821-051-73 Explanatory Note of Geological Map Ethiopia
821-051-73 Explanatory Note of Geological Map Ethiopia
821-051-73 Explanatory Note of Geological Map Ethiopia
EXPLANATION OF THE
Complied by
Published by EIGS
Technical publications Team, 1999
;.
I. fNTRODUCTION
The first 1:2,000,000 scale GElological Map of Ethiopia with an explanatory note
was compiled by V. Kazmin and published in 1972 by the Ethiopian Institute of
Geological Surveys. The map together with 'other regional compilation works (e.g.
DaneUl, 1943; Mohr, 1963; Merla et aI., 1973) has served Its purpose by providing a
broad overview of the country's geology.
Our knowledge of the geology of Ethiopia when the first edition was complied
was however at Its infancy. Although there still remains a lot of geological research to
be done in the country, a considerable amount of new geologic information has
accumulated in particular in parts of tl1e Great East African Rift valley in Ethiopia, and
in the western and southern parts of the country in the I~st two decades. This
accumulated data has inevitably led to the compilation of the present 2ndedition of the
Geological Map of Ethiopia at the same 1:2,000,000 scale. An attempt to give a
balanced emphasis to mappable geolc>gicspace-time units both as a geochronometric,
genetic and/or paragenetic units have enabled to take this compilation a step ahead
from its predecessors. The main sources of information used for the compilation of the
map are listed beside the geological map (pocket).
The geological units in Ethiopia fall into one of the following three major
categories; the Precambrian Basement, Late Paleozoic to Early Tertiary sediments and
the. Cenozoic volcanic and associated
.
sedimentary rocks. The task of compiling of the
2nded.itiongeological map and the acccmpanying brief explanatory note was entrusted
to a team consisting of three geologists. Mengesha Tefera was incharge of the task of
compilation of the map, and report on the Precambrian basement and overall
coordination and final a$.semblyof the map with the accompanying explanatory .note.
Workineh Haro. was responsible for the compilation of the map and note on the
. Late
Paleozoic to Early Tertiary glacial and marine sediments.. The responsibility of
preparing the map and Dote on the Cenozoic volcanic and the associated sedimentary
units and1he final editing of the explanatory note was given to Tadiwos Chernet.
---
I, INTRODUCTION
The first 1:2,000,000 scale Geological Map of Ethiopia with an explanatory note
was compiled by V. Kazmin and published in 1972 by the Ethiopian Institute of
Geological Surveys. The map tooether with 'other regional compilation works (e.g.
Danelll, 1943; Mohr, 1963; Merla 19taI., 1973) has served its purpose by providing a
broad overview of the country's gecllogy.
Our knowledge of the geology of Ethiopia when the first edition was compiled
was however at its infancy. Although there still remains a lot of geological research to
be done in the country, a considerable amount of new geologic information has
accumulated in particular in parts of the Great East African Rift valley in Ethiopia, and
in the western and southern parts of the country in the I~st two decades. This
accumulated data has.inevitably led to the compilation of the present 2ndedition of the
Geological Map of Ethiopia at tho same 1:2,000,000 scale. An attempt to give a
balanced emphasis to mappable gelologicspace-time units both as a geochronometric,
genetic and/or paragenetic units h,ave enabled to take this compilation a step ahead
from its predecessors. The main sources of information used for the compilation of the
map are listed beside the geological map (pocket).
The geological units in Ethiopia fall into one of the following three major
categories; the Precambrian Basement, Late Paleozoic to Early Tertiary sediments and
the. Cenozoic volcanic and associated
.
sedimentary rocks. The task of compiling of the
2nded.itiongeological map and the accompanying brief explanatory note was entrusted
to a team consisting of three geolo'lists. Mengesha Tefera was incharge of the task of
compilation of the map. and report on the Precambrian basement and overall
coordination and final a$.semblyof the map with the accompanying explanatory note.
Workineh Haro was responsible for the compilation of the map and note on t~e Late
Paleozoic to Early Tertiary glacial and marine sediments.. The responsibility. of
preparing the map and note on the Cenozoic volcanic and the associated sedimentary
units and1he final editing of the explanatory note was given to Tadiwos Chemet.
- -~
- --- - - --
The blocks of gneissic terranes which are considered to be generally older than
the v~lcano-sedimentary belts they commonly enclose, consist of high grade
heterogeneous orthogneisses and paragneisses, at upper amphibolite to granulite
facies metamorphism. Metamorphic facies in the low grade volcano-sedimentary
succession, however, typically ranges from greenschist to lower amphibolite facies.
The most conspicuous foliation trend is north-south with deviations to northeast and
northwest. These trends are characteristics of both the loWgrade volcano-sedimentary
succession and the high grade rocks. The boundaries between the gneissic and
volcano-sedimentary sequences are typi~lIy of tec;tonicorigin.
I
2
Sheared, mylonitized and highly tectonically deformed rocks occur particularly along
the contacts between the gneissic and the low grade volcano-sedimentary belts.
The Precambrian shield.ir Ethiopia ~ccupies a unique position in this part of the
African continent situated between the predominantly gneissic rocks of the
Mozambique Belt, to the south in eastern and southern Africa, and the volcano-
sedimentary-plutonic complexes along a strike to the north bordering the Red Sea
(Eritrea and Sudan, Fig. 2). The Mozambique Belt (Holmes, 1951) has been
recognized as a polycyclic complex comprising units of highly varying lithology,
metamorphic grade and age (Almond, 1984; Cahen et al. 1984). It contains gneisses
of at least Early Proterozoic and possibly Archean age, but most of the isotopic dates
reflect. the Late Proterozoic to Early Paleozoic ("Pan-African") tectono-thermal events
(Almond, 1984; Black, 1989). Loggo and Almond,(1984) however have authenticated
the Archaean age of at least SO'lle of the high grade gneisses (Archean gneisses of
northern Uganda). Recent studil3s have also shown that the Red Sea region consists
of at"least five northerly trending
..
volcano-sedimentary belts, largely separated by strips
that contain ophiolitic rocks. These are now generally accepted to be accreted arc
complexes of Pan-African age (ca. 1000-450 Ma) with intervening remnants of
dismembered oceanic crust (StoE!sseret aI., 1985; Vail, 1983 & 1985). Within Ettliopia,
the Precambrian basement contains elements that resemble both of the above
terranes.
-- - - - ---
- - - -- - - ---
-
(Adola Group PR2a) with its attendant intrusives occurs enclosed by gneissic terranes
of Early Proterozoic to Archaean age (ARk, AR1, ARa, ARy, PR1w and PR1r (see
geological map in pocket).
Early or pre-tectonic plutonic bodies (gb, dt, tn, gd, gt, and gt2) are internally
foliated and concordant with their host rocks. There are, however, equidimensional
and discordant intrusions (gt3 and gt4) of Late Proterozoic to Early Paleozoic age.
These intrusives mark the end of tectonic activity in Late Proterozoic to Early Paleozoic
and are known as post or late tectonic intrusions. Intrusions of alkaline magmas (alkali
granites and syenites; gt5) of Tertiary age are relatHd to the early phases of rifting in
the Afar are also remarkable.
Most of the known economic ore deposits such as auriferous quartz veins,
volcanogenic sulfide mineralizations are associated with the low grade volcano-
sedimentary assemblages. The distinction of this Jnit from the high grade gneissic
terrane is of paramount importance in the search for mineral deposits. Base-metal and
auriferous quartz vein mineralizations, characteristic of the volcano-sedimentary
complexes-are absent in the high grade terrane.
Two major transgression-regression cycles took place during the Mesozoic era
(Kazmin 1972). The first transgression started in the Early Jurassic or Late Triassic
from the Ogaden region in the Southeast towards northwest and reached its maximum
extent in Kimmeridgian. During this time Adigrat Formation (Ja) consisting mainly of
6
sandstone and minor lenses of silt~;tone;Hamanilei Formation (Jh) consisting mainly of
limestone and dolomite; Abay Formation (Jb) consisting of limestone, sandstone,
gypsum and shale; Urandab and Antalo Formation (Ju and Jt) consisting mainly of
fossilferous limestones were deposited. The transgressive sea appears to have
reached as far west as 36°E 10ngitLde.
The regression of the sea started towards the end of Jurassic depositing
lagoonal facies of the Agula Formation (Jag), which consists of black shale, marl and
claystone with beds of limestone, ~ypsum and dolomite in the Mekele area in northern
Ethiopia. The Gabredare Formation (Jg1 and Jg2) consisting of limestone and marl
marks the upper most part of thE! Jurassic sedimentary succession in the Ogaden
region, in eastern E~hiopia.
A third and less extensive transgressive event took place in Late Cretaceous
until Middle to Late Eocene depositing Jessoma Formation (Pj) consisting mai,nlyof
sandstone; Taleh Formation (Pt) consisting mainly of anhydrite, and Karkar Formation
(Pk) consisting qf limestone interbedded with shale and gypsum in the eastern extreme
corner of the Ogaden region. The Mesozoic sedimentary sequence contains many
prospective areas for hydrocarbon exploration. Exploration done in the-Ogaden region
during the last few decades has defined a gas field at Calub.
Following the Late Mesozoic-Early Tertiary transgression of the sea from the
south-east an epierogenic uplift of Afro-Arabia (East Africa together with Arabian
Peninsula and the intervening regions now occupied by the Red sea and Gulf of Aden)
--
----
occurred on an immense scale. According to Mohr (1962) the magnitude of the uplift
was such that nowhere in the world outside the orogEmicbelts have basement rocks
been uplifted to such an elevation as that associated with the East African swell. The
cause of the major uplift is related to a mantle plume!whose decompression melting
generated enormous quantities of basaltic magma in th,alithosphere and resulted in the
formation of a classic continental flood basalt province. The upraised and uparched
crust fissuring under tension permitted the ascension of voluminous basaltic magma
(estimated as 300,000 km3, Mohr, ~983) to form the Trc~pSeries of Ethiopia.
The first significant volcanic activity occurred in the Late Mesozoic along the
margins of the Proto-Afar. Alkaline and tholeiitic basalts are found interbedded with
Cretaceous regressive sandstones along the southern and western margins of the Afar
Depression, Kulibi and Sakota areas on the southeastern and northwestern plateau
re'spectively (Gouin and Mohr 1964; Merla et aI., 197'3)., Available geochronological
data strongly suggests that it was not until Late Eocene or Early Oligocene that basaltic
volcanism was widespread. These early flood basalts shown on the map as Ashangi
Basalts (P2a), Jima Volcanics (Pjb & Pjr), Aibe Basa ts (P3a), Arsi and Bale Basalts
(pNab), Makonnen Basalts (P~mb), Alajae Formation (PNa), Tarmaber Gussa and
Tarmaber.Megezez Formations (PNtb & Ntb) now cover extensive areas of the western
and southeastern plateaus.
running parallel to and west of the pre~ent Ethiopian Rift. The flood basalt volcanism
culminated by the formation of alkaline ba$altic_shield volcanoes some of which rise
8
ab,Ove 4000 m a.s.1. on both the western and southeastern plateau (ct. Tarmaber
Formation).
According to Kazmin et al. (1980), initial sagging of the MER started about 15
Ma and was followed by major episodes of rifting at 10, 5, 4 and 1.8 to 1.6 Ma. Each
stage of rifting and downfaulting was accompanied by a bimodal (felsic-mafic)
volcanism in the rift and formation of basaltic and trachytic shield volcanoes on the rift
shoulder and margins. The gE!neral consensus is that downfaulting of the Afar
Depression started at a much E!arlier age and that rifting was accompanied by a
voluminous flood basalt volcanisITI. Some workers consider initial phases of stretching
started as early as in the Early Miocene (Barberi et aI., 1975) and the opening of the
depression was preceded by alkaline granite intrusions (gt5) along its margins and was
followed by the deposition of detrital sediments shown on the map as the Danakil
Group (Nrs).
