1. No category

島根大学地球資源環境学研究報告 19，113∼125ページ
（2000年12月）
Geoscience Rept．Shimane Univ．，19，p．113∼125（2000）
GeologicalStructuresandTectonics ．
aroundtheKokaReservoir，NorthemMainEthiopianRift
ShuichiroYokota＊and SileshiMamoFantaye＊＊
Abstract
The Koka dam and its reservoir，located in the northem Main Ethiopian Rift，are the main facilities for irrigation and
hydroelectric power in Ethiopia．However，a considerable water leakage from the reservoir has observed since the
completion ofthe Koka dam in1960．Topographical and geological conditions around the reservoir have been studied to
obtainfundamentalinfomationonpathsofleakage．ThereservoirareaislocatedintheWonjiFaultBeltoftheno丘hem
Main Ethiopian Rift，and is characterized by small horst and graben stmct皿es with distinct fault sca甲lets．The structures
areelongatedNNE−SSWintheno質heastemhillsofthereservoir，which is theprobable leakage zone，andthey obliquely
intersect the course of the Awash River flowing out from the reservoir．The northeastem hills consist of rhyolitic
ignimbrites and various volcanic rocks intercalated with post−Pliocene pumiceous and lacustrine deposits。They dip
gently，but are partially displaced by faults。
Considering that uplift and tilting offault blocks continued d皿ing the Quatemary，such movements may have shifted
thecourseofthedver．Alowelevationzoneelongatedno質heastw肛dfromapoint35㎞southoftheKokadamwas
inferred to be an old river co皿se．Although this zone is partially divided by an uplifted horst mound，it is elongated
northeastward from the reservoir bank．In contrast，the c皿rent river course where the Koka dam was constructed is deep，
andmaybe anewerone．While thefomleris characterizedby remarkably weatheredrockmass，thelatteris fresh due to
rapid dissection．Based on these topographical and geological characteristics，the low elevation zone corresponding to a
paleo−chamel might have become a leakage path as water level rose after inundation．
Key words：Ethiopia，Koka dam，Leakage path，Reservoir，Rift，Ignimbrite
there are varied and geological structures are complicated．
lntroduction
One of the authors，Sileshi Mamo has attempted to
The Koka dam and its reservoir，10cated in the northem
detemine the leakage paths倉om the viewpoints of
MainEthiopianRift（MER，Fig．1）are the main facilities for
hydrogeology using isotope techniques．Geological conditions
irrigation and hydroelectric power in Ethiopia．The Koka
around the reservoir are not clear as to the leakage paths
reservoir covers an area of236km2with the capacity of
concem．Here，we attempted to compile a geological map
1，850×106m3．However，a considerable water leakage from
around the reservoir to understand geological stmctures and
the reservoirhas observed since the completion ofthe Koka
tectonic movements in addition to intelpretations of
dam at1960，and this is economically serious problem for
pemeabilityofrockmasses．
the region．Some attempts have been made to estimate the
leakage loss rate of reservoir water（Sogreah，1965；Ital
ToOographic an⊂l geologic characteristics around
Consult，1970；Halcrow，1989）．According to these reports，
theWonjiFaultBelt
the rate ranges ffom90to435×106m3／year（Mamo，1995；
Mamo andYokota，1998）．
1．NorthemMainEthiopianRiftandWo勾iFaultBelt
This area is topographically characterized by low rehef
The Kokadamanditsreservoirarelocated on the Wo可i
hills，whichhavebeenfomedashorstandgrabenstructures
Fault Belt（Mohr，1962，1967），which is one portion ofthe
in the northem MER．The hills around the reservoir
northem MER（Fig．1）．The MER is a part of East Afhcan
consist ofignimbrites and various volcanic rocks intercalated
Rift System，and comprises a series ofrift zones，extending
with post−Pliocene pumiceous and lacustrine deposits．
overadistanceofabout1，000㎞fromtheAf肛Thple
In generally，leakage path is closely related to topographic
Junction at the Red Sea−Gulf of Aden intersection to the
and geologic conditions including Physical properties of
Kenya Rift in the south（Fig．1）．In the Ethiopian Ri負zones，
rock masses around the reservoir．However，the leakage
theMER and the Southwestem Ethiopian Rift（SWER）are
