Chapter 2: THE BIOPHYSICAL ENVIRONMENT

Authors: Braam de Villiers (1) and Stuart Mangold (2)

1.Geography Dept.,Potchefstroom University, Potchefstroom
2.North West Dept. Agriculture, Conservation and Environment, Mafikeng


CONTENTS
  1. Geographical location and size
  2. Climate and Rainfall
  3. Topography
  4. Geology and Geomorphology
  5. Hydrography
  6. Soil types
  7. Vegetation types
  8. Conclusion
    9.
References

Maps:

Orientation of the North West Province (Map 1)
Mean annual rainfall for the North West Province (Map 2)
Morphology of the North West Province (Map 3)
Geology - Stratigraphy of the North West Province (Map 4)
Geology - Lithology of the North West Province (Map 5)
Geology - Rock types of the North West Province (Map 6)
Primary drainage regions of the North West Province (Map 7)
Secondary drainage regions of the North West Province (Map 8)
Tertiary drainage regions of the North West Province (Map 9)
Quaternary drainage regions of the North West Province (Map 10)
Rivers and dams in the North West Province (Map 11)
Wetlands and pans the North West Province (Map 12)
Groundwater storage rock types of the North West Province (Map 13)
Groundwater mean recharge rate of the North West Province (Map 14)
Soil types in the North West Province (Map 15)
Biomes of the North West Province (Map 16)
Vegetation types of the North West Province (Map 17)


Appendices:

Appendix 1: Detailed description of climate and rainfall of the North West Province
Appendix 2: Detailed description of the stratigraphy and geomorphology of the North West Province
Appendix 3: Detailed description of the soil types of the North West Province
Appendix 4: Detailed description of the vegetation types of the North West Province

2.1. Geographical location and size

The North West Province occupies a total area of 116 320 km2 (9,5% of the total area of South Africa) and is the sixth largest province in South Africa. It is geographically situated between 25 and 28 degrees south of the equator and 22 and 28 degrees longitude east of the Greenwich meridian. The North West Province borders the provinces of the Northern Cape in the west, the Free State in the south, Gauteng in the east and Limpopo (formerly Northern Province) in the north-east. It shares an international border with the Republic of Botswana in the north (Map 1).

2.2. Climate and Rainfall

Climatic conditions in the North West Province vary significantly from west to east. The far western region is arid (receiving less than 300mm of rainfall per annum), encompassing the eastern reaches of the Kalahari Desert. The central region of the Province, are dominated by typically semi-arid conditions, with the eastern region being predominantly temperate.

The rainfall pattern is highly variable both spatially and temporally and largely mirrors the prevailing climatic conditions of the Province. On average, the western region receives less than 300mm per annum, the central region around 550mm p.a., while the eastern and south-eastern region receives over 600mm per annum (Map 2). The dominant rainfall season in the central region is mid summer (peaking in January). Western parts of the province typically receive rain in the late summer (peaking in February), while the eastern parts typically peak in early summer (December). Significant variability in rainfall is observed in the province and this variability increases progressively from medium (3) in the east to high (5) in the west (Scale: 1-low, 6-high). Except for the south-eastern region, evaporation exceeds precipitation in the province, a further indicator and contributor to the arid and semi-arid conditions, which dominate much of the province. Hail from convective storms does occur sporadically in summer, with the southeast receiving an average of 3 - 5 hailstorms per year and the rest of the province approximately 1-3 per year. A major source of veld fires are lightning strikes and in the far east of the province the typical ground flash density is 8 - 9 flashes/km2/yr, reducing to around 5 - 6 flashes/km2/yr in the central parts and 2-3 flashes/km2/yr in the west.

The Province is also characterised by great seasonal and daily variations in temperature, being very hot in summer (daily average high temperatures of 32ºC in January) and mild to cold in winter (average daily minimum in July is 0.9ºC). Seasonal fluctuations in mean temperatures between the warmest and the coldest months exceed 15ºC in the western region, while the central and eastern regions experience a range between 12ºC and 15ºC. The mean number of days of frost in the province is 31. The north-eastern and north-western parts receive between 1 and 30 days of average heavy frost, while the rest of the province have between 31 and 60 days. These time frames refer to the period between the first and last day of frost, but the total number of days of frost is less. Occasional snow has been known to occur in the south and eastern regions.

Relative humidity is also typically low throughout the Province, being below 28% in the northern part of the Province in July and between 28-30% for the central and eastern regions. In February, the month with the highest relative humidity, the eastern and northern parts range between 66 and 68 % and the rest of the province ranges between 64-66%. This gives rise to high potential evapo-transpiration rates, affecting the resident flora of the region.

No long-term data is available for wind speed and dominant wind direction for the North West Province, although, the predominant wind direction is from a northerly direction. There is a trend for the windy months to occur between August and November.

For a more detailed description of climate and rainfall for the North West Province, see Appendix 1.

2.3. Topography

The North West Province is purported to have the most uniform terrain of all the provinces, with an altitude ranging from 920-1782 metres above sea level. The central and western regions are characterised by flat or gently undulating plains. Dunes associated with the arid environment of the Kalahari occur in the far western region. The eastern region (east and north-east of Zeerust) is of a more variable topography, giving rise to the Magaliesberg mountain range of the Transvaal Sequences Magaliesberg formation. Another prominent feature in the east is the Pilanesberg which consists of a formation of concentric hills or ring- dykes, remnants of an ancient volcano (Map 3).

2.4. Geology and Geomorphology

The North West Province has an interesting and ancient geological heritage, rich in minerals and palaeontological artefacts. The north-eastern and north-central regions of the Province are largely dominated by igneous rock formations, as a result of the intrusion of the Bushveld Complex. . Ancient igneous volcanic rocks dating back to the Ventersdorp age (more than 2 000million years) appear to be the dominant formations in the western, eastern and southern regions of the Province. Sedimentary rocks dating back to the Quaternary period (65 million years) occur in the north-western corner of the Province (Map 4, Map 5 and Map 6).

Archaean Granites

The oldest rock formations in the North West Province are the basement Archaean Granites (3 204 ± 65 Ma, million years); Kent 1980), which mostly form flat to slightly undulating landscapes. Outcrops of these granites occur in the south-eastern portion of the Province (in the Potchefstroom district) and further west as far as the north-central portion of the Vryburg and Ganyesa districts.

Kraaipan Group

The Kraaipan Group consist mainly of schist, amphibolite, jaspilite, lava, banded ironstone and a small proportion of dolomite (Kent 1980). This formation forms a series of narrow, more or less parallel, discontinuous ridges approximately 60km south-west of Mafikeng running north to north-north-west. Another series of parallel, discontinuous outcrops occur approximately 50 km further west (near Stella).

Dominium Group

The Dominium Group, dated at 2 800 Ma, is comprised of arenaceous sediments, conglomerates, grits, basic volcanics, tuffs, coarse pyroclastic rocks and quartzite. It is predominantly volcanic in origin, forming a series of hills to the west of Klerksdorp (in the vicinity of the old gold-mining town of Dominium Reefs). The thickness of the Group is 2 250 m in this area. A second important outcrop occurs at Ottosdal. The third outcrop (maximum thickness of 240m) occurs as a narrow band of rocks on the fringe of the younger Witwatersrand outcrops in the southern portion of the Potchefstroom district (Kent 1980).

Witwatersrand Super Group

The economically important Witwatersrand Super Group is dated at 2 800 and 2 300 Ma extends into the North West Province from the east and south as far as the Wolmaransstad district. It is comprised of a thick sequence (more than 5 km) of shales, quartzite and conglomerates, the latter often being associated with gold bearing reefs and uranium deposits. Such is the case of the gold bearing reef around Klerksdorp. Further east towards Stilfontein under a layer of dolomite and lava lie further gold bearing conglomerates. The source of this material is thought to be from sedimentation processes from ancient rivers.