Following} the. initiation of subsidence of the Afar Depression .and the MER;
subsequent volcanism was restricted at first to the evolving rifts and then to the axial
zones which later became a focus of Quaternary and recent volcanic activity. The East
African Rift System in Ethiopia comprises the elements of the following major structural
and physiographic features of the region.
1. The Gulf of Aden Rift which affects Ethiopia in that faults of this system form the
southern boundary of the Afar Depression and determined the trend of regional
~--- - --
- -- - - - --
5. The Lake Turkan~, Lake Chew Bahir Rifts and Heireba proto-rifts in southern
Ethiopia which are not directly connected but progressively displaced to the east
due north.
In the course of the development of parts of the Great East African Rift System
in Ethiopia, a variety of continental sedimentary basins were developed since Miocene.
In the Afar Depression, sediments originating from the rapid erosion of the steep
escarpm~nts together with abundant volcanic products tended to fill the depression but
tectonic deepening was more rapid than vo~cano-sedimentaryinfilling. Moreover, the
nature of the ~sedimentswas dependent on whether the basins are marginal or axial.
Evaporite beds have also been formed during the re:stricted marine invasion of the
northern Afar Depression (Danakil Depression). Plio-Pleistocene fluvo-Iacustrine
sediments are also widespread in the Ethiop'ian. Rifts. In the MER, lacustrine"
sedimentation is wide spread during the pluvial periods of the Quaternary. The present
Rift Valley Lakes are therefore remnants of larger ancestral lakes, which covered most
parts of the developing rift floor.
10
--
and other salts occur in the Danakil Depression related to a marine evaporite sequence
of Late Tertiary to Quaternary age. Economic reserves of diatomite are found
associated to Pleistocene lacustrine sediments in the central part of MER. There are
reported bentonite occurrences in central and southern Afar associated with a Plio-
Pleistocene lacustrine sediment~;in the region. Lignite seams have been reported from
many of the intertrappean fluvo-Iacustrine sediments. located on the northwestern
plateau.
11
The gneisses of the Archean complex have been grouped into the following five major
units.
1. Konso Group (ARk) which consists of a relativHly mafic rich hornblende gneiss,
amphibolite and a mafic rich granulite.
3. Awata Group (ARa) which comprises well layer~d gneiss and granulite containing
clearly recognizable metasedimentary components.
4. Yabello Group (ARy) made of pale pink to light gray quartzo-feldspathic gneiss and
granulite, generally leucocratic and having a granitic composition.
In Ethiopia, the Archean age of these gneisses has not been proven; however,
their metamorphic grade and other field evidence allowed correlation to other Archean
rocks in the African continent. Metamorphic facil3s in the gneissic terranes is mainly
. middle to upper amphibolite facies, but has attained granulite facies m~tamorphism in
many places.
12
preserved in many rocks and accounting for the occurrence of in close proximity, for
example, of two-pyroxene hornblende granulite and epidote green hornblende
amphibolite. In many rocks partial retrogression is witnessed by the growth of lower
grade minerals in and around mirerals such as hypersthene. Green amphibole rims
are very common around both 11yperstheneand augite grains. At some places
retrogressive amphibolite facies zones are common within granulite facies rocks.
These localized retrogressions are particularly associated with restricted zones of
ductile shear zones.
The Konso Group (ARk), formerly named as the Konso Gneiss (Kazmin, 1972 &
1975), is comprised of dark gneisses rich in hornblende and poor or lacking quartz.
These rocks occur in southern part of the country in the Konso and Hamar regions
(Kazmin, 1975; Davidson, 1983). They underlie a large area starting from the Segen
River north to the southern side of the Gidole highlands and reappear from beneath the
Tertiary volcanic cover west of Lake Chamo. Similar gneisses occupy several strips in
the area between Omo and Segen rivers, in particular close to Lake Turkana.
Biotite appears to be a stable phase in some rocks, but is secondary in others. Sphene I
and epidote are not stable phases. I
13
-- --
-- -- --- --
Amphibole and/or less mafic hornblende gneiss are interlayered in various ratios
and at different scale within most of this unit. Paragneisses including aluminous types,
calc-silicate gneiss and marble, are locally present as thin layers, particularly in the
area north of Fejej (Lat. 4° 35' N and Long. 36° 20' E). In other areas notably just
northwest of Chew Bahir, due north of Minogelti and southwest of Turmi, relatively
uniform masses of dark hornblende gneiss likely represent deformed and
metamorphosed gabbroic to quartz dioritic plutonic rocl<s.
The Alghe Group (AR1) former Alghe Gneiss of Kazmin (1972 & 1975) consists
of a rather uniform, gray, coarse grained gneisses. These rocks occur west of the
Negele meridian and east of the Awata valley where they were originally mapped as
Alghe Gneiss (Kazmin, 1978). Similar rock types have been mapped in the eastern
.
(Berhe 1978), western (Gore, Gimbi, Nekemte and Abu Ramla areas) and in the
southern (Hamar region) parts of the country (Amenti in preparation, Mengesha, 1990;
Mengesha and Berhe, 1989; Davidson, 1983).
The Alghe Group consists of gray gneiss with variable color index showing
development of layering. Much of this unit is relatively uniform and poorly layered
orthogneiss, representing deformed and metamorphosed plutonic rocks of dioritic,
quartz dioritic and tonalitic composition. In places this ';lr:aygneiss is moderately to well
layered and contains subordinate, narrow, interlayered units of mafic, quartzo-
feldspathic and metasedimentary gneisses.
14
The gray gneisses in areas of amphibolite facies are composed of andesine,
quartz, biotite and/or hornblendE~. The dark gray gneisses have similar mineral
assemblage to those of the mafic granulite of Konso Group (ARk). Biotite and
hornblende are the dominant mafic minerals that impart a dark color and garnet,
epidote and augite are also present locally in small amount. Color index ranges
between 10 and 20, and the rocks are overall distinctly less mafic than those of Konso
Group gneisses (ARk). Migmatitic phases are common, with mobilizate both as
irregular quartzo-feldspathic segrE!gationparallel to the foliation and layering with cross
cutting veins and dykes of pegmatitic character. In extreme examples of this process,
the rock has become essentially i';;Jneousin appearance. In the Hamar region the gray
gneisses attain granulite facies rretamorphism and are darker (greenish gray) in color
where fresh, and distinctly more nranular in texture. Sillimanite-garnet gneisses occur
along Gaba River north of Metu. A narrow belt of gneiss containing muscovite,
staurolite and abundant garnet is also found along the western border of the Alghe
Group (AR1) but muscovite is ram in the high grade terranes.
The Awata Group (ARa) originally identified in Sidamo region and named as
Awata Gneiss (Kazmin, 1975) consists qf well layered gneiss and granulite of
metasedimentary origin and occur mainly west of Negele and in the Hamar region
(Kaz'11in,1972 & 1975, Davidson. 1983). The group is characterized by well developed
layering and by the presence of tl,ick, and red-brown weathering zones.
15
- - - - --- -- - - - - -
-- - - - -
hornblende gneiss may represent original clayey sediments and the amphibolites may
represent basic intrusions (Qavidson, 1983).
The pelitic gneisses of this unit contain muscovite in a restricted region in the
western Hamar plains approaching middle ~mphibollte facies. There and elsewhere
biotite and garnet are the main mafic minerals, locally accompanied by sillimanite;
kayanite is rare, and andalusite, staurolite and corc;lie!ritewere not observed. Marbles
and calc-silicate gneisses at low grade contain tremolite, bl,Jtcloser to the granulite
zone of the Hamar range, they contain diop~id~Jgrossularite and scapolite, and only
rarely amphibole. Deformation and metamorphism h~ve been so intense that all
evidence of former sedimentary structures have been obliterated; this unit of diverse
paragneiss with large aerial extent indicates a moderate to shallow marine deposition
in a relatively.stable tectonic regime (Davidsqn, 1983).
The Yabello Group was originally recognized in the Yabello area of Sidamo
region and was named as Yabello Gneiss ~nd is characterized by the presence of
discontinuous layers of rather massive qu~rtzo-feldspathic rocks which merge into
grani~icgneisses (Kazmin,.1972). Mapping rarri~d in the Omo River area (Qavidson,
1983) showed that similar quartzo-feldspathic gneiss and granulite underlie 'Iarge tracts
of ground between Jinka and the north end of th~ Hamar range, between Beto and the
northern part,of the Chew Bahir rift and on th~ southern side of Mt. Gugu. The first two
lie in broad, northwest oriented strips, separated by gray gneiss of the Alghe Group
(AR1) east of Jinka, but converge in the vicini~yof Lake Weyto (Lat. 5° 25' N and Long.
36° 58' E). The third is separated from the r~st by tracts of units of Konso and Alghe
Groups that occupy tt-learea west of Lake Ch~mo.
1
Like the Alghe Group, the rocks of the Yabello Group vary from being relatively
uniform orthogneiss to well-layered gneiss, and probably contain rocks of both plutonic
and supracrustal origins. They are characteri~ed by their light gray, pink, buff or cream
colors and by low color indices. In the areas around Jinka, Beto and Balta, pink and
light gray quartzo-feldspathic gneisses are foliaJed, biotite-bearing and commonly
16
migmatitic with stromatitic form and pegmatitic leucosomes richer in K-feldspar. To the
Southeast, these rocks merge along strike with more granular leucocratic gneisses in
which magnetite is the dominant mafic mineral along with scattered garnet and rare
biotite in some rocks. This char:'lgecoincides with the elevation to granulite facies as
indicated by the appearance of hypersthene in the rock units, and is accompanied by
change in color to buff and cream hues.
Baro Group (ARb), originally referred to as the Baro Domain (Mengesha and
Berhe, 1980; TekleWolde an'ti Moore, 1989) consists primarily of ortho and
paragneisses metamorphosed to upper amphibolite facies. The group underlies a
large tract of land at the extreme western part of the country (Mengesha et aI., 1990;
Teklewolde et aI., 1989; Davidson, 1983). The southern end of the group has been cut
by northwesterly trending sinistral shear zone (Davidson, 1983).
The Baro Group (ARb) consists of predominantly uniform and layered quartzo-
feldspathic gneisses. Most of the area west of Bonga village consists of layered and
relatively uniform, lenticular gneisses. The predominant rock types are strongly
foliated, light gray to pink, medium grained biotite and hornblende-bi.otite gneisses.
Minor ferruginous quartZite and amphibolite layers occur within hornblende-biotite
gneisses.
17
-----
--- --- -
i~dex ranges between 5 and 15%: Some of the gneisses are migmatitic cut by
numerous subconcordant lenses of granitic and pegJratitic material. Discordant dykes
and pods of pegmatite and granite also occur, and some bear garnet. A uniform
granitoid composition with a lenticular structure over hundreds of square meters of
outcrop area suggests that the majority of these gneisses are tonalitic or granodioritic
orthogneisses. The more well layered, garnet-bearing varieties are, however, most
probably paragneisses.
Although biotite and hornblende-biotite gneisses are the dominant rock types in
the Baro Group (ARb), the group also contains laYHrs of garnet-amphibole, garnet-
sillimanite, and calc-silicate vaieties. The gneisses from west and south of Bonga (Lat.
8° 10' N and Long. 34° 9' E) contain layers of coarse, mafic-rich plagioclase-garnet-
hornblende and garnet-gedrite gneisses that locally predominate to form garnet-
amphibole gneiss. Deep red porphyroblasts of garnet reach 5 cm in diameters and
constitute up to 40% of the rock which makes it a potential source of abrasive material.
Interlayered aluminous gneisses and biotite-quartz-felclspargneiss are subordinate.
Paragneisses dominate the succession along the eastern side of the Baro
Group, north and south of the Baro River. The garnet-sillimanite gneiss unit underlies
the area immediately east of Bonga village. It consis~sof interlayered aluminous and
calc-silicate gneisses with subordinate garnet amphibole varieties and magnetite-
bearing quartzite. The main exposures occur near Bonga and consist of up to tens of
meters thick of sillimanite and/or garnet-bearing biotite-microcline-quartz-plagioclase
gneiss layers associated with gray to pink biotite gneisses and schists.
