Environmental and Social Impact Assessment - Gibe III

Gibe III – Environmental and Social Impact Assessment 300 ENV R CS 002 C - A9003099
UNWEATHERED TRACHYTE (lower layer)
The lower part (unweathered) of the trachyte body is less altered, or unaltered, as it is constituted by a sub
intrusive body and is basically not interested by the fracturation and/or alteration phenomena. It is generally
dark and hard and difficult to access for the atmospheric agents. The plutonic evolution typical of the
magmatic room has produced dark, compact and resistant phases. Some few meter thick brecciate “zones”
are met at various depths. It has not yet possible to adequately assess, basing on the available investigations,
their correlation. Decomposed trachyte levels are met at various depths. These are composed by a few meter
thick sand clay materials. The most important level is located from el. 680 to 650 m a.s.l.. The tallest level
could interfere with the upstream toe of the dam (see boreholes B1, A3a, C1, A2, A1). The thickness of this
level decreases moving downstream nearly disappearing in the dam axis area. The characteristics of these
levels are probably related to the hydrothermal alteration. For long periods, the stabilized ground water table
has deteriorated the rock by effect of the heat and chemism of the water. The river has eroded the upper
softer layer of the weathered trachyte and, as of consequence, the riverbed conform with the contact between
the lower harder unweathered layer grey trachyte.
SLIGHTLY WEATHERED TRACHYTE (upper layer)
This zone corresponds to the superficial and effusive part of the magmatic body. It shows sub-vertical
cooling joints that have supported the development of the alteration and the oxidation like concentric bands.
In the marginal areas along the scarps of the river Omo, or along the Tsida stream, the superficial flow is
subject to the tilting phenomenon, while the phenomenon of oxidation is accentuated and the joints result
open and often filled by plastic material. The passage from the inferior limit to the unweathered trachyte,
almost always abrupt, especially when the superior layer is thin, shows somewhere the evidence of the
“caterpillar track” phenomena together with the presence of breccia.
BASALT / TRACHYTE CONTACT
The contact is discordant. The basalt is constituted by subsequent flows and shows a columnar structure. It
fills the very uneven structure of the Paleo-morphology constituted by the underlying trachyte. The contact
surface in the depression zones is characterised by oxidized levels with the presence of paleosoil and also
important levels of conglomerates (ancient alluvium) as in the borehole C5. The trachyte appears altered and
oxidized (bordeaux colour). In the high contact zones (borehole A9, E5, etc.) the contact can be abrupt, as if
an erosion surface. It may also be intercalated with a 7-8 meter level of pyroclastic rocks characterised by
ignimbrite outcropping (RB and LB) or, as in the borehole A6, by pyroclastic sands.
COLUMNAR BASALT
Above the trachyte plug there is the columnar aphanitic basalts (Bac) involved in tertiary tectonic
movements.
COLUMNAR BASALT WITH TRACHYTE HABITUS (i.e. not involved in tertiary tectonic
movements)
There is a subsequent volcanic episode constituted by two or more 18-17m. thick horizontal trachytic
columnar basalts is not involved in the tertiary movements. Two lenticular few meter thick intercalations of
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