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

Abstract

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. Reflecting its polyphase evolution, the river is subdivided into segments with different geological and geomorphological character, a subdivision finally fixed by man's construction of large reservoirs and faithfully 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 of Lake Kariba and next downstream of Lake Cahora Bassa, documenting a stepwise decrease in quartz and durable heavy minerals. Composition becomes quartzofeldspathic 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 accurately reconstruct the evolution of this mighty river flowing across changing African landscapes since the late Mesozoic. 3"A river or a drainage basin might best be considered to have a heritage rather than an origin.It is like an organic form, the product of a continuous evolutionary line through time."

The Segmented Zambezi Sedimentary System from Source to Sink: 2. Geochemistry, Clay Minerals, and Detrital Geochronology

“…This study integrates the petrographic and heavy-mineral data illustrated and discussed in the companion paper (Garzanti et al 2021a) with new data on elemental geochemistry, Nd-isotope geochemistry, clay mineralogy, and detrital-zircon geochronology from the same sample set. The two articles in combination provide a multi-proxy characterization of sediment composition in the diverse tracts of the large Zambezi catchment from the Zambian headwaters to the Mozambican coast.…”

“…This section briefly summarizes petrographic and heavy-mineral data (provided in full in Appendix Tables A6 and A7, and illustrated in detail in the companion paper Garzanti et al 2021a) and presents new data on clay mineralogy (Table 1; Fig. 3), elemental geochemistry for mud and sand samples (Table 2; Fig.…”

The Zambezi deep-sea fan: mineralogical, REE, Zr/Hf, Nd-isotope, and zircon-age variability in feldspar-rich passive-margin turbidites

“…7) of the Zambezi sedimentary system from coastal Mozambique to the deep-sea fan, including detritus generated in SW Madagascar. The composition of sediment carried by the Zambezi River and its tributaries, together with the geological and geomorphological characteristics of the vast Zambezi catchment, are illustrated and discussed in full detail in two companion papers (Garzanti et al 2021a and2022a).…”

“…In many segmented river systems where all detritus generated upstream is impounded in large dams, the mineralogical signal can be nevertheless transmitted downstream by reworking of previously deposited channel, floodplain, and terrace deposits, thus damping the dam effect (e.g., Garzanti et al 2000Garzanti et al , 2015Vezzoli et al 2016;Malkovski et al 2019;Thomson et al 2022). This may well occur where the dominant erosional foci are located upstream of the dams and the river flows across open lowland landscape downstream, which is hardly the case for the Zambezi River, where all compositional signatures change radically and irreversibly downstream of Lake Kariba first, and of Lake Cahora Bassa next (Garzanti et al 2021a(Garzanti et al , 2022a. The narrow and steep river valley lacks floodplains downstream of the dams, where the youthful river course is largely carved in bedrock, forming particularly impressive rapids along the Cahora Bassa Gorge (Davies et al 2000).…”

“…The present article, intended as a complement of previous studies on land, focuses on the Zambezi deep-sea fan deposited in the Mozambique Channel between the African landmass and Madagascar Island (Fig. 1), thus completing the source-to-sink study of the entire Zambezi sedimentary system (Garzanti et al 2014a(Garzanti et al , 2014b(Garzanti et al , 2021a(Garzanti et al , 2022a(Garzanti et al , 2022b. Our primary aims are to illustrate and discuss the variability of petrographic, heavy-mineral, elemental-geochemistry, Ndisotope, and U-Pb detrital-zircon geochronological signatures of Middle Pleistocene to Holocene turbidite deposits, highlight provenance changes in space and time, reconstruct sedimentary and geochemical budgets, and assess the relative amounts of detritus supplied from Africa versus Madagascar as well as the changing contributions from different rivers of SW Madagascar.…”