Explanatory notes to the thematic geoscientific maps of Ethiopia at a scale of 1 : 50,000 Map Sheet 0637-D3 Arba Minch Kryštof Verner and Leta Megerssa (eds.) with co-authors Tomáš Hroch, David Buriánek, Karel Martínek, Jana Janderková, Jiří Šíma, Eva Kryštofová, Hadush Gebremariyam, Ezra Tadesse, Ferdawok Legesse, Ephrem Nisra, Befekadu Abateneh, Genet Assefa, Jan Valenta, Zoltán Pécskay, Petra Hejtmánková and Zuzana Krejčí Czech Geological Survey Geological Survey of Ethiopia 2018 Reviewed by Petr Hradecký (Czech Geological Survey, Prague, Czech Republic) and Ameha Atnafu Muluneh (Addis Ababa University, Ethiopia) © Kryštof Verner, Leta Megerssa, Tomáš Hroch, David Buriánek, Karel Martínek, Jana Janderková, Jiří Šíma, Eva Kryštofová, Hadush Gebremariyam, Ezra Tadesse, Ferdawok Legesse, Ephrem Nisra, Befekadu Abateneh, Genet Assefa, Jan Valenta, Zoltán Pécskay, Petra Hejtmánková and Zuzana Krejčí ISBN 978-80-7075-948-6 ACKNOWLEDGEMENT Compilation of the map as well as these explanatory notes were done by the team of geologists from the Czech Geological Survey (CGS), the Geological Survey of Ethiopia (GSE) and AQUATEST Ltd. within the framework of the Czech Official Development Assistance Program supported by the Czech Government through the Czech Development Agency. Thorough reviews by Petr Hradecký (CGS) and Ameha Atnafu Muluneh (Addis Ababa Univeristy) greatly improved this explanatory notes and are gratefully acknowledged. We would also like to thank our colleague Vladimír Žáček (CGS) for the constructive discussions. The comments and suggestions from the staff of GSE, in particular Bereket Fentaw (Hydrogeologist), Sisay Degu (Basic Geoscience mapping Directorate) and Samuel Hailu (Geologist) are also greatly appriciated. The SNNP Region President’s Office and SNNP Region Mines and Energy Agency are thanked for their welcoming and cooperative engagement. Due acknowledgment also goes to the Gamo Gofa Zone administration for issuing permissions and cooperation whenever needed. The administration of the Nech-Sar National Park is also greatly acknowledged for providing valuable information and access to the park. The Geology Department of Arba Minch University is thanked for the assistance during the field-work. This work also benefited from the support and facilitation by the management of the Geological Survey of Ethiopia, particularly Masresha G/Selassie (Director General), Hundie Melka (Deputy Director), Genet Asefa (acting head of the Geo-hazards Investigation Directorate), Almaz Mengiste (Secretary of Geo-hazards Investigation Directorate). We are also grateful to drivers from the Geological Survey of Ethiopia, namely Mekonen Hailu, Mulugeta Simegn, Getachew Tegene and Mulisa Legesse who were at the front of the demanding task of getting by, around the remote and rough terrain at different stages of the field work. Finally, the team would like to acknowledge the untiring support of the local people who assisted the team by all means possible and facilitated the data collection and those who helped us in various ways. Verner K., Megerssa L., Hroch T., Buriánek D., Martínek K., Janderková J., Šíma J., Kryštofová E., Gebremariam H., Tadesse E., Legesse F., Nisra E., Abateneh B., Assefa G., Valenta J., Pécskay Z., Hejtmánková P., Krejčí Z. (2018). Explanatory notes to the thematic geoscientific maps of Ethiopia at a scale of 1 : 50,000, Map Sheet 0637-D3 Arba Minch. 137 pages, 3 annexes, 4 maps. Czech Geological Survey, Prague; AQUATEST Ltd., Prague; Geological Survey of Ethiopia, Addis Ababa. LIST OF ABBREVIATIONS apfu: Atoms per formula units CES: Code of Ethiopian Standard CGS: Czech Geological Survey CZDA: Czech Development Agency DEM: Digital Elevation Model EARS: East African Rift System EMA: Ethiopian Mapping Agency FAO: Food Aid Organization FDRE: Federal Democratic Republic of Ethiopia GPS: Global Positioning System GSE: Geological Survey of Ethiopia IUSS: International Union of Soil Sciences JICA: Japan International Cooperation Agency Ma: Million years MER: Main Ethiopian Rift MoWIE: Ministry of Water Resources, Irrigation and Energy MoWR: Ministry of Water Resources NMA: National Meteorological Agency of Ethiopia RSG: Reference Soil Group RVLB: Rift Valley Lakes Basin SNNP Region: South Nations, Nationalities and People Region of FDRE TDS: Total Dissolved Solid UNDP: United Nations Development Program WFB: World Food Program WRB: world Reference Base (for Soil Resources) WWDSE: Water Works Design and Supervision Enterprise CONTENT SUMMARY . 