Idusi Fm
Type Locality and Naming
Ruhuhu Basin (Tanzania). Named after the Idusi River gorge in the Ruhuhu Basin of southwestern Tanzania. It was originally mapped and described by Stockley (1932) as the "K1" horizon or the "Basal Conglomerates and Sandstones." It was formally established as the Idusi Formation within the modern Songea Group lithostratigraphy by Wopfner (2002).
Synonym: K1 horizon, K1 beds, Basal beds
Lithology and Thickness
Composed of a classic basal glacial and periglacial sequence. It typically begins with a massive basal diamictite (tillite) containing large, often striated, basement clasts supported in a fine silty matrix. This grades upward into fluvioglacial conglomerates, cross-bedded sandstones, and finely laminated glaciolacustrine dropstone-bearing varved shales and siltstones. Lilangu Mbr(upper) consists of black kaolinitic shales and siltstones. Lisimba Member (lower) consists of Rhythmite, Sandstone, Diamictite
Relationships and Distribution
Lower contact
Rests with a profound angular unconformity directly upon the ancient Precambrian crystalline basement rocks (the Mozambique Belt). This contact is frequently marked by glacially striated pavements.
Upper contact
Conformably to transitionally overlain by the fluvio-deltaic and peat-forming Mchuchuma Fm (K2 / Lower Coal Measures).
Regional extent
Recognized at the base of the Karoo sedimentary fill in the Ruhuhu Basin and other Karoo grabens in southwestern Tanzania (e.g., Songwe-Kiwira and Ketewaka-Mchuchuma coalfields). It is the direct regional equivalent of the Dwyka Gr in the Main Karoo Basin of South Africa, the Musipizi Conglomerate Member in Zambia, and the Monte Lilonga Fm/Fubue Fm in Mozambique.
GeoJSON
Fossils
Age
Depositional setting
Glacial, fluvioglacial, and periglacial (glacio-lacustrine) environments.
Additional Information