The basal Idusi Formation of the Karoo succession in SW Tanzania represents deposits of the final Late Palaeozoic deglaciation event. It reflects the change from a cold to a temperate, more humid climate. The lower unit of the Idusi Formation (Lisimba Member) consists of periglacial deposits with glaciolacustrine diamictites, sandstones, dropstone-bearing mudstones and varvites as well as fluvial and deltaic conglomerates and sandstones. The upper unit (Lilangu Member) is dominated by organic-rich siltstones and sandstones which were accumulated in an eutrophic postglacial lake. The petrography of the Idusi Formation reveals an arkosic composition indicating provenance from cratonic basement rocks. The Lisimba Member is characterized by low mineralogical maturity with a high proportion of garnet. Fine-grained matrix (particles < 30 μm) within the glaciolacustrine diamictites and mudstones derived from suspended glaciogenic rock flour of the meltwaters. Climatic amelioration during the postglacial stage is demonstrated by high organic carbon and increased mineralogical maturity in the Lilangu Member. Higher proportions of quartz and stable heavy minerals reflect enhanced chemical weathering in the source areas due to raised humidity and temperature. Lack of fine-grained rock flour matrix within the siltstones and sandstones documents offset of glacial grinding in the source areas. In the Lisimba Member authigenic formation of chlorite, corrensite, and laumontite at a moderate stage of burial was due to alkaline pore fluid conditions with high ionic strength caused by restricted percolation in the matrix-rich sediments and their low mineralogical maturity. The postglacial sedimentary rocks are characterized by early diagenetic kaolinite, siderite, pyrite, and barite. This paragenesis formed under the influence of acidic, meteoric- derived porewater and the decay of organic matter. Such early diagenetic conditions are frequently met in humid continental environments.