The geology and coal potential of the Karoo supergroup and younger rocks in Zimbabwe

Abstract

Abstract by author:

Three rift basins, the Cabora Bassa, Mana Pools and Mid-Zambezi, occur along the Zambezi Rift Valley in Mozambique, Zambia and Zimbabwe. The studies completed established that the 050 [symbol] trending Mid-Zambezi Basin, together with the contiguous Mana Pools and Passarge Basins, lie along the Mid Zambezi Rift Zone, while the 095 [symbol] trending Cabora Bassa Basin forms the Lower Zambezi Rift Zone. The Chewore Accommodation Zone separates the Mana Pools Basin from the Cabora Bassa Basin, and the Chizadra and Kamativi Inliers divide the Mid-Zambezi half-graben into the Gwembe, Matabolo and Mlibizi Sub-basins

The Middle Jurassic to Early Cretaceous isotopic age dates reported on lavas collected from the Batoka Basalt Formation in the Mid-Zambezi Basin and the Dande Formation in the Cabora Bassa Basin, together with the palaeontological evidence available, indicate that these sequences were deposited during the Early Jurassic to Early Cretaceous. Based on this data and similar findings in Namibia and South Africa the end of Karoo Supergroup deposition has been extended to include Early Cretaceous strata. Following accepted guidelines to stratigraphic procedure the Lower and Upper sequences of the old Karoo System have been assigned Group ranking

Combined the two lithostratigraphic units form the Karoo Supergroup. Two principal fining upward cycles, attributed to subsidence and uplift possibly partly related to isostatic rebound following the Dwyka Glaciation, were identified in the Lower Karoo Group, while in the Upper Karoo Group four major intercontinental episodes of rifting, all of which are linked to regional tectonic events, are recognised. In addition, the Late Cretaceous isotopic age datings obtained on samples of lava in the Maria Pools Basin, which occur intercalated in a red-beds sequence visually different to the Dande Formation, together with the presence of the unconformity noted within Jurassic / Cretaceous sediments in the Cabora Bassa Basin, provides support for the deposition of the previously unrecognised Hurungwe Formation

In the Lower Karoo Group it was concluded that the existing threefold sub-division of the Madumabisa Mudstone Formation was difficult to apply throughout the Mid-Zambezi Basin. Consequently, the sequence has been re-correlated into two lithostratigraphic units, with the basal carbonaceous coal-bearing strata constituting the Lower Madumabisa Mudstone Member and the overlying, frequently carbonate-rich beds the Upper Madumabisa Mudstone Member. Additionally, the three basal units of the Upper Karoo Group in the Mid-Zambezi Basin, the Escarpment Grit, Ripple-marked Flagstone and Fine Red Marly Sandstone sequences, which were deposited in response to the first major episode of rift tectonism, have been downgraded in status from formations to members of the newly created Chete Formation. Inadequate correlation across the international border has resulted in the lithostratigraphic equivalent of the Lower Wankie Sandstone, Coal Measure, Upper Wankie Sandstone and Lower Member of the Madumabisa Mudstone Formations in the Mid-Zambezi Basin in Zimbabwe being combined to form the Gwembe Coal Formation in the Gwembe Sub-basin in Zambia. The involvement of geoscientists from both countries is required to establish the cross-border correlation of these beds and the equivalent of the Chete, Pebbly Arkose and Forest Sandstone Formations in the Upper Karoo Group

Peat formation, as supported by the general abundance of inertinite in the coal developed which indicates repeated episodes of oxidation, is interpreted as having occurred in alluvial floodplain environments. The basal section of the Main Seam at Wankie in the Mlibizi Sub-basin in Zimbabwe is generally richer in vitrinite than the overlying coals. These relatively low ash coals are interpreted as having formed under relatively stable anaerobic conditions within wetlands in which the peat, on desiccation following the lowering of the mildly acidic paludal groundwater level, was subject to periodic oxidisation. Such fusinitisation, as illustrated by the greater proportion of interlaminated dull to bright coal, resulted in the formation of coals generally rich in inertinite and poor in vitrinite. The rank of the coal seams in the Coal Measure Formation in the Mid-Zambezi Basin increases in a general north-westerly direction, from Lignitic at the Bari Coal Locality in the Matabolo Subbasin to Bituminous Medium to High Volatile in the vicinity of Hwange in the Mlibizi Sub-basin

The studies completed establish that potentially economic coal development is extensively confined to the Coal Measure Formation and equivalent strata in the Karoo Supergroup. The Entuba, Lubimi, Lubu, Lusulu, Sengwa, Wankie and Western Areas coal localities in the Mid Zambezi Basin were judged to possess the greatest economic coal potential