paths are still presently unknown because rocks distributed
thetwom勾orhftsectors．TheMERfomstheno質heastem
sector，and is subdivided into northem，centra1，and southem
segments（Chrowiczαα1．，1994）．Younger volcanic rocks
＊Depaltment ofGeoscience，Shimane Univ．，Nishikawatsu，Matsue690−
ofthe Pliocene to Pleistocene age predominate in the MER．
8504，Japan
＊＊
In contrast，the SWER foms the southwestem sector and
dthiopian Institute of Geological Surveys，P．0．Box2302，Addis Ababa，
links to the north−south trending Kenya Rift in south．The
Ethiopia
113
Geological Structures a．nd Tectonics
114
around the KokaReservoir，Northem Main Ethiopian Rift
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Fig．1．lndex map ofthe Kokareservoirlocatedin the northem Main Ethiopian Rift．
SWER is characterized by predominantly older volcanics
MER．It flows northeastward from the westem margin of
rangingfromtheEocenetoMiocene（8．8．，WoldeGabrie1α
theMER，fo㎜ing steep gorges up to25kmupstream ofthe
α1．，1990；Stewart and Rogers，1996；Kabeto and Sawada，
Koka reservoir．It meanders east up to the reservoir with
1997，1999；Boccalettiαα1．，1999；Kabetoαα1．，2000）．
low gradient，and it finally attains to the Lake Abe in the
Two stages of rifting：1ate Oligocene to early Miocene，
Afar depression zone．The location of the Koka reservoir in
and late Miocene to early Pliocene were proposed for the
the midstream of the Awash River is regarded as a local
MER and SWER（Davidson，1983；WoldeGabrielαα1．，
depression zone elongating in NE−SW trend，and the
1990；Ebingerαα1．，1993）．The northem，central and
reservoir shapes an ellipsoid elongating NE−SW trend in
southem segments of the MER，and their major ri負一
plan．The zone also might have been formed as a result of
boundingfaultsareorientedtoN45。E，N30。EandN5−
tectonic movements in theWonji Fault Belt．Water level of
101E，respectively（KabetoandSawada，1999）．TheNE−SW
thereservoiris aroundEL．1590．7m．Although the bottom
gross structuraltrendoftheMERis segmentedby NW−SE
topography ofthe reservoir is not clear，the water depth may
trending mostly dextral transverse faults（6．8．，Mohr，1967；
be mostly shallower than10m．
WoldeGabrie1αα1．，1990；Chrowiczαα1．，1994；Hayward
As shown in Fig．2，the Koka dam is located in the
and Enginger，1996；Kabeto and Sawada，1999）．The latest
northeastem side of the reservoir，outlet of the Awash
stage of development ofthe MER in late Miocene to early
River．The eastem bank（right bank）of the reservoir is
Pliocene is also followed by a na皿ow well−developed
characterized by low relief hills ranging from1，600to
Quatemaryfaultingthatismostlyrelatedto theWonjiFault
1，800m，and their hilly slopes directly face to the reservoir．
Belt（Mohr，1962，1967）．Therefore，the distribution and
However，there exist shallow zones where elevation is
structures of the Quatemary strata are commonly affected
below the water level ofthe reservoiL On the contrary，wide
by faulting in this Belt．
plain spreads over around the westem bank（le負bank），and
The age and geochemical characteristics ofthe volcanism
scattering distribution of small isolated mounds is seen，
changes along and across the MER，and its complicated
where villages are located．Awash River and its large
history and nature reflects the tectonic evolution of the rift
tributary，the M句。 Riverjoins each other near the westem
（6．8．，Hartαα1．，1989；WoldeGabrie1αα1．，1990；Wolde，
bank of the reservoir．Along the upstream course of the
1996；Boccalettiαα1．，1999；Kabetoαα1．，2000）．Ebinger
Awash River in the westem bank，small marsh and ponds
andSleep（1998）haveals・c・ncludedthatupli丘ing，
are found separated from the main reservoir．As a result，
magmatism and extension in Ethiopia and east Africa is
while the eastem bankofthe reservoiris nearly straightline，
attributed to simple plume activities beneath the Ethiopian
the westem bank is irregular．These topographical contrasts
Plateau（Afar plume）．However，Georgeαα1．（1998）and
may be attributable to faulting and tilting in the area．
Rogersαα1．（2000）suggested that the presence of two
A slightly dissected Gedemsa Caldera is recognized in
mantle plumes，Afar and East Afヤican．
southeast ofthe reservoir．Its diameter is8km long，and its
