|
| UNDERLYING
GEOLOGY The metropolitan area is underlain by various geological formations. Basement Complex: The Witwatersrand Supergroup The Ventersdorp Supergroup The whole area below the Klip River System and surrounding the Lenasia area, as well as a zone west of Meadowlands, Soweto, consists of dolomite chert of the Malmani Subgroup (Transvaal Supergroup). Lenasia south is inter-spaced with shales. Alluvium deposits (which are Tertiary of Quaternary in age) are found below the 1:50 year floodline along the Klip River. Quartsite, shale and conglomerate (10-30m thick) of the Black Reef Formation (Chuniespoort Group of the Transvaal Supergroup) underlies Eldorado Park, Protea, Dobsonville and Nancefield (west and south of Soweto). The central and western parts of Lenasia consists of sandstone, shale and coal of the Ecca Group of the Karoo Supergroup. These rocks immediately overlie dolomite. Dolerite intrusions are found between Lenasia and Ennerdale. The Ennerdale area as well as east of Orange Farm, consists of andesite of the Hekpoort Formation of the Transvaal Supergroup. The area between Ennerdale and Orange farm consists of alternate layers of shale and quartsite (Buttrick & Stapelberg, 1994). The state of the underlying geology of informal settlements in the SMLC, and some possible impacts thereof, are as follows (Rudolph, 1997):
|
SOIL TYPES, PROPERTIES AND AGRICULTURAL POTENTIAL The ultramafic rocks weather to fertile, deep red soils. The Halfway House granites weather to a gray, sandy soil containing angular quarts pebbles. The lower quartsite of the Hospital Hill Subgroup, forms prominent ridges. Residual soils, which are deep and compressible develop on the West Rand Group, while the Central Rand Group areas all have shallow excavatibility and instability. Active clays form from weathering of the Klipriviersberg Group. Shallow excavatibility could be expected in areas underlain by the Black Reef Formation. Soils from the Ecca group may have collapsible properties. Soils associated with the Hekpoort Formation are active clays. Dolerite intrusion lead to active and compressible clays (Buttrick & Stapelberg, 1994). The largest part of central to southern areas of Johannesburg, south of Roodepoort, surrounding and south of the Johannesburg CBD, Soweto, Lenasia, Ennerdale and Orange Farm, has a moderate agricultural potential. The northern areas, especially the areas adjacent to the northern boundary has a very high potential for agricultural. Other high potential areas include areas around the Klip River System in the south, the Anchorvill area (southwest of Lenasia), and the Wheilers Farm area (east of Ennerdale). Low agricultural potential can be seen in the west from Roodekrans, through Kloofendal towards Northcliff, and eastwards from Emmarentia to Bruma and Kensington. Other areas of low agricultural potential include the whole southeastern area of South Hills, Turffontein, Naturena, Glenvista and Bassonia, as well as Zakariyya Park (southeast of Lenasia) and the Lawley and Hiltonia areas (west of Ennerdale). CONSTRUCTION MATERIAL RESOURCES Various quarries are being exploited for various construction materials (Buttrick & Stapelberg, 1994). Brick-making clays include shales of the Hospital Hill Subgroup, Witwatersrand Supergroup (westwards from Northcliff), Johannesburg Subgroup (Roodepoort to Bedfordview), Ventersdorp lavas (Bedfordview area) and Karoo Supergroup (parts of Lenasia). Natural, fine building sand (0.5-2mm) is found from weathering of the Halfway House granites in the north. Manufactured sand is also mechanically created from these granites. Stone aggregates are rock crushed to specific sizes (37.5 – 75mm) after mining. Potential sources include the Ventersdorp lavas, Halfway House granites, Karoo Supergroup dolerites, and concrete stones from the gold mining rock waste dumps of the Witwatersrand Supergroup quartsites (Buttrick & Stapelberg, 1994). Irregular plains are associated with the Klip River Valley and tributaries like the Diepkloof Spruit (northeast Soweto) and the Bloubos Spruit (east of Kibler Park). The Klipriviersberg consists of low to high hills, while the whole Klip River System in the south has floodplains and marshes. The central and western