The oldest rock exposed in the island is the There is a trachyte hill between shigima and Pleistocene lacustrine sediments. These arura village. Some exposure of grayish lacustrine sediments include diatomite, brown trachitic rock with abundant bentonitic clay and diatomaceous silt, which are exposed in the uplifted blocks within the phenocrysts of feldspar are observed near island .the claybeds are mainly dominant in the shigima village lying on top of basalt. In the lower most part of the lacustrine some places this rock is slightly weathered. sequences with respect to the diatomite Holocene lake sediments are exposed beds.(Million et.al., 1991) on the shore of the present lake while recent West of arura village there are small alluvium deposits are restricted at the mouth outcrops of yellowish brown, massive silty of some gullies within the island. limestone which is sandwiched between the lacustrine beds. In some places within the lacustrine beds (specially Shigima area) Figure 6 Bentonite from Gewane area (geomuseum) photo. by mesfin there are gypsum anhydrite, manganese The reserve in the area is estimated to be nodules and some pistolites of carbonate 3462860 m3.and the chemical composition of origin. The upper part of the diatomite layer gidicho bentonite is almost comparable with in biaso area is partly covered by chert. the world bentonite producing countries The underlying beds of the lacustrine (Table 4). In some areas slight variation also formation are not exposed. The Pleistocene observed. lacustrine sediments are overlain by Tests conducted on Bentonite samples from volcanic rocks. Ignimbrite which is these and Gidcho deposits have shown them rhyolitic, greenish grey, fine grained and to be suitable for iron–ore pelletization and vesicular overlies the lacustrine beds. The drilling mud after proper beneficiation ignimbrite inturn is overlain by dark grey, (Tibebe et. al., 2002). Laboratory analysis of vesicular basalt and the vesicles filled with calcarious material.
The oldest rock exposed in the island is the Pleistocene lacustrine sediments. These lacustrine sediments include diatomite, bentonitic clay and diatomaceous silt, which are exposed in the uplifted blocks within the island .the claybeds are mainly dominant in the lower most part of the lacustrine sequences with respect to the diatomite beds.(Million et.al., 1991) West of arura village there are small outcrops of yellowish brown, massive silty limestone which is sandwiched between the lacustrine beds. In some places within the lacustrine beds (specially Shigima area) there are gypsum anhydrite, manganese nodules and some pistolites of carbonate origin. The upper part of the diatomite layer in biaso area is partly covered by chert. The underlying beds of the lacustrine formation are not exposed. The Pleistocene lacustrine sediments are overlain by volcanic rocks. Ignimbrite which is rhyolitic, greenish grey, fine grained and vesicular overlies the lacustrine beds. The ignimbrite inturn is overlain by dark grey, vesicular basalt and the vesicles filled with calcarious material. There is a trachyte hill between shigima and arura village. Some exposure of grayish brown trachitic rock with abundant phenocrysts of feldspar are observed near the shigima village lying on top of basalt. In some places this rock is slightly weathered. Holocene lake sediments are exposed on the shore of the present lake while recent alluvium deposits are restricted at the mouth of some gullies within the island. Figure 6 Bentonite from Gewane area (geomuseum) photo. by mesfin The reserve in the area is estimated to be 3462860 m3.and the chemical composition of gidicho bentonite is almost comparable with the world bentonite producing countries (Table 4). In some areas slight variation also observed. Tests conducted on Bentonite samples from these and Gidcho deposits have shown them to be suitable for iron–ore pelletization and drilling mud after proper beneficiation (Tibebe et. al., 2002). Laboratory analysis of
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