Ventersdorp Super Group

The Ventersdorp volcanic rocks also comprise an important geological formation in the Province. This covers an extensive area both within and around the Witwatersrand basin, ranging in thickness from around 5000m to 300m. It is especially prevalent near Ventersdorp, Vryburg, Klerksdorp and Mafikeng where it extends into south eastern Botswana. Other outcrops occur near Vredefort Dome near Parys. The Vredefort Dome near Parys is believed to be the result of a large meteorite impact that occurred some 2000 million years ago. The central core is composed of gneiss granite surrounded by a rim of Witwatersrand, Venterdorp and Transvaal rocks that contain shatter cones indicative of violent mechanical shock. The deformation of the rim layers surrounding the dome provides further evidence that a violent event was responsible for the formation of the Vredefort Dome.
The Ventersdorp formation is composed largely of volcanic andesitic lavas and related pyroclastics (metamorphic rocks formed by the extremely hot temperatures associated with volcanic activity). Various conglomerates, tuffaceous and calcareous shales and porphyries are also constituents of this formation.

Transvaal Sequence

The Transvaal sequence represents a large component of the Province's geology and consists of two main sedimentary groups: the Pretoria-Griquatown group and the Olifants River-Campbell Rand group. Both groups occur discontinuously in the North West Province. The western section occurs in an area bounded by Vryburg, Kuruman and Douglas known as the Ghaap Plateau. The eastern section extends from the south in the Potchefstroom district to the north of Ventersdorp, west of Mafikeng and to the Rustenburg-Brits section.

Lower strata of the Transvaal sequence comprise mostly of dolomite (with some chert and shales interspersed in places) while the upper strata appear to be more varied in constituents. Dolomite consists largely of calcium carbonate and is hence vulnerable to solution, especially by the carbonic acid found in rainwater percolating downwards. The dissolution of dolomite can lead to the formation of underground caverns and horizontal chambers often filled with large volumes of groundwater. Malmane Dolomite appears to be one of the main elements of the Transvaal sequence. It contains abundant algal stromatolites, evidence of an aquatic environment in ancient times. The algal stromatolites have a number of distinctive shapes such as domes, columns and spheres, their shape being governed by the environment in which they were formed. It is believed that the dolomites were laid down in shallow inter-tidal or sub-tidal zone of open water seas.

Near Zeerust is an overlying banded iron layer known as the Penge Formation as well as a mixed zone containing chert breccia, carbonaceous shale or mudstone and limestone. The basal sequence of the Malmane dolomite is known as the Black Reef that is made up of very thin sequence of quartzite, conglomerates and shales. In the western region of the Province is the dolomitic Ghaap Plateau that includes the Campbell Rand, Kuruman and Griquatown formations. The former consists of limestone, dolomite, chert, shale siltstone and quartzite. The Kuruman Formation consists of banded iron deposits, not unlike the Penge Formation. The latter formation, which overlies the other two, contains jasper in abundance as well as layers of iron.

The Pretoria Group is a formation running westward from Lobatsi past Rustenburg and Pretoria almost reaching Witbank in the east. This formation occurs in a thick layer ranging from 4000m to 7000m, consisting of chert conglomerates and quartzite. Between the Magaliesberg and Daspoort, there is a thick volcanic sequence as well as shales and other pyroclastic materials. The shales often contain metamorphic minerals such as cordierite, staurolite, garnet, andalusite and sillimanite. Slate is also prevalent in this area.

Griqualand West Sequence

The western portion of the North West Province is dominated by the rocks of the Griqualand West Sequence (similar in age than that of the Transvaal Sequence) (Kent 1980). The quartzite, shale, siltstone and lava of the Vryburg Formation (approximately 75-100 m thick) are the oldest rocks in the sequence and outcrop as to the east of Vryburg.

The dolomite of the Campbell Group, consisting of the Schmidtsdrif (3 - 275 m thick) and Ghaap Plateau Formations (alternating in thickness between 900 and 1600 m) are made up of alternate intercalated layers of dolomite, shale, limestone, calcareous sandstone, siltstone and chert. The main outcrop of this Group occurs in the area to the south west of Vryburg and stretches down to the border of the Province near Kudumane.

Bushveld Complex

The Bushveld Complex is largely igneous in origin and occurs in the north-eastern region of the Province, from Brits and Rustenburg in the east to north of Zeerust and Swartruggens into Botswana. Of the three suites making up the Complex, only the peripheral mafic intrusive phase occurs in the North West Province. This consists of four main layers: the upper zone of gabbro, olivine, diorite to grandiorite with some anorthosites and magnetites. The main zone of 5 200m which consists mostly of gabbros, which gives rise to topographic features such as hills. Below this is the critical zone consisting of norites, anorhosites, pyroxenites and chromites, below which is the basal zone mostly of pyroxenite and peridotite. Platinum and chromite from this zone is mined extensively in the Rustenburg / Brits region.

Pilanesberg Complex

The isolated ring of alkaline rocks at Pilanesberg, called the Pilanesberg Complex, is basically a series of concentric hills covering over 500 km2. It consists of an overlying layer of alkaline volcanic and pyroclastic rocks, the latter consisting of fragmental volcanic material that has been blown into the atmosphere by explosive activity. The top layers have largely been eroded. The intrusive base is composed of foyaite and syenite. It also consists of small outcrops of quartz-rich rocks. The complex was formed by the coalescence of a number of small volcanoes and the welling up of successive ring-dykes around the collapsed crown.

Karoo Sequence

Outcrops of the Karoo sediments occur between Lichtenburg and Mafikeng as well as to the south of Vryburg. Another small outcrop occurs to the east-north-east of Schweizer-Reneke and near Bloemhof (Kent 1980). Another outcrop of these rocks covers a portion of the Bushveld Complex rocks north of Brits. Karoo sediments are also known within sinkholes in the dolomite in the vicinity of Stilfontein.

Kimberlites

Kimberlites are diamond bearing vertical "pipes" or "fissures" which have intruded to the surface. They contain a number of minerals including mica, olivine, augite, red garnet and pyrite to name a few. Several kimberlite fissures occur in the North West Province, the most notable being around the Harts River 100km north-west of Kimberley.
For a detailed description of the geology and geomorphology refer to 2.

2.5. Hydrography

Water is one of the North West Province's most critical and limiting natural resources. The four sources of water available in the Province are surface water, groundwater, imported water and re-usable effluent.
2.5.1. Surface Water
The North West Province's surface water comprises rivers, dams, pans, wetlands and dolomite eyes fed by underground springs. Apart from highly variable precipitation from year to year, one of the most important factors affecting surface water in the Province is the highly variable but low actual runoff. Runoff as a percentage of the precipitation ranges from less than 1% in the west to approximately 7% in the eastern region. Average runoff for the Province is 6%, which is below the average of 9% for Southern Africa (Schulze, 1997).

2.5.1.1. Rivers
Being a predominantly dry province, the North West Province has very few perennial rivers. Of the six major drainage basins in South Africa, the Limpopo, Orange and the Vaal partly fall within the boundaries of the Province (Map 7). With the exception of the Vaal River, the highly variable runoff from the non-perennial water sources prohibits direct utilization by runoff-river abstraction on a large scale from major rivers in the Province.

Secondary, Tertiary and Quaternary Catchments

Map 8, Map 9 and Map 10 portray the secondary, tertiary and quaternary catchments in the Province respectively. The Limpopo River headwaters flow in a general northerly direction before veering eastward, forming the border between the Northern Province and Botswana. The entire catchment of the Groot Marico River, which is one of the major headwater tributaries of the Limpopo, is located within the Province. The source of the Ngotwane River as well as the catchments of a number of other tributaries of the Limpopo river system lie within the boundaries of the Province, namely the Elands and Hex Rivers which join the Crocodile River in the north-east. Other rivers of the Limpopo River system that lie within the Province are the Magalies, Olifants, Moretele and Tolwane rivers (Table 2.1).

