The Lower Karoo coal (k2-3) of the Mid-Zambezi basin, Zimbabwe: depositional analysis, coal genesis and palaeogeographic implications

摘要

Intensive lithological study and correlation of borehole records from the k2-3 coalfields or coal occurrences in the Mid-Zambezi basin led to the identification of two sedimentological types of coal: the Alluvial plain coal and the freshwater-lake shoreline coal. The Alluvial plain coal was found only at Gokwe and in the Nyamandlovu area. Its depth of more than 200 m below surface, the thinness and discontinuous nature of the seams, and the high ash content of the coal make the economic significance extremely small. In clear contrast, lithologically and economically, stand the lake shoreline coal fields at Wankie, Lubimbi, Lusulu. Lubu, Busi, and Sengwa. The pay-zone is the basal Main Seam, up to 17-m thick. The shoreline coal is either more or less massive (Wankie, Lusulu-Lubu) or is thin coal bands alternating with carbonaceous mudstone (Lubimbi, Sengwa). The clearest evidence for a lake shoreline environment comes from the lateral lithofacies change of the coal, e.g., at Wankie where it turns down-dip into sapropelic mudstone of the lake, and up-dip into terrestrial sediments of the coastal plain. The lake shoreline interpretation results finally in the delineation of a 20- to 40-km-wide coal-belt stretching from Wankie in the W to Sengwa in the E. The new model also opens up new perspectives for more coal within and between the coalfields. The study of quality and petrography of the shoreline coal supports the above depositional environment and reveals a standard maceral profile characterized by a basal vitrinite-rich coal passing upwards into inertinite-rich coal forming the major upper part of the sequence (typical Gondwana coal). The profile reflects an initial swamp phase generating a wet-forest swamp with Glossopteris trees, but this turned soon to a dry-forest swamp, with oxidation and decomposition of the vegetation, before it was finally overlain by fluviodeltaic sandstones of k4. The paludification is referred to an eustatic rise of the water-table caused by post-ice-age meltwater, but soon the water level dropped, due to the wanner climate. The local and regional controls of the peatswamp -formation were considered, as well as the autochthonous and diachronous nature of the coal. The two coal types led to a new palaeogeographic setting for the Mid-Zambezi basin which is in agreement with the new rift concept. It was more of a trough having a SW-NE trend axis which was in the centre filled by a shallow freshwater lake. The above coal-belt was formed, out of a peatswamp zone along its palace-shoreline. South of this stretched a ca. 100-km-wide shallow alluvial plain drained towards the NW by some meandering rivers, with adjacent-flood plains temporarily occupied by local swamps. The alluvial plain was bounded on the SE by crystalline highlands representing the source of clastic sediments for the basin.