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島根大学地球資源環境学研究報告 19,113~125ページGeoscience Rept.Shimane Univ.,19,p.113~125(2000)
(2000年12月)
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hemMain 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㎞southoftheKokadamwasinferred 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 torapid 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
lntroduction
The Koka dam and its reservoir,10cated in the northem
MainEthiopianRift(MER,Fig.1)are the main facilities for
irrigation and hydroelectric power in Ethiopia.The Koka
reservoir covers an area of236km2with the capacity of
1,850×106m3.However,a considerable water leakage from
the reservoirhas observed since the completion ofthe Koka
dam at1960,and this is economically serious problem for
the region.Some attempts have been made to estimate the
leakage loss rate of reservoir water(Sogreah,1965;Ital
Consult,1970;Halcrow,1989).According to these reports,
the rate ranges ffom90to435×106m3/year(Mamo,1995;
Mamo andYokota,1998).
This area is topographically characterized by low rehef
hills,whichhavebeenfomedashorstandgrabenstructures
in the northem MER.The hills around the reservoir
consist ofignimbrites and various volcanic rocks intercalated
with post-Pliocene pumiceous and lacustrine deposits.
In generally,leakage path is closely related to topographic
and geologic conditions including Physical properties of
rock masses around the reservoir.However,the leakage
paths are still presently unknown because rocks distributed
*Depaltment ofGeoscience,Shimane Univ.,Nishikawatsu,Matsue690-
8504,Japan** dthiopian Institute of Geological Surveys,P.0.Box2302,Addis Ababa,
Ethiopia
there are varied and geological structures are complicated.
One of the authors,Sileshi Mamo has attempted to
detemine the leakage paths倉om the viewpoints of
hydrogeology using isotope techniques.Geological conditions
around the reservoir are not clear as to the leakage paths
concem.Here,we attempted to compile a geological map
around the reservoir to understand geological stmctures and
tectonic movements in addition to intelpretations of
pemeabilityofrockmasses.
ToOographic an⊂l geologic characteristics around
theWonjiFaultBelt
1.NorthemMainEthiopianRiftandWo勾iFaultBelt The Kokadamanditsreservoirarelocated on the Wo可i
Fault Belt(Mohr,1962,1967),which is one portion ofthe
northem MER(Fig.1).The MER is a part of East Afhcan
Rift System,and comprises a series ofrift zones,extending
overadistanceofabout1,000㎞fromtheAf肛ThpleJunction at the Red Sea-Gulf of Aden intersection to the
Kenya Rift in the south(Fig.1).In the Ethiopian Ri負zones,
theMER and the Southwestem Ethiopian Rift(SWER)are
thetwom勾orhftsectors.TheMERfomstheno質heastemsector,and is subdivided into northem,centra1,and southem
segments(Chrowiczαα1.,1994).Younger volcanic rocks
ofthe Pliocene to Pleistocene age predominate in the MER.
In contrast,the SWER foms the southwestem sector and
links to the north-south trending Kenya Rift in south.The
113
114
Geological Structures a.nd Tectonics
around the KokaReservoir,Northem Main Ethiopian Rift
~“
「111一」
350E150N
500E
4oo
轡…(’\ヤ\.寓
吻/ノEthi・pia\鱒
!Awashl\…’一一 River,’贈tiGulfofAden
Addis メ し N Ababa ノー鹸」レ」ヤ」レ
、メm7ムnMaix手 メ/ Rese四〇1「/Σ!淋一MainEthi・岬i孟
▽て. ,〆ノ \.!!’一7/
Kenya) @/S。malia (
0 300kml lndian
一 1 0cean
350E
15。N
oo
500EFig.1.lndex map ofthe Kokareservoirlocatedin the northem Main Ethiopian Rift.
SWER is characterized by predominantly older volcanics
rangingfromtheEocenetoMiocene(8.8.,WoldeGabrie1α
α1.,1990;Stewart and Rogers,1996;Kabeto and Sawada,
1997,1999;Boccalettiαα1.,1999;Kabetoαα1.,2000).
