THE SEDIMENTARY SUCCESSION IN
CENTRAL AND EASTERN ETHIOPIA
stratigraphy, sedimentology, tectonic and palaeoflora of
the main sedimentary
studied in the Central Ethiopian Plateau and
the Western Harar Plateau. The surveyed areas are as
ETHIOPIAN PLATEAU: Central Blue Nile Bassin (Blue Nile Bridge, Mugher
Wodem Valley, Ambo-Guder area), Addis Ababa region and Eastern border
the plateau, Upper Omo Basin.
b) HARAR PLATEAU: Chercher
Mounts, Dire Daoua area.
The following points are
and palaeofloral dating of palaeozoic and mesozoic clastics
and palaeofloral dating of main volcanic events
- Structure of the surveyed
areas by ERTS imagery interpretation.
FORMATIONS OF ETHIOPIA
There are 5 major diachronous
FACIES: Carboniferous-Lower Permien
SANDSTONE FACIES: Permian-Triassic to Jurassic
ABBAI STRATA: Triassic(?)-Liassic
LIMESTONE FACIES: Middle-Upper Jurassic
SANSTONE FACIES: Upper Jurassic-Cretaceous
addition, lacustrine beds are disseminated in the cainozoic
2.1. PRE-ADIGRAT FACIES
description is used for various non-metamorphosed sediments
underlying the Adigrat SandstoneFacies. These layers are generally
correlated with Carboniferous glaciations. Numerous partches were
observed in Northern
Ethiopia, Blue Nile Basin and Chercher Mounts.
formations were mapped.
Their ages are unknown.
Tigre province, shale with boulders and gray sandstone were
described 40 years ago by Italien Geologists. Recently, they have
as glacial deposits. Due to the apparent conformity with the
Sandstone, a carboniferous age is inferred. Similar facies were
the Danakils Alps.
patches of gray sandstone are exposed in
the very bottom
of the Blue Nile canyon. The maximum thickness is probably several
100 m. The
lower beds constitute the filling of ancient
valleys in the metamorphic basement.
The upper beds are conformably covered by sub-horizontal Adigrat
Sandstone Facies. The sandstone is frequently cross-bedded,
grey-coloured, weIl sorted, medium to fine grained, and contains a
large amount of feldspar, biotite, chlorite, pyroxene and volcanic
fragments. The matrix is mainly comprised of illite and chlorite with
kaolïnite, dickite and corrensite. Some beds contain blocks of
yellow calcitic material rich i montmorillonite and interpreted as
reworked palaeosol fragments. At the top of the formation, there are
thin horizons of
Carboniferous-Lower Permian plants débris; the surface of some beds
fragments of haematitic crust and haematized wood: these observations
warmer environnent. Diagenetic evolution is important.
The poorly preserved prints of
leaves, cuticles, wood
débris, and rare spores refer to an Upper Carboniferous to Lower
Chercher Mounts, grey sandstone underlying Adigrat red
facies are less
thick, no more than 200 m. They a well-bedded, finely laminated
(varvoid aspect), moderately sorted and fine grained to silty. Lower
beds contain some angular
pebbles of amphibolite. Feldspars are abondant, and associated
with biotite and chlorite. Matrix is made of illite,chlorite and
and iron hydroxydes occur at the top of the formation. In some
are intensively folded although they are perfectly horizontal
disposition is interpreted as huge slumping of unconsolidated
material due to
faulting. Scarce spores refer to Carboniferous.
Pre-Adigrat sediments of Tigre represent true
tillites, Pre-Adigrat sandstones of Blue Nile and Chercher are
interpreted as outwash of moraines deposited in fluvial and deltaic
environnent. Carboniferous montain
glaciers were located in the North-Western Ethiopia, in
and provided detrital material which were transported and
accumulated in valleys and coastal plains.Climatic change took place
in the LowerPermian.
2.2. ADIGRAT SANDSTONE FACIES
cross-bedded red sandstone overlie the metamorphic
basement or the Pre-Adigrat sandstone and are generally covered by
Antalo limestones. It is of large extent in Ethiopia; its thickness
can exceed several
100 m, its age ranges from Permian to Middle Jurassic.
the Blue Bile Basin, the Adigrat Sandstone Facies is
about 200 m thick and can be divided in 2 members.
lower member consists of fine to medium grained red
sandstones alternating with silty shale. Their bedding is slighty
inclined and variably oriented; the basal beds are transitional with
grey sandstone. This lower member was deposited in a continental
upper member is generally made of coarse cross-bedded
sandstone, with channels filled by gravels. The general direction of
South-Eastward. Some beds are horizontally bedded and graded, with
and gravel at the base, and silt, ripple-marks, mud-cracks at the
sandstones are orthoquarzites; they are mainly composed
of quartz, with few feldspars and moscovite, and very little matrix.
made of kaolinite.
Calcite is present in weathered parts. Grains are generally coated by
a goethite film. The sorting and roundness are better at the top of
and suggest a deltaic and near shore deposition environment. The
evolution is generally not important, except for localized quartz
overgrowths giving a quartzitic structure to some beds.
Poorly-preserved silïcified trunks in a channel can be correlated
with Permo-Triassiu trees.
Chercher, red sandstones are similar, but the lover beds are coarse ,
and contain small terrestrial Gasteropods shells.
