Taken from the paper " A short introduction to the Geology of Ethiopia"
by Solomon Gerra
Introduction
The Orogenic belts of the Arabian Nubian Shield (ANS) and the Mozambique Belt (MB) are believed to be more prominent in outcrop in Ethiopia than in any other country of the Horn of Africa (Kazmin, 1972; Berhe, 1990). However, the rocks belonging to these orogenic belts are only exposed in a few areas, which have not been affected by Cenozoic volcanism and rifting, and where the Phanerozoic cover rocks have been eroded away (Tefera et al., 1996)
Generally, little work was done on the geology of Ethiopia until the early 1970s. Some more generalized ideas about the Geology of Ethiopia were derived from limited mapping and prospecting in various regions of the country (eg., Blanford, 1870; Dainelli, 1943). Dainelli (1943) translated this work from the Italian in to the English language and compiled the Geology of Ethiopia with limited modification, specifically for the rift valley volcanics and tectonics. Subsequently, the work by Kazmin (1972), Kazmin et al. (1978) and Merla et al. (1973) extended the previous studies of the geology of Ethiopia and East Africa.
Gilboy (1970) and Chater (1971) were first to propose a three-fold classification of the Precambrian basement rocks of southern Ethiopiain to Lower, Middle and Upper Groups. Each of these groups is characterized by different lithology and structure, with an overall decrease in metamorphic grade from the Lower to the Upper Group. Kazmin (1972) redefined these groups as the Lower, Middle and Upper Complexes, comprising Archean, Early Proterozoic, and Late Proterozoic lithostratigraphic sequences, respectively. The same author compiled the Geology of Ethiopia in the form of a 1:2,000,000 scale map, with an accompanying report (Kazmin, 1972).
In 1968, the Ethiopian Institute of Geological Surveys (EIGS) was established with the aim to conduct systematic and integrated geological mapping and mineral exploration. Since then, a large amount of mapping and exploration activities has been carried out in different parts of the country (e.g., Omo River Project, 1975; Kozyrev et al., 1985; Moore et al., 1987; Woldegebriel et al., 1994; Hambissa et al., 1997; Tadesse, 1997; Tadesse and Melaku, 1998). At present, about 30 % of the country has been mapped at the 1:250,000 scale. The cumulative regional mapping shows that the country is covered by about 18% Proterozoic
The proterozoic basement rocks are poly-deformed and poly-metamorphosed, high grade ortho- and paragneisses and schists, which are associated with less deformed and metamorphosed mafic-ultramafic suites and meta-sedimentary rocks. Mafic and felsic plutonic rocks, which are either deformed or undeformed, sporadically intruded crystalline basement rocks. The highest metamorphic grade, at granulite facies, is recorded in gneisses and schists of the southern and southwestern Ethiopia. This terrane is believed to form the northern extension of the Mozambique Belt (Gichile, 1991; Davidson, 1983). Outhcrops of Precambrian basement are restricted to the southern, southwestern, northern, eastern and western parts of Ethiopia. Therefore, mapping and exploration activities for Gold and base metal mineralization have also been largly limited to these regions (compare Selassie and Reimold, this volume).
Kazmin’s (1972; also Kazmin et al., 1978) classification scheme for Precambrian basment rocks has been utilized until the early 1990s. More recently, Amenti et al. (1992) introduced a new approach to the basement geology of Ethiopia, namely differentiation of the high-grade rocks, whereby the former are said to occupy the lowest stratigraphic positions.
Recent works by EIGS geologists, in collaboration with other institutions and universities, and based on geochronological an isotopic studies, indicated that the basement gneisses represent Late Proterozoic continental crust. This strongly suggests that the Ethiopian basement stratigraphy must be modified. Rocks, which have been believed to be of Archean age (Kazmin, 1972; Tefera et al., 1996), have been identified as Late Proterozoic continental crust (e.g., Ayalew et al., 1990; Ayalew and Gichile 1990; Gichile, 1991; Worku, 1996; Teklay et al., 1993, 1998). However, some workers (e.g., Gichile, 1991; Worku, 1996) encountered components of reworked Pre-Neoproterozoic continental blacks (1.2-0.5 Ga) and suggested that this might represent a root zone of thickened supracrustal sequences formed along the Pan-African collision belt.
The Precambrian of Southern Ethiopia can be classified into two major types of terrane:Granite gneiss and Meta-Volcano-Sedimentary-Ultramafic Sequences. Recent chronological work showed that the lithologies of the Granite Gneiss Terrane were emplaced between about 900 and 540 Ma ago. The meta-volcano-sedimentary-ultramafic sequences are interpreted as ophiolitic sequencesrepresenting marginal basins formed in a supra-subduction zone setting during roughly the same period, representing the development of the East African Orogen (e.g., Stern, 1994).