Two stages of rifting:1ate Oligocene to early Miocene,
and late Miocene to early Pliocene were proposed for the
MER and SWER(Davidson,1983;WoldeGabrielαα1.,
1990;Ebingerαα1.,1993).The northem,central and
southem segments of the MER,and their major ri負一
boundingfaultsareorientedtoN45。E,N30。EandN5-101E,respectively(KabetoandSawada,1999).TheNE-SW
gross structuraltrendoftheMERis segmentedby NW-SE
trending mostly dextral transverse faults(6.8.,Mohr,1967;
WoldeGabrie1αα1.,1990;Chrowiczαα1.,1994;Hayward
and Enginger,1996;Kabeto and Sawada,1999).The latest
stage of development ofthe MER in late Miocene to early
Pliocene is also followed by a na皿ow well-developed
Quatemaryfaultingthatismostlyrelatedto theWonjiFault
Belt(Mohr,1962,1967).Therefore,the distribution and
structures of the Quatemary strata are commonly affected
by faulting in this Belt.
The age and geochemical characteristics ofthe volcanism
changes along and across the MER,and its complicated
history and nature reflects the tectonic evolution of the rift
(6.8.,Hartαα1.,1989;WoldeGabrie1αα1.,1990;Wolde,
1996;Boccalettiαα1.,1999;Kabetoαα1.,2000).Ebinger
andSleep(1998)haveals・c・ncludedthatupli丘ing,magmatism and extension in Ethiopia and east Africa is
attributed to simple plume activities beneath the Ethiopian
Plateau(Afar plume).However,Georgeαα1.(1998)and
Rogersαα1.(2000)suggested that the presence of two
mantle plumes,Afar and East Afヤican.
2.Topographic features in and around the Koka reservoir
Fig.2shows the topography around the Koka reservoir
and the Awash River.The Awash River on which the Koka
dam was built is one of the m句or rivers nowing in the
MER.It flows northeastward from the westem margin of
theMER,fo㎜ing steep gorges up to25kmupstream ofthe
Koka reservoir.It meanders east up to the reservoir with
low gradient,and it finally attains to the Lake Abe in the
Afar depression zone.The location of the Koka reservoir in
the midstream of the Awash River is regarded as a local
depression zone elongating in NE-SW trend,and the
reservoir shapes an ellipsoid elongating NE-SW trend in
plan.The zone also might have been formed as a result of
tectonic movements in theWonji Fault Belt.Water level of
thereservoiris aroundEL.1590.7m.Although the bottom
topography ofthe reservoir is not clear,the water depth may
be mostly shallower than10m.
As shown in Fig.2,the Koka dam is located in the
northeastem side of the reservoir,outlet of the Awash
River.The eastem bank(right bank)of the reservoir is
characterized by low relief hills ranging from1,600to
1,800m,and their hilly slopes directly face to the reservoir.
However,there exist shallow zones where elevation is
below the water level ofthe reservoiL On the contrary,wide
plain spreads over around the westem bank(le負bank),and
scattering distribution of small isolated mounds is seen,
where villages are located.Awash River and its large
tributary,the M句。 Riverjoins each other near the westem
bank of the reservoir.Along the upstream course of the
Awash River in the westem bank,small marsh and ponds
are found separated from the main reservoir.As a result,
while the eastem bankofthe reservoiris nearly straightline,
the westem bank is irregular.These topographical contrasts
may be attributable to faulting and tilting in the area.
A slightly dissected Gedemsa Caldera is recognized in
southeast ofthe reservoir.Its diameter is8km long,and its
westem slope faces the reservoir directly.A large fluvial
plain,ofwhichelevation is around1,500m,spreads over
the northeastem side of the caldera,and this has been
utilized for a large sugarcane farm,the Wonji Sugar
Plantation.
ShuichiroYokotaandSileshiMamoFantaye 115
\
・十
\
誘
↓
ノ吃 吃ぞ \箋K・kat・wn
咳叙ノ \
Awash River V 悼
〃
一ノ\16。。
Koka resen/oir
(EL1590.7m》
転
↓
1 〆 〆 ,ぜ ・
如 6
M
K・kadam評OAw箏hRiver 囲 ノ
諾錦 Dヂ@ハ 翁…轡・
経筒酵 ・ ~へ
β』
つ δ 、
1妨
毒1喰
o
180D
や暦箋
○ 8 撃 ス
1㌔
鴫
八姫
5km
Fig.2.Topographic outline around the Kokareservoir.Koka dam is located at northeastem side ofthe reservoir.
