1 General
Area: 236,040 km²
Population: 23,318,000 (July 2000 estimate)
2 Summary of Geology
气质美女图片More than two thirds of Uganda are underlain by Archean and Proterozoic rocks. Apart from the vast Archean Gneissic-Granulitic Complex in the north there are at least three major Proterozoic belts expod in the country: the Paleoproterozoic Buganda-Toro System, the Mesoproterozoic Karagwe-Ankolean System and the Neoproterozoic Mozambique Belt. Tabular Neoproterozoic diments are also widespread. Tertiary to Recent diments fi lled parts of the downfaulted Western Rift. Tertiary carbonatites and Cenozoic volcanics are related to rift activities and occur along the eastern and western borders of the country.
3 Stratigraphy
About 60% of rocks outcropping in Uganda - especially in the northern and central regions - are high grade metamorphics, for which the name Gneissic-Granulitic-Complex has been propod (Schlüter, 1
997). The oldest unit embedded into the Gneissic-Granulitic Complex in the West Nile region compris largely granulite facies grade rocks, for which the name Watian Group is ud. The Watian is apparently of Mesoarchean age (2,910Ma). Similarly, some of the rocks of this grade appear to form the earliest unit in the Karamoja District, where they form enclaves in the gneissic quences. Rock types include acid and intermediate granulites and charnockites, quartz diorites, banded, porphyroblastic and quartz-feldspathic types. Basic granulites are less common and pure calcareous rocks are unknown. Retrogressive metamorphism is extensive. Folding is usually relatively simple. Rocks of the Aruan Group tectonically and stratigraphically succeed the Watian in the West Nile region. The group has a monotonous lithology, uniformity of metamorphic grade, and a structural style typifi ed by tight folding on steep axial planes with generally northerly directed axes. Rock types include biolite gneiss, banded, migmatitic and granite gneiss with lesr quantities of hornblende gneiss, amphibolite, quartzites and very few ultrabasic pods. In Karamoja the Central Karamoja Gneiss Group has very similar lithological, metamorphic and structural characteristics. Rocks belonging to the Mirian Group occur in the eastern West Nile region, where they tectonically post-date the Aruan rocks. They are characterized by a similar lithology to the Aruan rocks, but are isoclinally and intenly folded on recumbent axes trending approximately northeast and overturned to the northwest. The rocks are typically fl aggy and of epidote-amphibolite facies grade. Rocks ascri
bed to the Neoarchean Nyanzian System in Kenya also occur in southeast Uganda. They compri mainly rhyolites, porphyries, tuffs and basalts. This unit is not appreciably regionally metamorphod, but is well folded. The Buganda-Toro System (or Ruwenzori Fold Belt) is of Paleoproterozoic age and occupies much of the south-central and western parts of the country. Argillites predominate, but basal or near basal arenites are an important feature. Locally, as in the Jinja area, occur thick amphibolites, which are probably derived from basaltic material. Large tracts of the system are granitized; on the other hand low-grade phyllites also occur, particularly towards Lake Victoria in the southeast. Folding is fairly tight on predominantly ENE axes in the east, but varies in the west. Axial planes are steep and there is a tendency for this folding to decrea in intensity with lowering of metamorphic grade southwards. Included in the Buganda-Toro System are the Igara Schists compod mainly of quartzites, mica schists and gneiss, the Bwamba Pass Series of the Ruwenzori Mountains, which is made up of grits, sandstones, slates and phyllites, and the Kilembe Series of Toro. The rocks of the Mesoproterozoic Karagwe-Ankolean System (or Kibaran Belt) rest unconformably on the Buganda-Toro System in the southwest. Argillites predominate, but arenites and silty rocks are also regularly distributed as thin bands throughout. At the ba of the system metacalcareous rocks generally occur. Metamorphism is less highly developed than in the Buganda-Toro System and many parts are entirely unmetamorphod. Fairly
open folding in two main directions, i. e. approximately north-westerly and north-easterly on steep axial planes is typical, but the fold pattern is more complex in the lower parts, where earlier isoclinal structures are encountered. The age of the low-grade metamorphic rocks of the Madi Series in the northwest of Uganda has not yet been established. Tabular Neoproterozoic rocks are known from various sites: The Bunyoro Series occupies a narrow tract of country over 160km in an E NE-WSW direction in central Uganda. The rocks are predominantly argillaceous, but pebbly beds are locally abundant. Near the ba of this formation a tillite has been recognized. The Singo and Mityana Series of the Bukoban System occur in the
Uganda
southern and western central part of the country and are both arenaceous and unmetamorphod, probably reprenting molas-type deposits. An asmblage of gneiss, amphibolites, marbles, quartzites and ultramafi c rocks occupies a strip of about 200 by 40km along the Uganda/Kenya border in the Karamoja area and is called Karasuk Group. This unit is apparently part of the Neoproterozoic high-grade metamorphic Mozambique Belt. Intrusive rocks of various Precambrian ages occur in many parts of Uganda. Three small exposures of Karoo-aged strata are also recorded from Uganda. The occur in apparently down-faulted outliers near Bugiri, on Dagusi Island and underlying part of Entebbe, probably indicating an Ecca age.