The others beds are fluvial and deltaic, while the uppermost strata
clearly marine and constitute the transition to the overlying Antalo
Limestone. In some places, the clastic formation is very thin; it
consists only of the
Jurassic upper marine beds directly covering the metamorphic
have undergone the continental erosion during Permo-Triassic times
Adigrat Sandstone Facies is similar.
conclusion, the Adigrat Sandstone Facies is a typical molassic red
facies deposited under a tropical climate, in a first continental
grading to a near-shore
one at the top. Its age ranges mainly from Permian to Triassic but
can extend to Jurassic in some places (in Chercher for instance).
thick evaporitic formation (500 m) is restricted to the Blue Site
Basin. It consiste mainly of variegated shales, gypsum, dolomite and
some beds of sandstones. Clay minerals are predominantly illite and
distribution and shape of grains in sandstones indicate a near-shore
of small Pelecypods, Gasteropods and Ostracods débris were found in
beds but dating is not precise: a Triassic to Liassic age is
Abbai Strate were deposited in a shallow marine basin
connected to the open sea in the Northern part.
ANTALO LIMESTONE FACIES
reef limestone and marl a well represented in
Ethiopia and characterize the maximum
Mesozoic transgression (In Ogaden Basin the
description' Hamanlei Formation' is used
instead of 'Antalo'). They are fossiliferous
and have provided long lists of Gastropods, Brachiopods, Pelecypods,
and microfossils. Ammonites were found only in the Ogaden Basin.
from Bathonian to Kimmeridgian or Portlandian according te the
Tigre and Ogaden, a shaly unit occurs at the top of the
regressive facies ends the sedimentary series except
in SE Ogaden where the marine sedimentation continued into the
the Blue Nile Basin and Chercher Mounts, the lithologic
facies progressively changed at the top of the Antalo Limestone
first deposited sandstone beds are near-shore sediments, but the
major part is continental.
Sedimentological features are very similar to Adigrat Sandstone
Continental sondstones are red, cross-bedded and poorly sorted; they
on a piedmont slope ('Glacis'). They provided a lot of silicified
trnnks very useful for dating. The top of the formation was generally
eroded at the
end-Cretareous, prior to the Tertiary eruptions, but in some places,
there is no noticeable
unconformity between the Upper Sandstone and the volcanics,
that the first eruptions were contemporaneous there
the upper most sandstone
conclusion, the Upper Sandstone Formation is also a typical
molassic facies ranging from Upper Jurassic to end-Cretaceous
Plateau, but to early Tertiary in the Chercher Mounts.
SEDIMENTARY INTERCALATIONS OF TERTIARY VOLCANICS
associated with Cainoeoic basaltic
flows. Many pyroclastic beds were deposited in lacustrine environment
clay and lignite. Plant remains are frequently encountered (prints of
silïcified or carbonized woods). Silïcified wood is also present
Palaeobotanical determinations were useful for dating the main
events. They shown that episodes
are numerous and
in various regions.
activity started in the
on the central
Ehiopiau Plateau, but later (end-Eocene or
the Harar Plateau. The
flood basalts of the Trop Series were mainly
the Oligocene and Miocene
times. On the Ethiopian Plateau, shield volcanoes and acidic welded
Miocene and Pliocene.
EVOLUTION OF ETHIOPIA
geological studies made in East Africa
and Arabia Peninsula, the sedimentary history
of Ethiopia con be reconstructed
Southern part of Africa. They
the Ethiopian-Sudanese Massif too. They have deposited
in the North
(and West ?) Ethiopia. Elsewhere, moraines have been
fluvial and deltaïc environment
PERMIAN and TRIASSIC
to faulting, Ethiopia was
a pattern of
horsts and basins (hemi-grabens) roughly oriented NE-SW. The horsts
were the Ethiopian-Sudanese
Massif, the Harar-Yemen Massif and the Bur Acaba Massif. They were
and they provided detrital material to the basins of Tigre-Blue
Kenya and Indian Ocean coast.
Adigrat Sandstone was deposited on piedmont
slopes ("glacis") and in alluvial plain ("bahada').The
climate was tropical.
of subsidence of the Tigre-B1ue
Basin, and of
the Ogaden Basin, a shallow sea invaded these
and locally deposited evaporitic
facies (Abbai Strata).
Horn of Africa. There were reefs in shallow water (Antalo Limestone)
and little clastics along the
Western sea shore (Ambo Sandstone). The regression started at the
Due to the general uplift of
the Horn of Africa, the regression was completed, except in the SE
Ogaden where the sea persisted until the early Tertiary. The erosion
of the Ethiopian-Sudanese Massif, and secondarly of the North Somalia
one, provided clastics on pedmont slopes and alluvial plains up to
the sea shore (Upper Sandstone).
Faulting in the early Eocene
gave way to the Trap Series flood basalts. Lacustrine deposits were
accumulated during gaps in volcanic activity. During the Miocene and
the Pliocene, on the Ethiopan Plateau, short but thick basaltic flows
an pyroclasts from isolated centers (Shiels Group) and thick acidic
tuffs ( Post-Trappean Ignimbrites) were erupted. In that time, the
Arabic Peninsula split away and the Ethiopian Rift was generated.
Abstract from thesis work, université de Lyon 1, 1977.