--> The Provenance of Sands in Transcontinental Rivers: What Can Detrital K-Feldspar Tell Us?

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The Provenance of Sands in Transcontinental Rivers: What Can Detrital K-Feldspar Tell Us?

Abstract

Long-lived, continental-scale river systems dominate the sediment supply to many passive margin basins. Understanding how such rivers mix and transmit sediment downstream is key to understanding sediment fluxes, the stratigraphy of coastal and offshore deposits, and the output sand composition, a critical control on the reservoir quality in offshore sinks. Whereas both K-feldspar and zircon are well-placed to resolve the provenance of sands in big river systems, K-feldspar is important as a framework component that can help fingerprint different parts of the hinterland drainage basin and give insight into sediment budgets delivered to the trunk river from the main tributaries. In order to better understand how big rivers collect and mix sand from different regions of the drainage basin, the source(s) of sands in two modern continental-scale river systems are traced using the variable Pb isotopic composition of detrital K-feldspars. The Mississippi and Nile river systems are comparable in size (>3 million km2) but vary significantly in both the number of tributaries feeding the trunk stream and the climatic zones through which they flow. Over 2,300 detrital K-feldspars have been analysed from modern tributaries in both systems using LA-MC-ICPMS. Feldspars currently transiting the two drainage basins are dominated by contributions from single tributaries tapping parts of the upper drainage basin, the Missouri and Blue Nile. The Missouri input is distinctive and overwhelms the other major up-stream tributaries, producing a consistent mix of feldspars over much of the Lower Mississippi with little impact from further tributary additions or dams. The Mississippi feldspars can be related to the basement domains straddling the upper drainage basin and the resulting mix is consistent with estimated modern sediment fluxes and feldspar loads. Nile feldspars from the Blue and White Nile are indistinguishable, despite the headwaters of the White Nile tapping older basement rocks with a distinctive Pb signal that is not present in the modern detrital population. Furthermore, the dominant Blue Nile input is a Pleistocene-Recent phenomenon, and older sands delivered to the coast are known to be more feldspathic and to tap additional sources not represented by the modern sands or the upstream basement domains as currently characterised. Dynamic topography, knickpoint migration and tropical climate variability likely complicate sand transfer through the Nile system.