Miocene volcanics outcrop in veral areas of eastern Uganda, clo to the Kenyan border and are denoted topographically by prominent mountains of the southern Karamoja region. Carbonatitic ring
complexes, possibly formed since Cretaceous times and reprenting the eroded remnants of volcanoes of a similar geological suite, occur in veral eastern locations. Other Cenozoic rocks are either of dimentary or volcanic origin and are found in the western Rift Valley adjoining Rwanda and the Democratic Republic of Congo. The diments are often fossiliferous and sometimes exceeding 4,000m in thickness. The volcanics have been ejected from vents, but are now dormant. There are some hot springs in this region.竞争策略
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4 Tectonics
The major structural controls of Uganda include orogenic fold belts and shear zones within the Precambrian rocks, and the process of formation of the rift valley and later volcanic centres, followed by crustal warping during the Pleistocene that probably resulted in the formation of Lake Victoria. Shear zones occur in the Precambrian rocks in veral areas of the country. The Aswa Shear Zone is the most extensive,
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following a northwesterly trend for over 300km through northern Uganda and into southern Sudan. Other shear zones, probably all of Neoproterozoic age, run through the Karamoja and Acholi regions and the West Nile District. The Rift Valley extends along the western border with the Democratic Rep
ublic of Congo and encompass Lake Albert, Lake George, Lake E dward and the Ruwenzori Mountains horst block. Sediment thickness of 1,800 to 4,000m are estimated to lie within the Rift Valley.
5 Economic Geology
Extensive portions of Uganda have been expod to prolonged and inten weathering. The process have led to some materials being concentrated into ores, most notably gold, tin, pyrochlore (niobium, tantalum and rare earths) and apatite. Mineral occurrences include gold in Busia in the southeast, hosted by an Archean greenstone belt, whilst in the southwest at Buhweju and Kigezi, gold occurs in Paleo- to Mesoproterozoic metadiments. Gold mining has taken place at Busia in both alluvial and quartz vein occurrences. Recent exploration in the Buhweju-Mashonga area indicates that much of the gold being extracted by artisans derives from lateritic gravel lying under on kaolinid bedrock. Other potential gold areas include Mubende and Karamoja. The most prospective region for ba metals is in the Kilembe area, where volcano-dimentary rocks of Paleoproterozoic age outcrop for a distance of over 90km. Other copper ores have been recorded from Bobong and Kaabong in the Karamoja region. Chromite occurs in ultramafi c rocks at Nakiloro, also in Karamoja. An ultramafi c rock asmblage at Moroto has the potential for nickel, chromium, c死亡地带
opper and platinum-group mineralisation. Lead, zinc and gold have been found in the Buganda-Toro System at Kitaka, within the Buhweju gold district. The Muko iron ore deposit occurs in Mesoproterozoic Kibaran rocks, whilst magnetite occurrences in the east, such as at Sukulu, are found in Tertiary carbonatites. Most of the cassiterite, tungsten, columbo-tantalite, beryl and lithium mineralisation is hosted by pegmatites and granites of the Buganda-Toro and Karagwe-Ankolean Systems. Amongst industrial minerals, phosphates are found in the east in Tertiary carbonatites that also host limestone, titanium and rare earths elements. There is also limestone at Hima (Fig. 232) in the southwest, in a condary deposit derived from calcareous tuffs and hot springs. Other industrial minerals include clay, kaolin, feldspar, diatomite, silica sand and various types of dimension stone.
Fig. 232 Lacustrine Neogene lake deposits and quarry at Hima, western Uganda.
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6 Geohazards杜甫月夜忆舍弟
Various geoenvironmental hazards have been reported from Uganda: Earthquakes along the Wester
n Rift have been noted (Hampton, 1995), sometimes triggering landslides (Muwanga et al., 2001). Groundwater pollution caud by mining has for instance occurred in the Kilembe area (Muwanga, 1997).
7 Geosites
A preliminary inventory of potential geosites of Uganda was prented by Schlüter et al. (2001) and includes the following six locations: Nyero Rock Shelter and Paintings, Napak V olcano, Bukwa Fossil Site, Nkondo-Kaiso Fossil Site, Kigi-Nyabusosi Fossil Site and Nyakasura Caves with Speleothems.
8 References
Hampton, C. (1995): The Fort Portal E arthquake 5th February 1994, W Uganda.- Berliner Geo-wisnschaftliche Abh. A175, 147-158; Berlin.MacDonald, R. (1966): Uganda Geology, Scale 1:1,500,000.- Department of Geological Survey Mines; Entebbe.
禾苗简笔画Muwanga, A. (1997): E nvironmental impacts of copper mining at Kilembe, Uganda: A geochemical investigation of heavy metal pollution of drainage waters, stream diments and soils in the Kilembe
Valley in relation to mine waste disposal.- Braunschweiger Geowisnschaftliche Arbeiten, 1-140; Braunschweig.
Muwanga, A., Schumann, A. & Biryabarema, M. (2001): Landslides in Uganda, Documentation of a Natural Hazard.- Documenta Naturae 136, 111-115; Munich.
Schlüter, T. (1997): Geology of East Africa.- I-XII, 1-484; Gebrüder Borntraeger, Berlin, Stuttgart.Schlüter, T., Kibunjia, M. & Kohring, R. (2001): Geological Heritage in East Africa - its Protection and Conrvation.- Documenta Naturae 136, 39-50; Munich.
Fig. 233 A church hit by the Fort Portal earthquake of 5 February 1994 in western Uganda. Fig. 234 Mt Stanley and Lake Bujuka photographed by the Luigi Amedeo di Savoia expedition in 1906.Fig. 235 Alexandra Peak en from Margherita Peak in the Ruwenzori Mountains.
Fig. 236 The Murchison/Kabalega Falls descending into Lake Albert, western Uganda.
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