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Lithostratigraphy of Neoproterozoic syn-rift sedimentary megabreccia from Mwambashi, Copperbelt of Zambia, and correlation with olistostrome succession from Mufulira

Geology Department, University of Botswana, Private Bag 0022, Gaborone, Botswana email: wendorff{at}mopipi.ub.bw; mpw{at}it.bw

The Neoproterozoic sedimentary succession of the Katanga Supergroup at Mwambashi in the Zambian Copperbelt includes a 105 m thick interval of fragmental rocks, interpreted in the past as tectonic friction breccia. Re-examination of these rocks shows that their textural and structural features are indicative of sedimentary genesis. This coarse clastic sedimentary complex rests with an angular unconformity and a pronounced hiatus on the basal, siliciclastic unit of the Roan Group. The succession evolves from fluvial or marginal marine red-bed sandstones and fine-grained conglomerates to subaqueous coarse-grained debris flow conglomerates and includes large exotic blocks, up to 30 m thick, of gabbro, dolomite and quartzite. Both, the exotic pebbles and large blocks are derived from the Roan Group rocks. The upper boundary of this conglomeratic complex is transitional to the succeeding fine-grained siliciclastic and dolomitic strata of the Mwashya Subgroup. Based on the stratigraphic position in the Katanga Supergroup succession, composition, and the textural and structural features the conglomeratic complex at Mwambashi is here correlated with the recently identified new lithostratigraphic unit of coarse clastic strata defined at Mufulira, to the east of Mwambashi, as the Mufulira Formation. The Mufulira Formation marks a pronounced uplift in the southern part of the Roan rift basin, which terminated deposition of the Roan Group, and opening of a younger Mwashya rift, which expanded towards the north.

Lateral variations of the maximum clast-size, structural features of conglomerate beds and the associated finer-grained facies imply that the Mwambashi section occupies a position proximal relative to the source area uplifted in the southwest, whereas the succession intersected at Mufulira was deposited in a more distal and deeper setting within the Mwashya rift basin. An occurrence of oxidised Fe-rich, reddish coloured siliciclastic sediments within generally grey-coloured- successions enables correlation between Mwambashi and Mufulira. The correlation suggests that at the southern margin of the Mwashya rift the basin infill evolved diachronously, the marine flooding and associated retrogradation of subaqueous coarse-grained facies progressing towards the southwest.