18
(gedrite)-bearing rocks are locally interlayered with the sillimanite-garnet-biotite
gneisses.
East of Bonga, along the eastern boundary of the Baro Group within the
sillimanite gneiss unit, is a well-exposed section of calc-silicate gneiss. Similar
sections occur west of Bonga, north and south of Baro River, within both biotite and
hornblende-biotite gneisses. The rocks is fine to medium grained, green-gray, weakly
foliated and commonly layered on a centimeters scale. Mineral assemblages in these
rocks include garnet-epidote-hornblende-diopside, garnet-epidote-microcline-biotite
and biotite-plagioclase-hornblendl~, with epidote rich layers centimeters-scale clusters
of brown titanite grains and rare calcite.
A narrow belt of muscovite-bearing gneiss and schist also occurs along the
eastern border of the Baro Group biotite gneiss. This probably represents a retrograde
metamorphism due to shearing and may be equivalent to the biotite gneiss. A similar
rock, containing'staurolite and abundant garnet, is also found along the western border
-
of Alghe Group (AR1).
2.1 Introduction
Rocks of possibly Early or Middle Proterozoic age, Wadera Group (PR1w) and
Mormora Group (PR1r) have be,en identified in Sidamo and Hararghe regions and
possiblyoccur in the westernpart of the country(Kazmin,1972;Berhe, ~ 987). A more
detailed study (Kozyrev et aI., 1B85) however didn't agree to the distinction between
the Wadera ~md Mormora Groups (Middle Complex of Kazmin, 1972) and referred to
them as a Mormora Group. The unit(s) are represented by psammitic and pelitic
metasediments with a characteristics northeasterly, northerly or northwesterly opening
sometimes asymmetric folds. Migmatization has developed sporadically, and yet
primary sedimentary features are well preserved. Abrupt changes in metamorphic
grade and style of dislocation indicate a major unconformity between these rocks and
the underlying gneissic complex (Kazmin 1972 & 1975). Similar rocks which were
deposited unconformably on the Archaean basement in Early or Middle Proterozoic
19
- - - - --- -
- - -- - - - ~--
times have been reported from northeastern Africa but are metamorphosed and
deformed with the underlying gneisses (Almond, 1984).
Rocks of this group, originally named after thEIvillage of Wadera, are typically
developed near Zembaba village, and have so far been recognized only in Sidamo
region (Kazmin, 1972). In later studies of regional geology and prospecting in the Adola
gold field, these rocks have been referred to as Zembaba Formation (Kozyrev et aI.,
1985). The Wadera Group consists of several hundred meters thick of chiefly yellowish
I brownish or dark gray, fine grained feldspathic sandstone containing thin beds of
quartzite, biotite and biotite-muscovite schists, muscovite and biotite gneisses and
amphibolites. The metasandstones are composed mainly of potash feldspar, quartz,
and plagioclase.
In the course of field mapping within the Negeie Map sheet (NB 37-11) in 1991
by a team consisting of Amenti Abraham and Wolc:iegebriel Genzebu, the Wadera
rocks were re-examined. The team concluded that thE~Wadera rocks do not represent
low grade metasediments, (Amenti, personal communication), but are mylonites and
ultramylonites formed in a large N-S running shear zones and that the reported
(Kazmin 1972; 1975) fine layering, cross bedding etc. are not primary sedimentary
structures, but are formed due tv shearing.
.
20
2.3 Mormora Group (ffi1d
The Mormora Group is represented by several thousands of meters of thick
psammitic and pelitic assemblages (biotite gneisses, graphitic and micaceous schists
and marbles) with frequent devE!lopmentof kyanite, staurolite and garnet (Kozyrev et
al. 1985, Kazmin, 1972). There is little or no migmatization in the Mormora Group. The
Mormora Group has been divided into Zembaba (present and former Wadera Group),
Aflata and Kenticha Formations during regional mapping and prospecting carried out in
the Adola gold fields (Kozyrev at aI., 1985). The rocks of the Aflata Formation are in a
conformable relationship with the underlying quartz-feldspathic gneisses
(metasandstones) of the Zembaba Formation (Wadera Group in the present and former
works).
3.1 Introduction
21
-- ------- ------
- - - - -- - -- ---
The lower parts of the complex are- well developed in Sidamo and Wollega
regions (Kazmin, 1972; Kazmin.et aI., 1978). The Late Proterozoic Birbir (PR2b) and
Tulu Dimtu (PR2td) Groups crop out in Wollega and extend through Gojam into
northern Ethiopia. They form the youngest Precambrian basement units in northern
Ethiopia, where they are referred to as Tsaliet Group (PR21), Tambien Group (PR2t)
(Beyth, 1971), Didikama Formation (PR2d) and Shiraro Formation (PR2s) (Kazmin,
1975, Garland, 1980).
The Late Proterozoic rocks in Ethiopia from the oldest to the youngest have
tentatively been divided into the following units on the map. These are the Adola
Group (PR2a) Kajimiti beds (PR2k), Birbir Group (PR2b), Tulu Dimtu Group (PR2td),
Tsaliet Group (PR21), Tambien Group (PR2t), Didikama Formation (PR2d), Shiraro
Formation (PR2s) and as an undifferentiated Proterozoic rocks (PR2).
This group is typically developed in Adola area in Sidamo region (Kazmin, 1972
& 1975; Kozyrev et aI., 1985). It consists of an assemblage of basic and ultrabasic
rocks, amphbolites of intrusive and volcanic origin with beds of chlorite and graphitic
phyllites and schists, ferrigeneous and graphitic quartzites, metasandstones and
22
metasiltstones. The interfaces between the Mormora and Adola Groups are straddled
by the occurrence of ultramafic bodies. Similar rock types have been recorded south of
Negele where they are associated with mainly metavolcanic rocks (Tilahun, 1983).
In the Adola Gold Field tlie group has been subdivided into Chakata and
Fenkelcha Formations (Kozyrev et aI., 1985) which show a conformable relationship.
The lower part, Chakata Formation, consists of volcano-terrigenous sequence
(greenshist, amphibolite, graphite schist and phyllite, ferruginous and graphitic
quartzite). The upper part, Fenkelcha Formation, consists predominantly of terrigenous
rocks (phyllite, metasiltstone and metasandstone). Unlike the Chakata Formation, the
Fenkelcha Formation is essentially terrigenous and consists mainly of phyllites,
metasiltstones and metasandstones. The rocks of the Fenkelcha Formation often
exhibit well defined sedimentary structures such as rhythmic bedding, lenticular and
cross-bedding and intraformational breccias.
The tract of land stretching from around Surma area (Long. 35°15' E and Lat.
6°00' N) due north up to Chanka village is underlain by a conspicuous group of low-
grade, predominantly greenschist facies rocks in contrast to the adjacent high-grade
gneisses of Baro Group (ARb) and Alghe Group (ARI). These volcano-sedimentary
assemblages reappear in the area north of Asosa (UN, 1972; Metal Mining Agency of
23
- -- -- -- - - - - - -- -
- - - - - - - - --- - - - -
Japan, 1972; Davidson, 1983; Mengesha and Seife Michael, 1990; Mengesha, 1990;
TekleWolde.etaI., 1989). The group is named afterthe Birbir River,majortributaryof
Baro River in western Ethiopia. Further north, the group merges with similar
rocks in Sudan, Uffat Group .(Vail et aI., 1986). The Birbir Group includes rocks of the
Akobo (Davidson, 1983) and Birbir Domains (Teklewold et aI., 1989; Mengesha et aI.,
1990).
The Birbir Group (PR2b) lies between the high grade gneissic terrane of Baro
Group (ARb) in the West and Alghe Group (ARI) in the East. In the north, the group is
separated from the high grade terrane of Alghe Group by the Tulu Dimtu-Dalati-Yubdo
ophiolitic suture zone (Kazmin 1973; Kazmin et aI., 1979) referred in this work as Tulu
Dimtu Group (PR2td). The belt narrows towards soutr and finally around Surma area
rocks of Baro and Alghe Groups merge, where they are all truncated by the northwest
trending Surma shear zone (Davidson, 1983). The contact between the Birbir Group
and the high grade gneisses is of a tectonic nature and marked by mylonites and the
change in metamorphic grade is also abrupt (Mengesha, 1990; TekleWo:de et aI.,
1989; Vail et aI., 1986; Davidson,
.. 1983).
The predominant units are mafic schists and phyllites of basaltic and andesitic
composition with some rhyolites and subordinate volcanoclastic rocks and thin
metasedimentary units. The mafic rocks are dark gray, dark green
hornblende/actinolite-plagioclase schists, greenschists and amphibolites. The
metasedimentary units are biotite-muscovite-quartz and graphitic schists,
conglomerate, ironstones, quartzite, marble, phyllitl~ and sandstone. Primary
sedimentary and volcanic features are locally preserved.
24
series of tight to isoclinal folds. All these rocks have undergone intense mylonitization
(Vail et aI., 1986; Tekle Wolde et aI., 1989; Davidson, 1983).
The Tulu Dimtu Group (PR2td) is named after the Tulu Dimtu ultramafic body
(Mt. Dimtu) east of Nejo in Wollega region. Thi$ group is composed of a thick
succession of low grade metavolcanic and metasedimentary unit, 'abundant intrusive
bodies of mafic, intermediate and granitic composition and discontinuous but
conspicuouS'ultramafic bodies of probably ophiolitic association (Kazmin, 1978; Kazmin
et aI., 1979; Mengesha'and Beme, 1990; Amenti tn preparation). The rocks form a
relatively narrow belt along the western part of the country and has been named as the
Tulu Dimtu-Dalati-Yubdo ophiolite suture zone. Primary volcanic and sedimentary
features are locally preserved and all the rock types have suffered intense
mylonitization..
The ltulu Dimtu Group forms a belt that extends due north for about 100 km from
south of Yubdo town and disappears under the volcanic succession of the northwestern
Ethiopian plateau. In the northern part of the country these rocks merge with the
volcanic sedimentary assemblages of the Tsaliet Group (Kazmin, 1972- & 1978;
'Garland, 1980; Arkin et aI., 1971). To the South the belt apparently terminates against
the gneistes of Alghe Group (AR1), where it separates the Blrbir Group from the Alghe
Group. The contact between the Tulu Dimtu and the Birbir Groups (PR2b) is not
certain but that of the Birbir and Alghe Groups extends straight north.
25
- - - - -- -- --- --
- - - - - - - - -- - - - - --- -
Andesites and acidic volCanics and associated sediments also occur in Aisha
uplift where they are known as Abdul Ghadar Series (Vishnevsky et aI., 1975). These
rocks contain strongly deformed,bodies of early diorite~ .and granodiorites and large
late to post-tectonic intrusions of granite and granodiorite of batholithic dimensions.
The rocks have been faulted and tightly folded to byild anticlinoriums afld anticlines but
subjected to only low grade metamorphism, mainly to lower greenschist facies. The
~ssociated sediments show sedimentary features such as graded bedding, current
bedding and very well preserved ripple ma~ksthat the superimposition of beds could be
identified very easily (Kazmin, 1972; Garland, 1980; Beyth, 1971).
The youngest Precambrian units that underlie northern Ethiopia ari9 dominantl~
clastics with subordinate carbonates. One of these units described as the Tambien
- Group (PR2t) consists of thousands of meters thick of majnly slate and limestone with
interbedded phyllite (Beyth, 1971). It is named after TamBien area 'in central Tigray
region. Structures in these sediments suggest that they accumulated in relatively
shallow water. An important
, , unit in this group is' a black limestone containing algae,
including probable oolites (Beyth, 1~71). The Tambien Group (PR2t) has been divided
into the Arequa Formation (lower unit) and Mai Kental limestone (upper unit) inthe
26
- . - - - --- - --.- - --- --- -- -- - -
course of regional mapping carried out in map sheet (Aksum) ND 37-6, (Adigrat) ND
37-7 (Adiarkay), ND 37-10 and (Mekela) ND 37-11 mapsheets (Garland, 1980).
