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Zusammenfassung

This work aims to unravel the provenance, depositional tectonic setting and source area weathering of the cretaceous Debre Libanos Sandstone in the Blue Nile Basin, Ethiopia from its petrographic and geochemical signatures. Both the average chemical index of alteration (CIA) and chemical index of weathering (CIW) of sandstones (CIA = 91.08, CIW = 96.75), shales (CIA = 81.89, CIW = 98.17) and siltstones (CIA = 98.10, CIW = 99.35) of the Debre Libanos Sandstone indicate very strong chemical weathering. The low Th/Sc ratio of sandstones (1.46), shales (0.85) and siltstones (0.81), and Th/Sc versus Zr/Sc relationship indicate sediment recycling that leads to a progressive enrichment of heavy minerals such as zircon which is the major host for Zr and Th. The increase of quartz content in sandstones and dense minerals in siltstones and shales can be related to hydraulic sorting, grain-size controlled fractionation and source composition. Trace element concentration and their ratios (La/Th, Cr/Th, Th/Sc, Cr/V, Y/Ni) with rare earth element proxies (Eu/Eu*, LaN/YbN) of the studied samples revealed felsic to mixed source rocks of upper continental crust with some input from recycled old sediments. The source area of the Debre Libanos Sandstone is from the northern and northwestern Ethiopia during the time of marine regression, retreat of the Indian Ocean from NW Ethiopia back to SE Somalia. Tectonic discrimination diagrams revealed continental rift setting for the sandstones and collision setting for shales and most of the siltstones. Furthermore, the studied sandstones were partly deposited in a passive margin setting which had undergone post-depositional K-metasomatic alteration. The shales and siltstones were deposited in active margin setting.