zpravodajství životního prostředí již od roku 1999

Krátce ke geologii Lesotha

Krátce ke geologii Lesotha

Lesothské království má rozlohu cca 30 344 km 2.Veškeré území leží v nadmořské výšce vyšší než 1000 m n. m. (nejníže položené místo 1 388 m n. m. - tzv. "lowlands" a nejvyšší položené místo je současně vrcholem nejvyšší hory jižní Afriky Thabana-Ntlenyana 3 482 m n. m.)

Africa was part of the larger landmass called Gondwanaland during the Paleozoic and Early Mesozoic Eras, 570-240 million years. Lesotho is situated within the Karoo Basin where sediments of the original mountain building process or cratonization are subhorizontal, the basement rock of Lesotho, strata of continental origin.

The earliest rocks, the Burgersdorf Formation, Early Triassic Period of the Mesozoic Era, 240 million years, are exposed in the riverbeds of the lowlands. These are the polycolored mudstones, siltstones, and sandstones and may include reptile tracks and bones of reptiles in certain locations in the country. The bench forming Molteno Formation are the buff colored mudstones, siltstones, and sandstones that outcrop along the drainage systems. The receding hillslopes, the Elliot Formation, above the benches consist of red, polycolored mucstones, siltstones, and sandstones, the "Red Beds" which include an abundance of reptile remains, in particular dinosaurs. The cliff forming Clarens Formation is the youngest of the sedimentary deposits and is prominent as the tan colored cliffs and plateaus of the lowlands.

The breakup of Gondwanaland began during the late Triassic, early Jurassic, about 144 million years. This tectonic process initiated a series of magmatic episodes. The volcanic activity created a lava plateau of great extent with little relief. The weathering and erosion of this plateau formed the foothills and mountains of the Drakensberg. The lava is basic in composition, rich in magnesium and iron, and rose through long fissures in the earth. It was deposited contemporaneous with the cliff forming Clarens sandstone in certain locations in Lesotho. The mafic dolerite dykes, vertical intrusions of magma, and sills, horizontal intrusions, that crisscross the country were part of the magmatic, tectonic episodes of the Early Jurassic Period.

The last major event was the emplacement of the diamond bearing kimberlite from the mantle, during the Upper Cretaceous Period, less than 65 million years. The greatest concentration of the kimberlites occurs in the Butha Buthe - Mokhotlong Kimberlite belt and dies out as it approaches the border west of Butha Buthe. All deposition from the Late Tertiary, about 65 million years to the present is called "donga sediments" the sediment formed by actual erosion of the existing formations.

The structure of Lesotho has been relatively stable, horizontal, with the exception of the gentle folds of small domes and basins associated with the emplacement of the dolerite dykes and sills.

ZDROJ: www.friendsoflesotho.org

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