The Arequa Formation, ~onsisting mainly of slates, is in turn, subdivided into
Werei Slate, Assem Limestone and Tsedia slate, from bottom to top and all have
conformable relationships (Garland, 1980). The Werei Slate is about 1000 meters
thick of mainly interbedded black s~ate,phyllite and limestone which has gradational
contact with the underlying Tsaliet Group. The Assem
. limestone. is about 300 meters
'thick of well-bedded' limestone interbedded with slate and marble. The Tsedia slate is
about 600 meters thick of variegated slate interbedded with limestone, quartzite and
dolomite. The Mai Kentallimestone, the upper unit of the Tambien Group, consists of
about 800 meters thick of black, massive, fine grained limestone partly algal and oolitic
commonly weathering into karstic topography and .has thin interbeds of dolomite and
thinly bedded limestone.
3.8 Didikama Formation lPR2d
The Didikama Formation consists of more than 1500 meters thick of creamish to
white dolomite alternating with gray, black or variegated slates. This formation occurs
in Tigray region, in northern Ethiopia and named aft.erits outcrop at Didikama village in
the Shiraro area (Kazmin, 1975; Garland, 1980) and extends northwards into Eritrea
and alsocropsout on the Danakil Alps. It conformably overlies the Tambien Group
(PR2t) in central and eastern Tigray region. However, in the West where the Tambien
Group is not fully developed or the upper formations of the Tambien Group are missing,
it rests unconformably on older formations (Garland, 1980). In the Shiraro area, at
Negash Syncline, it is unconformably overlapped by the youngest Precambrian
formation in Ethiopia, Matheos Formation (Kazmin, 1975; Garland: 1980). The
Matheos FOrrT)ationconsists of less than 30 meters thick, gray to black, well laminated,
and partly detrital limestone.
27
- ----
- - -- - - - - ------- -- -
Also restricted t6 northern Eth.iopia,this formation overlies the older rocks with
marked angular unconformity (Kazmin, 1975). It consists mainly of sandstone and
conglomerate in which slate, phyllite and granite clasts predominate. The Shiraro
Formation is weakly folded as compared to the underlying rocks. The conglomerate is
greenish with abundant granite and other rock inclusions, some rounded, mostly
angular and well cemented. The granite boulders, which belong to the underlying
granite, reach up to 1 meters in diameters. The sandstone is coarse grained, pinkish,
massive with numerous cross-bedding. slightly metamorphosed with high content of
quartz and sericitized plagioclase.
3.10 Undifferentiated Late Proterozoic Rocks (PR2)
4. PRECAMBRIANAND PHANEROZOICINTRUSIVES
4.1 Introduction
28
masses,someofwhichhavebathlllticdimensions.Mostof theseIntrusivesarerelated
to the tectonicdeve10pment
of the LateProterozoic
'Volcano-sedimentary
successions
or wereemplacedsubsequent
to theearlierperlod(s)of Intensetectonismduringwhich
the gnelsllc complexdeveloped. SomeIntrusivesIn the high gradeterranesare
ancientandmayrelateto thetectonicdevelopment
of the hostrocks. Therefore,It Is
ulually difficult to dlstlngullh and map the feilic Intrusivesseparatelyfrom the
supracrustalgnelilic hOlt rockl. Muchyoungerand typicallyalkaline(gtS)Intrullve
bo~lel,conlldered.1 proeur.ourof riftingIro expo.cad
Iiong thowe.ternmarginofthe
AfarCoproilion.
4.2~
Several bodies of gabbroic rocks occur associated mainly with the Late
Proterozoic volcano-sedimentary successions (Kazmin, 1972; UN, 1972; Davidson,
1983; Mengesh&, 1990; Teklewold and Moore, 1987). There are also some gabbroic
rocks within the high-grade gneisses and granulites. They clearly intruded their
enclosing supracrustal schist and gneiss country rocks and are themselves intruded by
later granitoid in1rusives. Foliation in the enclosing gneisses and schists was deflected
around some 'of the gabbroic plutons, suggesting that they were emplaced before or at
the time of deformation for the intrusives to behave as relatively rigid units. Where as
some gabbroic intrusives cut through the youngest Precambrian units in the northern,
western and southern part of the country. Someother gabbroic bodies show clear
concentric structures and are presumed to be formed as a number of arcuate dykes.
Overall the Metagabbro masses are commonly elongated or circular and are
heterogeneous both in composition and degree of deformation. The outcrops range
from massive to fairly foliated, commonly coarse grained and melanocratic. The central
parts of most of these plutons contain relatively unaltered rock with primary igneous
mineralogy and texture. Most of the gabbroic rocks are true gabbro, but some are
noritic. Plagioclase has the composition of labradorite, olivine is a rare constituent and
pyroxenes have been replaced partly or wholly by hornblende around the. rims and
cores. Both augite and hypersthene are present in most fresh specimens, but augite
29
-- - ---- -----
- - -- --- - ----- - -- -- --
alone occurs in some specimen. The presence of these too pyroxenes in some implya
compositional range from gabbro to norite.
The plutonic bodies are commonly medium to coarse grained, although locally
porphyritic and fine grained varieties occur. A considerable structural I deformational
variety exists among the granitoid plutons and the development of foliation is variable.
However most of the plutonic bodies are weakly to strongly deformed and well foliated.
In places they are also highly sheared, with well developed augen texture that some
have been na"medas orthogneisses' (gt1) since they are structurally well developed are
evidently post-tectonic with respect to the deformation history of the host rock. Their
structural relations and partly or wholly recrystallized nature in some localities, verfies
that they were emplaced earlier or accompanied the latest deforming event (hence the
name pre and or syn-tectonic granitoids) of the gneissic terranes.
The metatonalites (tn) are generally weakly to strongly foliated, leucocratic and
mediumto coarse grained. Plagioclase(albite to oligoclase)and quartz are major
30
constituents and muscovite is a minor constituent. Biotite is typically the main mafic
mineral. Xenoliths of host roCksare locally present.
The granodiorites (gd) ate gray,. weakly to well foliated, and locally contain
small, lenticular xenoliths of the host rocks. They are essentially made of quartz,
oligoclase and subordinate microcline in various proportions. Most of the granodiorites
-contain both hornblende and biotite but some contain only biotite.
The granite plutons (gt1 and gt2) are light gray to pink, weakly to strongly
deformed or recrystallized, medium grained and generally equigranular. They are more
leucocratic than the diorite (dt) and granodiorite (gd). These granites contain biotite,
locally with muscovite and rarely with minor hornblende. K-feldspar (microcline)
content is equal or greater than that of plagioclase (oligoclase). Xenoliths are rare but
locally basic inclusions occur.
Small bodies of these igneous rocks have been known in Wollega (Asosa), Kefa,
Sidaroo (Borona), Hararghe, the western part of the Tigray (Shire) regions and in
northern Eritrea (Barka and ,Anseb valley). Several small bodies of ultramafic rocks
occur near the road between Yabello and Agre 'MarYam associated with presumably
Yabello' Group rocks in Sidamo region (Kazmin, 1972), and within the mafic-rich
gneisses and granulites (ARk) mapped northwest of Konso (Davidson. 1983). The
ultrmafic bodies are characterized by hiUs w.ith sparse vegetation and distincti\le
purplish gray to white soils. Other outcrop surfaces are covered by' brown weathering
product with the rocks underneath commonly being pyroxenite, peridotite, dunite and
serpentinite.
-- --
----
These ultramfic bodies are commonly strongly sheared and serpentinized and
generally composed of serpentinized dunite, peridotite, pyroxenite and talc-chlorite
schists. They are capped by a peculiar brown or yellowish-brown rock called birbirite
named after the Birbir River (Kazmin, 1972; d~ Wit and Berg, 1977; UN, 1972). The
birbirite is exposed in the Birbir valley anq Yubdo area in Wollega region and is
composed predominantly of hematite, chalcedony and quartz.
4.5 Late and Post-tectonic and anoroaenic Granites and Syenites lat3...1Jt4.at5)
32
constituents and muscovite is a minor constituent. Biotite is typically the main mafic
mineral. Xenoliths of host rocKsare locally present.
The granodiorites (gd) are gray,. weakly to well foliated, and 19cally contain
small, lenticular xenoliths of the host rocks. They are essentially made of quartz,
oligoclase and subordinate microcline in various proportions. Most of the granodiorites
contain both hornblende and biotite but some contain only biotite.
The granite plutons (gt1 and g12) are light gray to pink, weakly to strongly
deformed or recrystallized, medium grained and generally equigranular. They are more
leucocratic than the diorite (dt) and granodiorite (gd). These granites contain biotite,
locally with muscovite and rarely with minor hornblende. K-feldspar (microcline)
content is equal or greater than that of plagioclase (oligoclase). Xenoliths are rare but
locally basic inclusions occur.
Small bodies of these igneous rocks have been known in Wollega (Asosa), Kefa,
Sidaroo (Borona), Hararghe, the western part of the Tigray (Shire) regions and in
northern Eritrea (Barka and Anseb valley). Several small bodies of ultramafic rocks
occur near the road between Yabello and Agre'MarYam associated with presumably
Yabello' Group rocks in Sidamo region (Kazmin, 1972), and within the mafic-rich
gneisses and granulites (ARk) '11appednorthwest of Konso (Davidson. 1983). The
ultrmafic bodies are characterized by hiUs w.ith sparse vegetation and distincti"e
purplish gray to white soils. Other outcrop surfaces are covered by-brown weathering
product with the rocks underneath commonly being pyroxenite, peridotite, dunite and
serpentinite.
Meta-ultramafic
bodiesoccuras linearbeltsand mostlyas elongated'masses
conformable
withthe regionalstructureof the adjacentmaficschistswithinwhichthey
formsmallerslivers. TheTulu Dlmtu-Daletl-Yubdo ultramafics(Kazmln,1972; 1978;
UN, 1972), Akoboultr,maflcbodies(Davlqson
1983) Shlngu-Bambasl.Dul
ultramafic
bodies(Mengesha,1990), Anseb.Barka
ultramaficbodies(Kazmln,1972),Adola
ultramaficbeltsandMoyaleultramaficbodieshave8 ulmllaroccurrence(KazmJni.:,1972
,-
. I'
& 1978;KozyrevIt aI.,1986)andar.ea8soclated
withmaficschists.
Th.l. ultr.mflc bodI.. Ir. commonlyItrongly Ih.arod and .orp.ntlnizld and
gln8rlily compolld of 18rp8ntinizeddunitl, p8ridotlt8,pyroxeniteIiIndtlillc=chlorite
schists. Theyaree8ppedby a peculiar.brownor yellowi~t;-browri
rockeall@dbirbirite
named after the Birbir River (Kazmin, 1972; de Wit and Berg, 1977; UN, 1972).' The
birbirite is exposed in the Birbir valley and Yubdo area in Wollega region and is
composed predominantly of hemafite, chalcedony and quartz.
32
granular texture, in which large subhedral to anhedral microcline and subhadral
plagioclase grains are surrounded by a matrix of finer grained plagioclase, microcline,
q~artz, biotite and hornblende. The deformed ones contCJinquartz and feldspar
showing undulose extinction.
The post-tectonic granites (gt4) are massive, pink and porphyritic, containing
phenocrysts of K-feldspar set in a fine to medium grained, subhedral, granular
groundmass and usually contain randomly oriented angular xenoliths of the host rocks.