The Vaal River which forms the southern boundary of the Province with the Free State, which rises on the western slopes of the northern sector of the Drakensberg-range in Mpumalanga, flows about 900 km westwards across the interior plateau. (Including Gauteng and the North West Province) It joins the Orange River near Douglas in the Northen Cape. Apart from the Molopo River, it is the only west flowing river system in the North West Province and is known for its exceptional flood plains, wetland systems, dolomite eyes and natural riverine vegetation in the lower reaches. Major tributaries of the Vaal River which have entire catchments within the North West Province are the Harts, Dry Harts, Schoonspruit, Makwassiespruit and Bamboesspruit Rivers. The Mooi River another major Vaal River system tributary has its headwaters in the North West Province, but is joined by a number of tributaries flowing from Gauteng (Table 2.2).

The Molopo River, which rises from the Molopo Eye near Mafikeng, flows westwards to form the northern border of the North West Province with Botswana. The Molopo River was once a tributary of the Orange River system, but being blocked by high dunes, it no longer reaches the Orange (Midgley et al. 1994). It is currently non-perennial as its water is heavily abstracted at source. This river has a number of tributaries which fall within the Province, namely the Ramatlabamaspruit, Setlagolespruit, Ganyesaspruit and Pepanespruit, all of which are non-perennial (Table 2.3). The Mathlaawaringspruit, a tributary of the Kuruman River is the most south-westerly drainage line in the Province.

Table 2.1a: The Limpopo Drainage Region (source: Midgley et al. 1994).


Catchment Area (km2) Mean Annual Precipitation (mm) Mean AnnualRiver flow ( X106m3)
109 610 528 2 176


Table 2.1b: Major tributaries of the Limpopo Drainage Region occurring in the North West Province:

MAIN RIVER PRIMARY SECONDARY TERTIARY
Crocodile Groot Marico Klein Marico
Pienaars Apies
Elands Selons Koster
Brakfonteinspruit
Heks Waterkloofspruit
Sterkstroom
Magalies Scheerpoort Nouklip-oog
Maloneys-oog
Swartspruit
Hennops Rietspruit
Jukskei Klein Jukskei
Bloubankspruit


Table 2.2a: The Vaal River Drainage Region (source: Midgley et al. 1994)

Catchment Area (km2) Mean Annual Precipitation (mm) Mean AnnualRiver flow ( X106m3)
196 263 571 4 298


Table 2.2b: Major tributaries of the Vaal River Drainage Region in the North West Province:

MAIN RIVER PRIMARY SECONDARY TERTIARY
Vaal Harts Dry Harts  
Groot Harts  
Klein Harts  
Schoonspruit Taaiboschspruit
Rietspruit
Mooi Loopspruit


Table 2.3a: The Orange River Drainage Basin (Midgley et al. 1994).

Catchment Area (km2) Mean Annual Precipitation (mm) Mean AnnualRiver flow ( X106m3)
409 621 315 6 987


Table 2.3b: Major tributaries of the Orange River drainage basin in the North West Province:

MAIN RIVER PRIMARY SECONDARY TERTIARY
Molopo Ramatlabamaspruit
Setlagolespruit
Ganyesaspruit
Pepanespruit


Impoundments

Excluding farm dams, the North West Province has thirty seven large dams (Map 11) ranging in yields from 0,41 m3/yr (Feloana Dam) to 1264,40 m3/yr (Bloemhof Dam). See chapter 10.1 Surface water resources for more details.

Wetlands and Pans

The RAMSAR definition of a wetland is:
"areas of marsh, fen, peatland or water, whether natural or artificial, permanent or temporary, with water that is static or flowing, fresh, brackish or salt…"
Included in this definition are pans, springs, seeps, bogs, mires, marshes and riverine wetlands. Wetlands are considered to be one of the most threatened habitats world wide. They result from unique aquatic and terrestrial conditions and consequently they possess ecosystems that are among the most complex in the world (Bacon, 1996). Due to the variable hydrological processes in wetlands, the organisms that inhabit these areas tend to exhibit extensive variation. Wetlands play an important role in natural filtration and purification of water as well as providing habitat to waterfowl and breeding grounds for a number of migrant bird species.

The national wetland classification system divides South Africa into several regions. The North West Province is part of the Western Plateau of the Plateau Wetland Group which has the greatest concentration of pans in the country (Cowan, 1995). They are mostly endorheic pans, which are closed with no outlet and an average size of 8 ha (from a total of 636 pans located in the Western Plateau). Several of these pans in the North West Province are located on the watershed between the Vaal and the Limpopo river systems and on the Ghaap Plateau, while the remainder of these pans are located in the area drained by the Harts river system (Map 12).

Unpredictable seasonal changes lead to a range in physical and chemical properties of the substrata and water in pans, such as major changes in salinity. When inundated, the temporary pans are inhabited by a variety of aquatic invertebrate species, the majority of which are highly specialised and adapted to the conditions in these pans. Fish are only present in the few perennial pans. Most of the fish has been introduced, except for Barberspan, which has indigenous species. Birds use the pans for feeding, drinking, roosting, moulting and breeding (Allan, et al, 1995). The salt pans at Delareyville attract thousands of flamingoes in summer.
The largest pan in North West Province is Barberspan (26°35' S 25°35' E) situated about 17km north east of Delareyville. Barberspan is listed by RAMSAR as a site that is not only of national but international importance and accorded special conservation status. South Africa has 16 declared RAMSAR sites in total of which this is the only one in North West Province. Barberspan is unique in that it is a large perennial grass pan (covering an area of some 1050ha). The pan was proclaimed as a natural reserve in 1954 and it is a permanent fresh water lake. It is an important stopover for many species of migrating birds. Barberspan is the only locality in South Africa where the pintail Anas acuta has been recorded. Several waterfowl species breed in the wetland. Seven endangered bird species and two endangered mammal species occur here. Barberspan also supports a rich plankton community. (RAMSAR, 1994).

There are two other smaller non-perennial pans in the vicinity of Barberspan, namely Leeupan and Gelukspan.
Riverine wetlands occur in the Mooi, and Harts rivers and at the different eyes of the Molopo River catchments, as well as those of the Schoon and Bamboes spruits. Several large wetlands occur near Koster, one of which forms the source of the Elands River. A few smaller stream fringe wetlands occur along streams feeding into the Limpopo system. These wetlands are usually not permanent, like those fed by dolomite springs. The wetland occurring at Barberspan is one of the only perennial pan wetland systems in the Province. Artificial wetlands have been created on the sides of some of the reservoirs in the province.
Threats to wetlands in order of importance are agricultural development, road building, mining and industrial development. Other threats to these wetlands are the mining of peat and calcrete for fertilizer in the Schoonspruit, Mooi River and to a lesser extent in the Harts River system.
Dolomitic Eyes and Sinkholes
The dolomite area covers approximately 4022km2 of the North West Province and forms the main watershed of the east-flowing Limpopo River system and the west-flowing Molopo River. The sources of many rivers, which rise in the Province, are located in this dolomite area. These include the Molopo, Marico, Malmani, Mooi, Schoonspruit and Harts Rivers. The actual source of these rivers are known as dolomitic eyes, which are water bodies fed by groundwater originating from fractures in the underlying dolomite. The water from these dolomitic features is typically alkaline (pH 7.5-9.3) having picked up magnesium and calcium carbonates through solution from the parent dolomite. By their very nature, such dolomitic features are partial to both factors affecting surface water and those affecting groundwater. For this reason, dolomitic eyes are extremely sensitive and often possess unique ecosystems characterised by a high degree of endemicity (species which are found only there). The North West Province has a number of these unique natural features most of which are classified as springs while one, known as Wondergat, is considered to be a sinkhole (table 2.4). However, these unique natural features are also under a number of threats ranging from water abstraction, groundwater contamination from agriculture, industry and mining, habitat transformation and invasions by alien species (particularly exotic plants e.g. poplars and fish species e.g. black bass). See Skelton et al, 1994 for more information.

