Karoo main pool

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Karoo sediment (dense sandstone) with San petroglyphs in the grasslands of the Amathole Mountains , South Africa .
Stratigraphic structure of the main Karoo basin

The Karoo main basin (also known as the Karoo system in older literature ) forms a significant part of the geological structure in southern Africa and consists mainly of sediments . Its landscape forms are very different today. The Karoo main pool is a very large structural unit within the Karoo supergroup .

Definition

The word " Karoo " (originally kurú ) comes from the language of the Khoi Khoi and means "dry". The same term also applies to the national park and the surrounding landscape.

Derived from the South African Karoo landscape, the term has found its way into the geosciences . It describes sediment sequences that were first explored in the South African Karoo landscape. Geologists from Rhodes University played a key role in this in the 19th and 20th centuries . Spectacular fossil finds near Grahamstown , beginning with the work of Andrew Geddes Bain , drew worldwide attention to this region.
In 1867, British geologist and paleontologist Thomas Rupert Jones published an article entitled Synopsis of the Karoo Beds , which appeared in the Journal of the Geological Society of London . This essay is one of the earliest surveys of this area. A well-known fossil collector in the Karoo was James Kitching .

expansion

The Karoo main basin extends far beyond the Karoo landscape and covers an area of ​​around 1.56 million square kilometers in southern Africa. These are primarily the territories of South Africa , Lesotho and Swaziland . Further extensions of the system extend to the border with Mozambique and the Limpopo River in northern South Africa. From Botswana as well as from eastern and central Namibia , individual parts of neighboring Karoo structures spread to the area of ​​South Africa.

Its maximum length in South Africa is 1300 kilometers, and its greatest width it reaches between the Kimberley and East London with about 590 kilometers. The Karoo main basin is extraordinarily diverse in terms of landforms. There are flat plains, hilly landscapes, medium mountainous areas with pronounced altitudes and high mountain structures. Rivers form shallow meandering valleys or deeply cut canyons .

For the overall understanding, it is important to point out that the Karoo sediments already extend extensively from the African equatorial region in a southerly direction and that individual sequences of this geological unit can also be recognized north of the equator in Gabon and Sudan . In the continental context, the entirety of these sedimentary deposits bears the name Karoo Supergroup ( Karoo Supergroup ).
The following explanations are limited to the geographical area of ​​southern Africa, i.e. to the states of South Africa, Lesotho and Swaziland, the so-called Karoo main basin, and also to Namibia and Botswana.

Development, geological structure

The Karoo supergroup is mainly composed of sediment sequences that formed in the Carboniferous , Permian and Triassic . The Karoo period ends in the Mesozoic deposits of the Jura . At this time, extensive and powerful lava outlets were once again formed , which shape the face of the Karoo main basin in South Africa. Overall, the sedimentation processes lasted well over 100 million years and, in individual cases, created a layer structure with a thickness of over 10 kilometers.

At its outer edges, the Karoo main basin is bounded several times by older rocks. Examples of this are the Grünstein Complex in the Barberton region and the Bushveld Complex northwest of Johannesburg .

The formation of the Karoo main basin, as well as the Karoo supergroup, is based on mutually acting tectonic and climatic processes. In the Upper Devonian and into the Upper Carboniferous , the southern continent of Gondwana joined the northern continent of Laurussia and the Asian continent to form the last supercontinent, Pangea . Along the edges of what was then the Panthalassa and Tethys oceans off the coast of Gondwana, subduction , alluvial deposits and deposits as well as mountain formation created the basis for the later Karoo structures, for example the Cape Supergroup between Cape Town and Port Elizabeth .

The sedimentation processes in the Karoo supergroup began in the subsequent uppermost Carboniferous, i.e. about 300 million years ago, when there was strong inland freezing on the Pangea continent , which produced considerable debris deposits (Dwyka group). In addition to tectonic influences, climatic fluctuations from cold and semi-dry to warm and hot periods with alternating precipitation phases followed. During this period, there were large inland lakes on the continent, which left extensive sediments in their basins. The climatic amplitudes had a direct effect on the extent of the deposits in Pangea, and there was temporary contact with the open ocean.

Leakage of igneous rock in the Amathole Mountains (Eastern Cape)

The breaking up of Pangea and with it the former continent of Gondwana led to strong tectonic activities with accompanying volcanism at the end of the Jurassic and the beginning of the Cretaceous Period. In its course it created extensive penetrations of the Karoo sediments with basaltic masses in the area of ​​today's Eastern Cape Province , primarily as dolerite . In this way, dome-shaped magma outflows , horizontal intrusions between individual sediment layers and intersecting outlet crevices ( dykes ) emerged, as can be seen well in the Amathole Mountains . In the Drakensberg the basalt masses of the Karoo main basin reach their greatest extent and form its youngest lithostratigraphic unit. The entirety of the basalt masses is classified in the group of Magmatic Greater Provinces because of their enormous extent and geologically relatively rapid formation .