westem slope faces the reservoir directly．A large fluvial
2．Topographic features in and around the Koka reservoir
plain，ofwhichelevation is around1，500m，spreads over
Fig．2shows the topography around the Koka reservoir
the northeastem side of the caldera，and this has been
and the Awash River．The Awash River on which the Koka
utilized for a large sugarcane farm，the Wonji Sugar
dam was built is one of the m句or rivers nowing in the
Plantation．
ShuichiroYokotaandSileshiMamoFantaye
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Fig．2．Topographic outline around the Kokareservoir．Koka dam is located at northeastem side ofthe reservoir．
3．Geological setup around the Koka reservoir
According to the compilation of Damteαα1．（1992）and
Fig．3shows a regional geologic map compiled by the
Boccalettiαα1．（1999），the Gedemsa area is located in
authors based on previous investigations （Ethiopian
geologically well studied Nazareth−Dera area，where
Institute of Geological Surveys，1981；Damteαα1．，1992）
severaltectonomagmatic units are identified；Melaksa（＜0．2
and additional field survey．The volcanic rocks in this area
Ma），Boseti（0．21Ma），Gedemsa（0．21−0．85Ma），Dera−
are mainly of rhyolitic ignimbrite（pyroclastic rocks）and
Sodere（く0．44Ma），Bofa（0．44−0．61Ma），Boku−Tede（0．51
rhyolitic to basaltic lava flows intercalated with lacustrine
−0．83Ma），Keleta（1．5Ma），Nazareth（1．41−1．71Ma），and
or pumiceous deposits．Recent fluvial and lacustrine
Eastem Margin（1．8Ma）units，respectively．Ignimbrites
deposits cover these partially．Basaltic rocks are distributed
and various volcanic rocks ofboth the Nazareth and Wonji
mainly along the downstream of the Awash River，and in
Groups are widely exposed in this area．The former
the Belale Forest，southeastem bank of the reservoir．These
constitute many flow units interbedded with paleosol layers
basaltic lava are named as Bofa Basalt（Ethiopian Institute
and aphyric floodbasalts（8．8．Meyerαα1．，1975；Boccaletti
of Geological Surveys，1981）．They are porphyritic with
αα1．，1999）．They also sometimes intercalated with ash fa11
plagioclase，and are vesicular．This was dated between O．44
1ayers and lacustrine deposits．Some NNE−SSW trending
and O．61Ma by whole rock K−Ar method（Mortonα
faults and minortraverse faults cause displacements in these
α1．，1979）．Basaltic rocks are also appears as numerous
strata，resulting in horst and graben structures．
volcanic cones．Damteαα1．（1992）divided basaltic rocks
The Gedemsa unit is related to the products of Gedemsa
distributed here into two members；Bofa Basalt and others．
Caldera collapse（Peccedlloαα1．，1995；Baccolettiα
However，it is not easy to discriminate them based on their
α1．，1999）．The pre−caldera activity gave nse to pantelleritic
characteristics．
rhyolite ignimbrites associated with ash nows，pumice falls，
According to Meyerαα1．（1975），the faulting in the
and surge deposits．Pantelleritic rhyolitic lava domes are
Wonji Fault Belt（WFB）started at the beginning of
also present in the pre−caldera sequences．The post caldera
Pleistocene（1．6Macα），causingtheunconfomitybetween
products are composed ofresurgent pantelleritic lava domes
theoverlying“Wo可i Series”（Pleistoceneto Holocene）and
and basaltic spatter cones scattered within the Caldera．
underlying “Nazareth Series” whose recent rift floor
Basaltic dikes，though to be ffom these scoria cones are
products are about1．8Ma（Bigazziαα1．，1993；Boccaletti
aligned in the NNE−SSW trend，which is almost similar to
8∫α1．，1999）．
theWonjiFaultBelttrend．
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around the KokaReservoir，Northem MainEthiopian Rift
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Shuichiro Yokota and Sileshi Mamo Fantaye
117
Fig．4．S撮ellite JERS−1image ar〔）und the northeastem bank ofthe Koka reservoir．
Recent lacustrine and臼uvial deposits cover the rocks in
this shows the topographical contrast well in this area．
the area，especially along the main river courses、They are
Scarplets ofNE−SW trending faults are recognized clearly．
composed of fine to medium sand，and sometimes with
Reddish portions in Fig。4show relatively wet condition