Several smaller springs also occur in all the dolomitic areas some of which have ceased flowing in recent times, largely due to ground water abstraction. However, the following are still flowing: Buffelshoek, Doornfontein, Dinokana, Doornplaat, Molopo, Olievendraai, Paardevallei, Rietgat, Rhenosterfontein, Stinkhoutboom, Tweefontein, Welgedacht and others. In general, the flows range from 7 l/s (Olievendraai) to 19-146 l/s (Buffelshoek) (Bredenkamp (2000).

Table 2.4: Main Dolomitic Eyes and Sinkholes in the North West Province (Source: Fleisher, 1981; Bredenkamp, 2000; Kotze et al. 2000; Skelton et al. 1994)

Name Location Yield (l/sec)
Klerkskraal Springs 26º 12'S 27º 10'E
Bovenste Oog 26º 12'S 27º 10'E
Gerhard Minnebron 26º 28'S 27º 09'E Min: 666
Max: 920
Malmani Oog 25 º 49'S 26º 04'E Min: 19
Max: 309
Marico Oog 25º 47'S 26º 22'E
Molopo-/Grootfontein 25º 53'S 26º 01'E
Schoonspruit 26º 17'S 26º 51'E Min: 824
Max:1998
Wondergat 25º 52'S 25º 53'E


2.5.2. Groundwater
Although the North West Province has few surface water resources it has a large reservoir of subterranean water in the form of fractured aquifers and dolomitic compartments. According to Nel et al. (1995) Groundwater regions in the North West Province can be divided into the following areas:
  1. Ghaap plateau Dolomites
  2. Coetzersdam-Louwna Granite-Gneiss region
  3. Vryburg Basin
  4. Kalahari Basin: Penrith-Radnor
  5. Western Transvaal Dolomite
  6. Other groundwater reserves
The Province's groundwater storage occurs in a number of ways (Map 13). Most of the Province's aquifers comprise secondary fractured aquifers. Storage and flow in these aquifers occur in the permeable transition zone between weathered soil and hard rock, fracture and fissure zones in hard rock, and contact zones between intrusions and country rock. Yields from secondary aquifers are usually far below those found in dolomite. Usually these aquifers are semi-confined, which means that the pressure in the aquifer exceeds the general air pressure causing groundwater to rise in a borehole. Excessive dewatering of the fracture zones that yield the water of a well might cause the fracture zone to be compressed due to the high pressure. Therefore the yield of the well might decrease if it is pumped above its sustainable yield.

The groundwater from the basic igneous rocks is also liable to contain a certain amount of dissolved materials. It must be noted that in very highly weathered zones within the Ventersdorp rocks, a high degree of weathering produces a large amount of clay, with the consequent impeding of the underground water seepage. The water from areas underlain by the granites is usually very good, although the water may be slightly acidic.

The water in the Bankeveld is strongly controlled by the degree of metamorphism. The higher degree of metamorphism in the west has as a consequence that the shale are better aquifers then the quartzite, while the picture reverses in the east. The quality of the quartzite water is very pure, while that of the shale tends to vary.

The water from the Bushveld basin tends to vary in quantity and quality in accordance with the rock type as well as the degree of weathering. The noritic aquifers tend to display fairly high silica content. Water derived from underground sources in the Pilanesberg Complex has unusually high fluoride content.

The sources of groundwater in the areas covered by the Kalahari Group (in the Terra Firma area), underground water seems to be limited. Water is frequently brackish and the general yields are small. The depth of the water table is often in excess of 130-160 m.

Groundwater in the Province is also stored in caverns and dissolution chambers in dolomitic rock and karstic formations that have extremely high transmissivity values. The yield i.e. litres/second of an aquifer is determined by its storativity and transmissivity. When a borehole is pumped the groundwater level is drawn down in a cone around it. The depth of draw down and radius of the cone is determined by the physical properties of the aquifer. In dolomite the draw down is usually very small, because water flows so quickly towards the well due to the large transmissivity values. The water quality from dolomite sources is usually good (except where polluted by mining and other industries has occurred), although this water usually has a high carbonate content.

Although groundwater recharge varies from around 3 to 95 mm/yr, the average for the Province is less than 10mm/yr (Map 14), the lowest in South Africa (Schulze, 1997). See chapter 10.2 on groundwater resources for more information.

2.6. Soil types

Due to the dynamic nature of soils, they are constantly evolving and degrading by means of natural and man-induced processes. The weathering of rocks (one of the main processes responsible for soil formation) in deserts and semi-arid areas tends to be superficial and hence these soils tend to be shallow and stony. Erosion and deposition by the agents of wind and water are responsible for the transportation of soils from one location to another.

Due to the low rainfall generally experienced by the Province, soils in the North West Province (Map 15) are deemed to be only slightly leached over much of the western region. With high evaporation rates, there is a predominance of upward movement of moisture in the soils. This often leads to high concentrations of salts such as calcium and silica in soils, which sometimes lead to the formation of hard pans or surface duricusts. As a result, high levels of salinity or alkalinity may develop in these areas. Levels of organic matter tend to be low, governing the vegetation types which are able to grow there.

Large areas of yellow shifting sands occur in the north-western region of the Province, while a plinthic catena of yellowish-brown sandy loams are characteristic of the south and eastern region. The central region has areas covered by red or brown non-shifting sands with rock. This region also has weakly developed lime soils associated with the dolomite limestone formations. The south-western region also has areas characterised by undifferentiated rock and lithosols. Lithosols are shallow soils containing coarse fragments and solid rock at depths less than 30cm.

The north-eastern portion of the Province has been shown to have lithosols of arenaceous sediments. The southern and central regions have black and red clays as well as ferrisiallitic soils of sands, loams and clays. The drier western region is characterised by red and yellow arenosols while the south west has calcareous sands and loams and arenaceous lithosols.

For a detailed description on the soil patterns of the North West Province, refer to Appendix 3.

2.7. Vegetation types

Of the seven major recognized biomes of South Africa, only the savanna and grassland biomes occur in the North West Province (Map 16). Most of the Province (71%) falls within the Savannah Biome with its associated Bushveld vegetation. The remainder falls within the Grassland Biome comprising a wide variety of grasses typical of arid areas. Given the arid and semi-arid conditions of the western half of the North West Province, the vegetation of this region largely comprises plants that are adapted to these conditions (known as xerophytes). As a result, low biomass, productivity and species richness of plants tend to prevail in this region. With the east-west variation in climate and rainfall, there is a corresponding gradation in the vegetation types from xerophytic in the west to open grassland and savannah in the central region and bushveld in the eastern region (Map 17).

According to Low and Rubelo (1998), nine different vegetation types are known from the Province, belonging to the clay, Kalahari, Kimberley, mixed bushveld and highveld grassland categories (Table 2.5). Broadly speaking, there is a predominance of Kalahari deciduous Acacia thornveld (open savannah of Acacia erioloba and A. haematoxylon as well as desert grasses) and shrub bushveld in the dry western half of the Province. The rocky soil is conducive to Tarchonanthus veld on the dolomite Ghaap Plateau (Acocks, 1975).

