A Synopsis of the Regional Geology and Hydrogeology of Ethiopia - Notes

A Synopsis of the Regional Geology and Hydrogeology of Ethiopia

Participants

  • Czech Geological Survey, Czech Republic
  • Ministry of Water and Energy, Ethiopia
  • Charles University, Faculty of Science, Czech Republic
  • Geological Institute of Ethiopia, Ministry of Mines, Ethiopia
  • SG Geotechnika a.s., Czech Republic
  • AquaCon Engineering plc., Ethiopia
  • Authors: Kryštof Verner, Jiří Šíma, Leta Megerssa, David Buriánek, Štěpán Dvořák, Karel Martínek, Jan Jelínek, František Vacek, Tarekegn Tadesse, Ahmed Wolela, Tadesse Alemu, Dereje Ayalew, Roland Nádaskay, Bereket Fentaw, Tomáš Hroch, Muhuddin Abdella, Jan Valenta, Bedru Hussien, Jan Jelének, Ondřej Nol, Tomáš Vránek, Craig Hampson, Berhanu Bekele, Abraha Adugna, Tesfaye Demisse, Jan Franěk, Petra Hejtmánková, Petr Mixa, Vladimír Žáček, Zenaw Tessema, Barbora Paulusová, Irena Třískalová, Adam Pařízek, Mesfin Kidanemariam, Tsehay Amare, Yonas Wubshet, Zewdu Yoseph, Yosef Kebede
  • © Kryštof Verner, Jiří Šíma, Leta Megerssa, David Buriánek, Štěpán Dvořák, Karel Martínek, Jan Jelínek, František Vacek, Tarekegn Tadesse, Ahmed Wolela, Tadesse Alemu, Dereje Ayalew, Roland Nádaskay, Bereket Fentaw, Tomáš Hroch, Muhuddin Abdella, Jan Valenta, Bedru Hussien, Jan Jelének, Ondřej Nol, Tomáš Vránek, Craig Hampson, Berhanu Bekele, Abraha Adugna, Tesfaye Demisse, Jan Franěk, Petra Hejtmánková, Petr Mixa, Vladimír Žáček, Zenaw Tessema, Barbora Paulusová, Irena Třískalová, Adam Pařízek, Mesfin Kidanemariam, Tsehay Amare, Yonas Wubshet, Zewdu Yoseph, Yosef Kebede, 2025.
  • ISBN 978-80-7673-113-4
  • Reflects a collaborative effort between 2019 and 2024 as part of the Development Aid Project, funded by the Czech Republic through the Czech Development Agency.
  • Serves as an accompanying output “Geological and Hydrogeological Map of Ethiopia 1 :1,200,000”.
  • Reviewed by: Prof. Asfawossen Asrat, Prof. Benjamin van Wyk de Vries, Assoc. Prof. Radomír Grygar, Assoc. Prof. Dessie Nadaw, Assoc. Prof. Jiří Bruthans
  • Citation: Verner K., et al. (2025): A synopsis of the regional geology and hydrogeology of Ethiopia. Czech Geological Survey, Prague, 364 p.

Acknowledgement

  • Compilation aided by institutes and individuals.
  • Czech Development Agency provided financial support.
  • Ministry of Water and Energy of Ethiopia contributed in-kind.
  • The Embassy of the Czech Republic in Ethiopia played a crucial role.
  • Czech Geological Survey’s management and staff oversaw technical work.
  • Ethiopian Geological Institute provided crucial source data.
  • Yared Girma (Ministry of Water and Energy): unreserved effort in accommodating requests.
  • Drivers from the host organization, the Ministry of Water and Energy, are greatly acknowledged.
  • Zonal Offices of Mines and Water of Ethiopia are recognized for allowing access to data and field verification.
  • Individuals who greatly contributed: Dr. Zebene Lakew, Prof. Miruts Hagos, Prof. Giacomo Corti, Mr. Hambaye Alamgana, and Mr. Bawoke Birhan.
  • Deep regret at the loss of Dr. Tesfaye Demissie.
  • Reviewers: Prof. Asfawossen Asrat, Prof. Benajmin van Wyk de Vries, Assoc. Prof. Radomír Grygar, Assoc. Prof. Dessie Nadaw and Assoc. Prof. Jiří Bruthans.
  • Acknowledgment of the support of local people.