coarse sand，pumice，silt and tuff．These deposits may be
because the image is captures the spectral response to
thinnerthan20m．
infrared wavelength For example， reddish portion
Fig。4shows an image obtained by a satellite JERS−1，and
appearing in eastem part of the sub dam（earth dam）
Geological Structures and Tectonics
118
around the Koka Reservoir，Northem Main Ethiopian Rift
（d》
Fig．5．Koka reservoir and some exposures＆round it．
（a）Northem shoreline ofthe Kok＆reservoir viewed from Koka dam site．
（b）Jointed ignimbrite exposed along the shoreline ofthe Koka reservoir．
（c）Highlyweatheredignimbrite（upPer）andpumicefalldep・sits（1・wer）exp・sed＆ttherightbank｛）ftheK。kad乱m．
（d）Subdam（le負）andal・welevati・nz・neel・ngating・ute㎜ard，whichmaybeequivalentt・an・ldriverc〔）urse（right）．
indicatesthatthisareaisrelativelywetduet・alarge
quantity of water seeping from the sub dam（earth dam）．
Fig5（a）shows topography around the shoreline of the
Detail topography around the northeastern bank of
thereservoir
reservoir，and Figs5（b）and（c）show exposures of
Considering that reservoir water leaks toward the
ignimbrite with intercalated pumiceous sediments．Fig．5（d）
downstream of the Awash River，northeastem bank of the
shows the topographical features of a low elevation zone
reservoir and its surroundings may be the most probable
elongated northeastward from the sub dam．
zone on leakage，and therefore the area needs to be studied
in detaiL Fig．6shows a more detail topographic map of this
Shuichiro Yokotaand Sileshi MamoFantaye
119
〃
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Fig．6．Topography in the northeastern bank area．
area，which includes the main Koka dam and two sub dams．
other is at westem slopes of the Wonji Sugar Plantation
While one ofsub dams is located atthebank，3．5km south
（Fig．6）．
of the main Koka dam，the other one in the hollow zone
mentioned above．Both are small earth dam of5meters
Geological conditions in the northeastern bank area of
the reservoir
high，and were constructed just after the completion of the
main dam．
Fig．7shows a detail geologic map of the northeastem
As already stated，the alignment of the NNE−SSW
bank area，which the authors compiled based on their
trending horst and graben stnlcture are predominant in this
surveyed data and log data of boreholes investigated by Ital
area，and these are attributable to faulting of NNE−SSW
Consult（1970）．The hills are mostly composed of rhyolitic
trend．One of the horst blocks face to the reservoir，and the
ignimbrite and rhyolitic lava．Intercalations with some
otheris of5−8km east fromthe reservoirbank．The summit
pumiceous sediment were confirmed in log data also．
of such uplifted horsts shows a relatively low relief
Distribution ofbasaltic lava（Bofa Basalt）was confimed
The Awash River flowing out from the Koka dam
downstream of the Koka dam，and small scoria cones of
intersects these two uplifted horsts in this area，forming
basalt were also confirmed in the eastem hills near the
distinct gorges．One is the gorge just downstream of the
Wo可i Sugar Plantation。Although the lower and upper
Koka dam site，and the other is the Wonji Gorge，
strata ofthe Bofar Basalt have been defined as the Nazareth
downstream of the Hippo hot spring as shown in Fig．6．
and Wonji Groups，respectively，lithofacies however
There exist two small shallow grabens between these horst
resemble each other，and constmction of stratigraphy is
blocks．One is the zone elongating northeastward from
therefore not easy in this area．Then，we describe here only
reservoirbank at2km south ofthe Kokadam，and the other
the characteristics of individual lithofacies and their
is the zone elongating NNE−SSW trend around the Hippo
distribution as follows．
hot spring．Extension of the fomer is，however，not so
evident．
1．Ignimbrites（welded tuff）
Gentle slopes that have been formed as composite fan
Ignimbrites distributed here are welded tuff fo㎜ed as
deposits are recognized in some places．One is the left bank
pyroclastic now deposits，and characterized by lenticularly
of the Awash River in the Hippo hot spring area，and the
elongated pumices．They are intercalated with some thin ash