Although the presence of Archaean basment rocks had been suggested by early workers (e.g., Kazmin, 1972), no geochronological data have to date supported this argument. Only a few zircon xenocrysts of Archaean age have been detected in granitoids of upper Proterozoic age (e.g., Teklay et al., 1993, 1998).
In addition, significant contributions were made in establishing the arc-arc, arc-continent and continent-continent assembly history in the ANS and MB by various researchers of the Proterozoic basement rocks of Northeast Africa and Western Arabia (e.g., Shakleton, 1994, 1996; Gichile, 1991; Stern, 1994; Abdelselam and Stern, 1997). According to these authors, ophiolite belts in the Arabian-Nubian Shield mark suture zones, along which different terranes were welded together during the East and West Gondwana collision orogency.
Gichile (1992) related the origin of the southern Ethiopian Precambrian to high-grade Proterozoic regional metamorphism belived to have occurred as a result of a major overthrusting event similar to a Himalayan-style continental collision. The granulites of southern Ethiopia seem to correlate with the Samburan-Sabalachian charnockites of Kenya and perhaps to Mozambican granulites further south (Ayalew et al., 1990; Gichile, 1992). The Neo-Proterozoic evolution of Southern Ehtiopia was intimately associated with the break-up of a larger continental block into East and West Gondwana between about 880 and 550 Ma.
Mesozoic sediments
Late-Paleozoic to Early Mesozoic marine an continental sedimentary rocks are exposed in all parts of Ethiopia (compare Tables 2 and 3, after Tefera et al., 1996). They are mainly sandstone and minor tillite, shale, siltstone. Limestone and conglomerates, which filled channels in the Precambrian basement. The Enticho sandstone and Edaga Arbi glacial deposits in the Tigray region (a map giving the boundaries of the National Regional States of Ethiopia is provided in Fig. 3), Permian sandstone in the Ilubabor and Kefa regions, the middle Abay tillite exposures, Waju sandstone, Calub sandstone, Gumbro sandstone, Bikh shale from deep bore-holes in the Ogaden region (Tefera et al., 1996), and the Genale basin glacial tillites (Tadesse and Melaku, 1998) are some of these occurrences.
The Mesozoic sedimentary succession forms a package of rocks resting unconformably on the Precambrian basement rocks. The geodynamic event responsible for the deposition of these rocks coincided with the end of the transgression of the Indian Ocean, The Adigrat, Antalo, Agula, and Amba Aradom Formations are the known Mesozoic deposits in the Tigray region of northern Ethiopia. They also crop out throughout eastern and southeastern Ethiopia up to the Somalia-Ethiopia border. The ages of the Mesozoic sediments, especially of those in the Ogaden region, decrease to the east, where sediments as young as the Eocene have been reported (Tefera et al., 1996).Cenozoic Volcanics and associated sediments
A first classification of the volcanic rocks of Ethiopia was introduced by Blanford (1870) and, with some modification, was used for almost hundred years. Considerable contribution to the study of volcanic rocks of Ethiopia was made by Dainelli (1943), Mohr (1963), Kazmin (1972) and Merla et al. (1973). Mohr (1963) attempted to divide the Cenozoic volcanic rocks of Ethiopia into the Trap and Aden Series. The term Trap Series is still widely used to represent the whole pile of Teritary flood basalt sequences, which form the northwestern and southeastern plateau and attain a thickness of up to 3 Km (Tefera et al., 1996). The term Aden Series was used for post-gift (Middle Miocene-Quaternary) volcanic rocks of the Main Ethiopian Rift, Afar Depression, and some parts of the Ethiopian Plateau. Several pre- and post-rift volcanic episodes have been distinguished as a result of recent mapping and are summarized in Tables 4-6.
Most of the geological studies related to the understanding of the geodynamic evolution of the Proterozoic, Paleozoic to Mesozoic, and Cenozoic terranes of Ethiopia were intended to determine whether the Proterozoic basement and other rocks have potential for economic mineral deposits. Systematic regional geological mapping and mineral exploration activities by the EIGS and others were successful in locating economic mineral deposits in different regions of the country. These investigations showed that the most promising regions for gold and base metal deposits are the low-grade belts of volcano-sedimentary successions and associated intrusives in the north, south and west of the country.
Several metallic and non-metallic deposits and occurrences have been identified since then. Among the metals, Gold is considered the most promising resource. Industrail minerals such as rock salt, potash, phosphate, soda ash, feldspar, dolomite, bentonite, graphite, lime, silica and kaolin are found. Hydrocarbon deposits and occurrences, such as natural gas, oil shale, and lignite, have been found in the Mesozoic and Tertiary sediments in various parts of the country. Lastly, dimension stones including marble, granite, sandstone, limestone, and ignimbrite, raw materials for cement and aggregate manufacturing, and gemstones of different type and quality are also available in many parts of the country.
The Ethiopian Government recently subscribed to a policy that allows any exploration company (Domestic or foreign) to take a concession in an area of potential. Accordingly, many private exploration companies are currently conducting exploration activities in different part of the country.