Genetic aspects of the Katangan megabreccias: Neoproterozoic of Central Africa

The structure of the external, fold-and-thrust region of the Neoproterozoic Katangan orogenic belt of Central Africa is characterised by thrust sheets , gravitationally transported towards the foreland region in the north, and associated with prominent megabreccias. The thrust sheets contain a more o r less complete succession of the Katangan sedimentary rocks. The megabrecc ias form tabular and wedge-shaped, mostly chaotic bodies, underlying the al lochthonous sheets and composed of subangular to rounded fragments of Katan gan provenance. Fragments range from millimetres to many kilometres in size and are set in an abundant clay matrix with dolomitic cement. It is common ly accepted in the literature that the Katangan megabreccias are of entirel y tectonic origin; friction acting beneath the advancing thrust sheets is t hought to be the main process responsible for fragmentation and abrasion of clasts. However, the analysis of stratigraphical, textural and structural features presented in this paper implies a sedimentary genesis of the Katan gan megabreccias. It is shown that they resulted from subaqueous re-sedimen tation by mass gravity flows of clastic materials eroded from the emergent tops and fronts of the advancing thrust sheets, in front of which olistostr ome wedges were deposited. The deposition of each individual olistostrome b ody stopped when the approaching thrust block overrode it due to the accele ration of gravity gliding. These processes, and their products, define the Katangan foreland region. The olistostrome sedimentary model has significan t implications for our understanding of the structural and tectonostratigra phic evolution of this sector of the Katangan belt. (C) 2000 Elsevier Scien ce Limited. All rights reserved.