Sedimentology and Geochemistry of Coal Deposit in Geba Basin, Illubabor, Western Ethiopia

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Addis Ababa Universty


The sedimentary succession exposed in a386 sq. km Geba basin represents one of thick accumulations of continental sediments that are commonly known to associate the Ethiopian Tertiary volcanics. The succession is a coal and odshale bearing, and comprises about 150m Conglomerate, sandstone, siltstone, mudstone, reworked tuff, oilshale and coal sediments. They are deposited in an east-west trending intracratonic rift basin, that may be evolved locally, from other regional contemporaneous tectonic episodes between Middle Oligocene and Early Miocene. The whole sedimentary sequence has been distinguished into five major sedimentological units correlable with depositional environments ranging from fluviatiles to lacustrine. Under fluvial conditions a number of coal seams are formed in flood plain. Under lacustrine condition, several thick coal seams have been formed in marshy areas, developed at the margin of the lakes. Generally, the coal beds range in thickness from few tens of centimeters to about 4.0 meters. Chemically, they are high ash and medium sulfur coals. The maceral study reveals that huminite dominated the organic forms of Geba coals(>85%), with high proportion of humocollinite and minor humodetrinite sub groups. Humotelinite sub group is markedly rare. Low inertinite (<7%) and considerable liptinite, as high as 23%, are main features of the coals. The overall petrographic aspect of the coals suggests humic origin, possibly under shallow water condition. Reflectances measured on the huminite range from 0.32-0.39%R. This indicates an ASTM rank ranging from lignite to lower boundary of Sub bituminous.



Geba Basin