The northern and eastern regions reflect the greatest variability of vegetation types in the Province. Vegetation types include sourish mixed bushveld (open savannah dominated by Acacia caffra and grasses of the Cymbopogon and Themeda types), turf thornveld and isolated pockets of Kalahari thornveld and shrub bushveld. The mountainous areas of this region are covered by mixed bushveld. The northern edge of the Magaliesberg is characterised by dense short bushveld trees and grasses (Acocks, 1975).

The Rustenburg and Marico districts are said to have mixed vegetation types dominated by Acacia caffra and A.erioloba, with the latter tree being found growing on deep sand overlying limestone bedrock.

South Zambezian undifferentiated woodland and wooded grassland occur in the central region. Highveld grassland of the Cymbopogon-Themeda veld type and Acacia erioloba savannah predominates in the central and southern regions, while Bankenveld vegetation (sparse, sour and strongly tufted veld) is prevalent in the south eastern region. This region is also characterised by open savannah of Acacia erioloba and grasses (Acocks, 1975).

Table 2.5: Vegetation types of the North West Province (Source: Low and Rebelo, 1998).

Area (Km2) Proportion of Province (%) Proportion Conserved (%)
GRASSLAND BIOME
Highveld Grassland type
Dry Sandy Highveld Grassland 22 613 19.49 0.69
Moist cool Highveld Grassland 47 0.04 0
Rocky Highveld Grassland 10 595 9.13 0.94
SAVANNA BIOME
Clay Thorn Bushveld 6 978 6.01 0.01
Kalahari Mountain Bushveld 134 0.12 0
Kalahari Plains Thorn Bushveld 38 203 0.12 0.63
Kalahari Plateau Bushveld 10 973 32.92 0
Kimberly Thorn Bushveld 7 875 6.79 1.73
Mixed Bushveld 18 606 16.04 4.55


However, overgrazing and agricultural practices such as maize and sunflower production have transformed much of the natural vegetation of the central and south and south eastern regions of the Province. For a detailed description on the vegetation types of the North West Province, refer to Appendix 4.

2.8. Conclusion

References

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Kent, L.E. (Compiler) 1980. The Stratigraphy of South Africa. Handbook 8. Geological Survey. (SACS). Part 1. Lithostratigraphy of the Republic of South Africa, South West Africa/Namibia and the Republics of Bophutatswana, Transkei and Venda. Department of Minerals and Energy Affairs. Government Printer, Pretoria.
Kotze, J.C., Z.M. Dziembowski & L.J. Botha. 1995. Schoonspruit Eye, District Ventersdorp analysis of rainfall, eye flow and water levels. Drainage area C24. Technical report GH3842. DWAF, Pretoria.
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Zerwick, J. & J. Schutte. 2001. Die paleolope van Noordwes en Benede Vaal (Alluviale diamantdelwerye 3). Lichtenburg Museum, Lichtenburg.


Appendix 1: Detailed description of climate and rainfall of the North West Province.


The climate of the North West Province varies considerably between the west and the east, while the eastern section also displays three distinct zones.
In the south east of the province, which includes the districts of Potchefstroom, Ventersdorp, Klerksdorp, Coligny and Lichtenburg as well as the southern portion of the Koster and Marico districts, the climate is typical of the South African Highveld. The average annual rainfall is in excess of 550 mm, increasing from the west to the east. The average in the east is between 600 and 650 mm. The variability of annual precipitation is high as the maximum and minimum recorded figures indicate (Table 2.6). The absolute yearly maximums recorded are all in the order of 1000 mm, while the minimums are all in excess of 360 mm. Relative humidity for the area has a daily average of between 71% and 72% in summer, and 36% to 37% in winter.

Table 2.6: Temperature, precipitation and humidity levels for select weather stations in the North West Province (Source: South African Weather Bureau).

STATIONS: MEAN TEMPERATURES (ºC) PRECIPITATION (mm) MEAN RELATIVE HUMIDITY (%)
JAN JUL MEAN HIGH LOW JAN JUNE
KURUMAN 31,5 1,1 445 964 172 72 35
VRYBURG 32,6 -0,6 _ _ _ _ _
VAALHARTS 32,7 -0,5 442 1089 158 65 33
CHRISTIANA 32,3 3,0 _ _ _ _ _
MAFIKENG 30,4 3,0 553 868 265 65 35
POTCHEFSTROOM 29,1 0,3 625 980 365 71 36
VENTERSDORP 28,6 -1,3 _ _ _ _ _
CARLETONVILLE 27,6 -0,2 670 1112 421 72 37
LICHTENBURG 28,1 1,9 602 965 380 _ _
ZEERUST 30,8 -0,8 600 1002 390 69 36
RUSTENBURG 30,6 1,8 685 1067 370 70 40
BRITS 30,3 1,3 621 886 338 71 36
MARICO 31,2 3,5 657 1062 299 _ _
LINLEYSPOORT 29,6 4,1 677 1097 247 _ _
SAULSPOORT 31,2 3,2 659 947 367 63 36


The average minimum and maximum temperatures for a specific month all occur either in January or in June, and vary from between 27,6ºC and 29,1ºC (maximum), while the average daily minimum is between -1,3ºC and 1,9ºC. The actual extreme daily maximum for this region was recorded during January in Potchefstroom, and reached 39,2ºC, while the corresponding absolute minimum temperature (-12,2ºC) was recorded in Ventersdorp during July. The average amount of days per year with temperatures below 0ºC increases from Lichtenburg (23,9) eastward, and reaches 46,2 in Carletonville (30km east of the eastern border of the province). The same picture emerges if the average number of days with an absolute minimum of below -2,5ºC is considered (Lichtenburg 6,3 days and Carletonville 16,2 days).

The average monthly values of sunshine recorded for different months (Republic of South Africa 1965) reveals that August is the month with the highest possible amount of sunshine that can be received during a normal day, namely 88%. Months with low percentages are December and February with a possible 63% sunshine. In general between 63 and 71% sunshine can be expected during summer, while the winter average is between 75%and 88%.

The area known as the Bankeveld, which occur in portions of the Zeerust, Marico, Rustenburg and Brits districts, can be separated from the Highveld region on the grounds of the differences shown in its climatic statistics. As far as the temperatures are concerned it is noticeable that the daily average maximums are all more than 30,3ºC, while the minimum for Zeerust is the only one below 0ºC. The absolute maximum temperatures of Zeerust, Brits and Marico are all in excess of 40,6ºC. The absolute minimums recorded varies between -3,3ºC and -7,8ºC.The days with temperatures below freezing is still in the order of 23 to 32, but days with temperatures of less than -2,5ºC are less than on the Highveld.

As far as precipitation is concerned it is noticeable that the averages are all in excess of 600mm. Data for the extreme east of the area in fact states that it receives in excess of 675mm. The absolute maximum and minimum yearly rainfall received as well as the humidity ranges are in general similar to that of the previous sub-region. The occurrence of thunderstorms and hail are lower than the figures obtained for the Highveld region. Zeerust receives on average 57,1 days with thunder and only 1,1 days with hail, while Marico have on average 46,3 days with thunder and 1,2 with hail. No data is available for sunshine duration or wind direction and speed.

The third sub-region distinguished is comprised of the area located primarily on the Bushveld Complex. This area is at a lower altitude than both the previous sub-regions, and the climatic stations, which can be used to describe its climatic conditions, are those situated at Rustenburg, Brits, Lindleyspoort and Saulspoort. This region differs from the previous two regions mainly with regards to its temperature regime. The maximum monthly temperature is similar to that of the Bankeveld, but its average daily minimum is markedly higher. Lindleyspoort has an average minimum daily temperature during the coldest month of 4,1ºC. The lowest ever recorded minimum temperatures of Rustenburg, Brits and Lindleyspoort are between -3,3ºC and -6,1ºC. The number of times during a year that frost and or heavy frost can be expected is in general lower than that for the previous two regions. No distinct differences are observable in the humidity figures. A marked drop in average annual rainfall from the south to the north of this sub-region is however discernable. The percentage of sunshine received is during August (90%), with February receiving only 58% sunshine. The average summer figures vary between 58% and 65%, while the winter and spring season receives between 79% and 90% sunshine. No specific statistics were available for the area described as the Northern portion of the Bankeveld in the extreme north of the Province.

