Browsing by Author "Bekele, Barsisa"

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    Geochemistry of lower sandstone in Blue Nile Gorge Mesozoic sedimentary sequences: Implication for provenance composition and paleoclimate
    (Addis Ababa Universty, 2011-06) Bekele, Barsisa; Ayalew, Dereje (PhD)
    Mineralogy, major, traces, and rare earth elements geochemistry of Adigrat Sandstones Formation from the Blue Nile Basin (BNB), NW Ethiopian plateau have been analyzed to determine their provenance composition, paleoweathering and paleoclimaice scenarios. Samples of this sandstone are slightly variable in composition. Mineralogically the framework grains are quartz (Q), feldspar (F) and lithic fragments (L) and on QFL diagram; most o the samples are plotted in subarkose and lithic subarkose fields. Geochemically, Adigrat sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA, PIA and CIW values for this sandstone and the A–CN–K diagram agreeably suggest that source rocks experienced intense chemical weathering under hot and humid tropical to sub-tropical climate. Mineralogical evidence suggests that plutonic and metamorphic rocks were major contributing source rocks. Perhaps, multi-cyclic processes reworked the sediment. Relatively immobile elements have been selected for provenance studies. Ratios and plots of relatively immobile elements consistently suggest that the Adigrat Sandstone is derived from compositionally mixed source between mafic and intermediate igneous rocks. Discriminant function diagram constructed from major oxides, the Chondrite normalized REE plot, Cr/V vs. Y/Ni ratios, La vs. Th plot and Th-Hf-Co ternary diagram all suggest that the Adigrat Sandstone is derived from compositionally mixed source. Significant contributions of intermediate supply are also confirmed by Al2O3/TiO2 ratios and lower Eu/Eu* (0.552) values than that of PAAS (0.65). Provenance composition and degree of sorting between Adigrat Sandstone in BNB vs. Mekele Basin (MB) have been compared. The result shows that sediment of the former were highly sorted and compositionally derived from mafic and intermediate input, where as that of the later was mainly derived from felsic rocks. Keywords: Adigrat sandstones, provenance, paleoweathering, paleoclimate and Discriminant function diagram