regions, between the Klipriviersberg and the Witwatersrand, are associated with low relief, rolling plains, with the Klip River draining in a southerly direction. The Witwatersrand hills in the central sector of the area have a moderate to high relief, and strike in an east-west direction. The area north of the Witwatersrand have a fairly low relief, with slope less than 6º, with the Jukskei and Klein Jukskei River draining in a northerly direction. The Gatsrand is situated on the western boundary of the study area, and is approximately 1 751m above sea level. Various dams can be seen in the area, especially in the Klip River and its tributaries. The mine dumps, which occur in an east-west band across the central region of Greater Johannesburg, all show up on the map as angular shapes consisting of gentle slopes surrounded by moderate slopes. The hills of the Witwatersrand, running through central Johannesburg, form a large east-west band of unstable slopes (commonly >15º). To the south the slopes of the Klipriviersberg are areas of instability. All the hills and slopes in the area strike from east to west. The upper slopes of the Gatsrand, on the western boundary of the area, have slopes of greater than 15º. Smaller patches of unstable slopes are found in the north of the area, including Diepsloot, Northriding and Kleve Hill Park. South of Lenasia and south of Ennerdale some unstable slopes also occur. Dolomites can be seen in a band stretching from the Doornkop Agricultural Holdings west of Soweto, southeast including Lenasia, Protea South, and eastwards joining the southeastern boundary of the area. Another patch lies northwest of Soweto (next to Meadowlands) and a smaller area south of the Hiltonoa Agricultural Holdings (west of Ennerdale). The geology determining these dolomite formations are as follows: - The Black Reef Formation underlies the dolomites of the Chuniespoort Group in the
Eldorado Park, Protea, Dobsonville and Nancefield areas; GEOTECHNICAL PROPERTIES (CONSTRAINTS) Almost all areas in Greater Johannesburg consist of collapsible soils, including the unstable slopes previously indicated. Areas running parallel to the north and south of the Witwatersrand Ridges, mainly consist of compressible soils. Compressible soils are also found to the south of the Klipriviersberg, through central Soweto towards the northwest. Soils of the northern suburbs of Johannesburg, as well as the area surrounding the compressible soils of the mining zone, all show excavation problems. Dolomites pose geotechnical problems around the Lenasia area, south and northwest of Soweto. Active clays occur around the Klip and Jukskei River Systems, as well as along the tributaries in the Ennerdale vicinity. In between the active clays in the south, some excavation problems exist. ANALYSIS OF ENGINEERING GEOTECHNICAL DEVELOPMENT POTENTIAL All the above natural components were combined to produce an anticipated geotechnical development potential of the area. Areas of low geotechnical development potential (areas regarded as least favourable to unfavourable for development, due to severe geotechnical problems) include areas of high agricultural potential, landforms such as crests, sand banks, drainage features, excavations, mine dumps and landfills, high risk dolomite zones, and areas including geotechnical constraints such as shallow excavatibility, steep/unstable slopes (> 15º) and drainage channels (Buttrick & Stapelberg, 1994). Such areas include an east west belt stretching from Roodekrans in the west of Roodepoort to Linksfield, northeast of Johannesburg. The area has excavation problems due to the quartsite rocks, many unstable slopes, some dams and floodplains. Other low development potential areas are found in an east-west band along the mining belt of Greater Johannesburg, where compressible soils, excavation problems and unstable slopes inhibit development. Mining activities further cause erosion, toxic soil pollution and alteration of the land. Geotechnical constraints in the Mondeor, Glenvista and Bassonia area to the southeast of Johannesburg include marshes, excavation problems, active clays, unstable slopes and collapsible soils. The whole zone along the Klip River System in the south, the south