The western region of the North West Province is the fourth sub-region that can be distinguished. This region is, however, also not homogenous, and it should be possible to subdivide it into two or more smaller units if the necessary statistics were available. It is also very difficult to draw the exact line between the western districts of the eastern portion of the Province and the western portion of the Province (also due to lack of statistics). A general description will however be given and differences within the region will, where ever possible, be pointed out.

From the maximum temperatures for a month, it can be deducted that this region's temperature increases from the northeast (Mafikeng) towards the south (Vryburg and eventually Christiana). Kuruman is slightly cooler. The months with the average lowest minimum temperature is only below freezing at Vryburg and Christiana, while the other three stations have temperatures exceeding 0ºC. The absolute maximums is in general higher than in the rest of the province, while the absolute minimums is (with the exception of Kuruman) between -8,3ºC and -9,7ºC. Days with frost and or heavy frost is similar to that experienced on the Highveld region. Vaalharts and Kuruman both falls within the Northern Cape, but was used due to lack of other stations in their vicinities.

The main distinction however between this region and the rest of the Province is in the amount of precipitation. With the exception of the Mafikeng region (with an average of 553mm) the rest of the region displays a very marked decrease in rainfall towards the west and northwest. The absolute maximums are similar to that of the Highveld region, but the absolute minimum is distinctly lower (as low as 158mm for Vaalharts, and 172mm for Kuruman). Rainfall on the western border of the Province is reduced to an average of as low as 244mm per year.

The relative humidity for January mornings is on average in the order of between 61% and 66% (average 34% to 47%), while the winter values varies between a maximum of 77% to 84% and 34% and 36%.

No wind statistics are available for this sub-region, but figures obtained for Kimberley show a marked increase in winds coming from a south westerly direction (in contrast with the rest of the North West Provinces' wind directions). Another distinct difference in the climate of the extreme west of the Province is seen in the percentages of sunshine received. Figures obtained for the Tsabong weather station in Botswana highlight these differences. The yearly average percentage of sunshine received is 88%, while the month with the lowest percentage is December (with 78%) and the month with the highest figure being August with 95%.

Appendix 2: Detailed description of the stratigraphy and geomorphology of the North West Province.



Stratigraphy
In this section the stratigraphic sequence of the Province will be discussed in chronological order, with some reference to landscape elements associated with the different units.

Archaean Granites
The oldest rocks in the North West Province for which an age determination is available are the basement Archaean Granites (3 204 ± 65 Ma, million years); Kent 1980). Archaean Granites mostly form flat to slightly undulating landscapes. Only a few typical castle koppies, that are normally associated with granites, are to be found on these rocks (Pritchard 1986). The Archaean Granites outcrop from the south-eastern portion of the Province (in the Potchefstroom district) and further west as far as the north central portion of the Vryburg and Ganyesa districts. A large portion of the western granites is, however, covered with younger unconsolidated material.

Kraaipan Group
The age of the Kraaipan Group's rocks outcropping in the Mafikeng and Vryburg districts are provisionally regarded by Kent (1980) as one of the oldest units in the stratigraphic column of South Africa. The true age of this group has not yet been determined. These rocks form a series of narrow, more or less parallel, discontinuous ridges approximately 60km south-west of Mafikeng running north to north-north-west. Another series of parallel, discontinuous outcrops occur approximately 50 km further west (near Stella). The rocks in this group consist mainly of schist, amphibolite, jaspilite, lava, banded ironstone and a small proportion of dolomite (Kent 1980).

Dominium Group
The Dominium Group (Kent 1980:95-101), dated at 2 800 Ma, forms the next age group of rocks in the Province. It is comprised of arenaceous sediments, conglomerates, grits, basic volcanic, tuffs, coarse pyroclastic rocks and quartzite. The rocks of this Group forms a series of hills to the west of Klerksdorp (in the vicinity the old gold-mining town of Dominium Reefs). The thickness of the group is 2 250 m in this area. A second important outcrop occurs at Ottosdal. The third outcrop (maximum thickness of 240m) occurs as a narrow band of rocks on the fringe of the younger Witwatersrand outcrops in the southern portion of the Potchefstroom district.

Witwatersrand Super Group
The economically important Witwatersrand Super group (Kent 1980) is dated at between 2 800 and 2 300 Ma, and the thickness of the Super group is in excess of 5 km. These rocks are underlying an area from the south eastern boundary of the Province as far as the western portion of the Wolmaransstad district. The rocks of the Super group are mainly comprised of quartzite, shale, and conglomerates. The gold and uranium deposits of this Super group are mainly contained within the conglomerates. The quartzite formations of this Super group form prominent ridges (with intervening valleys cut into the shale formations) fringing the granite outcrop in the south of the Potchefstroom district. A series of prominent hills strikes primarily north east from the Adamay suburb of Klerksdorp towards the area directly to the west of Ventersdorp. Smaller less prominent outcrops occur in the Wolmaransstad district.

Ventersdorp Super Group
The predominantly volcanic Ventersdorp Super group is dated at between 2 620 ± 50 to 2 300 ± 100 Ma (Kent 1980). The rocks of the Super group are composed mainly of andesitic lavas and related pyroclastics, conglomerates, tuffaceous and calcareous shale and porphyries are also constituents of this formation. The thickness of the rocks varies from approximately 1800 m in the Klerksdorp area, to more than 3 350m in the Potchefstroom district. This Super group outcrops extensively in the south eastern portions of the province, and is known to occur in all but the north eastern districts. This Super group forms undulating hilly areas in the central and eastern portions of the province, while extensive flat to slightly undulating plains predominates the landscapes towards the west.

Zoetlief Group and Bosch Kop Formation
The Zoetlief Group consists of both volcanic and sedimentary rocks, dated at between 2 470 and 2 310 Ma (Kent 1980). It occurs in a few small outcrops to the north and west of Vryburg. The Amalia Group, occurring to the north of the town, consist of a quartz porphyry and a basal tuff formation (Kent 1980:145-149), while the conglomerate and silty quartzite of the Bosch Kop Formation outcrops on the farm, and surrounding farms of Bosch Kop 306 that is located approximately 50 km to the north of Vryburg (Kent 1980).

Transvaal Sequence
The Transvaal Sequence covers extensive areas in the eastern portion of the Province (Kent 1980). The Black Reef Quartzite Formation is only about 25 m in thickness, and outcrops as narrow bands at the contact between the dolomite and the underlying older formations. A maximum age of 2 318 ±17 Ma can be deducted from volcanics underlying the Black Reef Formation in the north-west of the Province. The Chuniespoort Group consists of the Malmani Subgroup (dolomite) - in the order of 1880 m in thickness, and the Penge Formation (iron formation) 320m in thickness. The Malmani Subgroup was originally deposited in a shallow inter- to sub-tidal zone in a marine environment, and is comprised of five formations distinguished on the basis of their chert content as well as by the presence and or absence of the algal growths (stromatolites) found in the dolomite. As dolomite normally consists of calcium-magnesium carbonate, it is highly susceptible to solution by acidic water, especially by the carbonic acid found in rainwater. The dissolution of dolomite can lead to the formation of underground cavities where large volumes of groundwater can be stored.