3.Geological setup around the Koka reservoir
Fig.3shows a regional geologic map compiled by the
authors based on previous investigations (Ethiopian
Institute of Geological Surveys,1981;Damteαα1.,1992)
and additional field survey.The volcanic rocks in this area
are mainly of rhyolitic ignimbrite(pyroclastic rocks)and
rhyolitic to basaltic lava flows intercalated with lacustrine
or pumiceous deposits.Recent fluvial and lacustrine
deposits cover these partially.Basaltic rocks are distributed
mainly along the downstream of the Awash River,and in
the Belale Forest,southeastem bank of the reservoir.These
basaltic lava are named as Bofa Basalt(Ethiopian Institute
of Geological Surveys,1981).They are porphyritic with
plagioclase,and are vesicular.This was dated between O.44
and O.61Ma by whole rock K-Ar method(Mortonα
α1.,1979).Basaltic rocks are also appears as numerous
volcanic cones.Damteαα1.(1992)divided basaltic rocks
distributed here into two members;Bofa Basalt and others.
However,it is not easy to discriminate them based on their
characteristics.
According to Meyerαα1.(1975),the faulting in the
Wonji Fault Belt(WFB)started at the beginning of
Pleistocene(1.6Macα),causingtheunconfomitybetween
theoverlying“Wo可i Series”(Pleistoceneto Holocene)and
underlying “Nazareth Series” whose recent rift floor
products are about1.8Ma(Bigazziαα1.,1993;Boccaletti
8∫α1.,1999).
According to the compilation of Damteαα1.(1992)and
Boccalettiαα1.(1999),the Gedemsa area is located in
geologically well studied Nazareth-Dera area,where
severaltectonomagmatic units are identified;Melaksa(<0.2
Ma),Boseti(0.21Ma),Gedemsa(0.21-0.85Ma),Dera-
Sodere(く0.44Ma),Bofa(0.44-0.61Ma),Boku-Tede(0.51
-0.83Ma),Keleta(1.5Ma),Nazareth(1.41-1.71Ma),and
Eastem Margin(1.8Ma)units,respectively.Ignimbrites
and various volcanic rocks ofboth the Nazareth and Wonji
Groups are widely exposed in this area.The former
constitute many flow units interbedded with paleosol layers
and aphyric floodbasalts(8.8.Meyerαα1.,1975;Boccaletti
αα1.,1999).They also sometimes intercalated with ash fa11
1ayers and lacustrine deposits.Some NNE-SSW trending
faults and minortraverse faults cause displacements in these
strata,resulting in horst and graben structures.
The Gedemsa unit is related to the products of Gedemsa
Caldera collapse(Peccedlloαα1.,1995;Baccolettiα
α1.,1999).The pre-caldera activity gave nse to pantelleritic
rhyolite ignimbrites associated with ash nows,pumice falls,
and surge deposits.Pantelleritic rhyolitic lava domes are
also present in the pre-caldera sequences.The post caldera
products are composed ofresurgent pantelleritic lava domes
and basaltic spatter cones scattered within the Caldera.
Basaltic dikes,though to be ffom these scoria cones are
aligned in the NNE-SSW trend,which is almost similar to
theWonjiFaultBelttrend.
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Geological Stmct皿es and Tectonics
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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
the area,especially along the main river courses、They are
composed of fine to medium sand,and sometimes with
coarse sand,pumice,silt and tuff.These deposits may be
thinnerthan20m.
Fig。4shows an image obtained by a satellite JERS-1,and
this shows the topographical contrast well in this area.
Scarplets ofNE-SW trending faults are recognized clearly.
Reddish portions in Fig。4show relatively wet condition
because the image is captures the spectral response to
infrared wavelength For example, reddish portion
appearing in eastem part of the sub dam(earth dam)
118
Geological Structures and Tectonics
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・alargequantity of water seeping from the sub dam(earth dam).
Fig5(a)shows topography around the shoreline of the
reservoir,and Figs5(b)and(c)show exposures of
ignimbrite with intercalated pumiceous sediments.Fig.5(d)
shows the topographical features of a low elevation zone
elongated northeastward from the sub dam.
Detail topography around the northeastern bank of
thereservoir
Considering that reservoir water leaks toward the
downstream of the Awash River,northeastem bank of the
reservoir and its surroundings may be the most probable
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
N手
Koka resen’oir
(EL1590.7m)
sub dam ゴ ノ(earth daml,’
’ ,
4卿欝δ h
落
裟9
虜
Koka dam
愛
蘭
麟 〃
ヂ
、。5,彪
~4ち
多1625嬬
~翻
、625
08h
翻
8~
1625
照.留
75z5
臥 warm sprhgs zone
も\、,,
% W・njiG。rge箆 忽.