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Shuichiro Yokota and Sileshi Mamo Fantaye
121
1ayers with pumiceous and lacustrine deposits．Based on
Pumiceous and tuffaceous deposits are widely distributed
mapped data，at least4to5now units are distinguished in
here．Some of them are ash fall layers with pumices，but
this area，and they may belong to the Nazareth or Wonji
others are tuffaceous sandstones or mudstones formed as
Groups．While they are relatively hard in fresh exposures
lacustrine deposits，and are relatively consolidated．The
（Fig．5（b）），soft in weathered portions（upper portion of
latter are exposed around the Hippo Hot Springs and others．
Fig．5（c））．Especially，ignimbrites exposed around the sub
They appear to be stratigraphically below the ignimbrite in
dam are highly weathered．The ultimate product of their
some exposures．Probably，they may be same age as the
weathering comprise of soft and loose sand with brownish
pyroclastic and volcanic rocks mentioned above．
color．
Systematic joints are developed in exposures of
5．Surficial sediments
ignimbrite．Joint planes are generally tightly closed，but
Surficial deposits distributed along rivers and low land．
some of them are open．Based on the distribution of
They are divided into alluvial deposits and fan deposits．
ignimbrites exposed just to the downstream of the main
Especially，fan deposits may develop around the northem
Koka dam，its foundation too may be composed of these
side of the Hippo Hot Spring and westem slope of the
ignimbrites．Therefore，rock mass condition on permeable
Wonji SugarPlantation．Theyaremainlycomposedofloose
properties is considered to be relatively good．
sand，silt，and gravels．Around the shallow valley，gravelly
deposits are observed in well walls．Some breccias covering
2．Rhyolitic to dacitic lava（trachyte）and lava dome
gentle slopes are recognized around the sub dam．
The lavas distributed here are mainly rhyolite to trachyte．
They form small lava domes，and they show a monadnocks
topography．These rocks are highly vesicular，and rich in
Geological structure in the northeastern bank area of
the reservoir
Plagioclase and quartz phenocrysts． Various kind of
VOICaniC rOCkS are alSO eXpOSed a．10ng the Caldera Wa110f
L Horst and graben structures and faults
the Gedemsa Caldera．They are mostly rhyolitic to dacitic
As shown in Figs．6and7，this northeastem bank area is
lava，and some portions are surge deposits．The greenish
characterized by alignment of NNE−SSW trending faults．
welded tuff associated with Gedemsa Caldera was dated on
At least，three m勾or faults are topographically recognized
whole rock K−Ar method gave about O．85Ma（Mortonα
here，and they form horst and graben structures．However，
α1．，1979）．However，a fission track age on glass of a pre−
we could not confinm their fault planes in exposures．Based
caldera unit gave a more reasonable age of O．21Ma for the
on topography，m勾or faults show sense of easterly uplift．
Gedemsa unit（Bigazziαα1．，1993）．
They are numbered F−1，F−2and F−3from west to east．
One of them（F−1），having an easterly sense of uplift
3．Basaltic rocks
elongates in NNE−SSW trend intersectingjust upstream of
Basaltic rocks are mainly exposed as lava（Bofa Basalt）
the Koka dam．The other two faults，F−2and F−3extend
along the Awash River，especially downstream of the Koka
1．5to2km east and4km east ofthe shoreline，respectively
dam and around the Hippo hot spring．It was dated to O．44−
（Fig．7）．Each blocks bounded by faults slightly tilts
0．61Ma（Mortonαα1．，1979）．They include plagioclase
eastward．Most of strata also dip eastward almost
phenocryst，and show dark gray and dark greenish gray．
concordant with the topography．These faults and blocks
They may overly the ignimbrites mentioned above．Basaltic
elongate inNNE−SSWtrends intersecting thecourseofthe
lava is vesicular，and slightly to moderately jointed．Top of
Awash River．
the lava is highly weathered in there．
Fig．8shows geologic sections in A−A’and B−B’1ines as
Basaltic scoria cones are recognized at eastem hills．On
shown in Fig．7．Rocks in this area are mainly composed of
thewall ofanew well attheWo可i SugarPlantation，it was