This Group outcrops mainly in five distinct areas. The first narrow outcrop lies to the north of the Ventersdorp Super group's outcrop in the southern portion of the Potchefstroom district. This outcrop forms the flat semi-circle of ground between the undulating surface exposures of the Ventersdorp Super group and the lower most hills of the quartzite of the Pretoria Group. The second group of dolomites are exposed in a 10 -25 km wide band starting near Orkney and striking roughly north-eastward towards the Provincial border with Gauteng (to the north-east of Potchefstroom). The third (and major) outcrop of this formation strikes roughly east to west from the area between Lichtenburg and Mafikeng, and the provincial border with Gauteng (east of Schoemansville). The western surface area of this outcrop is in excess of 60km wide, while it narrows down to approximately 5 km on the Gauteng border. Another outcrop is a small isolated occurrence approximately 22 km to the south of Mafikeng. The last major outcrop is found in the northern portion of the north eastern districts of the Province. The landscapes on the dolomite are predominantly flat to undulating.

The approximately 3 000 m thick group of quartzite, conglomerates, siltstones, metamorphosed shale, slates and hornfelses (containing metamorphic minerals such as cordierite, staurolite, garnet, andalusite and sillimanite) and amygdaloidal andesitic lavas of the Pretoria Group forms the most prominent landscape features of the central and eastern portion of the Province. It must also be noted that this Group has at a later stage been intruded by diabase. These rocks outcrop between the narrow band of dolomite in the Potchefstroom district and the second dolomite outcrop, described in the previous paragraph. The second exposure of these rocks are found to the north of the major dolomite outcrop, while the third outcrop occurs on the west and northern sides of the small outcrop to the south of the fifth main northerly dolomite outcrop, where it forms a series of southwards dipping homoclinal ridges. The quartzite are mainly responsible for the hills occurring in the east and south of the province, while the other rock types described previously are mainly responsible for the formation of valleys and or level plains. Another very important landscape feature in the Province developed in this Sequence is the so-called Bankeveld (stretching from the Gauteng border to the south of Brits, up to the Botswana border in the west, and its northern extension on the edge of the northern districts of the province).

Griqualand West Sequence
The western portion of the North West Province is dominated by the rocks of the Griqualand West Sequence (similar in age than that of the Transvaal Sequence) (Kent 1980). The quartzite, shale, siltstone and lava of the Vryburg Formation (approximately 75-100 m thick) are the oldest rocks in the sequence and outcrop as to the east of Vryburg. The major exposure of this formation is, however, the approximately 5 km wide east to west trending outcrop found to the west of the town. The dolomite of the Campbell Group, consisting of the Scmidtsdrif (3 - 275 m thick) and Ghaap Plateau Formations (alternating in thickness between 900 and 1600 m) are made up of alternate intercalated layers of dolomite, shale, limestone, calcareous sandstone, siltstone and chert. The main outcrop of this Group occurs in the area to the south west of Vryburg and stretches down to the border of the Province near Kudumane. These rocks outcrop further north through the Morokweng area and up to the Molopo River where the river changes its main westward course towards the north. The only outcrop of the Asbestos Hills Banded Ironstone Formation in the Province is found to the north of Kuruman and further north past Heuningvlei as a series of isolated hills.

Bushveld Complex
The Bushveld Complex (Kent 1980) underlies the northern section of the Province to the north of the Bankeveld. The Rustenburg Layered Suite (2 095 ± 24 Ma) and the Rashoop Granophyre Suite (2 090 ± 40 Ma) occurs mainly in the western portion of the northern part of the province, while the Lebowa Granite suite (1670±30 Ma) occurs in the extreme northeast around Mabopane. The following subdivision developed for the Suite in the western portion of the Complex. The lowermost unit in the sequence is called the Kolobeng Norite. This layer is easy to distinguish in the field as it represents the first horizontal plain to the north of the quartzite of Bankeveld ranges. The Eerlyk- and Makgope Bronzitite is followed by the Groenfontein Harzburgite, which in turn is followed by the Tweelaagte Bronzitite. The platinum group minerals containing the Merensky Reef that is part of the Mathlagame Norite-Anorthosite follow the economically important unit of the Ruigtehoek Pyroxenite (containing chromite layers). The Pyramid Gabbro-Norite layer follows this unit. This unit is characterised by the series of hills visible to the north of Rustenburg and Brits. This range is of economic importance as it is quarried for excellent quality gabbro. The Bierkraal Magnetite Gabbro is an inconspicuous outcrop occurring at the top of the Layered Suite in the west. The Rashoop Granophyre Suite is composed mainly of homogenous granophyre, granophyric granite and pseudogranophyre and outcrops to the west and east of the settlement of Rashoop.

Mokolian Intrusives
Intrusives of Mokolian age in the Province are provided in Table 2.7.

Table 2.7: Mokolian Intrusives (Source: Kent 1980).

Name Nature Locality Age
Losberg Complex
Pilanesberg Complex
Pilanesberg Dykes
Rietfontein Complex
Roodekraal Complex
Schurwedraai Alkali granite
Tweerivier Carbonatite Complex
Ystervarkkop Carbonatite Complex


The Pilanesberg Complex deserves a fuller description as this complex covers an area of more than 5 000 km2. This complex consists of lavas, tuffs, agglomerates, breccias, different foyaites and syenite. These different rocks form a series of concentric hills. The mostly eroded overlying alkaline volcanic rocks and some of the other pyroclastic material overlies an intrusive base composed of foyaite and syenite. The Complex consists mainly of a succession of ring-dykes around a collapsed crown.
Karoo Sequence
The extent of Karoo sediments in the North West Province is limited to outcrops between Lichtenburg and Mafikeng as well as to some of the Dwyka tillites to the south of Vryburg. Another small outcrop occurs to the east north east of Schweizer-Reneke, as well as another small occurrence near Bloemhof (Kent 1980). Another outcrop of these rocks cover a portion of the Bushveld Complex rocks in an area to the north of Brits. Some of the older geological maps (Bray 1974) show outcrops of Karoo formations in the extreme north-western corner of the Province and outcrops of Waterberg in the area. Karoo sediments are also known within sinkholes in the dolomite in the vicinity of Stilfontein.
Kimberlites
A few diamond-bearing Kimberlite fissures occur in the North West Province, the most notable being around the Harts River 100km north-west of Kimberley.
Kalahari- and other Sediments (Tertiary to recent deposits)
Various younger sediments are shown on the geological maps of the Province. No accurate data as to the age of most of these deposits are available, although it is generally accepted that these sediments vary in age from mid-Tertiary (22,5 Ma) for Lichtenburg diamondiferous gravels), down to the Quaternary deposits (0,15 Ma) along rivers and in some cases in the redistributed sands of the Kalahari (Kent 1980) which occur over large tracts of land in the far west of the Province. Some of these sediments consist of well-sorted fine-grained alluvium, while others are made up of unsorted colluvial materials. Another group of these materials is the so-called river gravel deposits as quite a few of these deposits contain alluvial diamonds. Various studies have so far been conducted into the locality as well as composition of these gravels. Some of the better-known deposits occur at various levels on the banks of the Vaal- and Harts-River systems. A large amount of surface limestone and calcrete deposits also occur, especially in the western section of the Province. Depending on the nature of the deposit and the floor rock itself the thickness of these deposits varies between a few centimetres to depths in excess of 30 m (Marshall 1987).
Geomorphology
Various events were responsible for the formation of the landscapes. For the scope of this study it is deemed unnecessary to give a complete review of the whole of the sedimentation and tectonic history of the area. The most important events will however be highlighted in order to explain the major landscape features presently controlling the configuration of the landscapes of the Province. Certain landscape features have already been mentioned in the previous section on the geology of the Province.