The predominant minerals are intergown microcline and quartz (perthite), plagioclase
and mafic phases include biotite and hornblende with accessory minerals 'such as
allanite, apatite, magnetite, titanite and zircon. U-Pb zircon dating conducted on late to
post tectonic-granites from western part of the country yielded ages of 541 Ma. and 570
Ma. (Teklewold and Moore, 1989). These dates probably mark the end of tectonic and
igneous activities associated with .the Precambrian basement rocks in the country:
Abolonge alkali granites and syeriites (Kazmin, 1972; Mengesha, 1990; UN,
1972; Davidson, 1983). No age data is available for these intrusions, however,
analogous intrusions elsewhere in northeast Africa suggest that they possibly are of
Paleozoic age.
The Abu Ramal and Abolonge intrusions have a ring structure and are
composed of alkali syenite, which consists mainly of alkali feldspars, alkali hornblende,
biotite, rare quartz and apatite, and show trachytic texture (Kazmin, 1972; Davidson,
1983). The Abu Ramla and Abologe ring complexes are not associated with extrusive
33
--- - - - - --
-------- ~ - -
rocks; however, it is likely that these co!'1lplexesare deeply eroded feeder intrusions to
now eroded felsic flows similar to those that are still preserved. The Gangan Mt. is built
of both alkaline syenite and granite (UN, 1972; Mengesha, 1990). The northern slope
of the mountain is built of alkali syenite consisti.ng of mainly microcline, perthite
anorthoclase and albite, with little riebeckite, quattz and sphene. The southern slope
of the mountain is made of alkali granite consisting of orthoclase, perthite, oligoclase,
quartz and hornblende.
Both isolated and swarms of narrow dykes and sills of dolerite, trachyte and
.granite are common. In the northern part of the country clusters of dolerite dykes and
sills that occur within the Mesozoic sediments (Arkin et al. 1971) were named as
Mekele dolerite and are black andesine dolerite with ophitic texture. They usually
occur as sills
and dykes intruding the Adigrat Formation (Ja), Antalo Formation (Jt) and Agula
Formation (Jg) in the Mekele outlier and occur as small stocks intruding crystalline
basement and sedimentary rocks in the escarpment.
Most of the hypabyssal felsic rocks are massive. Locally xenoliths of both earlier
intrusives and the wall rock, including Precambrian basement rocks are carried up from
a deeper level. Many of the hypabyssal rocks are somewhat porphyritic, with' ubiqitous
phenocrysts of plagioclase, but quartz, amphibole and pyroxene phenocrysts are also
34
present in some of the rocks. Many of these intrusions are related in age to nearby
felsic dxtrusive rocks or have the same age as the flows within which they have been
emj:'laced.
"
II
35
---
i
j
IV. PHANEROZOIC COVER ROCKS
The Enticho Sandstone and Edaga Arbi Glacial (Dow et aI., 1971) occur in the
northern part of Ethiopia. The Enticho Sandstone is named after the town of Enticho on
the Aksum-Adigrat road, and the Edaga Arbi Glacials is named after the Edaga Arbi
type section. The Enticho Sandstone is about 160 meters thick and composed of white,
calcareous, coarse grained, cross bedded sandstone containing lenses of siltstone, grit
and polymict conglomerate with subrounded to well rounded pebbles, cobbles and
boulders. Scattered erratics, mainly granite and gneiss are common in places. The
Edaga Arbi Glacials, tillite and other glacial rocks, have a thickness in the range of 150
to 180 meters. The rocks have been correlated to either of the Late Carboniferous
glacial. rocks of southern Africa or to the Ordovician glacial rocks of Hoggar district of
.
southern Algyria (Dow et aI., 1971). The tillite overlies the Enticho Sandstone,
however, the latter is reported to interfinger with the glacial rocks at several places
between Adwa and Adigrat towns.
In the Abay River Canyon due south of Lake Tana, sandstone, siltstone and
shale fill channels in the Precambrian rocks and are unconformably overlain by the
Adigrat Formation (Ja) (Jepson and Athearn, 1961). Another occurrence of sandstone,
36
I.
silt and shale with fossil evidence of possible Silurian age have been reported in the
Fincha Valley, Wollega region (Mesfin, personal communication).
Medium to coarse grained yellow sandstone, grit and shale of the Waju
Sandstone fill channels in the Precambrian basement extending across the present-day
Jerjertu, Ramis and Soca Valleys in Hararghe region (Mohr, 1963; Kazmin, 1975).
They are unconformably overlain by the Adigrat Formation. The presence of silicified
wood suggests Late Paleozoic continental erosion and deposition. In western and
southern Ethiopia, scattered occurrences of mainly sandstone with minor siltstone and
conglomerate of originally more extensive nature occur in Gilo and Waka basins in
lIIubabor region; west of Tepi town and in Kari River in Kefa region (Davidson, 1983)
and in Gura River in Bale region (Belay, unpublished report). Palynological analysis
has proven a Permian age for the Gilo Basin and Kari River sandstones. (Davidson,
1983; Davidson and Mc Gregor, 1976). Several hundred meters thick of sandstones
and shale, Calub Sandstone, Gumburo Sandstone and Bokh Shale, overlain by the
Adigrat Formation have also been found in deep bore holes drilled for natural oil and
gas expioration in the eastern Ogaden region.
The Adigrat Formation (Ja) which varies in thickness from a few meters to 800
!lleters, was originally named as Adigrat Sandstone after Adigrat town in Tigray region
(Blanford, 1870), and includes the whole succession of clastic rocks resting
unconformably on the Precambrian basement and overlain unconformably by the.
Antalo Formation (Jt); (Dow et aI., 1971; Garland, 1980). The Adigrat. Formation in
some parts of northern Ethiopia rests with a slight disconformity on the Late Paleozoic
to Early Mesozoic sedimentary rocks of Entichd Sandstone and Edaga Arbi Glacials
(Pzt) and is overlain by Antalo Formation (Jt). However, in Harar:-DireDawa area and
deep boreholes in the Ogaden region, it is overlain by the Hamanilei Formation (Jh)
(Mohr, 1965: Beyth, 1971; Dow et aI., 1971; Kazmin, 1972; Garland, 1980).
37
-
The Adigrat Formation chiefly comprises sandstone with minor lenses of
siltstone and conglomerate and laterite up to two meters thick (Garland, 1972). The
formation is typically yellowish to pink with a laterized top to a depth of about 20 meters
and composed of fine to medium grained, well sorted, cross bedded quartz sandstone.
It is non-calcareous except at the top near the contact with the overlying Antalo
Formation (Jt) or Hamanilei Formation (Jh), where thin beds of limestone have
developed.
In the central part of the country, in the Abay Canyon, variegated sandstone,
siltstone and shale correlable with the Adigrat Formation (Ja), occur between the
Precambrian Basement and the Abay Formation or Tertiary volcanics. In Harar-Dire
Dawa area, the sandstone, which lies between the Precambrian Basement rocks and
Hamanilei Formation (Jh), is conglomeratic, pporly to moderately sorted. Along the
Danakil Alps, the sandstone becomes fine grained and calcareous and lacks cross
bedding, and is intercalated with siltstone and shale indicating deeper water conditions
of deposition.
38
~
.--
2.3 Abav Formation (J.Q1
The Abay Formation previously named as Abay Beds and the lower unit of the
former Antalo Group (Kazmin, 1972; 1975) occurs in the Abay River lying between the
Adigrat and the Antalo Formations. The formation consists of a 196 meters thick of
sandy limestone and calcareous sandstone; a 257 meters thick unit of gypsum, as well
as a 138 meters thick upper unit of a sequence of alternating shale and limestone
bringing the total thickness of the formation to 580 meters. The Abay Formation (Jb) is
middle Jurassic age.
2.4 Urandab and Antalo Formation (Ju and Jt)
The Antalo Formation was first named by Blanford (1870) at the type locality as
Antalo Limestone in Tigray region, and later described in detail by Mohr (1963), Beyth
(1971), Kazmin.(1972 & 1975) and Merla (1973 & 1979). The Antalo Formation is a
750 meters thick sequence which consists predominantly of fossiliferous yellow
limestone containing thin beds of marl and calcareous shale, and occasionally
arenaceous bands near the top.
In the Mekele area, it conformably overlies the Adigrat Formation (Ja) and
grades upward into Agula Formation (Jg) (Garland, 1980). In the Adigrat area, the total
thickness is more than 1000 meters. In the Danakil Alps, where the unit consists of
thinly bedded, pale fossiliferous limestone, the succession reaches a thickness of up to
1420 meters.
The marginal parts of the Mekele outlier consist of sandy oolitic facies
suggesting a near shore environment. While in the escarpment black limestone and
shale indicates deeper water conditions of deposition. In the Abay River canyon the
Antalo Formation, wtiose age is considered to be Middle to Late Jurassic .is restricted to
the upper 288 meters thick former Antalo Group (Kazmin, 1975) and lies between the
Abay Formation and the Tertiary volcanics being separated from the Adigrat Formation
by the Abay Formation.
39
-- -
- ---
The Urandab Formation rests on the washed out surface of the upper part of the
Hamanilei Formation (Jh) Over most of the area, Urandab Formation is represented
mainly by greenish gray to black, poorly carbonaceous organogenic shales. Gray to
black argilaceous limestones are often found within the lower part of the sequence and
are .usually minor than the shales. Up the section lies a member of limestones which is
dark-gray wjth greenish shades, often changing.to calcareous argillites. This grades
upward into light gray to green, clastic argillites.
In Tigray region, in the Mekele outlier and around the escarpment a gray-green
and black shale, marl and claystone interlaminated with finely - crystalline black
limestone containing disseminated pyrite with some gastropods and brachiopods was
previously named as Agula Shale after Agula village and overlies the Antalo Formation
(Jt) (Arkin et aI., 1971; Kazmin, 1975). The sequence which is 60 to 250 meters thick,
contains some thin beds of gypsum and dolomite and a few beds of yellow coquina.
Near Antalo village, a 200 meters thick unit of a correlable red and greenish silty marl
occurs. The Agula Formation (Jg) is of Late JurassIc (Kimmeridgian) age, and is of
probable lagobnal origin representing a regression phase of the Jurassic sea.
40
- - --< - - - - - - ~-
2.8
The Korahe Formation, formerly named as the Main Gypsum Unit (Kazmin, 1972
& 1975), represents the lowermost part of the Cretaceous succession in the Ogaden
region, in eastern Ethiopia; where it is exposed in the Wabi Shebele and Genale
Valleys. It varies in thickness from 100 to 500 meters. It has gradational contact with
the underlying Gebredarre Formation (Jg2 and Jg1) and is overlain by the Mustahil
Formation (Km).
The upper part of the formation \(Kg2) consists of massive, white and gray anhydrite
with beds of dolomite and shale.. The Korahe Formation has been assigned
41
----
-- --- --
Neocomian age on the basis of choffatellaand orbitolina found in the unit (Merla,
1979).
2.8 MustahiJ Formation (Km)
Rocks of the Mustahil Formation (Km), formerly known as the Mustahil Series
(Kazmin, 1972 & 1975), crop out on both sides of the lower Wabi Shabele valley near
the village of Mustahil after which the unit is named, and in Fafan Valley in the eastern
part of the country. The lower part of the formation consists of light gray limestones
interbedded with shales and marls. The upper part of the sequence is made of cream
organogenic, partly reef limestones. The Mustahil Formation has been assigned the
Aptian to Albian age on the basis of paleontological evidence (Kazmin, 1975).
The Ferfer Formation formerly known as Ferfer Gypsum (Kazmin, 1975), occurs
in southeastern Ogaden outcropping in the lower part of thE. l\Jabl ShatJele valley near
the. S6malia border. It has conformable relationship with both the underlyings Mustahil
(Km) and overlying Belet Uel1 (Kb) Formations. The Ferfer Formation (Kf) consists of
alternating grayish brown, commonly peletal dolomite, IJray clayey limestone, olive gray
",.. \ I _ ~~,~ ;:...,:.' -" "_.' ~ _'__ ;.., -:_ ' _ -"--," r" '-, --...
marl, shale and white-'anhydrite:' In' the lazgaf and Marehan' sections anhydrite"
predominates. The formation varies from 100 to 200 meters in thickness. An Albian to
Cenomanian age has been deduced for the Ferfer Formation on the basis of the ages
of the ,underlying and overlying Formations.
