The geology of Uganda extends back to 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
Archean eon (4 billion - 2.5 billion years ago) 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.
Mesoarchean (3.2 - 2.8 billion years ago) The oldest rocks in Uganda are the Mesoarchean gneisses and granulites of the Uleppi Group (or ‘Complex’) together with the associated Goli charnockitic gneiss, both reckoned to have been formed around 3,080 million years ago, in the extreme northwest of the country between Nebbi and Koboko. The slightly younger Karuma Complex dated to 2,990 million years ago is found in a tract of country extending north-east from the eastern shores of Lake Albert to the Victoria Nile.
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.
Neoarchean (2.8 - 2.5 billion years ago) The Neoarchean is represented by rhyolites, porphyry, tuff and basalt in the Nyanzian System, close to the Kenya border and in southeast Uganda. Most are between 2650 - 2590 million years old.
Proterozoic eon
Paleoproterozoic (2.5 billion – 1.6 billion 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 granitized, 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 Buganda Group of rocks comprising orthoquartzites, conglomerates, metavolcanics, slates, phyllites, mica schists and metasandstones is found around the shores of Lake Victoria from Jinja westwards and across as far as the largely faulted eastern edge of the Western Rift.
Mesoproterozoic (1.6 billion - 1 billion years ago) 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.
A mix of quartzites, slates, shales, conglomerates and shales constitute the Muyage and Ruvubu groups grouped as the Kagera-Buhweju Supergoup in the region between Rakai and Mbarara, north of Busheyri and near Kamwenge. An arcuate suite of dykes were emplaced around 1,370 million years ago within the largely Neoarchaean and Palaeoproterozoic rocks from Kampala westwards. The Mityana Group of sandstones and conglomerates occur around Lake Wamala and nearby Mityana as well as forming the larger part of the Ssese Islands in Lake Victoria.