Unity University Faculty of Engineering Department of Mining Engineering GENERAL GEOLOGY (Geol 2081) Chapter 7: GEOLOGY OF ETHIOPIA Tadesse Alemu Director Basic Geoscience Mapping Directorate Geological Survey of Ethiopia ([email protected]) December 2012 Addis Ababa 1 CONTENTS SUMMARY OF THE GEOLOGY OF ETHIOPIA .......................................................................................1 Precambrian rocks.......................................................................................................................................1 Palaeozoic – Mesozoic sedimentary rocks .................................................................................................2 Cenozoic volcanic rocks and associated sediments ....................................................................................2 SUMMARY OF TECTONICS OF ETHIOPIA .............................................................................................5 PRECAMBRIAN TECTONICS.................................................................................................................5 PHANEROZOIC TECTONICS .................................................................................................................7 SUMMARY OF THE GEOLOGY OF ETHIOPIA The geology of the country underlies by rock types range in age from Precambrian to Recent. These rocks are categorized into the following geological formations:- 1. Precambrian rocks 2. Paleozoic – Mesozoic sedimentary rocks 3. Cenozoic volcanic rocks and associated sediments Precambrian rocks Precambrian metamorphic and associated intrusive igneous rocks make up 25% of the country’s landmass. They are exposed in the northern, western, southern and eastern parts of the country and have a fundamentally important tectonic position in that they occupy the interface between the Mozambique Belt in the south and the Arabian-Nubian Shield to the north. They are dominantly north-trending linear belts of low-grade volcano-sedimentary rocks and mafic-ultramafic rocks, sandwiched between medium- to high-grade gneisses and migmatites. The high-grade gneisses and migmatites are referred to as Lower Complex is part of the Mozambique Orogenic Belt and generally consist of amphibolites facies (locally granulite facies) orthogneisses, parag-neisses, migmatites, and amphibolite with bands of marble. The low- grade volcano-sedimentary rocks with associated mafic to felsic intrusives, which is referred to as Upper Complex, on the other hand, belongs to the Pan-African Arabian-Nubian Shield. The linear belts of mafic and ultra-mafic rocks are commonly confined to major shear zones and often mark the contacts between the high-grade gneisses and migmatites and the low-grade volcano-sedimentary rocks. Most ages obtain from these rocks range between 900 and 500 Ma with exception of older Archean and Mesoproterozoic ages obtained from some of the rock units. The Precambrian rocks have received attention in the current exploration activity for base and precious metals. The belts of mafic-ultramafic rocks and major shear zones bounding the two contrasting stratigraphic complexes are potential targets for gold, base metals, nickel, platinum and other mineralization. 1 Palaeozoic – Mesozoic sedimentary rocks A thick succession of Palaeo-Mesozoic sediments covers 25% of the country’s landmass are represented by three distinct sedimentary basins; namely: the Ogaden Basin (350,000 km2), the Abay (Blue Nile) Basin (63,000 Km2), and the Mekele Basin (8,000 km2). The development of the basins can be thought of as relating to the polyphase break-up of Gondwanaland, which was accomplished, in general by reactivation along the preexisting Precambrian structures. Rifting associated with the break up of Gondwanaland was dominant from the late carboniferous onwards, and basins developed as a result of this extension both at the margins and within the continental African plate. The Ogaden Basin contained at places over 10,000 meters thick sediments consist of non-marine to deep marine clastics, very thick, shallow-to-deep marine carbonates and evaporites. The Abay Basin consists of Paleozoic and Mesozoic sedimentary succession exceeding 2000 m, which are represented by pre-Adigrat clastics, equivalent to the Karoo system in the Ogaden, the Antalo limestone, and the Amba Aradom Formation (the Upper Sandstone). The sedimentary succession of the Mekele Basin comprises 2000 m thick sediments ranging from fluvio-lacustrine to shallow and deep marine types. The Paleozoic-Mesozoic sediments have high potential for hydrocarbon exploration and development. Discoveries in the Ogaden Basin indicated the gas-condensate fields of Calub and Hilal, with estimated reserves of 2.7 Tcf and 1.3 Tcf respectively. An indication