Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
5.1.2 Geology
5.1.2.1 Regional Geology 1
The regional geology of southern Ethiopia consists mostly of metamorphic rocks of green schist, amphibolite
and granulite facies that represent the southern margins of the Arabian-Nubian shield.
The Precambrian basement rocks are the result of a complex tectonic and metamorphic history during the
Proterozoic times. During the Palaeozoic, the Precambrian rocks were eroded to a vast peneplan, apparently
a continuation of the Kenya -Sudan plains to the South and West.
In early Mesozoic times, a major marine incursion from the east deposited the Triassic and Jurassic Adigrat
sandstones and limestones that cover much of the eastern part of Ethiopia and Somalia. The central and
western parts of Ethiopia remained a continental land area throughout the Mesozoic, with a thin unit of
presumed continental sediments being the only record of deposition.
Marine sedimentation was brought to an end by widespread regional uplift starting in the Eocene period,
associated with the initial development of the Red Sea and the East African rift systems. This uplift was
accompanied by extensive volcanic activity. At first, this took the form of huge outpourings of basalt lava
(flood basalts) from long fissures that opened along the line of the rifts. These flood basalts erupted for
almost 20 million years and accumulated to a total thickness of several thousand metres. They now form the
foundation of the Ethiopian Plateau.
However, the details of the geological history between about 30 and 15 millions years ago has some
uncertainties, owing to the scarcity of absolute ages for the felsic volcanic rocks, and it is likely that basaltic
and more acid lavas erupted contemporaneously from different volcanic centres (Davidson 1983) with a
progressive evolution from dominantly basaltic lavas to mainly trachytic and rhyolitic lavas.
It is inferred that volcanic activity declined about 20 million years ago, leading to the localised formations of
a series of shallow lakes and swamps which developed on top of the flood basalts. This resulted in the
deposited series of shallow-water lacustrine sediments containing carbonaceous shales, lignite and coal
deposits.
Around 13 million years ago, the development of the main Ethiopian Rift Valley commenced, reaching
something similar to its present form by the early Pliocene, about 5 million years ago (Davidson, 1983). This
inaugurated a period of felsic volcanism, including rhyolites and trachytes, where the available dates indicate
a duration between 10 and 3 million years ago; these felsic volcanics extended over much of the central
plateau of Ethiopia.
1 Main data on which the present geological study is referred are:
· 300 GEN R BR 001 Seismic Hazard Assessment
· 300 GEO R SP 001 Geological Report
· 300 GEO R SP 003 Hydrogeological Report
· 300 GEO R SP 002 Reservoir seepage risk analysis
In particular a large campaign of site investigations has been carried out and is presently in progress. The investigations
already completed include:
· Boreholes (also with lugeon tests) > 13000 m, 120 No
· Geophysical Investigations
· Geostructural Surveys > 5000 joints
· Laboratory Tests
· Inspection Adits > 350 m, No. 7
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