Environmental and Social Impact Assessment - Gibe III

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Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 5.3.13 NGOs working in and around the affected Weredas There are few NGOs involved in the affected weredas in the area of rural road construction, soil conservation, rural water supply, provision of health service and other conservation activities. The lists of NGOs involved in the weredas are shown below. Table 5.43: NGOs working in the affected Wereda NGOs available in the affected wereda Name of NGOs Kindo Koyisha Gena Bossa Loma Soro Omo sheleko Bolo Soro Kacha Birra Kale Hiwot � Inter Aid � � World Food Programme � � Catholic Mission/church � Action Aid � � � Acts of compassion � African humanitarian action � World Vision � � Mekane Eyesus � � 5.4 Baseline Information: Downstream Area Additional studies on downstream area has been carried out and presented in a separate report 19 and summarised in this section. 5.4.1 Geology The lower Omo Basin geology can be divided into two large groups: • Rock formations crowning the lower Omo basin • Sediments forming the lower Omo basin 5.4.1.1 Rock formations crowning the lower Omo basin “Hammar Domain” rocks form the crystalline basement. The domain in the study area underlies the lower part of eastern Omo river area and the northern part of Lake Turkana. This area contains two Precambrian major rock groups: a complex of older gneiss and granulite, which is highly deformed, re-crystallized and partly migmatized, as well as a suite of younger plutonic rock not necessarily related to the other and far less or not deformed / re-crystallized. The geological structure is oriented NNW-SSE and develops in subsequent plates whose width varies from 6 to 12km. Subsequently, the structure bends and develops northwards encircling the plutonic masses. The principal faults are oriented ENE-WSW. Traces of faults and fractures are also observed in the sediments that fill the Omo River valley. On the crystalline basement are the Pre/post-rift sediments/volcanic, characterised by four principal formations: Main volcanic sequences, basalt, rhyolite, trachyte, tuff, ignimbrite (Eocene, Oligocene), Omo group (Mursi formation basalt), Shungura formation (Pliocene), Nakwa formation basalt (quaternary). The Main formation sequence rims the western side of the lower and middle Omo valley reaching the Jbai range 19 300 ENV RAG003A Additional Study on Downstream Impact – April 2008 CESI SpA - Mid-Day International Consulting Engineers Page 155
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 and the inner zone of the eastern part, to the east of the Mursi plain, while the Nakwa formation generates the Korath ranger located in the lower part of the western Omo, and the Mursi formation covers the eastern part of the mid-Omo valley (Mursi plain). The undivided part of the pre-rift volcanic succession is separated almost everywhere from the crystalline basement by the basal residual sandstone. Basalt dominates the lower part of most sections, followed by a thick unit of salic rocks. However, salic flows and breccias lie directly on the sandstone in some places. The thickest salic succession lies astride a north-north-east axis along the mid-Omo plain trend. The main faulting trend is oriented NNE-SSW according to the rift valley orientation. The Mursi Fm is divided into two units, the lower being sedimentary and the upper one formed by flood basalt. The well-studied lower-sedimentary unit, divided into three members, is exposed at the west side of the southern part of the Nyalibong range. It lies uncomfortably on west-tilted pre-rift rhyolite and is composed of some 150m of clays, silts and sand with subordinate tuff and pebble beds. These sediments are conformably overlain by flood basalts having a total thickness probably less than 100m. The Mursi basalt underlies much, if not all, of the cover of the middle Omo plains, (Mursi people plains) and its extension could continue southward beyond the Nyalibong ridge. The Shungura formation is exposed for about 60m along the west side of the Omo river north of Lake Turkana, and includes 760 m of brown, grey, and buff clays, silts, sand, gravel, tuffs, marls and freshwater limestone. These sediments are tilted gently to the west and are overlain by the shallow unconformity of the Kibbish formation. The faults develop both parallel and oblique to the north with respect to the axis of tilting and have an offset bedding of several metres locally. The sedimentation records fluctuating fluvial and lacustrine cycles. Nakwa Formation (quaternary) Mont Nakwa is located in the middle of a chain of very recent and wellpreserved volcanic cones known as the Korath range which lies 55Km north of the Turkana Lake. These cones stand 400m above the monotonous surrounding plains that are effusive events for volcanic rocks with basic alkaline affinity. The main lavas are basanites with minor thephirite. The cones are aligned north northeasterly, i.e. the same trend as the Turkana rift and many of the prominent faults in the region. 5.4.1.2 Sediments forming the lower Omo basin Local lacustrine, fluvial and fluvio-lacustrine sediments divide into five formations which all belong to the Pleistocene Holocene. These include Kibish formation; Undivided alluvial, fluviatile, lacustrine sediments; Fluviatile sand and silt; Lacustrine silt and clay; Alluvium. A sequence of horizontal sediments, related to the major fluctuations in the ancestral Lake Turkana level, lies unconformably above the various formations of the Omo Group while dissected, and in its turn overlain, by the late Holocene deposits. The sequence is termed “Kibish” Formation and is well exposed where the Omo river and its ephemeral side tributaries have cut through the otherwise flat and unravelling plain that CESI SpA - Mid-Day International Consulting Engineers Page 156
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