Environmental and Social Impact Assessment - Gibe III

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
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