EXPLANATION OF THE GEOLOGICAL MAP OF ETHIOPIA
SUMMARY OF GEOLOGY
By V. KAZMIN, Geologist.
Summarized by A.J. W ARDEN, Geologist
The basement upon which all the younger formations were deposited contains the oldest rocks in the country, the Precambrian, with ages of over 600 million years. They are exposed in areas where the younger cover rocks have been eroded away, namely in parts of Hararge, Sidamo, Bale, lllubabor, Gojam, Welega, Begemdir, Tigre and Eritrea provinces (Figure 1).
The Precambrian contains a wide variety of sedimentary, volcanic and intrusive rocks which have been metamorphosed to varying degrees. The basement in the south and west of the country, where granitic rocks and gneisses predominate, has been more strongly metamorphosed than the Precambrian sequences in the north. Though in many cases strongly folded and foliated, the rocks in the north, which include the youngest formations yet known in the Precambrian, have generally undergone only weak metamorphism, reflecting the relatively low temperatures to which they have been subjected since their deposition.
The Precambrian or Basement rocks contain most of the presently known metallic deposits of Ethiopia. In particular, the gold deposits of the northern, western and southern provinces all occur in these rocks, where they are associated with fracturing, quartz reefs or sulphides. Under favorable conditions these deposits have given rise to major placer deposits along river beds, as in the Kibre Mengist-Adola area. Copper, zinc and lead sulphides have been found in the weakly metamorphosed volcanic facies (greenschists) of these old rocks. Similarly occurrences of potentially exploitable talc and nickel deposits are associated with ultrabasic bodies, and it is in a layered body of this type that platinum mineralisation occurs at Yubdo in Welega Province. There are considerable possibilities of finding other economically exploitable deposits of these minerals in the Precambrian, while major intrusions and dykes hold good potential for other minerals. Most of these Precambrian rocks are relatively impermeable, and have been subjected to several orogenic episodes since their formation. This process, combined with the rifting associated with the development of the Red Sea and the East African-Ethiopian Rift Valley, has resulted in considerable fracturing and shattering. Major water resources are associated with these fracture zones.
At the end of Precambrian times uplift occurred, which was followed by a long period of erosion. Any sediments which were deposited during the Palaeozoic interval, which lasted some 375 million years, have been largely removed by erosion, except for shales and deposits partly of glacial origin laid down in northern Ethiopia towards the end of this period.
Subsidence occurred in the Mesozoic, which began some 225 million years ago, and a shallow sea spread initially over the Ogaden and then extended farther north and west as the land continued to subside. Sand, now sandstone, was deposited on the old land surface. Deposition of mudstone and limestone followed as the depth of water increased.
The subdivision of the Mesozoic sequence and younger sediments on the map into approximate age categories is largely based on palaeontological evidence.
In the west of the Country sedimentation ended with the deposition of clay, silt, sand and conglomerate brought in from the land as the sea receded due to uplift of the landmass. In the southeast gypsum and anhydrite were precipitated on inter-tidal flats. In the Ogaden there was a fresh invasion of the sea in Lake Mesozoic times during which the sequence of Sedimentation was repeated, ending again with the precipitation of gypsum and anhydrite.
The same cycle of sedimentation was repeated yet again in the Ogaden during the Tertiary period. It ended with the deposition of conglomerates. sandstones and mudstones with some interbedded marls, and finally erosion as the area was uplifted. Mesozoic rocks are considered to have the greatest potential for oil and gas deposits. The presence of such deposits is dependent on the original existence of organic materials, mostly of marine origin, in sediments which were buried to form the so-called source beds. In these beds, which may now be represented by shales or stromatoloid reef limestones, the organic material was altered, by pressure of the overlying sediments and a build-up of heat, into oil and gas which migrated into nearby permeable (reservoir) rocks. The Mesozoic of southern Ethiopia contains few favorable structures for oil, being only weakly folded, but nevertheless there remains a reasonable possibility of the occurrences of stratigraphic traps. Outcrops of the Mesozoic in the north and west are considered as less favorable due to the possible terrestrial nature of the rocks in the north, and the probable thinness of the rocks in the west. The map emphasizes the necessity for additional studies in these areas.
Metallic minerals have not been discovered to date in the Mesozoic rocks, but the occurrences; shown on the map of limestones and saline deposits point out the need to prospect these formations further for lead, zinc and silver ores. Rocks of this age also represent major reserves of glass sands, cement limestones and other industrial minerals. Extensive fracturing occurred early in the Cenozoic, the earliest rocks of which are dated at 65 million years, although major displacement along the fault systems which approximate to the alignment of the Red Sea, Gulf of Aden and East African rift systems did not occur until later in the Tertiary. Faulting was accompanied by widespread volcanic activity and the two processes, which are partly related, have largely determined the form of the landscape in the western half of Ethiopia and in the Afar Depression. The outpouring of vast quantities of basaltic lava over the western half of the country was accompanied by, and alternated with, the eruption of large amounts of ash and coarser fragmental material, forming the Trap Series. Several shield volcanoes, also consisting of alkali basalts and fragmental material, then developed around the eastern edge of the Lake Tana depression and southwest of Goba. More recent volcanism is associated with the development of the Rift Valley, activity being concentrated within this structure and along the edge of the adjoining plateau.
Volcanism has persisted into the present time in the Afar region within small eruptive centers. The composition of the lavas produced ranges from basalt to siliceous types. The youngest sediments are of Quaternary age. These include conglomerate, sand clay and reef limestone which accumulated in the Afar Depression and the northern end of the main Rift Valley. Sediments which accumulated in former lakes occur in the south end of the Afar, in the main Rift Valley, and in the Omo valley. Undifferentiated Quaternary sediments and superficial deposits occur intermittently along the Sudanese and Kenyan borders.
Oil and gas deposits may be present in some of the Tertiary deposits, but in general the rocks of Cenozoic age in Ethiopia are of potential economic importance because of their extremely large salt deposits. These include not only the common rock salt traditionally worked in the Afar, but also deposits of potassium and magnesium salts. Furthermore the geothermal activity associated with many of the Cenozoic deposits has given rise to metallic mineral occurrences, copper and manganese in particular, concentrated as a result of leaching of the rocks by saline solutions. Iron ores, and possibly even aluminum ores (bauxite), developed on the volcanics as a result of intense weathering, and may prove to be of economic interest.
The underground water resources of the Cenozoic are of great importance. The often lenticular nature of the beds necessitates detailed investigations, but in general, subject to suitable recharge areas, the deposits can be considered as highly favorable for water exploitation.