Geology of Uganda
The geology of Uganda formed beginning in the Archean and Proterozoic eons of the Precambrian, and much of the country is underlain by gneiss, argillite and other metamorphic rocks that are sometimes over 2.5 billion years old. Sedimentary rocks and new igneous and metamorphic units formed throughout the Proterozoic and the region was partially affected by the Pan-African orogeny and Snowball Earth events. Through the Mesozoic and Cenozoic, ancient basement rock has weathered into water-bearing saprolite and the region has experienced periods of volcanism and rift valley formation. The East Africa Rift gives rise to thick, more geologically recent sediment sequences and the country's numerous lakes. Uganda has extensive natural resources, particularly gold.
Stratigraphy and geologic history
The oldest rocks in Uganda date to the Archean and are more than 2.5 billion years old, forming a gneiss and granulite complex in the north and center of the country. The Watian Group rocks in the West Nile region date to the Mesoarchean, 2.9 billion years ago and metamorphosed to granulite grade on the sequence of metamorphic facies. Other rocks, similar to the Watian Group form enclaves within gneiss sequences in the Karamoja District.
These Archean rock formations tend to also include quartz diorite, acid and intermediate granulites and less commonly, basic granulites. Aruan Group rocks overly the Watian Group in the West Nile area, with biotite gneiss, hornblende gneiss, migmatite and granitic gneiss and quartzites. Karamoja Gneiss Group rocks share a very similar lithology to the Aruan Group, along with the eastern West Nile Mirian Group, although this group has experienced intense isoclinal folding.[1]:264
The Neoarchean is represented by rhyolites, porphyry, tuff and basalt in the Nyanzian System, close to the Kenya border and in southeast Uganda.[1]:264
Proterozoic (2.5 billion – 541 million years ago)
The Ruwenzori Fold Belt (also known as the Buganda-Toro System) dates to the Paleoproterozoic and covers much of western and south-central Uganda. The fold belt is mainly argillite, with thick amphibolite in the Jinja area and basal arenite in some locations. Parts of the belt are grantized, with low-grade phyllite close to Lake Victoria. Overall, the metamorphic grade tends to decrease southwards. The Ruwenzori Fold Belt includes the Igara Schist gneiss, quartzite and mica schist, as well as the sandstones, slate and phyllite of the Bwamba Pass Series and the Kilembe Series.
The Kibaran orogeny impacted the region in the Mesoproterozoic, forming the arenites, argillites, siltstones and metacalcareous rocks of Karagwe-Ankole Belt, which unconformably overlies the Buganda-Toro System in the southwest. The Madi Series in northwest Uganda may date to a similar time period, but its age remains poorly understood.
The Neoproterozoic Bunyoro Series a 160-kilometer section of central Uganda, built on a bottom unit of tillite, likely related to the Snowball Earth glacial deposits from the period. The unmetamorphosed, molasse related Singo Series and Mityana Series of the Bukoban System are common in the west-central parts of the country. Karasuk Group rocks including gneiss, amphibolite, marble, quartzite and ultramafic rocks form a 200- by 40-kilometer strip near the Kenyan border, related to the Neoproterozoic Pan-African orogeny.
Mesozoic-Cenozoic (251 million years ago – present)
During the Paleozoic, between 541 and 251 million years ago, the supercontinent Gondwana, including Uganda, joined with Euramerica to form the new supercontinent Pangaea. A large rift valley formed in southern Pangaea spanning southern Africa and southern South America and filled with sediments, depositing the Karoo Supergroup, the most extensive stratigraphic unit in Africa. Compared with neighboring countries, Uganda has comparatively few Karoo rocks. Down-faulted outliers are situated beneath Entebbe, Dagusi Island and Bugiri.
Throughout the late Mesozoic into the Cenozoic, Uganda experienced the regional effects of the East Africa Rift, which caused deep layers of sediments—often laden with fossils--to accumulate as sedimentary rocks in rift valleys. In the Miocene, volcanic activity near the Kenyan border created the mountains in the Karamoja region. Carbonatite ring complexes formed due to the regionally unique geochemistry.[1]:264
Structural geology & Tectonics
Structurally, the geology of Uganda is strongly influenced by fold belts and shear zones, as well as the East Africa Rift related crustal downwarping in the Pleistocene that formed Lake Victoria. The Aswa Shear Zone is one example of several Neoproterozoic shear zones in Precambrian rocks, running for 300 kilometers on a northwestern trend.
The East Africa Rift defines the western border of Uganda with the Democratic Republic of the Congo. The rift is occupied by Lake Albert, Lake Edward and Lake George, along with the horst block of the Ruwenzori Mountains. Modern sediment depths in the Rift Valley are believed by 1.8 to 4 kilometers. Although volcanoes are no longer active in Uganda, the country has some remnant hot springs.[1]:265
Hydrogeology
Unconsolidated and unconfined fluvial aquifers, with an average water table depth of 20 meters are common along rivers in Uganda. However, most of the country is underlain by igneous and metamorphic crystalline basement rock from the Precambrian. Water in these semi-confined units occurs five to 20 meters below ground, either in weathered saprolite or fractures in the rock.[2]
Natural resource geology
Because of long-running, intense weathering, Uganda has deposits of gold, niobium, tantalum, tin, rare earth elements and apatite. Gold has been mined from quartz vein and alluvial deposits near Busia, hosted in an Archean greenstone belt, and occurs in Paleoproterozoic and Mesoproterozoic metasediments at Buhweju and Kigezi. Exploration in the Buhweju-Mashonga area, indicates that artisanal gold mining is taking place in laterite gravels, overlying kaolin bedrock.[1]:265
References
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