Geology of Lesotho

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The geology of Lesotho is built on ancient crystalline basement rock up to 3.6 billion years old, belonging to the Kaapvaal Craton, a section of stable primordial crust. Most of the rocks in the country are sedimentary or volcanic units, belonging to the Karoo Supergroup. The country is notable for large fossil deposits and intense erosion due to high rainfall and a rare case of southern African glaciation during the last ice age. Lesotho has extensive diamonds and other natural resources and has the highest concentration of kimberlite pipes anywhere in the world. [1]

Contents

Stratigraphy & Geologic History

Lesotho shares many commonalities with the geology of South Africa, with Kaapvaal Craton crystalline basement rock that formed 3.6 to 2.5 billion years ago. Geologists have gleaned insights into Lesotho's deep past through extrapolation from South Africa, xenoliths extracted from kimberlite pipes and other data from drilling. Above the ancient basement rocks, almost all of the rocks in Lesotho are part of the Karoo Supergroup, the most widespread stratigraphic unit in southern Africa.

The Karoo Supergroup began to form 510 million years ago in a rift valley in the south of the supercontinent Gondwana, spanning into what is now southern South America. The Burgersdorp Formation is the top unit of the Beaufort Group and the oldest Karoo-related unit in Lesotho, with red and maroon sandstones interspersed with thin coal seams. The Stormberg Group is divided between the Molteno Formation, Elliot Formation and Clarens Formation. The Molteno Formation is 15 to 300 meters thick and records a Late Triassic lake bed environment, with abundant plant and insect fossils. There is an unconformity between the Molteno and the overlying 70 to 250 meter thick sandstones, mudstones and siltstones of the Elliot Formation, which contains silicified wood and dinosaur fossils. The 15 to 250 meter thick Clarens Formation lies atop the Elliot Formation and has sandstones, siltstones and thin bands of chert. [2]

The Early Jurassic, amygdale enriched volcanic rocks of the Lesotho Formation (part of the Drakensberg Group) cap the sedimentary sequence and form Lesotho's mountain tops, with a 1.6 kilometer thick layer. For the most part, the Lesotho Formation is basalt that cooled from a tholeiitic magma series. The numerous layers of ropy plateau basalt are intruded by 70 volcanic vents and over 1000 dikes and sills. Kimberlite pipes may have formed during eruptions in the Cretaceous marking an end to volcanic activity.

In the Cenozoic Lesotho experienced high rainfall and was one of the few places in southern Africa glaciated during the Pleistocene. As a result, extreme erosion formed cobble beds with sand and clay layers in lower elevation valleys, with material up to 20 meters thick. Donga is a local term for the steep-sided ravines common in Lesotho that formed in this material. [3]

Structural geology and tectonics

Lesotho forms a syncline within the larger southern African Karoo Basin and the sediments of the Karoo Supergroup were influenced by deformation at three different times before, during and after the formation of the Drakensberg mountains. The Hellspoort fault is the only large fault in the country, running northeast–southwest with a vertical displacement of 300 meters near Mafeteng, although there are other smaller faults. [4]

Hydrogeology

More than 50% of Lesotho's territory is underlain by fractured igneous rock aquifers, such as the low permeability basalt of the Lesotho Formation. Dolerite-dike related aquifers have high water productivity in fractures that formed in the surrounding rock due to the high temperature of the intrusion. The sandstones of the Clarens Formation, Elliot Formation and Burgersdorp Formation are small or low quality aquifers, but the Molteno Formation is regarded as the best aquifer in Lesotho with high yield and frequent springs. Steep terrain, rain and snow melt mean that Lesotho has a high risk of landslides. [5]

