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 are affected by sets of undulated cooling joints that create a 1-1,5 meter wide and >20 meter high vertical prismatic column. The dip direction of main joints are sub-parallel and perpendicular to the river flow while the dip is subvertical. These regulate the general structure. Toppling features occur along the scarps and in correspondence of the diversion tunnel portal.. In the right bank, a fault with a vertical movement brings trachyte and columnar basalt into contact. In this bank, the sub-vertical joints are cut by a joint trend with a 45-50° dip. An important NNE-SSW fault crosses the entire study area in correspondence of the West end of the R3 plateau. Such fault divides the trachytes and basalts zone within the study area from a series of loose or semi-loose volcanic materials zone with small embedded flows. This fault seems to belong to an important NNE-SSW movement that cuts the entire area. 5.1.2.3 Geology of the project area The main Geological Units present in the area are: • Tertiary highland Volcanic Units (Jimma Formation Pjb-Pjr, according to Merla, or Felsic volcanics PNv, according to Omo Gibe river Master Plan and Davidson.); • Quaternary-Tertiary Rift sediments & Volcanics with undifferentiated Quaternary cover; • Salic intrusions. The river bed crosses the following main units: • the Tertiary Volcanic Unit named Jimma Volcanics Lower Unit (Pjb), • the Western part of the overhanging Plateau with Jima Volcanics Upper Unit (PJr), and • the Eastern part of the same plateau with one unit of Quaternary-tertiary Volcanics named Nazret Series (Nn). According to the L1D Geological Report, the geology of the project is characterized by four major lithostratigraphic groups; these units are separated by unconformity where faulting has caused tilting and the subsequent erosion of the older rocks, spaced out by pyroclastics events. 1) Tertiary volcanic units (Jima Formation, Eocene to Miocene pre-rift volcanic rocks) including flood basalts (lower unit, Pjb) and rhyolites (upper unit, Pjr). The Jima Volcanics comprise trachybasalts and rhyolites covering most parts of south-western Ethiopia. They form a thick succession of basaltic and felsic rocks with basalt dominating the lower part of most sections. The basalt flows form a succession up to several hundred meters thick, with felsic lavas intercalated towards the base of the succession. The basalts crop out mainly along the main riverbed and over a vast area west of it; the rhyolites crop out mainly along the western overhanging plateau. 2) Tertiary to Quaternary volcanics of the so-called Nazret Series (Nn), cropping out along the eastern overhanging plateau (Mio-Pliocene, 9 to 3 M.y.). The Nazret series includes a thick succession of ignimbrites with composition ranging from sub-alkaline rhyolites and trachytes to (rare) peralkaline varieties, characterized by flame textures, pumice and ash levels and rhyolite flows and domes with rare intercalations of basaltic flows. The Nazret ignimbrites are considered the product of eruptions from marginal rift centers. CESI SpA - Mid-Day International Consulting Engineers Page 81
Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099 3) Tertiary to Quaternary syn- and post-rift sediments and interlayered volcanics (late Miocene to Holocene) and hypoabyssal intrusive rocks. The Quaternary units overlying the Nazret Series include: • the Dino Formation (Qd), formed by ignimbrites and associated (locally water-laid) pyroclastics, occasionally interbedded with lacustrine beds and aphyric basalts with a maximum thickness of 50 m. • Peralkaline (rhyolitic) lavas and pyroclastics (Qr) derived from volcanic complexes situated along the axial zones of the main Ethiopian Rift and Afar, characterized by obsidian flows, pumice, ignimbrite, tuffs and scoriaceous basaltic lavas. Most of the basaltic flows have fissural origin from the late fractures of the Wonji Fault Belt,which is a narrow shear zone along which most of the deformation and magmatism related to the northern Ethiopian Rift is concentrated (Boccaletti et al. 1999). • Quaternary sediments (Q) including fluvial, lacustrine, eolian, and eluvial deposits are widespread all over the area. 4) A Precambrian basement unit, found in the study area south of the Gibe III reservoir, is represented by the Archean Alghe Group (AR1). The bulk of this unit consists of a relatively homogeneous and poorly layered orthogneiss, representing deformed and metamorphosed plutonic rocks of dioritic, quartz-dioritic and tonalitic composition. Orthogneisses are locally interlayered with thin units of mafic schists, quartzo-feldspathic gneisses and paragneisses. According to the L1D Geological Report, the riverbed and in general the lower part of the reservoir is constituted by basalt flows and by minor ignimbrite and tuff levels, while the upper part of the reservoir would rest on trachytes, rhyolites, ignimbrites and tuffs. Plugs and dikes of rhyolite, trachyte, phonolite and microgranite are reported along the valley flanks. 5.1.2.4 Geology of the Dam site The dam site area is characterized by a deep gorge with sub-vertical walls. The river alluvium is constituted by fine sands and gravel of variable thickness, with a maximum depth up to about 15 meters. From the river banks start the 30-50 m slopes of a coarse angular colluvium in silty-sandy matrices. The slightly weathered trachyte walls, outcropping are mainly characterized by two sub-vertical joint families, being parallel and orthogonal to the river. The walls are affected also by a relevant transverse fault system with NW-SE orientation. The Left bank is about 200 m high and appears almost intact, while the right bank is affected by a NNW-SSE fault system. The trachyte flow appears in some parts weathered and altered by fractures and hydrothermal fluids. At the top of the right bank a main NNW-SSE fault has caused a vertical displacement and put basalt and trachyte structures into contact. Near the contact with the colluvium, the rock assumes a more plutonic look. The ends of the walls are sometimes characterized by basalt columnar flows (two at least) upset by a structural movement. At the top of this flows start a columnar basalt and vacuolar basalt flows series with horizontal asset interbedded by a metric series of pyroclastic rocks and continental erosional deposits constituted by a etherogenic pebbles. CESI SpA - Mid-Day International Consulting Engineers Page 82
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Environmental and Social Impact Assessment - Gibe III

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