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The Segmented Zambezi Sedimentary System from Source to Sink 1. Sand Petrology and Heavy Minerals

The Zambezi River rises at the center of southern Africa, flows across the low-relief Kalahari Plateau, meets Karoo basalt, plunges into Victoria Falls, follows along Karoo rifts, and pierces through Precambrian basement to eventually deliver its load onto the Mozambican passive margin. The river is subdivided into segments with different geological and geomorphological character, a subdivision fixed by man’s construction of large reservoirs and testified by sharp changes in sediment composition. Pure quartzose sand recycled from Kalahari desert dunes in the uppermost tract is next progressively enriched in basaltic rock fragments and clinopyroxene. Sediment load is renewed first downstream Lake Kariba and next downstream Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzo-feldspathic in the lower tract, where most sediment is supplied by high-grade basements rejuvenated by the southward propagation of the East African rift. Feldspar abundance in Lower Zambezi sand has no equivalent among big rivers on Earth and far exceeds that in sediments of the northern delta, shelf, and slope, revealing that provenance signals from the upper reaches have ceased to be transmitted across the routing system after closure of the big dams. This high-resolution petrologic study of Zambezi sand allows us to critically reconsider several dogmas, such as the supposed increase of mineralogical “maturity” during long-distance fluvial transport, and forges a key to unlock the rich information stored in sedimentary archives, with the ultimate goal to reconstruct the evolution of African landscapes since the late Mesozoic.


Eduardo Garzanti1, Guido Pastore1, Alberto Resentini1, Giovanni Vezzoli1, Pieter Vermeesch2, Lindani Ncube3, Helena Johanna Van Niekerk3, Gwenael Jouet4, Massimo Dall'Asta5
1Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano-Bicocca, 20126 Milano, Italy; 2London Geochronology Centre, Department of Earth Sciences, University College London, London, WC1E 6BT, UK; 3Department of Environmental Sciences, University of South Africa, Florida, South Africa; 4Unité de Recherche Geosciences Marines, Ifremer, CS 10070, 29280 Plouzané, France; 5TOTAL E&P, CSTJF, Avenue Larribau - 64018 Pau Cedex Pau, France
GeoKarlsruhe 2021
Southern Africa