construction of the road extensive cracking occurred, some of which was found to be due to slope instability after the normal rainy season 1971/72. These cracks mixed with a variety of stabilisation cracks of the lime-stabilised basecourse which normally cracks in a very distinct block pattern. The slope-instability cracks took the form of longitudinal cracks up to 20 mm wide on the road shoulders close to the edges of the bituminous pavement.
The maximum dry density of the schist was to be found 2.007 kg/m3 at Modified AASHO compaction (1.942 kg/m3 Proctor compaction) with optimum moisture contents of 8,5% (9,5% Proctor) and a minimum air void content of 12% for both. Samples compacted at less than 90% Modified AASHO showed significant collapse settlement on inundation in laboratory odometer tests. Analyses of many of the failed embankments using Bishop's method of slices for circular failure at zero pore water-pressure revealed that the slope stability was sufficient for a density of 90%, but the factor of safety was too low in cases where the densities reached only 80%. Remedial measures adopted were the removal of all materials deep into the inside of the road prism, cutting out all cracked sections and benching down to the roadbed. The sides have then be reconstructed to the specified densities and side slopes of 1:1,5. No further problems have been encountered to date.
4.3.4 THE GARIEP COMPLEX
The Namibian Gariep Complex has the same age than the Damara Sequence. It comprises a sequence of sedimentary and volcanic rocks which are accumulated in a basin that formed on rocks of the Namaqua Mobile Belt, the Orange River Group, the Vioolsdrift Suite and the Richtersfeld Intrusive Complex. Miogeosynclinal deposits such as quartzites, conglomerates, dolomites and shales, together with volcanic tuffs and felsite, occur mainly in the northern and eastern parts of above mentioned basin. Phyllites, schists, epidote-amphibolite facies, porphyric lavas and minor quartzites, arkoses and carbonaceous rocks are common in the areas around Oranjemund and Alexander Bay. The Rosh Pinah Formation is forming the base of this complex.
The isotopic ages, locations and properties of road building materials of the Namibian and Cambrian rocks including those of the Nama Group are summarised in table 16 .
TABLE 16 LOCATION AND PROPERTIES: NAMIBIAN AND CAMBRIAN ROCKS
4.3.5 THE NAMA GROUP
The Nama Group consists of three Subgroups: Kuibis, Schwarzrand and the Fish River Subgroup. The Fish River Subgroup is subdivided into four formations: Stockdale, Breckhorn, Nababis and Gross Aub formations. It is situated in the southern and south-western parts of Namibia. The early era of the Nama Group reaches still into the late Namibian age but the largest part of this group is taken by the Cambrian age. It consists mainly of a virtually unmetamorphosed succession of flat-lying marine, sedimentary rocks overlying a stable platform. A transgressive basal white quartzite is overlain successively by black limestone, green shale and sandstone, a second black limestone horizon, and red sandstone and shale. Several intrusions of granite, syenite, bostonite, alnoitic tuffisite and carbonatite cut Nama rocks in places and form a north-easterly line south-west of Grünau. In the eastern part of the Nama Group these rocks are partly overlain by the Dwyka Formation of the younger Karoo Sequence and partly covered by the unconsolidated to partially consolidated sediments of the Kalahari Group .