ignimbrite，and small rhyolitic lava caps the fo㎜er
reddish scoria layer below the unconsolidated lacustrine
partially．Probably，they dip eastward at low angles．Based
deposits（alluvial deposits）．Basaltic dikes are also observed
On tOpOgraphy and diStribUtiOn Of lithOfa．CieS ShOWn in
along the area of shoreline．They intrude ignimbrite，and
Fig．8，vertical displacements are estimated to be50−60m in
elongate in N40。to50。E with3meters width at l km south
F−1andF−2，and70to100minF−3，respectively．Insome
ofthe main Koka dam．Several dykes with O．5m width and
places，we confirmed that strata dip steeply along these
N20。一30。E are also observed near the sub dam．Chilled
faults．In addition to these major faults，NW−SE trending
margin were recognized in these exposures．There may be
transverse faults are also estimated in individual blocks．
recent dikes from basaltic且ows of Melkasa unit（Damteα
α1．，1992），whose age is younger than O2Ma（Mortonα
2．Tectonic movements and river course changes
α1．，1979）．
Considering that the shorehne of the right bank almost
correspondto the scarplets ofthe NNE−SSW faults（F−1），
4．Pumiceous deposits and lacustrine deposits of tuffaceous
this depression domain of the reservoir might have been
sandstones and mudstones
fomedbyupliftingandtiltingoffaultblocks．Fig．9shows
Geological Stmctures and Tectonics
122
aroundthe KokaReservoir，Northem Main Ethiopian Rift
NE
SW← →
A Fg2
A’
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EL（m）
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9
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ゼ ヤ
Wonji Sugar Plantation
や ｝ レ 』、、 L L し
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1400 Pumicefa”Iaye「
50m
L25。m
Fig．8．Geologic sections ofthe northeastem bank area．
Locations of sections A−A’and B−B’are shown in Fig．7．
端諺
N→下
一，，》5τ5
・一．葦葺z5嘘（
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ぷ
Fig．9．Lowland elongating from the sub dam（earth dam）．
DottedportionshowstheareabelowEL．1590m（waterlevelofthereservoir）．Arrowsshowtheprobableoldriver
course before uplifting of fault blocks．
123
ShuichiroYokotaand SileshiMamoFantaye
a
d
b c
避 ㌦．動・w
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Awash River，ノ
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㌧ 一
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K。kaRese四。魯・
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（inundation） ＝コゼ
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Fig．10．Schematic image showing river course change ofthe Awash RiveL
a：Before faulting，the old Awash River flows from west to east．
b：NNE−SSW faulting and uplifting ofblocks begin．The old nver course（paleo＿chamel）was partially divided by uplifting of the
horst．
c：Fomationofthecun・enthvercourse．
d：The Koka dam was constnlcted along the new river course．After inundation，water level increased and water leakage occurred along
the old river course（paleo−channel）．
the disthbution of the area below the water level of the
efforts are necessary to dete㎜ine the paths more precisely．
reservoir（EL．1590m）．Although，a low elevation zone
elongates northeastward from the vicinity of the sub dam
Conclusions
（Fig．5（d）），this zone is not same as the graben，and it is
rather obliquely intersected by the NNE−SSW trending
To obtain fundamental info㎜ation on paths of leakage
horstandgrabenstmct皿es．Conside血gthatupliftingand
waterfromthe Kokareservoir，topographical and geological
tilting of blocks have continued up during the Quatemary
conditions around it have been studied． Results are
Period，such movements might have disturbed and shifted
highlighted as follows：
the river course．Based on topographic features，a shallow
（1）The reservoir is located along the Wor“i Fault Belt of
hollow zone elongating northeastward from the vicinity of
thenorthem Main Ethiopian Rift，and is topographically
the sub dam is estimated to be an old co皿se of the Awash
characterized by NNE＿SSW trending horst and graben
River．It elongates northeastward，but it is partially divided
stnlctures with distinct fault scarplets．
by an uplifted horst mound．
