THE SEDIMENTARY SUCCESSION IN CENTRAL AND EASTERN ETHIOPIA






1. LOCATION AND TOPICS


The stratigraphy, sedimentology, tectonic and palaeoflora of the main sedimentary formations are studied in the Central Ethiopian Plateau and the Western Harar Plateau. The surveyed areas are as follows:
a) ETHIOPIAN PLATEAU: Central Blue Nile Bassin (Blue Nile Bridge, Mugher Valley, Zega Wodem Valley, Ambo-Guder area), Addis Ababa region and Eastern border  of the plateau, Upper Omo Basin.
b) HARAR PLATEAU: Chercher Mounts, Dire Daoua area.
The following points are particularly emphasized:
- Sedimentology and palaeofloral dating of palaeozoic and mesozoic clastics
- Stratigraphy and palaeofloral dating of main volcanic events
- Structure of the surveyed areas by ERTS imagery interpretation.



2. SEDIMENTARY FORMATIONS OF ETHIOPIA


There are 5 major diachronous formations:
- PRE-ADIGRAT FACIES: Carboniferous-Lower Permien
- ADIGRAT SANDSTONE FACIES: Permian-Triassic to Jurassic
  - ABBAI STRATA: Triassic(?)-Liassic
- ANTALO LIMESTONE FACIES: Middle-Upper Jurassic
- UPPER SANSTONE FACIES: Upper Jurassic-Cretaceous
In addition, lacustrine beds are disseminated in the cainozoic volcanic formations.


2.1. PRE-ADIGRAT FACIES
This 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.
In Eritrea, several non-metamorphic formations were map­ped. Their ages are unknown.
In Tigre province, shale with boulders and gray sandstone were described 40 years ago by Italien Geologists. Recently, they have been interpreted as glacial deposits. Due to the apparent conformity with the overlying Adigrat Sandstone, a carboniferous age is inferred. Similar facies were observed in the Danakils Alps.
Some 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 have fragments of haematitic crust and haematized wood: these observations suggest a 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 Permien age.
In 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 vermiculite. Kaolinite and iron hydroxydes occur at the top of the formation. In some places, strata are intensively folded although they are perfectly horizontal elsewhere. This disposition is interpreted as huge slumping of unconsolidated material due to faulting. Scarce spores refer to Carboniferous.
Although 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 the so-called 'Ethiopian-SudaneseMassif', 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
These cross-bedded red sandstone overlie the metamor­phic basement or the Pre-Adigrat sandstone and are generally covered by the juras­sic Antalo limestones. It is of large extent in Ethiopia; its thickness can exceed several 100 m, its age ranges from Permian to Middle Jurassic.
In the Blue Bile Basin, the Adigrat Sandstone Facies is about 200 m thick and can be divided in 2 members.
The 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 the underlying Pre-Adigrat grey sandstone. This lower member was deposited in a continental basin.
The upper member is generally made of coarse cross-bedded sandstone, with channels filled by gravels. The general direction of transport was South-Eastward. Some beds are horizontally bedded and graded, with clay balls and gravel at the base, and silt, ripple-marks, mud-cracks at the top.
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 the formation, and suggest a deltaic and near shore deposition environment. The diagene­tic 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.
In Chercher, red sandstones are similar, but the lover beds are coarse , poorly-sorted and contain small terrestrial Gasteropods shells. The others beds are fluvial and deltaic, while the uppermost strata are 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 basement: These areas have undergone the continental erosion during Permo-Triassic times until the Jurassic transgression.
In Northern Ethiopia The Adigrat Sandstone Facies is similar.

In conclusion, the Adigrat Sandstone Facies is a typical molassic red facies deposited under a tropical climate, in a first continental environment 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).


2.3 ABBAI STRATA
This 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 kaolinite. Size distribution and shape of grains in sandstones indicate a near-shore environment. Shells of small Pelecypods, Gasteropods and Ostracods débris were found in dolomitic beds but dating is not precise: a Triassic to Liassic age is inferred. The Abbai Strate were deposited in a shallow marine basin connected to the open sea in the Northern part.


2.4 ANTALO LIMESTONE FACIES
Jurassic 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, Corals and microfossils. Ammonites were found only in the Ogaden Basin. Dating ranges from Bathonian to Kimmeridgian or Portlandian according te the localities. In Tigre and Ogaden, a shaly unit occurs at the top of the formation.


2.5.UPPER SANDSTONE FACIES
This regressive facies ends the sedimentary series except in SE Ogaden where the marine sedimentation continued into the Tertiary.
In the Blue Nile Basin and Chercher Mounts, the lithologic facies progressively changed at the top of the Antalo Limestone Formation. The first deposited sandstone beds are near-shore sediments, but the major part is continental. Sedimentological features are very similar to Adigrat Sandstone Facies. Continental sondstones are red, cross-bedded and poorly sorted; they were deposited 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, indicating that the first eruptions were contemporaneous there to the upper most sandstone beds.
In conclusion, the Upper Sandstone Formation is also a typical molassic facies ranging from Upper Jurassic to end-Cretaceous on the Ethio­pian Plateau, but to early Tertiary in the Chercher Mounts.


2.6 SEDIMENTARY INTERCALATIONS OF TERTIARY VOLCANICS
Tuffs and agglomerates are associated with Cainoeoic basaltic flows. Many pyroclastic beds were deposited in lacustrine environment with clay and lignite. Plant remains are frequently encountered (prints of 1eaves and silïcified or carbonized woods). Silïcified wood is also present in terrestrial deposits. Palaeobotanical determinations were useful for dating the main volcanic events. They shown that episodes are numerous and not synchronous in various regions. Volcanic activity started in the early Eocene on the central Ehiopiau Plateau, but later (end-Eocene or Oligocene) on the Harar Plateau. The flood basalts of the Trop Series were mainly erupted during the Oligocene and Miocene times. On the Ethiopian Plateau, shield volcanoes and acidic welded tuffs are Miocene and Pliocene.


 3. PALAEOGEOGRAPHIC EVOLUTION OF ETHIOPIA

Considering the recent geological studies made in East Africa and Arabia Peninsula, the sedimentary history of Ethiopia con be reconstructed as follows.


3.1 CARBONIFEROUS-LOWER PERMIAN
Mountain glaciers covered the Southern part of Africa. They covered the Ethiopian-Sudanese Massif too. They have deposited moraines in the North (and West ?) Ethiopia. Elsewhere, moraines have been reworked,transported and finally deposited in fluvial and deltaïc environment (Pre-Adigrat Facies).
3.2 PERMIAN and TRIASSIC
Due to faulting, Ethiopia was fragmented by 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 eroded and they provided detrital material to the basins of Tigre-Blue Nile, Ogaden-North Kenya and Indian Ocean coast. Adigrat Sandstone was deposited on piedmont slopes ("glacis") and in alluvial plain ("bahada').The climate was tropical.
3.3 LIASSIC
Because of subsidence of the Tigre-B1ue Nile Basin, and of the Ogaden Basin, a shallow sea invaded these areas and locally deposited evaporitic facies (Abbai Strata).
3.4 MIDDLE-UPPER JURASSIC
The transgression was generalized over the 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 end-Jurassic.
3.5 CRETACEOUS
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).
3.6 CAINOZOIC
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.

J. Beauchamp, Abstract from thesis work, université de Lyon 1, 1977.