0 、
亀HipP・H・tSpring
グ
。
、5π5
1575
諺
2
罐
灘鯵.
鱗汐
観望
騨
驚ゲ
7
グ ロ ヨ ロ ド め
集
麟,α51。km
Fig.6.Topography in the northeastern bank area.
area,which includes the main Koka dam and two sub dams.
While one ofsub dams is located atthebank,3.5km south
of the main Koka dam,the other one in the hollow zone
mentioned above.Both are small earth dam of5meters
high,and were constructed just after the completion of the
main dam.
As already stated,the alignment of the NNE-SSW
trending horst and graben stnlcture are predominant in this
area,and these are attributable to faulting of NNE-SSW
trend.One of the horst blocks face to the reservoir,and the
otheris of5-8km east fromthe reservoirbank.The summit
of such uplifted horsts shows a relatively low relief
The Awash River flowing out from the Koka dam
intersects these two uplifted horsts in this area,forming
distinct gorges.One is the gorge just downstream of the
Koka dam site,and the other is the Wonji Gorge,
downstream of the Hippo hot spring as shown in Fig.6.
There exist two small shallow grabens between these horst
blocks.One is the zone elongating northeastward from
reservoirbank at2km south ofthe Kokadam,and the other
is the zone elongating NNE-SSW trend around the Hippo
hot spring.Extension of the fomer is,however,not so
evident.
Gentle slopes that have been formed as composite fan
deposits are recognized in some places.One is the left bank
of the Awash River in the Hippo hot spring area,and the
other is at westem slopes of the Wonji Sugar Plantation
(Fig.6).
Geological conditions in the northeastern bank area of
the reservoir
Fig.7shows a detail geologic map of the northeastem
bank area,which the authors compiled based on their
surveyed data and log data of boreholes investigated by Ital
Consult(1970).The hills are mostly composed of rhyolitic
ignimbrite and rhyolitic lava.Intercalations with some
pumiceous sediment were confirmed in log data also.
Distribution ofbasaltic lava(Bofa Basalt)was confimed
downstream of the Koka dam,and small scoria cones of
basalt were also confirmed in the eastem hills near the
Wo可i Sugar Plantation。Although the lower and upper
strata ofthe Bofar Basalt have been defined as the Nazareth
and Wonji Groups,respectively,lithofacies however
resemble each other,and constmction of stratigraphy is
therefore not easy in this area.Then,we describe here only
the characteristics of individual lithofacies and their
distribution as follows.
1.Ignimbrites(welded tuff)
Ignimbrites distributed here are welded tuff fo㎜ed as
pyroclastic now deposits,and characterized by lenticularly
elongated pumices.They are intercalated with some thin ash
LEGEND
一一T欝Pleistocene
to Pliocene?
騒理B犠臨,匪垂ヨBa潔臨、
囲Rhyditicr・cks
匪調Pumice・usdep・sits
N
圃1gnimbrite(weldedtu行)
/ Faults/ ● Bore holesB・1
・ 、、、し
イを・
<・、」
へ4
《
虜じ“k
色・ill’ !、、7 、 ! 、 ’ 1『’ 1, ” 一’ ノ ロ ド ヤ ノ ラ
、2P}mlcequsて一×ノ
磯辮§itl影S誉、 /、禦娩篠
/Kokadam!
孝繊}警簑 り .」」 一L J」
一己_ ユー 一L. ソ し
v ムbasaltic Iava J.
=一L・ _L_ 一_」一 _』ぐ ノしむ ム ム ユ ノ ソ
v肖 一L一L→1/》ソv》 v 》」 一」↓‘ ) v
〉 V ) 、!t、 ) V い V V 》 V v ’こ、 .,、
/ Ignimbrite
影の“f1ヤ/(糊t砦1<一’わd轍諺 籔感傷,. ・、ll~へ㌔一’2・》) τつ5751、ム
シ鷲’鳩・ヤ 〆プイ⊃,一臨,・今・、㌧・ム・無偏塞㌧.》v鷲鶴舞.1今・㎞藤・1 熱讃麓薩§繭ぴド、.