The strata of the main Karoo Basin in South Africa have a very slight slope, the dipping of the stratification is directed to the northwest, north and east. At its edge in the southwest and south, the layer structure is graduated and folded . There, they will contact the folding zone of the Cape fold belt ( Cape Fold Belt ) in contact.

With the exploration of the Karoo main basin, it was divided into the following groups . The group names are derived from the places and regions that played an important role in their discovery or geological exploration. The order of the following individual representation goes from young to old.

The Maletsunyane waterfall in the Drakensberg basalts

Drakensberg Group (Drakensberg Group)

This youngest and thus final part of the Karoo supergroup is strongly characterized by basalt rocks and has only a few, thin layers of sandstone. This magmatic event only lasted a few million years and therefore represents only a relatively short period of time in the history of the main Karoo Basin. The Drakensberg basalts have been dated to an age of around 180 million years and therefore chronostratigraphically fall into the Jurassic period.

The lava flows of this epoch are among the greatest volcanic events in the history of the earth. The continuation of these basalts can be found today in Patagonia and the Falkland Islands as well as in Tasmania and parts of Antarctica due to the continental drift . Since the lava layers are significantly more resistant to erosion than the older sedimentary rocks that underlie them, they now form the peaks of the Drakensberg.

In older geoscientific literature, the Drakensberg basalts are still regarded as a subgroup of the Stormberg group. According to more recent views, they represent an independent lithostratigraphic unit.

Mountain group east of the Sani Pass with sediment sequences from the Stormberg group

Stormberg Group (Stormberg Group)

This group is dominated by sandstones and is also one of the youngest sections in the Karoo main basin. It got its name from the Stormberg massif south of the city of Aliwal North . Their formation extends from the Middle Triassic to the Jura (230–183 million years). It occurs mainly in the foothills of Lesotho and the South African province of KwaZulu-Natal .
In the Elliot Formation, 12 meter long dinosaur skeletons ( Melanorosaurus ) were found. The name is derived from the small town of Elliot . The Molteno and Clarens formation also belong to the Stormberg Group. Molteno is a village in the Stormberg massif and Clarens is a settlement on the northern edge of the Drakensberg.

Beaufort Group (Beaufort Group)

Plain with sediments of the Upper Permian (Beaufort group)

The first description of the group goes back to the British geologist Thomas Rupert Jones (* 1819 in Cheapside, † 1911 in Chesham), who examined the sediment layers at Beaufort West around 1867 . Their origin lies between the late Permian and the Middle Triassic (255–237 million years).

Typical rocks of this group are sandstones and pelite (English mudstone ). In the landscape, their layer structure can often be clearly observed, and in this way they shape the relief of the respective regions. The dolerite intrusions and crests in the Amathole Mountains between Alice , King William's Town and Cathcart are particularly striking . Another distinctive form of landscape in the Beaufort group are the wide-open plains with only slight undulations.

The sediment sequences of the Beaufort group are particularly susceptible to weathering. Under these circumstances they break up easily and form only thin soils of sands and silts . Intensive land use in rural areas intensifies this development in a particular way. As a result of these processes, so-called gully formation occurs in numerous areas . By soil erosion valuable ecosystems and agricultural land destroyed. In addition, the soil washed away by surface water pollutes the rivers with large amounts of mineral suspended matter that can render dams and other water management systems inoperable.

Important fossil reptile species found in this group are Cistecephalus ( Dicynodontia ), Cynognathus , Eodicynodon , Lystrosaurus , Pristerognathus , Procolophon , Tapinocephalus and Tropidostoma .

Ecca Group (Ecca Group)

The Ecca group is the second oldest unit in the Karoo supergroup. It got its name from the Ecca Pass near Grahamstown , where its type profile is located. The period of their deposition falls mainly in the lower Permian (approx. 290–260 million years). Its outcrop lines the entire Karoo core area with the sediments of the Beaufort and Stormberg groups. Along the southern edge of the Karoo Basin, the Ecca sediments are in direct contact with the adjacent folding zone of the Cape region ( Cape Fold Belt ).

The most important rocks are bluish to greenish clay stones and fine-grained conglomerates . Sandstones are occasionally turned on. South of Aberdeen , the Ecca strata are more sandy and siltstones and claystones are more subordinate. Lenticular limestones are also turned on there . In this southern zone, the sandstones are hard, dark, blotchy in color, medium to fine-grained and contain feldspars . In the north of the Karoo main basin, in the highveld , coal seams can be found in the middle layers (there: Vryheid formation ) of the Ecca group.

In this group, plant fossils occur more frequently ( Gangamopteris , Glossopteris , Phyllotheca ).