Natural resource geology

Lesotho has extensive diamond deposits in both kimberlites and alluvial gravel. The country has the highest concentration of kimberlite bodies in the world—a total of more than 400, including 343 dikes, 39 pipes and 23 blows. Northern Lesotho has an average of one kimberlite body for every 10 square kilometers. In spite of the large deposits, very few sites are mined and research is ongoing into alluvial deposits. The Karoo Supergroup hosts low-grade uranium and likely coal and coal-bed methane, although the Lower Karoo which hosts the coal seams does not outcrop in Lesotho. The country has quarrying of Quaternary clay deposits, Drakensberg Group fine-grained basalts and sandstones in the Elliot, Molteno and Clarens Formations. [4]

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Geology of the Democratic Republic of the Congo

The geology of the Democratic Republic of the Congo is extremely old, on the order of several billion years for many rocks. The country spans the Congo Craton: a stable section of ancient continental crust, deformed and influenced by several different mountain building orogeny events, sedimentation, volcanism and the geologically recent effects of the East Africa Rift System in the east. The country's complicated tectonic past have yielded large deposits of gold, diamonds, coltan and other valuable minerals.

Geology of Ivory Coast

The geology of Ivory Coast is almost entirely extremely ancient metamorphic and igneous crystalline basement rock between 2.1 and more than 3.5 billion years old, comprising part of the stable continental crust of the West African Craton. Near the surface, these ancient rocks have weathered into sediments and soils 20 to 45 meters thick on average, which holds much of Ivory Coast's groundwater. More recent sedimentary rocks are found along the coast. The country has extensive mineral resources such as gold, diamonds, nickel and bauxite as well as offshore oil and gas.

The geology of Malawi formed on extremely ancient crystalline basement rock, which was metamorphosed and intruded by igneous rocks during several orogeny mountain building events in the past one billion years. The rocks of the Karoo Supergroup and newer sedimentary units deposited across much of Malawi in the last 251 million years, in connection with a large rift basin on the supercontinent Gondwana and the more recent rifting that has created the East African Rift, which holds Lake Malawi. The country has extensive mineral reserves, many of them poorly understand or not exploited, including coal, vermiculite, rare earth elements and bauxite.

The geology of Mozambique is primarily extremely old Precambrian metamorphic and igneous crystalline basement rock, formed in the Archean and Proterozoic, in some cases more than two billion years ago. Mozambique contains greenstone belts and spans the Zimbabwe Craton, a section of ancient stable crust. The region was impacted by major tectonic events, such as the mountain building Irumide orogeny, Pan-African orogeny and the Snowball Earth glaciation. Large basins that formed in the last half-billion years have filled with extensive continental and marine sedimentary rocks, including rocks of the extensive Karoo Supergroup which exist across Southern Africa. In some cases these units are capped by volcanic rocks. As a result of its complex and ancient geology, Mozambique has deposits of iron, coal, gold, mineral sands, bauxite, copper and other natural resources.

Geology of Namibia

The geology of Namibia encompasses rocks of Paleoproterozoic, Mesoproterozoic and Neoproterozoic and Paleozoic to Cenozoic age. About 46% of the countryʼs surface are bedrock exposure, while the remainder is covered by the young overburden sediments of the Kalahari and Namib deserts.

Geology of Sierra Leone

The geology of Sierra Leone is primarily very ancient Precambrian Archean and Proterozoic crystalline igneous and metamorphic basement rock, in many cases more than 2.5 billion years old. Throughout Earth history, Sierra Leone was impacted by major tectonic and climatic events, such as the Leonean, Liberian and Pan-African orogeny mountain building events, the Neoproterozoic Snowball Earth and millions of years of weathering, which has produced thick layers of regolith across much of the country's surface.

Geology of Tanzania

The geology of Tanzania began to form in the Precambrian, in the Archean and Proterozoic eons, in some cases more than 2.5 billion years ago. Igneous and metamorphic crystalline basement rock forms the Archean Tanzania Craton, which is surrounded by the Proterozoic Ubendian belt, Mozambique Belt and Karagwe-Ankole Belt. The region experienced downwarping of the crust during the Paleozoic and Mesozoic, as the massive Karoo Supergroup deposited. Within the past 100 million years, Tanzania has experienced marine sedimentary rock deposition along the coast and rift formation inland, which has produced large rift lakes. Tanzania has extensive, but poorly explored and exploited natural resources, including coal, gold, diamonds, graphite and clays.