懸で,、≧斜 》 1 /で』・、沖ノ, 。
ロ ヤノ諺)vどミ㌧しし1》
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ゾ ノレ
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、た婆“レ ソ
ノ門 4∠煩∠ ム』曳一』一
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1575,ム
’A
.一△ ’
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輪潅;1蒋燦
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》
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ろ.’ .』z懲 ..ノ 擦 ’h {’
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レ ソ ソ マ ソ
脅)vvvゾv▽謁62もQし
雛辣礁、㌔v
ヤ ・ 蕊加齢
へゆ% ゐ 4’雛噸 ~6◎◎
ψら’
ぷ
鎌㍗α51.Okm
Fig.7.Geological map ofthe northeastem bank area.
筒O
OOO一〇〇Q一〇包GつqgO一仁賊Oの”昌O↓090三〇〇〇
弩。=昌O旨O】〈O国”刃OのO目<O一おZO昌『Oコ」≦巴コ団浮一〇℃置目刀一陣
Shuichiro Yokota and Sileshi Mamo Fantaye 121
1ayers with pumiceous and lacustrine deposits.Based on
mapped data,at least4to5now units are distinguished in
this area,and they may belong to the Nazareth or Wonji
Groups.While they are relatively hard in fresh exposures
(Fig.5(b)),soft in weathered portions(upper portion of
Fig.5(c)).Especially,ignimbrites exposed around the sub
dam are highly weathered.The ultimate product of their
weathering comprise of soft and loose sand with brownish
color.
Systematic joints are developed in exposures of
ignimbrite.Joint planes are generally tightly closed,but
some of them are open.Based on the distribution of
ignimbrites exposed just to the downstream of the main
Koka dam,its foundation too may be composed of these
ignimbrites.Therefore,rock mass condition on permeable
properties is considered to be relatively good.
2.Rhyolitic to dacitic lava(trachyte)and lava dome
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
Plagioclase and quartz phenocrysts. Various kind of
VOICaniC rOCkS are alSO eXpOSed a.10ng the Caldera Wa110f
the Gedemsa Caldera.They are mostly rhyolitic to dacitic
lava,and some portions are surge deposits.The greenish
welded tuff associated with Gedemsa Caldera was dated on
whole rock K-Ar method gave about O.85Ma(Mortonα
α1.,1979).However,a fission track age on glass of a pre-
caldera unit gave a more reasonable age of O.21Ma for the
Gedemsa unit(Bigazziαα1.,1993).
3.Basaltic rocks
Basaltic rocks are mainly exposed as lava(Bofa Basalt)
along the Awash River,especially downstream of the Koka
dam and around the Hippo hot spring.It was dated to O.44-
0.61Ma(Mortonαα1.,1979).They include plagioclase
phenocryst,and show dark gray and dark greenish gray.
They may overly the ignimbrites mentioned above.Basaltic
lava is vesicular,and slightly to moderately jointed.Top of
the lava is highly weathered in there.
Basaltic scoria cones are recognized at eastem hills.On
thewall ofanew well attheWo可i SugarPlantation,it was
reddish scoria layer below the unconsolidated lacustrine
deposits(alluvial deposits).Basaltic dikes are also observed
along the area of shoreline.They intrude ignimbrite,and
elongate in N40。to50。E with3meters width at l km south
ofthe main Koka dam.Several dykes with O.5m width and
N20。一30。E are also observed near the sub dam.Chilled
margin were recognized in these exposures.There may be
recent dikes from basaltic且ows of Melkasa unit(Damteα
α1.,1992),whose age is younger than O2Ma(Mortonα
α1.,1979).
4.Pumiceous deposits and lacustrine deposits of tuffaceous
sandstones and mudstones
Pumiceous and tuffaceous deposits are widely distributed
here.Some of them are ash fall layers with pumices,but
others are tuffaceous sandstones or mudstones formed as
lacustrine deposits,and are relatively consolidated.The
latter are exposed around the Hippo Hot Springs and others.
They appear to be stratigraphically below the ignimbrite in
some exposures.Probably,they may be same age as the
pyroclastic and volcanic rocks mentioned above.
5.Surficial sediments
Surficial deposits distributed along rivers and low land.
They are divided into alluvial deposits and fan deposits.
Especially,fan deposits may develop around the northem
side of the Hippo Hot Spring and westem slope of the
Wonji SugarPlantation.Theyaremainlycomposedofloose
sand,silt,and gravels.Around the shallow valley,gravelly
deposits are observed in well walls.Some breccias covering
gentle slopes are recognized around the sub dam.
Geological structure in the northeastern bank area of
the reservoir
L Horst and graben structures and faults
As shown in Figs.6and7,this northeastem bank area is
characterized by alignment of NNE-SSW trending faults.
