4.4 THE KAROO SEQUENCE

 

The Karoo Sequence stretches from the carboniferous to jurassic and cretaceous isotopic ages. It consists of the older, carboniferous Dwyka Formation and thereafter the Ecca Subgroup with four formations: Prince Albert, Whitehill, Aussenkjer and Amibberg. Following are five formations: Gai-As and Omingonde, further Etjo, Kalkrand and Etendeka. The main Karoo rocks are tillites, dolerites, mudrocks, shales and sandstones. Large parts of Namibia are underlain by this sequence. Basal glaciogenic rocks of the Dwyka Formation are overlain by shales, sandstones, mudstones and carboniferous shales of the Ecca Subgroup. The succeeding red beds and aeolian sandstones are followed by basalts, latites and quartz latites. Extensive dolerite sills and swarms of dykes are related to the volcanic rocks.

 

4.4.1  TILLITE AND OTHER STRATA OF THE DWYKA FORMATION

  

Tillites form the major component of the Dwyka Formation [20]. Other components are boulder shales and shales. Sandstones, conglomerates and mudrocks are also present in places but rarely exposed and are of only local significance from a geotechnical viewpoint. In Namibia the tillite overlays a complex pre-Karoo topography and is predominantly of variable thickness.

 

4.4.2  KAROO SANDSTONES AND MUDROCKS

 

A large portion of the Permian Ecca Subgroup is comprising sandstones and mudrocks. However, the different facies and local environments of deposition of the Karoo sediments have resulted in highly variable stratigraphic sequences in different parts. This is particularly occurring within the Ecca Subgroup where thin, poorly sorted sandstones may be found in one area while thick quartzitic sandstone of the same age occur in the neighbouring region.

The Ecca Subgroup consists of sandstones, mudrocks and shales. The arenaceous members of the Ecca Subgroup, though broadly categorised as sandstones, range in fact from arkose through subarkoses to greywackes. Mudrocks include, however, all sedimentary rocks which are predominantly of silt-sized or smaller particles. Detailed classification of mudrocks is difficult owing to the fine-grained nature of these rocks. For simplicity it can be assumed, as Namibia's road builder does, that mudstone or mudrock respective shale and even sandstone have all similar properties as far as road construction properties are concerned. For general purposes only, the terms mudrock or mudstone are used for the fissile variety and the term shale is used for the massive variety.

 

TABLE 17  LOCATION AND PROPERTIES OF KAROO ROCKS

 