The Belet Uen Formation (Kb), also previously known as Belet Uen Series
occurs in southeastern Ogaden and varies in thickness from 87 meters to 232 meters
(Kazmin, 1975; Merla: 1979). The formation named after the Belet Uen type locality in
northern Somalia (Merla, 1979) extends as far north as around Shilabo area following
the Wabi Shebele Valley into Ethiopia.
The Belet Uen Formation conformably overlies the Ferer Formation (Kf) and is in
turn cOFlformablyoverlain by the Jessoma Formation (Pj). The formation consists of
42
-.- --- --
cream to light gray colored limestones, some of which are of reef origin, with
intercalations of greenish gray ~~Iauconiticshales and green or brown sandstone. It has
been assigned an age of Cenomanian to Turonian on the basis of fossil evidence
(Merla, 1979).
The Amba Aradom Formation (Ka.), formerly known as the upper sandstone
(Mehade, 1968; Beyth, 1971; Arkin et al. 1971; Kazmin, 1972 & 1975) occurs in
. eastern, central and northern (Tigray region) parts of Ethiopia. The formation consists
( of sandstone, shale and marl. It lies conformably on the Jurassic Antalo Formation (Jt)
) in central part of the country, whereas in the Mekele (Tigray) and Garamulata
(Hararghe) areas, it lies unconformably on the Jurassic sediments, namely, Jt and/or Ja
and Ju and/or Jh (Mahadi, 1968; Greitzer, 1970; Kazmin, 1975).
In northern part of the Mekele outlier, the formation consists of claystone and
siltstone with interbedded massive, sometimes cross bedded, white to pink sandstone
(Arkin et aI., 1971; Beyth, 1971). The rocks become progressively coarser in grain size
southwards. Further south, on the southern rim of the outlier, the conglomerates and
sandstones give way to fine grained sediments as the formation thickens again. In
most of the area the formation is laterized.
In the Abay River canyon the thickness ranges from 450 to 600 meters, in
Garamulata, Haraghe region the unit is more than 300 meters thick whereas in the
Danakil Alps, the unit is only 1E,Ometers thick. The Amba Aradom Formation (Ka) is
probably of Late Cretaceous agE!and represents a regressive facies of the Cretaceous
sea (Kazmin, 1975).
This formation, formerly known as Jessoma Sandstone (Kazmin 1972 & 1975)
covers a wide area in the eastern most part of the Tertiary sedimentary basin in eastern
Ethiopia (Ogaden). The Jessor'la Formation (Pj) is a transgressive unit which rests
43
- --
--- - - - - - --
The Taleh Formation (Pt), formerly named as Taleh Series, occurs. at the
eastern most corner of the Ogaden region along the border with Somalia (Kazmin,
1972 & 1975). It lies conformably on the lower sequence of the Auradu Formation (Pa),
but interfingers with its upper part. It has gradational relationship with the overlying
Karkar Formation (Pk) with the upper part of the Taleh Formation grading laterally into
chalky limestone of the Karkar Formation (Pk).
44
II
of clastic deposits. It is similar to the Gypsum-anhydrite series of Somalia which attains
a maximum thickness of 350 meters and thins rapidly towards the West. Early to
Middle Eocene age is inferred for the Taleh Formation from the accurate age of
interfingering with upper Auradu Formation and gradational contact with the Karkar
Formation (Pk).
The Karkar Formation also known as Karkar Series covers a relatively very small
area in the extreme eastern part of Ogaden along the northern Somalia frontier where it
covers much wider ground and attains a thickness of 230 meters (Kazmin, 1972 &
1975). The Karkar Formatior' consists of white cavernous chalky limestone
interbedded with brown fissile shale and bands of fibrous gypsum, which are
particularly common near the base. Lithological and faunal evidence suggest
deposition under deeper water cOr'lditionsthan those of the Taleh evaporites. On the
basis of micropalaeotological evidence, the Karkar Formation (Pk) has been assigned
an age of Middle Eocene (Kazmin 1972 & 1975).
45
- - - - - -- -
- - -- -- -
4.1 Introduction
Mohr (1962) divided the Cenozoic volcanic rocks of Ethiopia into the Trap
Series and Aden Series. The term Trap Series is still widely used to refer to the whole
pile of the Tertiary flood basalt sequence with intercalation of felsic lava and pyroclastic
rocks (commonly on the upper part) which form the northwestern and southeastern
plateaus and attain a thickness of up to 3 km. The name Aden Series was used for
post-rift (Middle Miocene to Quaternary) volcanic rocks of the Main Ethiopian Rift, Afar
Depression and some parts of the Ethiopian plateaus. Recent studies have enabled to
identify several distinct volcanic pulses in both the Trap Series and Aden Series The
following criteria were used to make the classification.
46
et al. (1979) which showed that volcanism has migrated to the axial zones of the
developing rifts (e.g. Wonji Fault Belt) which later became the locus of Quaternary
volcanism. Abbate and Sagri (1980) also using other structural criteria have divided
the volcanics of t.he Ethiopian and S~malia plateau into three main successions
(namely, the north Ethiopian plateau volcanics, the Wollega and southwestern
Ethiopia volcanics and the Somali plateau volcanics). The main structures
controlling the distribution of. these successions are the escarpment of Afar
Depression, the Main Ethiopian Rift and transversal lineaments such as Adwa-
Aksum, Addis Ababa-Ambo-NI3kemte,and Bonga-Goba and Sagan.
3. The type of volcanic activity has been used to distinguish I classify the different
volcanic episodes. e.g. shield forming basaltic volcanism (Pntb & Ntb) versus
fissural flood basaltic volcanism (P2a& P3a).
4. Several attempts have been done to characterize gechemically the different pulses
of the Cenozoic volcanic roc,o(Sin Ethiopia (Zanettin et al. 1974, 1976 & 1980;
Piccirillo et aI., 1979; Mohr, 1983;). As an example the Ashangi and Aiba pulses
consist of dominantly basalts whereas the Alajae Formation is represented by a
bimodal rhyolite (ignimbrite) and basaltic volcanism. Moreover a change in the
alkalinity of the flood basalt sequence of the western plateau was observed; the
youngest shield forming stagt~ (Tarmaber Formation) was found to be typically
alkaline. An increase in alkalirity in the younger plateau flood basalt sequence and
of the flood basalts of the western plateau from north to south has been suggested
by aerhe et al. (1987). Most of the Quaternary basalts (Qb), of the Main Ethiopian
Rift and Ethiopian Plateaus an3 transitional in nature whereas basalts of the axial
zones of the Afar Depression stlowa distinctive tholeiitic affinity. Furthermore, most
of the Quaternary silicic volcanic products (Qr & Qd) in the axial zones of the Main
Ethiopian Rift and the Afar Depression are characteristically peralkaline whereas
Mio-Pliocene silicic volcanic pr::>ducts(Nn & Nc) from centers along the rift magin
are predominantly sub-alkaline (Di Paola, 1972; Gibson, 1972, Chernet, 1995).
5. The type of substratum has been used to distinguish between and classify the
differe[1t volcanic pulses. The! different volcanic formations shown on the map
---
47
--- - - - --
- - - - - ------
.
typically overlie the Precambrian basement (Pjb, PNmb); Paleozoic or Mesozoic
sediments (P2a)and/orpreexistingvolcanicrocks(P:!a,Ntb).
The Cenozoic volcanic and sedimentary covers Gonsist of the followingunits on
the accompanying geological map.
4.2 Ashanai Formation (P2~
The Ashangi Formation represents the earliest fi:;sural flood basalt volcanism on
the northwestern plateau. The basalt flows are several hundreds of meters to a
kilometers thick of strongly weathered, crushed, tilted basalts which lie below the major
Pre-Oligocene unconformity (Zanettin et aI., 1980). Thl3Ashangi Formation consists of
predominantly mildly alkaline basalts with interbeddecl pyroclastics and rare rhyolites
and is commonly injected by dolerite sills and dykes. The upper part of the Ashangi
Formation is more tuffaceous and contains interbedde1jlacustrine deposits with lignite
seams. It was believed that these early flood banalt flows are restricted to the
northwestern plateau (Zanettin and Jusetin Visentin, 1975, Merla et al. 1973, 1979)
until a group of early flood basalt were found in southwestern Ethiopia (Davidson,
1983) with KfAr ages between 49 to 36 Ma. Among tl1ese the Akobo Basalt (49 to 46
Ma.) are here considered to be analogous with the basalts of the Ashangi Formation.
The age of the Ashangi Formation in the type area remains uncertain (Zanettin
et ~I., 1980). The oldest reported age for a volcanic rock on the northwestern plateau
is 54 Ma. (Kazmin, 1979 and references cited therein). The general consensus
remains that the Ashangi Formation have a Eocene to Oligocene age. However, basalt
flows and tuffs interbedded in Jurassic and Cretacemus sediments have also been
mapped in the Kulubi-Dire Dawa area on the southeastern plateau (Merla et aI., 1973)
and in Sakota area (Wello region) on the western plateau and appear to be evidence
"for an earlier basaltic volcanism related to the mantle plume which resulted in the
J~rassic uplift of Afro-Arabia.
48
4.3 Jima Volcanics (Pib and Pir)
The name Jima volcanics was given by Merla et al. (1973) to trachybasalts and
rhyolites which cover most parts of southwestern Ethiopia. The Jima volcanics which
are considered analogous with t.le Main Volcanic Sequence of Davidson (1983), form a
thick succession of basalts and felsic rocks with basalts dominating the lower part of .
most sections. Davidson (1983) has reported K/Ar age of 42.7 to 30.5 Ma for the Jima
Volcanics. Two units (Jima Basalts - Pjb and Jima Rhyolites - Pjr) which show a
conformable relationship were identified. The Jima Rhyolites being the younger of the
two units in southwestern Ethio::)iaare equivalent to the Magadala Group of Kazmin
(1972). The Jima volcanics almost always rest on the Precambrian basement, the
unconformity being marked by a basal residual sandstone. The basalt flows form an
unbroken succession several hundred meters thick in some places. In others felsic
rocks are intercalated with basalt flow close to the base or form a thick succession just
above the basal basalts.
The Aiba Basalts represent the second major pulse of fissural basalt volcanism
on the northwestern plateau. Tl1ey are generally aphyric, compact rocks, in place"',
showing stratification and contain rare interbedded basic tuffs. The Aiba Basalts (P3a)
unconformably overlie the Ashan!~iFormation (P2a)and attain a thickness of 200 to 600
meters. The basalts show a distinctive tholeiitic nature with transitions to mildly
alkaline varieties. The absolute age of the Aiba Basalts (P3a)ranges from 34 to 28 Ma.
placing them in Oligocene (Zanettin et aI., 1980; Kazmin, 1979).
The name Arsi and Bale Basalts (Pab) was given to the flood basalt succession
of the southeastern plateau whem the flood basalt activity culminated by the formation
of large volcanic edifices which fcrmed some of the highest peaks on the southeastern
plateau. The Arsi and Bale Basalts (Pab) are also commonly felsic on the upper parts.
Merla et al. (1973) gave an age range between Oligocene to Miocene to these basalts
which makes them in part correlable to the Makonnen Basalts (Pnmb) of southwestern
49
-
-- - - -
Ethiopia. The Arsi and Bale Basaltsare overlainby the Ghinir Formationin the east
and post-riftvolcanicsof the NazretSeries(Nn)in the northwestand by the Quaternary
Basaltsof the BatuMountainsin the south.
The name Makonnen Basalts was coined by Davidson (1983) for up to 700
meters thick of sub-horizontal flood basalts which cap the plateau in southern and
southwestern Ethiopia (Le. those of the Makonnen, Gecha, Nano and Gura Ferda
plateaus). The Makonnen Basalts (PNmb) mostly directly overlie on the crystalline
basement in the type area and yielded an obsolute K'Ar age range of 34.8 to 23.1 Ma
(Late Oligocene to Early Miocene) by Davidson (1983). The Makonnen Basalts are
physically separated and are readily distinguishabh3 on the basis of similar basal
elevation, columnar form, thickness of flows and t 1e fact that they have a basal
paleosol rather than a residual sandstone.
