The Zambezi deep-sea fan: mineralogical, REE, Zr/Hf, Nd-isotope, and zircon-age variability in feldspar-rich passive-margin turbidites
|Author(s)||Garzanti Eduardo1, Bayon Germain2, Vermeesch Pieter3, Barbarano Marta1, Pastore Guido1, Resentini Alberto1, Dennielou Bernard2, Jouet Gwenael2|
|Affiliation(s)||1 : Laboratory for Provenance Studies, Department of Earth and Environmental Sciences, University of Milano–Bicocca, 20126 Milano, Italy
2 : University of Brest, CNRS, Ifremer, Geo-Ocean, F-29280 Plouzané, France
3 : London Geochronology Centre, Department of Earth Sciences, University College London, London WC1E 6BT, U.K.
|Source||Journal Of Sedimentary Research (1527-1404) (Society for Sedimentary Geology), 2022-11 , Vol. 92 , N. 11 , P. 1022-1043|
We here present the first comprehensive provenance study of the Zambezi deep-sea fan, based on integrated petrographic, heavy-mineral, elemental-geochemistry, isotope-geochemistry, and detrital-zircon-geochronology analyses of middle Pleistocene to Holocene turbidites. The Zambezi Valley and Fan represent the submarine part of an ∼ 5000-km-long sediment-routing system, extending from the heart of the South African Plateau to the abyssal depths of the Indian Ocean. Sediment is derived not only from the African side, but also from Madagascar Island mostly via the Tsiribihina Valley. Being shed by two dissected rifted margins, detritus supplied from opposite sides of the Mozambique Channel shares similar feldspar-rich feldspatho-quartzose composition, although with significant differences in heavy-mineral and geochemical signatures. The εNd values of Madagascar sand are markedly more negative and TNd model ages notably older. Zircon grains yield mostly Irumide (late Stenian) U-Pb ages in African-derived sand and mostly Pan-African (Ediacaran–Cryogenian) U-Pb ages in Madagascar-derived sand, which also yields a few grains as old as Paleoarchean and many discordant ages reflecting Pan-African reworking of Archean cratonic rocks. Lower Valley and Lower Fan deposits have intermediate fingerprints, indicating that sediment supply from Madagascar is not much less than from Africa despite a much smaller catchment area, which can be explained by deposition of a conspicuous part of Africa-derived sediment in the Intermediate Basin confined between the Zambezi Shelf, the Beira High, and the Îles Éparses. By assuming that compositional differences between Quaternary submarine deposits and modern Zambezi River sands primarily resulted from sediment impoundment by large dams, we could evaluate the anthropogenic impact on natural sediment fluxes. Quaternary turbidites are somewhat higher in quartz and poorer in heavy minerals with higher relative amounts of durable ZTR species, and yield more Ediacaran, Neoarchean, and Carboniferous detrital-zircon ages than modern river sands. The Orosirian peak characterizing the Intermediate Basin sample points to prominent supply from the middle and upper parts of the Zambezi catchment in the middle Pleistocene. Rough calculations suggest that pre-dam Zambezi sediments were generated ≤ 10% in the upper catchment, ∼ 60% in the middle catchment, and only ≥ 30% in the lower catchment that provides the totality of sediment reaching the Indian Ocean today.