|=================================================================================================================|
| AGE/     | GROUP/   | FORMATION/| LITHOLOGY: SEDIMENTARY,       | PROPERTIES: APPLICATIONS AND EXPERIENCES      |
| SEQUENCE | SUBGROUP |COMPL/SUITE| VOLCANIC AND INTRUSIVE ROCKS  |                                               |
|-----------------------------------------------------------------|-----------------------------------------------|
| CARBONIFEROUS/CRETACEOUS ERATHEMS:ISOTOPIC AGES: 400 Ma TO 70 Ma|                                               |
|-----------------------------------------------------------------|-----------------------------------------------|
| Karoo    |          | Dwyka (Cd) | Tillite,boulder shale,shale, |Tillites:Good for natural basecourses: TR 1/3, |
| Sequence |          |            | sandstone,limestone.         |TR 4/1, also favourable for subgrade layers:   |
| 400 to   |          |            |                              |TR 1/3 and TR 5/1 also for gravel wearing c.:  |
| 120 Ma   |          |            |                              |MR 27 but not for surfacing chips (flakiness). |
|          |----------|------------|------------------------------|-----------------------------------------------|
|          | Ecca     | Prince     | Shale,mudstone,sandstone.    |Karoo sandstones: Unfavourable as coarse       |
|          | sub group| Albert(Pp) | Nossob (Pn); Auob (Pau).     |concrete aggregates, baked shales are more     |
|          |          | Whitehill  | Carbonaceous shale.          |suitable: bridges 205,211 on TR 1/3. Sandstone |
|          |          | (Pw)       |                              |can be used as natural basecourse provided it  |
|          |          |Aussenkjer  | Shale, limestone, siltstone. |has a 10% FACT (Fines Aggregate Crushing Test) |
|          |          | (Pa)       |                              |larger than 140 kN.Mudrocks play a great role  |
|          |          | Amibberg   | Sandstone, mudstone, shale.  |in the south.Conglomerates of sandstones,til-  |
|          |          | (Pm)       |                              |lites and shales were used favourably as base  |
|          |          |            |                              |course on TRs 1/3,1/2 and 4/1. Sandstones,     |
|          |          |            |                              |shales,mudrocks favourable as subgrades: TR 3/1|
|          |          |            |                              |Shales and mudrocks favourable as gravel       |
|          |          |            |                              |wearing courses: MRs 25,30,39. Problems:TR 1/3:|
|          |          |            |                              |Itzawisis:weak subsurface drainage. Sandstone  |
|          |          |            |                              |gravel wearing courses due to weak PI and lack |
|          |          |            |                              |of binder on isolated gravel roads:south-east. |
|          |          |            |                              |Baked shales:not suitable for surfacing chips. |
|          |----------|------------|------------------------------|-----------------------------------------------|
|          |          | Gai-As(TRg)| Red mudstone,siltstone,grit, |TRo sandstone: good as subbase/basecourse on   |
|          |          | Omingonde  | sandstone,conglomerate       |TR 1/11 and MR 92 and rehabilitation:TR 1/7.   |
|          |          | (TRo)      | From Ecca to Omingonde:      |It was also favourably used for surfacing      |
|          |          |            | CTR*                         |chips (indurated mudrocks).                    |
|          |          |------------|------------------------------|-----------------------------------------------|
|          |          | Etjo Je    | Aeolian sandstone.           |                                               |
|          |          |            |                              | Not systematically tested as yet.             |
|          |          |            |                              |                                               |
|          |          |            |                              |                                               |
|          |----------|------------|------------------------------|-----------------------------------------------|
| 180 Ma   | - (**)   |(Jd)and(Jr) | Extrusive dolerite+rhyolite   |Volcanic Karoo rock good for surfacing chips  |
|          |          | (**)       |into Karoo Sequence and older |and coarse concrete aggregate:TR 5/1 and MR 61 |
| 180 Ma   |          | Kalkrand   | Basalt, minor sandstone.     |Ke basalts with high PI are good as wearing    |
| 120 Ma   |          | (Jk)       | (Je)*                        |course for salt-gypsum roads along the coast   |
|          |          | Etendeka   | High-silica basalt,latite,   |of the Damaraland, although not systematically |
|          |          | (Ke)       | quartz latite,minor aeolian  |tested yet but unfavourable as gravel wearing  |
|          |          |            | sandstone.                   |course for district roads in the interior.     |
|          |          |            |                              |Karoo dolerite successfully used:coarse concr. |
|          |          |            |                              |aggr. all over the south, surfacing chips for  |
|          |          |            |                              |nearly all paved roads in the south, as        |
|          |          |            |                              |crusher-run and natural basecourse on TR 1/1   |
|          |          |            |                              |and TR 1/7, weathered dolerite as subbase/sub- |
|          |          |            |                              |grade on many paved roads in the south, also   |
|          |          |            |                              |used with good results as gravel wearing       |
|          |          |            |                              |course: MR 21, 22 and 30 and many others.      |
|-----------------------------------------------------------------|-----------------------------------------------|
|CARBONIFEROUS/CRETACEOUS ERATHEMS: ISOTOPIC AGES: 400 Ma TO 70 Ma|                                               |
|-----------------------------------------------------------------|-----------------------------------------------|
|+-< 100 Ma|          | Post Karoo | Granite (Kgr),syenite (Ksy), |                                               |
|          |          | Complexes  | foyaite(Kfo),granophyre(Kgy),| Not systematically tested as yet.             |
|          |          |            | rhyolite (Krh),tuff (Ktu),   |                                               |
|          |          |            | breccia (Kbr),diorite (Kdi), |                                               |
|          |          |            | picrite (Kpi),gabbro (Kg),   |                                               |
|          |          |            | pyroxenite(Kpy),diatreme (*) |                                               |
|          |          |            | intrusive into older rocks.  |                                               |
|----------|----------|------------|------------------------------|-----------------------------------------------|
|          |          | Pomona     | Marl,grit,conglomerate,      |                                               |
|          |          | (Kp)       | chalcedonic limestone.       | Not systematically tested as yet.             |
|          |          |            | Breccia of Brukkaros Mountain|                                               |
|          |          |            | Kimberlite intrusive         |                                               |
|          |          |            | into older rocks.            |                                               |
|=================================================================================================================|
NOTA: '* ' means " stratigraphy undifferentiated"; 'COMPL' means COMPLEX;
'**' means no specific subgroup or formation has been identified so far
10% FACT: The "10% Fines Aggregate Crushing Value" is the force in kN required to crush a sample of
-13,2 -9,5 mm aggregate so that 10% of the total test sample will pass a 2,36 mm sieve.