of oil seep from Were-Ilu locality in the northeastern margin of the basin makes the Abay Basin one of the promising areas to undertake petroleum exploration. Moreover, the presence of significant volume of pre-Adigrat source rocks towards the southwest and eastern part of the Abay basin is speculated from geophysical studies may suggest that the Abay basin could be one of the promising areas to undertake petroleum exploration. Cenozoic volcanic rocks and associated sediments Cenozoic volcanics and sediments cover 50% of the country’s landmass, and range in age from the late Eocene up to historical times. Volcanism started during the Eocene-late Oligocene with the eruption of flood basalts that have generally been related to either one or two mantle plumes impinging the base of the lithosphere under Afar or Afar-Northern Kenya rifts. The flood basalts with intercalated silicic volcanics built a subaerial volcanic pile typically 500-1500 m thick and locally attaining 3000 m. The total area covered by these volcanics has been estimated as presently 600,000 km2, and not less than 750,000 km2 before erosion. These flood basalts contribute to an estimated volume of 300,000 km3. The earliest flood basalts forming the Ethiopian Plateau apparently erupted in a rather short time interval (<5 Ma) with the greatest eruption rates occurring from 31 to 28 Ma. This strong eruption was concomitant with the onset of continental rifting in the Red Sea-Gulf of Aden systems by 29 Ma, but predates the main rifting phases associated to the development of the Main Ethiopian Rift (MER). However, limited volumes of basalts as old as 45 Ma have been described in southern Ethiopia, in the Broadly Rifted Zone separating the MER from the Kenya rift. Immediately after the peak of volcanic activity related to the flood basalt emplacement, a number of large shield volcanoes developed on the surface of the volcanic plateau. This subsequent less voluminous volcanic activity formed some of the highest relief of the plateau (such as Mts. Ras Dashen, Guna and Choke), rising 1000-2000 m above the top of the surrounding flood volcanics and being characterized by a basal diameter of 50-100 km. A second episode of flood basalt volcanism has been described in the southern Ethiopia at 18-11 Ma and in the MER-Afar transition zone at about 10-11 Ma. Time correlative basaltic units are widespread both in the western (Wollega and Lake Tana basalts, 11-9 Ma) and in the southeastern Ethiopia Plateau at about 10.5-9 Ma. After these episodes of widespread flood basalts and subordinate silicic volcanics, volcanism is closely associated with the tectonic development of the MER. Rifting in the various MER sectors was characterized by volcanism with fundamentally bimodal character. Widespread late Miocene-Pliocene rhyolitic ignimbrites (~7-3 Ma) with intercalated minor mafic lavas occur throughout the Northern and Central MER. In the Quaternary (<1.6- 2 1.8 Ma), bimodal volcanic rocks (lava, pyroclastics and volcanoclastic strata) were generally closely associated with Wonji Fault Belt affecting the rift floor. Cenozoic volcanics contain various semi-precious stones and industrial minerals including potash, bentonite, opal, diatomite and sulphur. Even though much of the Cenozoic volcanic terrain is not explored, there are good indications of gemstone and coal occurrences within intra-volcanic sedimentary rocks. Moreover, the Rift Valley has tremendous potential for geothermal energy. 3 Simplified geological map of Ethiopia 4 SUMMARY OF TECTONICS OF ETHIOPIA PRECAMBRIAN TECTONICS Tectonic development of the Precambrian of Ethiopia involves by subduction-accretion processes between arc terranes of the Arabian-Nubian Shield and predominantly gneissic terranes of the Mozambique Belt, which resulted in collisional amalgamation of lithotectonic terranes across sutures. It represents a plate tectonic cycle spanning a time-period of 350 Ma, beginning by about 900 Ma with rifting and continental break-up and ending by about 550 Ma subsequent to a continent-to- continent convergence between East and West Gondwana. In Precambrian of Ethiopia, lithotectonic terranes that comprise high-grade gneisses of the Mozambique Belt and low-grade metavolcano-sedimentary sequences of the Arabian-Nubian Shield are juxtaposed along north-south sheared thrust contacts, which are marked by arc-like ophiolite sequences, which are overprinted by post accretionary structures including north-trending shortening zones and major northwest-trending sinistral and minor northeast-trending dextral strike-slip faults. These lithotectonic terranes are mainly represented by Moyale, Adola, Bul Bul (southern Ethiopia), and Tulu Dimtu (western Ethiopia) belts. The Moyale Belt The Moyale Belt is formed part