At least,three m勾or faults are topographically recognized
here,and they form horst and graben structures.However,
we could not confinm their fault planes in exposures.Based
on topography,m勾or faults show sense of easterly uplift.
They are numbered F-1,F-2and F-3from west to east.
One of them(F-1),having an easterly sense of uplift
elongates in NNE-SSW trend intersectingjust upstream of
the Koka dam.The other two faults,F-2and F-3extend
1.5to2km east and4km east ofthe shoreline,respectively
(Fig.7).Each blocks bounded by faults slightly tilts
eastward.Most of strata also dip eastward almost
concordant with the topography.These faults and blocks
elongate inNNE-SSWtrends intersecting thecourseofthe
Awash River.
Fig.8shows geologic sections in A-A’and B-B’1ines as
shown in Fig.7.Rocks in this area are mainly composed of
ignimbrite,and small rhyolitic lava caps the fo㎜er
partially.Probably,they dip eastward at low angles.Based
On tOpOgraphy and diStribUtiOn Of lithOfa.CieS ShOWn in
Fig.8,vertical displacements are estimated to be50-60m in
F-1andF-2,and70to100minF-3,respectively.Insome
places,we confirmed that strata dip steeply along these
faults.In addition to these major faults,NW-SE trending
transverse faults are also estimated in individual blocks.
2.Tectonic movements and river course changes
Considering that the shorehne of the right bank almost
correspondto the scarplets ofthe NNE-SSW faults(F-1),
this depression domain of the reservoir might have been
fomedbyupliftingandtiltingoffaultblocks.Fig.9shows
122
Geological Stmctures and Tectonics
aroundthe KokaReservoir,Northem Main Ethiopian Rift
SW← →A Fg2 F-1EL(m)
17・O B3 B7B81gnimbr旋e(weldedtu貿)B11,6舗d㌦1他↓AII蹴麟醐“ IRh四雁F3
レ へたミヤソ ト レソ サ サ ヤ ソい レ サ
評謙総締鷲雛1☆」遡論調蔑鰯 1400
NE
A’EL(m)
1700
1600
1500
1400
→ E W ←BEL(m) B2
隙hdam↓1600↓A“uvialdep。sits
F・2
1gnimbrite(weldedtuff》
レ v v 噌
レ レ レ ソ
v 》 }
Rhyolite
し しゼ ヤ
ず ) v ぜ ㌔、L L
9
B↓
1500 ィ撃;謂1400 Pumicefa”Iaye「
1
F・3
LししL・し Rhyolite 、、 L L し 』 く v v ~、一、、l L し
や } レ 』、、 L L し
} V V 》 ソ 、、』、と』しし》 v ) ソ V v 》 }、一-・
) v レ v v ■ ) ) v v ザ ソ い v 》
EL(m) 1700
Wonji Sugar Plantation
v} A A”uvial 1600・11ヤdep。sits
l ロ し じロ じ ゴロ ニ1ド1黛※・
り り むの
t
1400
B’
H:Vニ1:5
50mL25。m
Fig.8.Geologic sections ofthe northeastem bank area.
Locations of sections A-A’and B-B’are shown in Fig.7.
N↑下端諺
購}
プ
/Koka dam.
)9賜4 〃
諺…蛎箔.総
ヤヘ625
ぺ .=
ン 〃暢
.遼 避Koka resen’Olr
/ ヨ じ ‘ .:な=・ 壷鮪’・・ ひ ■ ‘・ ・
. ●
‘軸 ■ ■● o ●
∠
/,急≦灘蔽
eadhdaソ1勝浦溢噸 御 翻 7霧, =三二・い~ゆ 』 浮 h
/1625 、慶
瀞…
繍
@
A鱒
鰯 ・一.葦葺z5嘘( 一,,》5τ5 り へ り い し し 樋 ・一1・・ご●●’一’●・・●∴二∵・一・、・.. … ’・i’㌔・’.’●・1.ご:・,’1ご,..も∵・.,
}.ヒ1難敵三1、い::・・’ドしll‘1』:●㌧’・
灘難●鑓驚欝,麟’
髪纂響灘…考騨鞭
1625
拠.硬
@
灘鍵 等〃
ぷ
2
窮
》裂借
7
グ
野馳・ ■ 畠
麟。。5t。㎞
Fig.9.Lowland elongating from the sub dam(earth dam).
