Petrographic and diagenetic signatures of climatic change in peri- and postglacial Karoo Sediments of SW Tanzania

Abstract

Abstract 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 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 pore-water and the decay of organic matter. Such early diagenetic conditions are frequently met in humid continental environments.