of Soweto and most of Lenasia, has a very low development potential. Unstable dolomites, marshes and flood plains, unstable alluvium and chert deposits, active clays and collapsible soils characterize the zone. Informal settlements and poverty in these areas lead to more land and vegetation being cleared for housing, heating and cooking, which leaves soil more prone to erosion. The south and southeastern areas of Greater Johannesburg, including Soweto, Kibler Park and surroundings, areas to the east of and south Lenasia, and the whole Ennerdale area, all have moderate development potential. Geotechnical constraints include poor drainage, shallow groundwater levels, active clays, moderate slopes (6º-15º) and some dolomite. This makes these areas prone to erosion and preferably suitable for formal residential, industrial and commercial developments (Buttrick & Stapelberg, 1994). Areas with similar characteristics are found north of the Johannesburg CBD including Bruma, Dewetshof, parts of Houghton, Linksfield north, and westwards towards Honeydew, Weltevreden Park, Northcliff, parts of Rand Park Ridge and the Zandspruit informal settlement. Areas of high development potential can mostly be seen in the northern parts of Greater Johannesburg including Randburg and Sandton and most of the northern suburbs. The central Johannesburg industrial/commercial/ residential zone, from Tulisa Park and Heriotdale in the east, westwards through City Deep, Industria and Lea Glen, Meadowlands in north Soweto, to Davidsonville and Witpoortjie in the west, also has a high development potential. These areas have low agricultural potential, gentle concave or convex slopes (< 6º), plains and collapsible or compressible soils. Old, stable granites also dominate the north. The areas are suitable for all development types including semi-formal, formal residential, commercial and industrial. South Africa, and thus the Greater Johannesburg area, lies in a semidesert/desert latitude zone of almost no weathering and soil formation, due to the drop in precipitation and vegetation cover, and higher temperatures and evaporation rates. This leads to the lack of soil development in the areas (Selby, 1985). Increased population growth, urbanisation, poverty, development and harmful human activities will only enhance soil degradation .Between 80 and 85% of the surface area of South Africa is devoted to agriculture, with some 1.5 million people dependent on this production (Fuggle & Rabie, 1992). Agricultural Holdings in Greater Johannesburg are most prominent in the northwest and north from Poortview, Aanwins and Honedew AH, through Ruimsig AH towards Chartwell and Diepsloot AH. Smaller AH are Modderfontein and Linbro Park (east of Alexandra), Doornkop AH (west of Soweto), Rispark and Patlynn AH (east of Kibler Park), small areas along the Klip River (north and south of Lenasia), Unaville and Alithea AH (east of Ennerdale) and the Hiltonia AH (northwest of Ennerdale). The mining belt stretches from the southwest of Roodepoort eastwards to the south of Heriotdale. Environmental degradation occurs here in the form of leachate from slimes dams and erosion from excavations and exposed mine dumps. Undermining is currently active in the west in portions of Doornkop, Durban Roodepoort Deep and areas around Main Reef Road. No active mines exist in the far south, northeast and north of Greater Johannesburg, and mining impacts are therefor limited to the mining reef in central, western and southern areas. Some old mine dumps are being reworked, and the natural decomposition of iron pyrites of these tailings lead to acidic leachate into waterbodies. INFORMAL SETTLEMENTS AND POVERTY URBANISATION, POPULATION GROWTH AND DEVELOPMENT REFERENCES Buttrick, D. & F. Stapelberg 1994: A report to the Central Witwatersrand Regional Services Council on an Engineering Geological Study of the Central Witwatersrand Regional Services Council area. The Council for Geoscience. Fuggle, R.F. & M.A. Rabie 1992: Environmental Management in South Africa. Juta: Cape Town. Rudolph, J. 1997: Status Quo: Informal Settlements in the Southern Metropolitan Local Council. Urban Dynamics: Vanderbijlpark. Selby, M.J. 1985: Earth's Changing Surface. Oxford: New York. |