Dwyka Group (Dwyka Group)

The Dwyka group is the oldest section of the Karoo main basin and formed in the upper Karbon (approx. 300-290 million years). It occurs mainly in the northwest, west and south as a coherent part. There are also narrow zones in KwaZulu-Natal, island-shaped units east of Pretoria and in the eastern part of Namibia and the South African Kalahari . Their name is derived from the Dwyka River or the settlement Dwyka in the southern Karoo. To the south of the scattered settlement, the canyon cuts through these sediments several times.

The most striking rock the Dwyka group is the Dwyka- Tillit (also called Dwykakonglomerat), a dense blue-gray granular mudstone with attachment and smaller debris of various origin rocks. It emerged from the ice age rubble during the permocarbon inland glaciation on the Gondwana continent. In addition to the tillites, there are siltstones and quartzites .

The important fossils found here include fish (Palaeoniscus), crustaceans (Pygocephalus, Anthrapalaemon) and plants (Lepidodendron). Echinoderms and marine mollusks were found in Namibia . Silicified tree trunks are found only very rarely , which were torn away by large masses of water when the glacier melted and deposited elsewhere.

Usable raw materials

Bishops Cathedral in Grahamstown
Sandstone building in Aliwal North
Church building in Cradock from Karoo sandstone

The Karoo main basin offers relatively few recoverable resources. In particular, it lacks significant ore deposits.

coal

South Africa is the sixth largest world producer of coal and has different qualities of coal. Lignite and hard coal are available. Anthracite is also present in small quantities. Coal deposits are extensive in the region east of Johannesburg (Mpumalanga Province), accounting for 83 percent of South Africa's coal reserves. The coal is mined in their northern sectors. There are other deposits in the provinces of Free State, Limpopo, KwaZulu-Natal and in the Eastern Cape. However, they are only partially subsidized industrially.

The discovery of the coal deposits by European immigrants is dated to the beginning of the 18th century and was near Franschhoek , but mining is only proven from 1864 near Molteno . The local population is said to have obtained a small amount of coal even before the settlement by Europeans.

Natural stone

The natural stones from the Karoo main basin, most of which are extracted for building and decorative purposes, are sandstones with a variety of rock formation. They are particularly architectural features in the cities of Aliwal North , Queenstown , Cradock , Graaff-Reinet and Grahamstown .
Intrusive rocks, such as the dolerite in the Eastern Cape, are only used to a minor extent as rubble stones in rural house construction, for garden design or in road construction (heavy masonry). Their high strength makes processing difficult.

The internationally known sandstone Naboomspruit comes from an isolated sediment sequence of the Clarens Formation (Stormberg Group) in the Northern Province . Also worth mentioning are numerous sandstone quarries in the western parts of Lesotho, which supply inexpensive masonry blocks for the inland and South African market.

On the northern edge of the system, west and east of Pretoria, igneous rocks such as gabbros and granites are extracted . The world-famous commercial varieties Belfast Black and Nero Impala as well as African Red come from these mining locations .

Individual evidence

  1. ^ T. Rupert Jones: Synopsis of the Karoo Beds . In: Quarterly Journal of the Geological Society of London, XXIII. Pp. 142-144. London, 1867
  2. ↑ List of Mesozoic and Cenozoic basalt magmatic events , Richard E. Ernst and Ian H. Campbell, site of Large Igneous Provinces Commission - International Association of Volcanology and Chemistry of the Earth's Interior
  3. ^ FW North: Colonial Mining Engineers Report on the Coalfield of the Stormbergen. Parliamentary Report of the Cape of Good Hope, G47., 1878
  4. ^ Nick Norman, Gavin Whitefield: Geological Journeys . Cape Town (Struik Publishers) 2006, p. 153 ISBN 1-77007-062-1
  5. ↑ Jasmina Stanimirovic: Geological controls on no. 4 seam roof conditions at New Denmark Colliery, Highveld Coal Field, Karoo Basin, South Africa. ( Memento of the original from February 21, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. Chapter 2: Stratigraphy. Master's thesis, University of Johannesburg, 2002, pp. 18–27 @1@ 2Template: Webachiv / IABot / ujdigispace.uj.ac.za
  6. ^ Energy profile of Southern Africa. at www.eoearth.org
  7. ^ AW Rogers, AL Hall, PA Wagner, SH Haughton: The Union of South Africa. Handbook of Regional Geology. VII. Vol. Section 7a, Heidelberg 1929
  8. ↑ Map of the South African coal deposits  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / www.geoscience.org.za  
  9. ^ Ernst Klimm / Karl-Günther Schneider / Bernd Weise: The southern Africa . Scientific Country Customers; Vol. 17. Darmstadt (Wiss. Buchgesellschaft) 1980, p. 139 ISBN 3-534-04132-1
  10. ↑ Overview map of South African natural stone deposits, unfortunately incomplete in the case of sandstone  ( page no longer available , search in web archives )  Info: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.@1@ 2Template: Dead Link / www.geoscience.org.za  

literature

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