Part I: The Regional Geology of Ethiopia

Preface
  • Need to update and interpret geological and hydrogeological data in Ethiopia; based on Kazmin (1972) and Chernet (1982).
  • Comprehensive knowledge of the geological environment is an essential basis for the efficient and sustainable use of natural resources.
  • Ethiopia is one of the priority countries for the Czech Republic in the field of development cooperation.
  • The book presents the results of a collaborative effort by a broader research team conducted from 2019 to 2024.
  • Comprehensive summary of the geological, lithological, petrological, geochemical, and tectonic characteristics, along with a newly proposed lithostratigraphic framework of its geological environment.
  • Assessment of Ethiopia’s hydrogeological resources.
  • National geological and hydrogeological maps of Ethiopia have been compiled at a scale of 1:1,200,000, based on existing maps at various scales, published literature, remote sensing data, gravity modeling, extensive field verifications, and a new set of analytical data.
  • Comprehensive hydrological factors influencing the distribution and availability of groundwater have been integrated.
  • Bridge significant knowledge gaps in the broader fields of geology and hydrogeology, providing up-to-date, relevant, and comprehensive information.
  • Valuable resource for numerous scientific studies and natural resource management efforts, both in the near and distant future.
Introduction
  • Ethiopia’s past and present geological phenomena are intriguing.
  • The oldest known event is the mountain building process during the East African Orogeny.
  • The youngest is represented by the active volcanic centers of the Ert’ Ale – Tat’ Ale’ ranges
  • The intervening period saw active dynamics involving a series of uplift, subsidence, marine incursion, and extensive flood basalt volcanism
  • Efforts were made to extract all accessible analytical data and previously compiled geological maps from various sources.
  • The main source of information and data was the archives of the Geological Survey of Ethiopia (recently renamed Geological Institute of Ethiopia).
  • When compiling the geological map, the geological setting was systematically divided into four major geological provinces:
    • The Neoproterozoic, low to high-grade metamorphic rocks and the related intrusive rocks of Neoproterozoic to Middle Ordovician ages
    • The platform lower Paleozoic to Paleogene sedimentary units.
    • The Eocene to Holocene volcanic rocks and volcaniclastic deposits belonging to the East African Rift System.
    • The Quaternary sedimentary deposits of Pleistocene to Holocene age.
  • Covering the entire territory of Ethiopia at the scale of 1 :1,000,000 (on-line interactive version) or at the scale of 1 : 1,200,000 (printed version).
Previous Geological Mapping and Map Outputs
  • The earliest attempt at geological mapping in Ethiopia was by Blanford (1870).
  • Blanford proposed a simple stratigraphy of the region’s volcanic rocks which comprises of the lower lying Ashangi Basaltic Unit and an overlying Magdala Unit.
  • A later, more systematic map, covering the entire territory of Ethiopia, was eventually compiled by V. Kazmin at the scale of 1 : 2,000,000.
  • In the present work, much of the data came from the existing sets of maps produced by the Geological Survey of Ethiopia over the last 5 decades
  • However, these maps lack consistency, where adjacent map sheets often show significant discrepancies in lithologic distributions, lithologic boundaries, structural trends and the details of the mapped units across the map sheets.
  • Furthermore, a number of analytical data pertaining to the composition, the genesis and the ages of the different rock units have accumulated since the publication of the last geological map of Ethiopia at a 1 :2,000,000 scale.
  • Prompted an endeavour to gather all the information from various sources and harmonize and present them in an updated digital format which increases their accessibility through online interactive portals.
  • Taking advantage of the relatively rich data, it was also possible to increase the scale of the map by two- -fold, i.e. to a 1:1,000,000 scale.
Data Sources
  • The existing series of geological maps at scales ranging from 1 : 50,000 through 1 : 250,000 to 1 : 500,000 were used as the bases for compiling the current map.
  • Most of these maps at the scale of 1 :250,000, cover a regular grid of 1° by 1.5° quadrangles, are also accompanied by detailed explanatory notes.
  • Given the large number of geologists, with diverse levels of expertise, involved in producing the maps over an extended period of time (more than 50 years), some inconsistency among the different map sheets is unavoidable
  • A geological map compiled for the exploration of oil in the Ogaden Basin at a scale of 1:1,000,000 (BEICIP, 1985) has also been a valuable source of information for the south-eastern and eastern sedimentary terrains of Ethiopia.
  • For the southwestern Ethiopia, where only a limited number of field campaigns were conducted, the current compilation depended on the geological map of the Omo Basin at a scale of 1 :500,000 (Davidson, 1983).
  • Commensurate to the existing knowledge of the plutonic bodies, a compositional classification of the intrusive rocks has been adopted in the current work.
Remote Sensing Analysis
Digital Elevation Model (DEM)
  • The digital elevation was prepared using the Shuttle Radar Topography Mission 1-arc second V3.0 (SRTM1) digital elevation model (DEM; NASA JPL, 2013).
  • To seamlessly cover the entire Ethiopian territory, a total of 108 SRTM1 tiles were processed and mosaiced.
  • Since the target map scale is 1:1,200,000 (Verner et al., 2025), the original 30-m resolution was down-sampled to 90-m, which provided sufficient resolution.
  • After processing the DEM layer, a multi-directional hillshade (Fig. 2-1b), which served as a base for visual geomorphological interpretation, was obtained.
  • These gaps were filled during post-processing using the interpolation procedure.
Optical Satellite Data
  • The geological mapping and map processing were supported by processing and analysing appropriate optical multispectral satellite data.
  • The Landsat 8 OLI/TIRS was chosen as the optimal sensor/platform for the current mapping scale and the size of the mapping area.
  • The original Landsat 8 resolution of 30-m has been further down-sampled to 90-m in order to complement the digital elevation products and enable processing of a seamless product for the entire territory of Ethiopia.
  • The Ethiopian territory is fully covered by 11 adjacent Landsat 8 orbital tracks (tracks No. 162 to No. 172).
  • The dry season (mid-December to end of February) was selected as an optimal season for acquiring optical satellite data for geological mapping.
  • The downloaded data were of a Level-2 product collection, meaning that the data represent surface reflectance and surface temperature.
  • The data were first mosaiced within each track on a per-band basis which did not raise any histogram issues due to same-day acquisition.
  • The mosaiced imagery was further processed using Principal Component Analysis (PCA) transformation and various spectral indices for the mapping quadrants
Regional Gravity Field of Ethiopia
  • The geology of Ethiopia has been shaped by processes ranging from Precambrian orogenic belts to active continental rifting.
  • One of the most recent models is the XGM2019e global gravity field model (Zingerle et al., 2020) which combines ground, satellite, altimetry, and topographic data.
  • The complete Bouguer anomalies (CBA) derived from this model are used in discussions in this chapter.
  • The Bouguer anomalies are, generally speaking, gravity effects (positive and negative) of lithologies whose density differs from the reduction density.
  • The CBA in the Ethiopian territory is generally characterised by a distinct gravity low (even less than –250 mGal), which extends over a significant part of the country
  • The Ethiopian gravity minimum (characterised by CBA values lower than approximately –100 mGal) generally coincides with the highlands which constitute the majority of the country.
  • In contrast, the highest values of the CBA (nearly 0 mGal) are found in the Afar Triangle and generally along the Main Ethiopian Rift (MER).
  • The gravity high within the MER is interpreted to be connected with a solidified high-density upper mantle intrusion.
  • The intrusion is located roughly in the centre of the MER and interpreted as a relatively thin (~20–30 km wide), NE–SW striking dyke with its upper boundary within the range of ~10 km depth.