 

4.4.3 VOLCANIC ROCKS OF THE KAROO SEQUENCE

 

At the end of the period of the Karoo Sequence, during the transition of the Jurassic to the Cretaceous age, a marked magmatic ascent of high magnitude has taken place with the result that the whole of the sub-continent of southern Africa became a magma province on its own [20]. These volcanic rocks of the Namibian Karoo Sequence are mainly represented by two formations, the Jurassic Kalkrand and the Cretaceous Etendeka Formation.

 

4.4.4  KAROO DOLERITE

  

The Karoo dolerite (Jd) is of Jurassic age and is intruding the whole series of the Karoo Sequence from the Carboniferous Dwyka to the Jurassic Etjo Formation. Karoo dolerite can be found in many parts of Namibia, mainly in the south-east but also in the north. Dolerite weathers in a similar manner than all crystalline rocks. Namibia's deposits of dolerites can be found in areas with N greater than 10, and disintegration is the only mode of weathering.

The isotopic ages, locations and properties of road building materials of the Karoo rocks are summarised in table 17 [18].

 

4.5  THE TERTIARY TO QUATERNARY AGES: KALAHARI SEQUENCE

  

The Kalahari Sequence of Tertiary to Quaternary (Recent) ages forms an extensive cover of terrestrial origin in the eastern and northern parts of the country. A lime-cemented sand and conglomerate or grit at the base is followed by green sandy clay, white, partly calcareous sand and the, for any road building purposes in Namibia, extremely important calcrete. Unconsolidated aeolian sand covers large areas of the Kalahari succession and forms stationary longitudinal dunes in many parts of the east of Namibia. In the west the sand seas of the Namib Desert contain both Tertiary and Quaternary dunes.

Due to the importance of sands and pedocrete materials from the Tertiary and Quaternary ages as road building materials they will be dealt with in two specific sections. The isotopic ages, locations and properties of road building materials of the Tertiary to Quaternary materials are summarised in table 18 [18]:

 

TABLE 18  LOCATION/PROPERTIES OF TERTIARY AND QUATERNARY ROCKS

 

|=================================================================================================================|
| AGE/      | GROUP/     | FORMATION/| LITHOLOGY: SEDIMENTARY,     | PROPERTIES: APPLICATIONS AND EXPERIENCES     |
| SEQUENCE  | SUBGROUP   |COMPL/SUITE| VOLCANIC UND INTRUSIVE ROCKS|                                              |
|------------------------------------------------------------------|----------------------------------------------|
| TERTIARY TO QUATERNARY ERATHEMS: ISOTOPIC AGES: FROM 65 Ma       |                                              |
|------------------------------------------------------------------|----------------------------------------------|
|           |             |Tertiary(T)| Marine clay,marly sandston | Phonolite is a hard rock which in its fresh  |
|39 to 32 Ma|             |Auas Mount.| Phonolite,trachyte (Tr)    | state is extensively quarried at Aris, south |
|           |             |Klinghardt | intrusive into older rocks.| of Windhoek. It is used as a high quality    |
|           |             |Mountains  |                            | surfacing aggregate for most of the paved    |
| Kalahari  |             |(Tk) & (Pp)| Suboutcrops under Tk and Qn| roads in the vicinity of Windhoek as far     |
| Tk        |             |           |+ other superficial deposits| south as Mariental and as far north as Sukses|
|           |             |           | (e.g.: Pp).                | between Okahandja and Otjiwarongo.           |
|           |             | (Tn)      | Calcrete terraces+red,part.| Kalahari sands: uniform grading with good    |
|           |             |           | consol.dune deposits:Namib | CBRs and binding properties: suitable for    |
|           |             | (Tk)      | Sand,calcrete and gravel of| road layers and asphalt aggregate but        |
|           |             |           | Kalahari.                  | collapsing fabric.                           |
|           |-------------|-----------|----------------------------| Namib sands: single graded and fine with no  |
|           |             | (Qn)      | Sand sea of the Namib.     | CBRs: unsuitable for road layers and asphalt |
|           |             |           | Alluvium,sand,gravel and   | aggregates: See Section 4.6                  |
|           |             |           | pedocretes like calcrete.  | Pedocretes like calcretes: Namibia's natural |
|           |             |           |                            | road building material number one due to     |
|           |             |           |                            | self-stabilising effects: It has been used   |
|           |             |           |                            | with great success for load bearing layers   | |           |             |           |                            | for paved roads and for wearing courses for  |
|           |             |           |                            | unpaved roads: Even sub-standard calcrete    |
|           |             |           |                            | has been proved successful for bearing and   |
|           |             |           |                            | wearing layers: See Section 4.7.             |
|=================================================================================================================|
NOTA: '* ' means " stratigraphy undifferentiated"; 'COMPL' means COMPLEX;
'**' means no specific subgroup or formation has been identified so far

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