Summary

Two groups of Pre-Cambrian rocks from North-West Ankole, Uganda, are described. The Igara Group consists of schists, quartzites and granitic rocks, representing a sedimentary sequence which has been metamorphosed and granitized. The group is folded isoclinally about steeply dipping, northerly trending, axial planes. In the south-west a refolding about horizontal, west-north-westerly trending axes is apparent. The Buhwezu Group is a sequence of recrystallized psammitic and pelitic rocks lying unconformably on the Igara Group. Part of the basal formations have been granitized contemporaneously with the Igara Group. The lower psammitic formations of the Buhwezu Group were deposited as deltas by swift-running rivers flowing southwards into a shallow lake. These rocks were then covered by shallow-water pelitic and psammitic sediments. The rocks are folded about horizontal, west-north-westerly trending axes, the intensity of folding increasing from north to south, and from top to bottom. Some north-easterly trending structures are also found. Two periods of orogenesis are deduced. The first folded the Igara Group about northerly trending axes, and was accompanied by low-grade regional meta-morphism. The Buhwezu Group was then deposited. The second orogenesis folded both groups about west-north-westerly trending axes and was accompanied by re-crystallization and metasomatism.