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The Evolution of Kalahari Basin

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The Kalahari Basin in the continent of Africa lies in the south and covers a vast lowland area in six different countries, it is a semi-arid, sandy savanna. Soil over the Kalahari basin is produced by the breakdown of rock due to physical or chemical changes, also known as weathering. Most of the basin is formed on the Karoo group but, now this basin comprises of marls, gravels, Kalahari sands, cretes, diamonds and pan deposits because of physical weathering, such as rifting and chemical weathering, such as silicification.

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To understand the evolution of Kalahari basin, important tectonic events that effected its formation should be understood. Over 510 Million years ago, rift is developed at south of Gondwana land mass (now south of Africa) forming sag in central Africa and mountains Cape fold mountains in south. Cape super group sediment is accumulated at foothills of these mountains while karoo-kalahari group sedimentation was deposited into the sag forming Kalahari and karoo basins.

Over the period these rifts continued to disrupt drainage, create uplifts and down warps. The basal unit of the Kalahari Group consists of gravels deposited by such disrupted drainage systems. As down-warp of the basin continued, more gravels were deposited and finer sediments are carried away by rivers.

Karoo deposition started with the Dwyka tillite around 330 Ma This dwyka group is the lowest karoo deposit. Because of inland sea formed by melting of glaciers, peat is formed which turned into coal, which is now being mined in Highveld of South Africa. The lower karoo in Namibia and south-west bostwana contains shelly fossils showing the marine origin. Deltaic and aeolian sediments continued to fill the Kalahari-Karoo basin but, the Karoo cycle was terminated by widespread volcanism dated by Rundle at between 200 to 160 Ma.

After volcanism, tectonic activities were reasonably subsided but, weathering of rocks and depositions of sands took priority. Along the stream, sandstone was deposited. Most of the Kalahari sands contain silica with organic origin. Sand grains at depth are cemented by carbonate and silica, which can be explained by the capillary rise of ground water containing CaCO3 and MgCO3 and SiO2. In the south east, transported polygenetic sands are mixed with grains derived from bed rock (regolith) forming sandstone which upon secondary silicification formed silcretes. Quartzite formed from such silica-cemented sandstone are commonly found near pans and old drainage patterns. In fact, different types of cretes (concretionary deposits like calcretes, silcretes, ferrocretes) are common in Kalahari region. Uplift in the Pliocene possibly elevated Kalahari Group and Karoo Supergroup sedimentary rocks above the basin floor and exposed many of them to erosion. The eroded sand was washed into the basin and reworked into dunes during drier periods.

Terrace gravels infilling Ancient River valleys incised into the pre-Kalahari surface in southern Kalahari are also discovered. These gravels form important aquifers in North Cape. Poorly sorted gravels and Red or pink marls of lime and magnesia bearing are also identified.

Top most surface, most exposed sand of this basin is Kalahari sands. They are fine-grained and well-natured. Reddish-brown and buffish-white aeolian sands. They form the mantle of large portions of the Kalahari basin. Other common deposits are clay and pan deposits. During downwarps, rivers are tilted as inland water bodies and clayey and sandy pan deposits are formed as the lakes dried up.

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