The Wollega Basalts of Merla et al. (1973) which were later subdivided into the
Lower Aphyric Basalts and Geba Basalts by Berhe et al. (1987) are here considered to
be analogous with the Makonnen Basalts. However tile KIAr age range of Middle-Late
Miocene given to the Wollega Basalts by BerhE! et al. (1987) are apparently
questionable since the ages are equal and even younger than those the overlying
Upper Aphyric Basalt. The Lower stratoid basalts maoped in the Dodola map sheet, in
southern Ethiopia, with an absolute age range from :m to 23 Ma (Berhe et aI., 1987),
are also correlable with the Makonnen Basalts.
The Alajae Formation (PNa) mainly consists of aphyric flood basalts associated
with rhyolit~ (ignimbrites) and subordinate trachytes. This formation (PNa) ranges in
age between 36-13 Ma (Kazmin, 1979; Zanettin et aL, 1980). A migration of Alajae
type volcanism from north to south is indicated by the occurrence of the older volcanics
of this formation on the northern part of the northWE!sternplateau. Alajae Formation
makes the bulk of the volcanic succession on both tt-e northwestern and southeastern
plateaus. On the northwestern plateau the Alajae Formation rests conformably on the
50
I
Aiba Basalts (P3a) but in some places (e.g. Kassem Gorge, Mugher Canyon and in
most outcrops on the southeastern plateau) it directly overlies on the Mesozoic
sediments.The Alajae Formation contains basalts transitional to tholeiitic in nature and
an increase in alkalinity is observed in the younger members of the formation. Thus
the Mio.cene members of the Alajae Formation are more alkaline and are associated
with sub-alkaline acidic members.
The classification Tarmaber Gussa Formation (PNtb) for the shield volcanoes of
the northern Ethiopian plateau with an absolute age range of 26 to 16 Ma and the
name Tarmaber Megezez Formation (Ntb) for the younger shield volcanoes with an
absolute age range from 16 to 13 Ma in the southern part of the northwestern plateau
and the southeastern plateau has been ~idely used and the latter is believed to mark
the intiation of rifting of the Main Ethiopian Rift (Kazmin, 1979 and references cited
therein). The upper age limit of the Tarmaber-Megezez Formation (Ntb) is lowered to 7
to 8. Ma since the large basaltic center of Arba Gugu with similar alkaline affinity is
considered to be the youngest episode of Tarmaber type volcanism. Other dominantly
basaltic units erupted within the age intervals frol1) 14 to 10 Ma (Anchar basalts of-.
Kazmin and Berhe, 1981) mapped on the eastern escarpments of the MER and
southern Afar and Miocene basaltic volcanism in western Ethiopia with an age range of
9 to 10 Ma (Upper aphyric basalt of Berhe et aI., 1987) are also considered with the
Tarmaber-Megezez Formation (Ntb) on chronological grounds.
51
- -
:'1
~
--- -- -
4.9Adw~
The Adwa Formation (Nad) consists of alkaline!volcanics and plugs which have
penetrated the Trap Series basalts of the area (Aiba Basalts, P3a). Blanford (1870) first
noted and considered the Adwa Formation separately from the Trap Series since then
the Adwa-Senafe area was considered as the type locality for Adwa Formation (Plugs)
(Garland, 1980). The Adwa Formation is composed of pale colored, fine grained lavas
and plugs which are of alkaline trachytic and phonolitic composition. The alkaline
plugs form a youthful and peaked topography with flow structure, columnar jointing,
concentric exfoliation and commonly form mountains of bare rock. The plateau from
which they rise is about 2,400 m a.s.1..and one of the highest peaks of the plugs
reaches 3,014 m a.s.1. In the wider age range between Oligocene and Miocene, similar
trachyte and phonolite plugs were mapped in many parts of the northwestern plateau
(e.g. Adis Zerhen-Gondar) and southwestern Ethiopia (Davidson, 1983) and are
considered analogous to Adwa Formation plugs (Nad) on the map.
The type locality of the Surma Basalts is furthE!rto the west on the international
boundary with Sudan. The Surma Basalts overlie older pre-rift volcanic successions
(Jima Volcanics; Pjb I Pjr). The Teltele and Surma basalts are of Early Miocene age
with reported KIAr ages between 21.2 and 18.2 Ma (Davidson, 1983).
52
--
The Adwa Formation (Nad) consists of alkaline volcanics and plugs which have
penetrated the Trap Series basalts of the area (Aiba Basalts, P3a). Blanford (1870) first
noted and considered the Adwa Formation separately from the Trap Series since then
the Adwa-Senafe area was considered as the type locality for Adwa Formation (Plugs)
(Garland, 1980). The Adwa Formation is composed of pale colored, fine grained lavas
and plugs which are of alkaline trachytic and phonolitic composition. The alkaline
plugs form a youthful and peaked topography with flow structure, columnar jointing,
concentric exfoliation and commonly form mountains of bare rock. The plateau from
which they rise is about 2,400 m a.s.1..and one of the highest peaks of the plugs
reaches 3,014 m a.s.1. In the wider age range between Oligocene and Miocene, similar
trachyte and phonolite plugs were mapped in many parts of the northwestern plateau
(e.g. Adis Zemen-Gondar) and southwestern Ethiopia (Davidson, 1983) and are
considered analogous to Adwa Formation plugs (Nad) on the map.
The type locality of the Surma Basalts is further to the west on the international
boundary with Sudan. The Surma Basalts overlil~ older pre-rift volcanic successions
(Jima Volcanics; Pjb I Pjr). The Teltele and Surma basalts are of Early Miocene age
with reported KIAr ages between 21.2 and 18.2 Ma (Davidson, 1983).
52
4.11 Mabla and Arba Guracha Formations (Nmr)
Middle Miocene felsic volcanism is confined to the margins of Afar and northern
part of the Main Ethiopian Rift. There were probably several centers in these areas
which produced thick sequences of ignimbrite and rhyolite flows and domes. Rhyolite
flows and domes and ignimbrites of slightly commendetic composition with rare
intercalations of basaltic flows, pumice and cinerites were mapped along the margins of
central and southern Afar as Mabla Rhyolites (CNR-CNRS, 1975). In the Danakil Horst
the rhyolites rest unconformably emthe Adolie Basalts (Varet, 1978) and are overlain
by Dalaha Basalts (Ndb) with a sharp unconformity. The absolute age range of the
Mabla rhyolites is from 14.2 to 9.7 Ma. (Varet op cit.).
The Dalaha Formation (Nclb) consists of fissural flows of basalts and hawaiites,
with some intercalated detrital and lacustrine sediments, rhyolite flows and pyroclastics
near the upper part. The Dalaha Formation is well developed in the neighboring
Djibouti and Northern and Central Afar attaining a thickness of up to 800 m. The
Dalaha Formation was given an absolute K/Ar .age of 8.6 Ma (Varet, 1978). The
basalts of Dalaha Formation are transitional basalts ranging from rare picritic type to
more frequent iron rich varieties (ferrobasalts).
53
--- -
-- ---- -- ----
Trachytes with subordinate basalts forming shield volcanoes at the Tulu Wollel
and Sayi areas in western Ethiopia formed during the Late Miocene are named as the
Tulu Wollel Trachytes (Ntt). The Tulu Wollel Trachytes overlie the Upper Aphyric
Basalts of Berhe et al. (1987) which on this map are correlated with Tarmaber-Megezez
Formation (Ntb) which form the base of the shield volcanoes. An absolute KIAr age of
ca. 8 Ma. was given to the Tulu Wollel Trachytes (BHrhe et aI., 1987). Trachytic and
phonolitic plugs of probably the same age occur in association with this formation.
The name Nazret Series was given to a thick succession of welded ignimbrites
with fiamme, pumice, ash and rhyolite flows and domes with rare intercalations of
basalt flows which occur in the MER, rift margins and adjacent plateaus (Meyer et al. I
1978). In the rift proper the Nazret Series attains a thicknessof up to 200 to 250
54
meters and tend to thin out on the 13scarpments.On the plateau margins a thickness of
1 to 30 meters was reported at many localities. Ignimbrites of the Nazret Series are
considered to be products of eruptions mainly from marginal centers in the rift
(Morbedelli et aI., 1973). In composition the ignimbrites are sub-alkaline rhyolites and
trachytes with rare peralkaline varieties. An age range of 9 to 3 Ma has been given to
the Nazret Series on the basis of its relation to the Mio-Pliocene lacustrine sediments
of the Chorora Formation and some absolute K/Ar age determinations (Tiercelin et aI.,
1980; Kazmin and Berhe, 1978).
The oldest sediments found within the Danakil Depression are represented by a
detrital formation named as the Danakil Group (Red Series) (Garland, 1972; CNR-
CNRS, 1973). Sediments of the Danakil Group (Nrs) crop out only in two parallel
bands along the margins of the Danakil Depression. The formation consists of several
hundred meters of conglomerate, sand, sandstones and red, green or multicolored
clays, with interbedded basaltic lavas and acidic intrusions with some marine beds
locally present. The Danakil Group lies unconformably on the Mesozoic limestones
and sandstones or directly on the epi-metamorphic basement that constitute the foot of
the Ethiopian escarpment and the Danakil Alps. K/Ar dates for interbedded basalts at
the base and top of the Red Series gave absolute ages of 24 and 5.4 Ma respectively
giving the series an Early MiocenE~-Plioceneage (Tiercelin et aI., 1980 and references
cited therein). This indicates that the Danakil Depression was already in existence in
the Early Miocene. Sandstone and Conglomerate which form a major part of the Red
Series represent alluvial fan sediments deposited by rivers issuing from the
escarpments, and pass laterally into lacustrine and marine clays with associated
gypsum, marine limestone and subaqueous lavas. Tectonic events are the controlling
factors of this coarse detrital sedimentation, and appear to have continued until 5 Ma,
which is the upper age limit of the Red Series.
55
----
predominantly trachytic volcanoes interfinger with tt'e upper part of the Nazret Series
(Nn) and in fact constitute part of the series. The upper unit (Ncb) which commonly
forms the top part of the shield volcanoes is invariably represented by fresh flows of
porphyritic alkaline basalts.
56
In some of the volcanoes like Daro on the eastern shoulder of the Main
Ethiopian Rift only the lower unit is present and in others like Wechecha and Yerer the
mafic products are scarce and a cle~arsubdivision is not apparent and are shown as the
lower unit (Nc). KIAr ages on most of these volcanoes range from 8 to 4 Ma (Kazmin et
aI., 1980b, Chernet, 1995). A rela::ivelyyoung age of 1.4 Ma has been reported by Di
Paola (1976) on a mugeritic flow on the western slope of Mt. Chilalo which implies that
some of the volcani.q activity has probably continued into the Pleistocene. Recent
geochronologic works based on the geologic mapping of the Dodola Sheet in
southeastern Ethiopia (Berhe et aI., 1987) has sugested that some of the shield
volcanoes in Bale are probably correlable to this unit.