（2）Rhyolitic ignimbrites and various volcanic rocks
On the contrary，the current river course where the Koka
intercalated with pumiceous and lacustrine deposits
dam was constructed represents the newer one．While the
since the Pliocene constitute the hills around the
fomerco皿se is shallow and characterized by remarkably
reservOlr．
weathered rock mass，the latter deep and f士esh by rapid
（3）These structures mentioned above intersect the course of
dissecting．Based on these topographical and geological
the Awash River flowing out ffom the reservoir．
characteristics，a hollow zone colTesponding to a paleo−
（4）A low elevation zone elongating northeastward f士om the
channel also have become as one of leakage paths as water
point3．5km south of the Koka dam was infe皿ed to be
level rose after inundation．A model of the river co皿se
an old river course （paleo−channe1） on the basis of
shifts is schematically shown in Fig．10．
topographical features．
The course of the old Awash River was divided by
（5）In contrast，the c皿rentriver course where the Koka dam
uplifting of a horst from the downstream，which was
was constructed is newer．
accompanying NNE−SSW faulting along the present Koka
（6）Whilethepaleo−chamel is low elevation andcomposed
reservoir．However，ve並ical displacement is not unifom
of highly weathered rocks，the latter deep with f士esh by
along the fault generally．As a result，a new hver course
rapid dissecting．
appears in northem side corresponding to the current main
（7）Based on these features，the low elevation zone corre−
dam site．Fresh rock masses exposed along the newer river
sponding to a paleo−chamel might have become one of
mouse may be suitable forconstruction ofconcrete dam．
leakage paths as water level rose after inundation．
After inundation，a paleo−channel might have become a
leakage path as water level rose，because the shallow zone is
remarkably weathered with high pemeablerock mass and
alluvialloosedepositsdisthbutedalongit．Consequently，
Acknowle“gements
We sincerely thank Mr．Kurkura Kabeto Fello of the
the shallow zone corresponding to an old river co皿se might
graduate school of Shimane University for kindly help our
have also become as one of leakage paths．However，detail
field surveys and discussions on tectonics．We also would
paths are unknown at the present step．Another methods and
like to thank Dr．Venkatesh Ragahvan，Osaka City Univ．for
GeOlOgiCal StmCtUreS a．nd TeCtOniCS
124
around the KokaReservoir，Northem Main Ethiopian Rift
reading the manuscript，in addition for valuable comments．
Analysis of the satellite image of JERS−1（Fig．4）was
main Ethiopian rift stmctures，Proo．φfh88∫h ln∫εrnα∫ionol Co喰r8ncθ
‘ゾE孟h∫olρ’αn S伽4∫8s，κyoオ。，」ゆon，No．3，603−604．
Kabeto，K．and Sawada，Y．，1999，Precambrian basement stmctures：
supported by DL Shir中Masumoto of Osaka City Univ．
Implications for the Cenozoic Ethiopian Rift Stmctures，Proc．げ
Satellite image data used here were supplied by the Remote
1n∫6rnα∫ionαl Coψr8n680n G80みnα’n’cs‘∼プCon∫∫n8n如1R昴in8，1GCP
Sensing Data Center，Tokyo．
400，No．1，80−86．
Kabeto，K．，Sawada，Y．，Roser，B．O．，Abebe，T．，Chemet，T．，Masunaga，T。and
Wakatsuki，T．，2000，Compositional differences between evolved
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（Received：130ct．2000，Accepted：20Nov．2000）
Shuichiro Yokotaand SileshiMamoFantaye
（要 旨）
横田修一郎・SiIeshiMAMOFantaye，2000，北部エチオピア主地溝帯，Koka貯水池周辺における地
質構造とテクトニクス，島根大学地球資源環境学研究報告，19，113−125．
北部エチオピア主地溝帯に位置するKokaダムとその広大な貯水池はエチオピアにおける灌漑と
水力発電のための主要な施設である．しかしながら，Kokaダムが1960年に完成して以来，そこで
『は多量の漏水が指摘されてきた．貯水池からの漏水経路に関する基本的情報を得るため，貯水池周
辺の地形・地質的状態について研究してきた．
この貯水池は北部エチオピア主地溝帯のWonji断層帯に位置しており，明瞭な断層崖をもった小
規模な地累一地溝構造によって特徴づけられている．漏水の可能性が高い貯水池北東側の丘陵では
この構造はNNE−SSW方向に伸びており，貯水池から流出するAwash川の河道を横断している．鮮
新世以降の流紋岩質火砕流堆積物や多様な火山岩類が軽石質堆積物や湖成堆積物を挟んでこれらの
丘陵の岩盤を構成している．それらの地層は緩やかに傾斜し，断層によって部分的に変位されてい
る．
断層ブロックの隆起や傾動が第四紀を通じて継続してきたことを考えれば，そのような構造運動
は河道を移動させてきたことも考えられる．Kokaダムから約3．5km南の地点から北東方向に伸び
る低標高のゾーンは地形的特徴から旧河道の1つと推定された．この浅い低標高ゾーンは隆起した
地累ブロックによって分割されているが，貯水池の東岸から北東方向に伸びている．これに対して，
Kokaダムが建設された現在の河道は深く，新しいものであろう．前者は著しく風化した岩盤によっ
て特徴づけられているのに対して，後者は急速な下刻によって新鮮な状態である．このような地形・
地質的特徴に基づけば，旧河道に相当するこの低標高ゾーンは貯水による水位上昇にとともに漏水
経路の1つになってきたと考えられる．
125