Browsing by Author "Zinaye, Bahru"

Now showing 1 - 1 of 1
  • No Thumbnail Available
    Item
    Geologcal and Geomorhological Characterization of The Dendi Caldera, West Central Ethiopia: Implications for Paleo-Environmental Reconstruction Using Lake Sediment Cores
    (Addis Ababa Universty, 2014-05) Zinaye, Bahru; Asrat, Asfawossen (PhD)
    The Dendi Caldera is located on the West Central Ethiopian Plateau, approximately 120 kilometers to West of Addis Ababa. It is located between 0804810011 - 0805210011 North latitude and 3705810011 - 3800410011 East longitude in Dendi kebele, Ginchi woreda, western showa zone, Oromiya regional state of Ethiopia. The caldera has almost elliptical shape with about 8.5 km diameter in its longest direction and 6.5 km in the shortest direction. There are two crater lakes making the shape of ‘8’ within this caldera, each with a circumference and an area of about 7.5 km and 4 sq. km respectively. The altitude varies between 2830m - 3270 m a.s.l.. The main objective of this research work is to study the geology, geochemistry and petrology of the Dendi caldera rocks, to construct the geological evolution of the caldera and to study the effect of the geology on the composition of the lake sediments in order to help reconstruct the paleoclimate using lake sediment core. The methods involved to achieve the objectives are desk studies about the area, fieldwork for mapping and sampling, petrographic studies, XRF analysis both on the lake sediment and rock samples, analysis and interpretation of data using different computer softwares. The caldera comprises different felsic volcanic products including trachyte, rhyolite and obsidian overlain by the pyroclastic deposits ignimbrite, volcanic surge and tuff. The rhyolite and trachyte are formed both as lava flow and lava dome. In some areas they show columnar joints. The surge deposit shows sedimentary structures like bedding and X-bedding. In thin section, the rhyolite, trachyte and ignimbrite has aphanitic and porpyritic aphanitic textures with alkali feldspar phenocrysts. The Trachyte also shows trachytic texture. On TAS diagram, the lavas are rhyolitic and trachytes/trachydacite and the pyroclastics are trachyandesite and basaltic andesite. In the spider diagrams (MORB normalized), Ba, Sr, P, Ti and Cr shows strong negative anomaly which verifies fractionation. The lava shows high Th and Nb with very low Sr. Rb/Nb and La/Nb ratios are very low while Rb/Sr ratio is very high in the Dendi volcanic products. These conditions show the only possible mechanism for the formation of the Dendi caldera is fractional crystallization of basaltic magma with limited or no crustal contamination. Five different sections have been identified based on the variation of different chemical and physical parameters applied on the lake sediment core. Section 4, with high grain size, MS, K, Si/Ti and Mn and low inc/coh, Sr/Ca, Zr/Rb and Rb/K, shows the driest period in the stratigraphy while section 2 shows the most humid period in the stratigraphy. The remaining sections spans between these two extremes. Key words: Dendi, Caldera, volcanic, felsic, fractional crystallization, paleoclimate