57
- - - - --- - - -
- - - - --
Other Pliocene basalts of Turkana Rift namE3das the Mursi Basalts have a
similar KfAr age of 4.2. Ma (Davidson, 1983). The Mursi Basalts consist of a relatively
few, thin, columnar flows of basalt and in most sections have a total thickness of less
than 100 meters. The basalts rest on a tuffaceous volcano-sedimentary sequence, and
field evidence shows that the basalts underlie much of the Middle Omo plain which is
covered by the Omo Group Sediments (Davidson, 1983). The Mursi and. Bofa Basalts
(Nb) are here considered analogous because of their similar age, occurrence and
predominantly transitional! alkaline nature.
sa
(b) Omo Group
The Omo Group sediments are also Plio-Pleistocene continental deposits which
cover most parts of the Kibish, Omo and Usno branches of the Turkana Rift. According
to Davidson (1983), four formations, Mursi, Nkalabong, Usno and Shungura comprise
the Omo Group Sediments. The sediments of the Mursi Formation lie unconformably
on a tilted pre-rift rhyolite of probably Miocene age and is composed of about 150
meters of clays, silts and sands with subordinate tuff and pebble beds. These
sediments are conformably overlain by the Mursi Basalts which are shown as Mursi and
Bofa Formation (Nb) on the map. The Nkalabong Formation overlies the Mursi Basalts
and is about 90 meters thick gray-brown fluvial clastic sediments, eolian sands, water
lain tuff and tuffaceous sediments. The Usno Formation comprises of about 200
meters thick of alternating, fluvial, lacustrine sediments with the tuffaceous horizons
and a single basalt flow (dated 3.31 Ma, Davidson, 1983) at the bottom of the section.
The Shungura Formation being the youngest of the Omo Group sediments (between 3
and 1.3 Ma) includes at least 750 meters of brown gray and buff colored clays, silt,
sand, gravel, tuff, marl and fresh water limestone deposited from a fluctuating fluvial
an9 lacustrine cycles. Other Plio-Pleistocene sediments in southwestern Ethiopian e.g.
lIeret sediments (Davidson, 1983) alrealso included in this unit.
59
-- - - -
6. QUATERNARY VOLCANIC AND SEDIMENTARY ROCKS
In the Main Ethiopian Rift the Bofa Basalts (Nb) and the Nazret Series (Nn) are,
in most places, overlain by green and gray ignimbntes with well developed fumme and
associated unwelded pyroclastics and waterlain pyroclastics with occasional
intercalated lacustrine beds and aphync basalts which have a maximum reported
thickness of 50 meters were named as the Dino Formation (Kazmin and Berhe, 1978 &
1980). In the Awash Gorge an ignimbrite member of the Dino Formation (Qd) was
isotopically dated to be 1.5 Ma old (Kazmin and Berhe, 1978). The pyroclastics of the
Dino Formation may have sources frolT)axial felsic volcaic eruptions complexes such
as the partly eroded volcanc;>known as Tinish Fantale and/or ealier of the other axial
center(Qr). The felsic lava of the Dino Formation an3 peralkaline in composition and
Kazmin et aI., (1980b,c) have observed that the ignimbrite members are not confined
only to the rift floor but are extensively developed on the escarpments.
The Ghinir Forrnation was mapped on the southeastern plateau by Meral et al.
(1973) to represent the Quaternary rhyolite volcanism in the Ghinir area (Bale Region).
The Ghinir Formation is mainly made up of rhyolites with subordinate basalts.
The Quaternary central volcanic complexes which are situated along the axial
zone~ of the Main Ethiopian Rift (Wonji Fault Belt) and Afar have produced peralkaline
lavas and pyroclastics. On most of the volcanoes recent stages of activity are marked
by obsidian flows, pumice, ignimbrite, tuff and scoriaceous basalt eruptions. However,
most of the basalt flows have fissural orgin from later fractures of the Wonji Fault Belt
which in some instances dissect even the volcanoes. The products of the volcanoes
range from trachytes to pera~kaline rhyolites (pantellerites and commendites). The
oldest reported KIAr age from the central volcanoes of the Wonji Fault Belt are, from
Bosete Volcano is 1.6 Ma (Morbeddelli et a!., 1975). Some strata volcanoes in the Afar
60
Depression (Varet, 1978) situated along the margins of the ,depressior'1!1ave erupteq
felsic lava (trachytes - rhyolites) and pyroclastics similar,to tli:t(3centers of the, VVonji
Fault Belt and are considered as the same unit on t~ map. , ,>,;. 9:111, "":)""":5
The other younger analogous 'unit is the relati',}§!y fresh: Tepi Basalts;lprodueed
by::Central type eruption in sOUthwestern Ethiopia with a 'HOlbcene 28ge (Davidson,
1983) and is considered tb'be an analogO'us2Grlit.The~af{!}U~t~A1Myr6asalt.flow§28(-e
characteristically alkaline and may represent the final pulsej~Jba~aHjC'lvOfc§aPtiSmei!fP1
the Ethiopian plateau. . .00 .0) eJn9mlDe2 V1Srl''''.78UO
~ - -- a.e
61
which produced the differentiates of basalts through intermediate iron-rich varieties
(ferro-basalts, hawaiites, mugearites and dark quartz rich trachytes) to a very limited
amounts of alkaline and peralkaline silicics (Qbt) (Varet, 1978).
basalts' par1i~larlY;I18'k>Qg;)the
Wonji Fault Belt whid;Ls!3Pw,AholeiiJictendencies like
those of the axial ranges in Afar. However basalts)pfJhe transverse volcanic ranges of
~far
u9')l)
(mainl~
)01q
~JPapak!1
"II,_o,, J
Ho]:st.th"at
I T 1:'-::1 ,
are controlle,d by east - northeasterly
,
lineaments) are
alkaline b~salts and inl roanYrl places, contain ultramafic xenoliths. Similar alkaline
.n02bIV60) 9Q6 . ') r ., G I ii'll "
~~~aJt~~~~~~R9nt~in ~J~~P)I~ric ,~roliths are also .found in the M~yale-Mega area and
are include d with this unit.
: to . J'I I6
~
no (r' lS.J 0..
1
8
6.6 Quaternary Sediments lQ. Qp. Qh)
SeQiments are mostlv of lacustrine origin. Lacustrine beds are interbedded with Plio-
r,u ems 3 en! ~s rlwOi1? 916 ,Co , .
Pleistqgene("\;pvrocla~tics
en! J.)rn
iDjthe ,lakes' region and ,on the rift shoulders (Mohr, 1966;
1::>9106,!' gl, .", ? l.> ; .It
Lloyd, 1980). The lacostrine beds are mostly redeposit~d volcanic s~nds, tuff wi~
.
calcareous material and diatomite. According to Mohr (1966) at the beginning of the
Quaternary an ancestral lake which was almost certainly continuous from the Abaya -
Ct}MAfuLakes 1Sjtne-'~b1Jtn
to the Awash basin to the north existed until it shrinked to
smaller ones by lat~-:Pleistocene tectonic movements. Pleistocene-Hblocene parts of
':A.faris represented by a number of sheet flood terraces mainly composed of salts,
Ii
62
sand, gravel, gypsiferous and fossiliferous limestone of lacustrine origin which are
found in the Lakes Aferera and Assai area.
Marine Pleistocene existed only in the northern part of the Afar Depression
(Danakil Depression) and the Red Sea Coast. In the northern Afar marine deposits of
the Enkafla Beds (White Series) (Brinckmann and Kursten, 1968) and part of the salt
formations of Dallol are of Quaternary age. Along the Sudan bord8r also large areas
are covered by undifferentiated fluvial outwash. The thickness of these sediments of
undifferentiated continental origin (gravel, sand, silt and clay) reachs 10 to 20 meters at
places increasing towards the plateau escarpments to the East (Kazmin, 1972).
iT'
.I' ...J
~ -:,'
_.
, I .J
63
---- - - --
1. Introduction
good potential for the development of a wide range of mineral resources. / A wide
variety of mineral and construction material deposits have.been identified (Fig. 1). Gold
remains by far the most enciting and promising resource. Other proven economic
metallic mineral deposits include those of copper, zinc, nickel, tantalum and platinum.
Economic deposits of industrial minerals in the country include rock-salt, potash,
phosphate, soda ash, feldspar, diatomite, bentonite, kaolin, sulphur and lime. Proven
reserves of hydrocarbons such as natural gas,.oil, oil-shale and lignite have also been
found in many parts of the country. Construction materials-such as marble, granite,
etc. and raw materials in cement and aggregate manufacturing are also found in many
suitable locations.
~. METALLIC MINERALS
The most promising region for base metal mineral prospecting is the low grade
belt in the north, west and south of the country. All the known metallic mineral
64
/
2. Base metal showings in the west, particularly in Wollega where base metals are
found associated with magnetite, in a copper-gold prospect, and under a gossan
cover.
3. Iron ore occurrences in the Gulliso-Yubdo and Bikilal areas, in Wollega region and
at Melka Arba in Bale region. The iron ore reserves at Bikilal and Melka Arba are
still under investigations, preliminary estimates of the Bikilal iron ore has indicatea
that there is a reserve of over 20,000,000 tons of ore.
2.2 Precious Metals
Geologically, the most promising regions for gold and other metallic minerals are
those terranes underlain by Precambrian rocks, primarily along the volcanogenic belts
of Proterozoic age (Fig. 1).; Presently known gold deposits, mostly placer, are largely
.
found around the Adola gold field in Sidamo region where mining have been conducted
for over forty years. The primary deposit has been discovered at Lega Dembi and
nearby Sakaro, Wollena and Korkoro and the potential for discovery of additional
economic primary deposits is high. Placer gold has been mined for centuries in
Wollega and Kefa regions by the local people. Primary deposits have also been
identified in Wollega. The geological environment in the other Precambrian terranes in
northern Ethiopia (eg. Tigray) also have excellent potential for primary gold
occurrences.
65
-- - - -
3. NON METALLIC MINERALS
Potash deposits in the Danakil Depression near Dallol, have been known since
the turn of the century. The potash-bearing strata consists of three units, they are
sylvite on top and kainite towards the base, with carnalite throughout the strata. A total
reserve of 172,897,000 tons of potash has been estimated.
Other major industrial minerals, which could offer a possibility for development
for local consumption are bentonite
..
and diatomite. Sufficient tonnage of quality
material for the filler industry as well as for usa in filters and for other specialized
industrial requirements (binders, bricks, insulating materials) is available. Bentonite
occurs in lagoonal/lacustrine deposits in the rift valley. In addition, a number of
diatomite deposits have been identified in a similar geological setting as the bentonite.
.
Ceramic raw materials like the Bomba Woha kaolin deposit has been discovered
north of Kibre Mengist in Sidamo region with a proven reserve estimated to be ~
250,000 tons. Pegmatites at Kenticha have also been examined as possible source of
feldspar and quartz. Testing has shown that both these minerals have a quaflty
suitable for use "in the ,ceramic and glass industry: Suitable raw materials for sheet
glass industry such as silica sand, dolomite, feldspar, limestone and soda ash are also
in ample supply.
66
Bikilal area (Wollega region), phosphate occurs in the form of apatite within a gabbro-
hornblendite complex several kilometers in length, with an average grade of 4 to 5%
P20S.
Sulphur deposits have been mined in the past between 1955 and 1953 in the
Dofan area, at Dallol and 20 km north of Dallol. Deposits of sulphur from fumaroles and
solfataras are common in the Danakil Depression. Reserves of 12,000 m3 of sulphur
rich rock materials have been reported for the sulphur field near the young Kebrit Ale
volcano. Semi precious stones peridote and garnet are found in the pegmatite I
ultramafic complexes of Sidamo region. The latter could possibly support cottage scale
industry.
3.3 Hydrocarbons
Lignite, coal and oil shale deposits within the Tertiary volcanic succession, 50
km east of Jima at places called Delbi and Moye in Iliubabor region, are under
investigation. Other occurrences of lignite within the intertrappean sediments of the
Ethiopian plateau, near Chilga (Gonder), Debre Brehan (Shewa) and Wuchale (Wello)
have been investigated for their calorific value and the feasibility economic recovery.
Petroleum exploration in the Ogaden and Red sea areas indicate that there are
promising prospects for discovery of hydrocarbons. The Calub gas field discovered in
67
- - - -- - ----
-- ----- ---
the Ogaden area has 76 BillionMetric ton. The feasibility of the development of the
Calub gas field is also under investigations.
3.4 Water and other Resources
68
-- - - - -- -~----
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VII. ACKNOWLEDGMENTS
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