The first event that had a major impact on the landscape was the so-called Vredefort event. This event probably occurred sometime just after the Ventersdorp Sequence was emplaced. The result of the event is the feature in the southern section of the Potchefstroom district consisting of a series of semi-circular ridges and rocks of the Witwatersrand, Ventersdorp and Transvaal sequences. The inner fringe rocks of the Witwatersrand Super group are overturned in places and dips at high angles towards the central core. This has resulted in the formation of a series of homoclinal and isoclinal ridges. The dip angles gradually lessens and results in cuesta formation on the hills furtherest away from the core. The formation of the Gatsrand between Potchefstroom and Carletonville may also have had its origin during this event. This landscape is unique in the sense that it preserves the roots of the ancient event. In fact it is considered to be one of the most ancient landscapes preserved anywhere in the world (De Villiers 1989).

The next major tectonic event, which had a major impact on the landscape in the province, was the emplacement of the Bushveld Sequence. This event has resulted in the formation of the Bankeveld structure. This is a structure exhibiting a series of parallel to sub-parallel ridges in the rocks of the Transvaal Sequence. This feature can be traced from the western border of the Province with Botswana in an easterly direction as far as the eastern Gauteng border. The steep northerly dipping Magaliesberg Quartzite Formation forms the so-called Magaliesberg, while the decreasing dip angles, further away from the emplaced Bushveld rocks, have resulted in the formation of the homoclinal ridges and cuestas that have formed on the other quartzite's of the Pretoria Group as well as on the Malmani dolomite. A portion of the northern section of the Bankeveld is found in the northern portion of Lehurutse, Madikwe and Mankwe, Brits and Odi districts where the Transvaal sequence dips southward underneath the rocks of the Bushveld.

It is possible that sedimentation may have taken place between the Bushveld event and the beginning of the Dwyka glaciation during the beginning of the Karoo Sequence of events. The Dwyka glaciation has had a major impact on the landscapes that are at present exposed in the Province. This event has re-shaped a major portion of the landscapes in the eastern and central parts of the Province. Large tracts of land have been bevelled into plains by this enormous continental glacier, and subsequently buried underneath the rest of the rocks of the Karoo Sequence. With the break-up of Gondwanaland between 250 and 125 Ma ago, a new cycle of denudation followed, and the old bevelled land surfaces re-exposed (and subsequently further denuded, but still retaining their major features). This resulted in the Lichtenburg and Ventersdorp plains and the valley to the south of Vryburg stretching into the Taung district and further south (see Wellington 1955).

The eastern rim of the Ghaap plateau and the step like feature bordering the western side of the area to the south of Vryburg is also the result of a broad shallow valley that was created by the glacier. Mayer (1974), postulates that the crustal movements associated with the Griqualand -Transvaal axis of uplift along a north-east to south-west Precambrian lineament has resulted in the disruption of the drainage systems of the Harts and Schoonspruit drainage and that of the Vaal River. It caused disruption of the paleo-Nossob, the paleo-Dry Harts and the paleo-Harts rivers. The disruption of these rivers, which were southward draining tributaries of the Vaal River during the tertiary times, culminated in the evolution of the present landscape features of the area to the northwest of the Vaal between Klerksdorp in the east and Barkley West (in the Northern Cape Province). Other major landscaping forces active during the Late Tertiary were the formation of the surface limestone and calcrete deposits distributed over a large percentage of the western section of the Province. The Quaternary deposits of windblown material covers a large portion of the underlyning materials in the northwest portion of the Province.

Appendix 3: Detailed description of the soil types of the North West Province.

The soil of the far west and north-west of the Province (Vryburg 1, Ganyesa and Kudumane) are arid and semi-arid brown soils not differentiated. On the map compiled by the Dept. of Agriculture these soils are described as Red-Yellow apedal soils. Van der Merwe distinguishes a small portion of soil in the north of the present Ganyesa district to which he adds a further distinguishing factor, namely, that these sandy brown soils have developed on calcareous crusts. The same patch of soil is depicted as Glenrosa and Mispah on the Agriculture map. The latter map also distinguishes another land type that has developed on hilly terrain. Soils developed on such terrains can usually be classified as lithosols (Mispah and Glenrosa). In general it can be stated that the soils of this area is mostly sandy and that it was derived from windblown material except in those patches of land were the underlying bedrock outcrops.

The other soils of the Ghaap plateau as well as the soils of the Harts River valley is described by Van der Merwe (1941) as Arid and sem-arid brown soils developed on calcareous crusts. His distribution area for these soils includes the present districts of Vryburg 2, Taung and portions of the Molopo, Vryburg 1, Ditsobotla, and Lichtenburg districts. According to the map produced by Agriculture the major portion of the area is classified as having Red-Yellow Apedal soils. Soils with a plintic catena are differentiated in the northwest and north of the Vryburg 2 district. The central, and southwestern portions of this district, and the western portion of Taung are covered by Glenrosa and Mispah soils. A portion of land in the south eastern portion of Taung is depicted as having soils with prismacutanic and pedocutanic characteristics. Soils in these categories usually have high clay content in the B-horizon.

Van der Merwe (1941) describes the soils of the rest of the Highveld and Bankeveld regions of the North West Province as "brown to reddish brown ferrigenous lateritic soils". According to the Agriculture map four main land types are distinguishable in the same area. The soils of these zones are the mainly Glenrosa, Mispah types developed in the hilly terrain and also on certain portions of the dolomite plains in the Potchefstroom-, Klerksdorp-, Ventersdorp-, Koster-, Rustenburg-, Swartruggens-, Marico-, Lichtenburg-, and Molopo districts. Soils with a politic catena are the dominant soils for the rest of the Highveld, with Red-Yellow apedal soils in the north of Taung, the whole of Christiana, most of Bloemhof, the south of Schweizer-Reneke and the western portion of Wolmaransstad and the far north-eastern portion of the Ventersdorp districts. Vertic, melanic and red structured soils are shown as important features on the floodplains of rivers and spruits in the Lichtenburg, Delaryville, Ditsobotla, Schweizer-Reneke, Vryburg and Wolmaransstad districts.

According to Van der Merwe (1941) soils similar to those developed on the Highveld region is distributed on the northern border of the Province (in the northern section of the Bankeveld) in the Lehurutse, Marico and Madikwe districts. The Agriculture map agrees in some respects with this classification although the map extends the distribution along the western border as well as through the central section of the Lehurutse district. In the steeper portions of this area as well as on the dolomite outcrops, areas where Glenrosa and Mispah soil types can dominate, is added to the general picture.

"Vertisols: Crystaline basic rocks Black Clays", "Lithosols and lithosolic soils: miscellaneous rocks" and "Fersialitic soils: not differentiated" are according to van der Merwe the main soil types that can be distinguished for the Bushveld Complex as well as the areas where the Complex's rocks are covered by younger materials. He also distinguishes a portion of Fersialitic soils developed on sandy material in an area to the west and north east of the Crocodile River in the Bafokeng-, Brits-, Odi and Moretele districts. In the classification by Agriculture similar soil patterns are deductible. The degree of differentiation is more detailed. Red-Yellow apedal soil types are described in all the districts in the area, while vertic- melanic- and red structured soils are also widely distributed. Glenrosa and Mispah soils occur primarily in portions of the Rustenburg-, Brits, Mankwe-, Odi and Moretele districts. Lithosols associated with hilly areas and with the Pilanesberg, completes the picture.

Appendix 4: Detailed description of the vegetation types of the North West Province.

The broad patterns of geology, soil types and climate are the major governing factors in the distribution of the Province's vegetation types. The following description of the vegetation patterns are based on Low & Rebelo (1998) map. However, between 50% and 60% of natural vegetation types in the North West Province have been transformed through anthropogenic activities.

Approximately 71,5% of the Province falls within the Savannah Biome, with the following major vegetation types (percentage cover shown in parentheses):



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