Conceptual density model: (see Fig. 3-4 for visualization)

  • A conceptual gravity model was constructed across a 1,430 km long profile, crossing the major geological units.
  • The Linsser indications mark the positions and inclinations of major fault zones and provide an estimate of their depth reach.
  • The western part of the profile exhibits a set of parallel fault zones, roughly following the direction of the rift.
  • Their orientation and inclination may suggest that the large crustal blocks (approximately 100–200 km wide) between them rotated, causing the western part to drop and the eastern edge to uplift by several kilometres.
  • The fault zone at approximately 180 km from the west forms a prominent deep-seated suture zone connected with the formation of the Arabian-Nubian Shield (ANS).
  • The gravity low and derived indications generally show a significant deepening of the MOHO (down to approximately 45km) in the vicinity of the MER, with an increased thickness of relatively low-density crystalline units in the upper crystalline basement.
  • In the centre of the MER, the increase in the CBA values suggests a high-density body, most likely an upper mantle intrusion, reaching a depth of approximately 10km below the surface.
Geodynamic Evolution and Paleogeography
Neoproterozoic (ca. 890 to 540 Ma)
  • During the Neoproterozoic era, the continent Rodinia broke up and intermediate continents of Pannotia, Pangea, Laurentia, Baltica, Siberia, and Gondwana were formed.