The Idusi Formation forms the basal depositional unit of Karoo basins of southwestern Tanzania. It is defined as the rock unit limited by the unconformable contact with underlying Precambrian metamorphics and the base of the overlying, fluviatile Mpera Sandstone Member of the Mchuchuma Formation. The Idusi Formation is subdivided into the Lisimba Member, comprising diamictites, lutites with dropstones, slump masses and laminites, and the overlying Lilangu Member, consisting of black, pyritic and kerogenous lutites. The type section is at Idusi Gorge, situated 6 km east of Lake Nyasa on 10°17′50″ S. The thickness of the formation at the type section is 240 m, with the maximum observed thickness being 715 m. Plant fossils and palynological assemblages indicate an Asselian age, probably extending down into the Late Carboniferous. The basal diamictites contain striated and facetted clasts, which identify them as glacial deposits. They are mainly wash-out and slurried tillites and also, less frequently, lodgment tillites. These are overlain by proximal and distal deposits of proglacial lakes, which were laid down during the retreat of the glaciers. Laminates of the upper Lisimba Member demonstrate progressively stronger seasonal control. Further amelioration of the climate, with substantially increased biological production both on land and in the water, is demonstrated by the black lutites of the Lilangu Member. They represent euxinic lake sediments formed during the final deglaciation phase. Deposition of the overlying, coal-bearing Mchuchuma Formation occurred under a temperate climate. The duration of the Late Palaeozoic glaciation is estimated at about 20 to 25 Ma. The last quarter of this time span was characterized by climatic amelioration. Global warming was accompanied by an eustatic rise in sea level and a marked decrease in atmospheric CO2.