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Karoo National Park

Geology

The Karoo Supergroup of Permian age consists of the Dwyka Formation, Ecca Group and Beaufort Group.

The Beaufort group overlies the Ecca group and consists of alternating mudstone (red in places) and sandstone. It sub-divides into the lower Abrahamskraal Formation (1 500m-2 000m thick) and an upper Teekloof formation (±1400m thick) with the boumdary arbitrarily at the base of the so-called “Poortjie Sandstone”.

This formation consists of mudstone (red in places), sandstone and thin greenish cherty beds. The sandstones represent river channel deposits and the mudstones, floodplain deposits. The cherty beds were originally ashfall tuffs, although the presence micro-cross-lamination suggests subsequent transportation by water onto the floodplains. Jurassic-age dolerite extensively intrudes the Beaufort Group as dykes and sheets. Inclined sheets form crescentshaped or concentric intrusions that are clearly visible on the Middle- and Upper Plateau.

The sills range from a few metres to over 100m in thickness. The dolerite causes a metamorphosis effect on the adjacent host rocks. Mudstone altered to hornfells and the sandstones develop a quartzitic appearance. Deposits of Quaternary age include river terrace gravel, calcrete, alluvium and debris. Terrace gravel consists of fairly well-rounded cobbles and boulders composed largely of dolerite. The terrace gravel shows clearly along the banks of the Gamka River where sharp profiles occur.

These gravels are partly calcrete cemented and occur on terrace remnants that now lie from a few metres to a maximum of 30m above the general land surface. Calcrete occurs directly on bedrock and has also formed extensive deposits within some larger areas of alluvium, attaining a maximum thickness of a few metres. The alluvium embraces both alluvial slopes (sheet wash) and alluvial valley (channel-related) deposits, with the former predominating in the Lammertjiesleegte and the latter dominating in the Stolshoek areas of Karoo NP.

Vegetation types are closely linked to soil type, soil depth, rockiness, slope and aspect. Finer soil types with smaller grains (such as clay) generally retain moisture better than coarse-grained soils. In soils with very high clay content water may be retained so well that it is unavailable for absorption by plant roots. Clay also forms a compacted surface more easily, causing water to run off rather than penetrate deeply into the soil. Deep soil, if it is too sandy does not retain water as well as rocky soils. Vegetation growing on rocky soils is therefore more resilient to the effects of drought and grazing. Deep Karoo soils are very easily eroded by wind and water if denuded of a healthy vegetation layer, leading to even more degraded veld with poor fodder production for herbivores.

Soils

The dominant soil-forming processes have been rock weathering, the formation of orthic topsoil horizons and, commonly, clay alleviation, giving rise typically to lithucutanic horizons. Soil forms that are typical of these processes are Glenrosa and Mispah. Any other soil form can however also be found in these land types. Oakleaf soil forms, deep or shallow, developed by rock weathering also occur in upland sites.

The steep slopes, middle plateau and Puttersvlei (upper plateau) areas of Karoo NP, excluding the northern most corner, fall into the Ib land type. Surface rock with underlying soil or rock covers sixty to eighty percent of these areas. The parent material of the slopes consists of mudstone, siltstone and sandstone with some dolerite intrusions, and typically Mispah or Glenrosa soil forms. Dolerite covers most of the middle plateau, with an influence of mudstone, siltstone and sandstone closer to the upper slopes.

Fertile soils occur on this flat plateau with little erosion save where the deep red soils gradually erode from a natural basin. Dolerite rocks cover most of the Puttersvlei section of land type Ib, with the underlying sandstone appearing in terraces, descending in a northerly direction. The northernmost corner of the upper plateau occurs in land type Db. Prismacutanic and/or pedocutanic diagnostic horizons characteristically dominate this land type. Non-red B horizon, duplex soils cover more than half the land area.


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