Petrogenetic Evolution of the Melka Wakena Pyroclastic Deposits: Implications for the Depositional History of the Intercalated Volcano-Sedimentary Rocks

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Addis Ababa Universty


The study area is located east of the Assasa town in the Arsi zone on the shoulder of the rift-plateau escarpment and close to the Gadeb archaeological sites. The main objective of the study was therefore to determine the petrogenetic evolution of the pyroclastic rocks using standard field (mapping and description), geochronological (stratigraphic logging, correlation and Ar-Ar dating of some selected samples), petrographic (thin section analysis) and geochemical (major and trace element geochemistry) methods, and constraining the depositional history of the intercalated volcano-clastic sediments in the area. A total of 26 samples were analyzed in this study: 9 for petrography, 13 for geochemistry and 4 for Ar-Ar dating. The pyroclastic rocks have been formed 1.617 to 0.696 Ma ago in a topographically complex area resulting in non-continuous and erratically distributed layers. They are generally of rhyolitic (the ignimbrite, ash fall and pumiceous ash units) and dacitic (the ash flow unit) composition. The geochemical characteristics and modeling suggest that the rhyolites were formed by fractional crystallization of basaltic magmas involving crustal input, while the dacites were formed by Assimilation Fractional crystallization followed by post depositional leaching of the mobile elements. The Melaka Wakena archaeological site was a geomorphologically active area during the Early to Middle Pleistocene where overbank sedimentary sequences (conglomerates and sands) formed by a big meandering river, dotted with possible temporary ponds and ox-bow lakes (depositing clays), have been covered by episodic volcanic eruptions and the products thereof. A dense network of channels and streams have been subsequently down cutting through the older volcanic and sedimentary sequences before the deposition of the younger volcanic and reworked volcaniclastic rocks. This episodic eruption of pyroclastic rocks on actively depositing and eroding sedimentary sequences containing cultural sequences allow constraining the age of the archaeological remains in the area as well as their depositional history. Key words: Melka Wakena, Petrogenetic evolution, Stratigraphy, Geochronology, Fractional Crystallization, Crustal contamination



Melka Wakena, Petrogenetic evolution, Stratigraphy, Geochronology, Fractional Crystallization, Crustal contamination