DottedportionshowstheareabelowEL.1590m(waterlevelofthereservoir).Arrowsshowtheprobableoldrivercourse before uplifting of fault blocks.
ShuichiroYokotaand SileshiMamoFantaye 123
a
’~・、 餉 ’ } ~ プー ’ 、’一1 ,夢 な、.!〆
Awash River,ノ ’ ノ ノレ で
ノ し N f∫、
牟 ノ ■ 3 ’ ●鴨 ■ ■ ・ し .. 0 コレ
㌧ 一
b c
避 ㌦.動・w
膨オ.轟
AwasXp腋)Aw一腹ζ釦p㈹ r , ま ミ ノ す5km ‘, ご1
d
Koka Dam/ Awash River
’暑ミ、 、 ’ ノノ 一_→ 一一」 ,’ 畦 ・タ/御
章 ! おノ のノ 罰 Wate「leakage?
要Subdam
K。kaRese四。魯・
コ (inundation) =コゼ
量
ゴ ゴ
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
reservoir(EL.1590m).Although,a low elevation zone
elongates northeastward from the vicinity of the sub dam
(Fig.5(d)),this zone is not same as the graben,and it is
rather obliquely intersected by the NNE-SSW trending
horstandgrabenstmct皿es.Conside血gthatupliftingandtilting of blocks have continued up during the Quatemary
Period,such movements might have disturbed and shifted
the river course.Based on topographic features,a shallow
hollow zone elongating northeastward from the vicinity of
the sub dam is estimated to be an old co皿se of the Awash
River.It elongates northeastward,but it is partially divided
by an uplifted horst mound.
On the contrary,the current river course where the Koka
dam was constructed represents the newer one.While the
fomerco皿se is shallow and characterized by remarkably
weathered rock mass,the latter deep and f士esh by rapid
dissecting.Based on these topographical and geological
characteristics,a hollow zone colTesponding to a paleo-
channel also have become as one of leakage paths as water
level rose after inundation.A model of the river co皿se
shifts is schematically shown in Fig.10.
The course of the old Awash River was divided by
uplifting of a horst from the downstream,which was
accompanying NNE-SSW faulting along the present Koka
reservoir.However,ve並ical displacement is not unifom
along the fault generally.As a result,a new hver course
appears in northem side corresponding to the current main
dam site.Fresh rock masses exposed along the newer river
mouse may be suitable forconstruction ofconcrete dam.
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,
the shallow zone corresponding to an old river co皿se might
have also become as one of leakage paths.However,detail
paths are unknown at the present step.Another methods and
efforts are necessary to dete㎜ine the paths more precisely.
Conclusions
To obtain fundamental info㎜ation on paths of leakage
waterfromthe Kokareservoir,topographical and geological
conditions around it have been studied. Results are
highlighted as follows:
(1)The reservoir is located along the Wor“i Fault Belt of
thenorthem Main Ethiopian Rift,and is topographically
characterized by NNE_SSW trending horst and graben
stnlctures with distinct fault scarplets.
(2)Rhyolitic ignimbrites and various volcanic rocks
intercalated with pumiceous and lacustrine deposits
since the Pliocene constitute the hills around the
reservOlr.
(3)These structures mentioned above intersect the course of
the Awash River flowing out ffom the reservoir.
(4)A low elevation zone elongating northeastward f士om the
point3.5km south of the Koka dam was infe皿ed to be
an old river course (paleo-channe1) on the basis of
topographical features.
(5)In contrast,the c皿rentriver course where the Koka dam
was constructed is newer.
(6)Whilethepaleo-chamel is low elevation andcomposed
of highly weathered rocks,the latter deep with f士esh by
rapid dissecting.
(7)Based on these features,the low elevation zone corre-
sponding to a paleo-chamel might have become one of
leakage paths as water level rose after inundation.
Acknowle“gements
We sincerely thank Mr.Kurkura Kabeto Fello of the
graduate school of Shimane University for kindly help our
field surveys and discussions on tectonics.We also would
like to thank Dr.Venkatesh Ragahvan,Osaka City Univ.for
124
GeOlOgiCal StmCtUreS a.nd TeCtOniCS
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
supported by DL Shir中Masumoto of Osaka City Univ.
Satellite image data used here were supplied by the Remote
Sensing Data Center,Tokyo.
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Shuichiro Yokotaand SileshiMamoFantaye 125
(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.