Geology of Zambia

The geological history of Zambia begins in the Proterozoic eon of the Precambrian. The igneous and metamorphic basement rocks tend to be highly metamorphosed and may have formed earlier in the Archean, but heat and pressure has destroyed evidence of earlier conditions. Major sedimentary and metamorphic groups formed in the mid-Proterozoic, followed by a series of glaciations in the Neoproterozoic and much of the Paleozoic which deposited glacial conglomerate as well as other sediments to form the Katanga Supergroup and rift-related Karoo Supergroup. Basalt eruptions blanketed the Karoo Supergroup in the Mesozoic and Zambia shifted to coal and sandstone formation. Geologically recent windblown sands from the Kalahari Desert and alluvial deposits near rivers play an important role in the modern surficial geology of Zambia. The country has extensive natural resources, particularly copper, but also cobalt, emeralds, other gemstones, uranium and coal.

The geology of Eswatini formed beginning 3.6 billion years ago, in the Archean Eon of the Precambrian. Eswatini is the only country entirely underlain by the Kaapvaal Craton, one of the oldest pieces of stable continental crust and the only craton regarded as "pristine" by geologists, other than the Yilgarn Craton in Australia. As such, the country has very ancient granite, gneiss and in some cases sedimentary rocks from the Archean into the Proterozoic, overlain by sedimentary rocks and igneous rocks formed during the last 541 million years of the Phanerozoic as part of the Karoo Supergroup. Intensive weathering has created thick zones of saprolite and heavily weathered soils.

Geology of Sudan

The geology of Sudan formed primarily in the Precambrian, as igneous and metamorphic crystalline basement rock. Ancient terranes and inliers were intruded with granites, granitoids as well as volcanic rocks. Units of all types were deformed, reactivated, intruded and metamorphosed during the Proterozoic Pan-African orogeny. Dramatic sheet flow erosion prevented almost any sedimentary rocks from forming during the Paleozoic and Mesozoic. From the Mesozoic into the Cenozoic the formation of the Red Sea depression and complex faulting led to massive sediment deposition in some locations and regional volcanism. Sudan has petroleum, chromite, salt, gold, limestone and other natural resources.

The geology of Nigeria formed beginning in the Archean and Proterozoic eons of the Precambrian. The country forms the Nigerian Province and more than half of its surface is igneous and metamorphic crystalline basement rock from the Precambrian. Between 2.9 billion and 500 million years ago, Nigeria was affected by three major orogeny mountain-building events and related igneous intrusions. Following the Pan-African orogeny, in the Cambrian at the time that multi-cellular life proliferated, Nigeria began to experience regional sedimentation and witnessed new igneous intrusions. By the Cretaceous period of the late Mesozoic, massive sedimentation was underway in different basins, due to a large marine transgression. By the Eocene, in the Cenozoic, the region returned to terrestrial conditions.

Drakensberg Group Jurassic geological group in Lesotho and South Africa

The Drakensberg Group is a geological group named after the Drakensberg mountain range where in its uppermost sections the rocks are found. The Drakensberg Group lies over most of Lesotho and localities in the Eastern Cape, KwaZulu-Natal, and Free State provinces of South Africa. It forms part of the greater Karoo Igneous Province, which occurs over an extensive area of southern Africa.

References

  1. ↑ Schlüter, Thomas (2008). Geological Atlas of Africa. Springer. pp. 140–142.
  2. ↑ Schlüter 2008, p. 140.
  3. ↑ Schlüter 2008, pp. 140–142.
  4. 1 2 Schlüter 2008, p. 142.
  5. ↑ "Hydrogeology of Lesotho". British Geological Survey.
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