11 SUMMARY IN AMHARIC . 13 1) INTRODUCTION . 17. 1.1 Background ............................................................ 17 1.2 Objectives.............................................................. 18 1.3 Location and accessibility ............................................... 18 1.4 Methods ............................................................... 19 1.5 Previous studies . 24 2) ENVIRONMENTAL SETTING . 26 . 2.1 Topography and Morphology........................................... 26 2.2 Hydrometeorological characteristics .................................... 27 2.3 Land use and Land cover................................................ 28 2.4 Prominent natural features.............................................. 29 3) REGIONAL GEOLOGICAL SETTING . 30 4) REMOTE SENSING ANALYSIS . 33 . 4.1 Data and methods...................................................... 33 4.2 Morphotectonic analysis................................................ 33 5) GEOLOGY OF THE ARBA MINCH MAP SHEET . 37 . 5.1 Lithology and Petrology ................................................ 37 5.2 Geochemistry .......................................................... 45 5.3 K-Ar dating ............................................................. 47 5.4 Structural and tectonic patterns......................................... 48 6) SOIL ENVIRONMENT . 52 . 6.1 Methods and samples .................................................. 52 6.2 Reference Soil Groups in the study area ................................. 53 6.3 Opportunities for and threats to the soils in the studied area ............. 63 7) HYDROGEOLOGY . 67 . 7.1 Hydrometeorology and hydrology ...................................... 67 7.2 Hydrogeological surveys................................................ 73 7.3 Hydrogeological Classification/Characterization . 75 7.4 Elements of Hydrogeological System of the area surveyed (Aquifers) ..... 76 7.5 Hydrogeological Conceptual Model ..................................... 78 7.6 Hydrogeochemistry .................................................... 81 7.7 Water resources . 85 VI MAP SHEET 0637-D3 ARBA MINCH 8) ENGINEERING GEOLOGY . 89 . 8.1 Engineering geological characterization ................................ 89 8.2 Engineering geological classification .................................... 90 9) GEOLOGICAL HAZARDS . 96. 9.1 Endogenous hazards ................................................... 97 9.2 Exogenous hazards ..................................................... 99 10) LOCAL PREVENTION AND MITIGATION MEASURES OF GEOLOGICAL HAZARDS . 107 10.1 Manageable geomorphic hazards ......................................107 10.2 Preventive and mitigation measures for rehabilitation of natural environment . 114 10.3 Suggested hazard monitoring and emergency response systems . .115 REFERENCES . 117 ANNEXES . 129 Annex 1 : Geochemical data and the subsequent analytical methods ...........129 Annex 2: Soil texture and chemical analyses ...................................133 Annex 3: Hydrogeological data ................................................134 CONTENT VII LIST OF FIGURES Fig. 1-1. Administrative boundaries on the Arba Minch map sheet........................19 Fig. 1-2. Location of key analyzed samples. ..............................................21 Fig. 2-1. Geomorphological scheme of the Arba Minch map sheet........................26 Fig. 2-2. Precipitation pattern at the Arba Minch and Chencha meteostations ............27 Fig. 3-1. Digital elevation model showing the East African Rift in Ethiopia and location of the studied area. Black rectangle is a frame of the larger Dila map extent at a scale of 1 : 250,000 scheme which coversthe red rectangle extent representing the map of Arba Minch at a scale of 1 : 50,000 scheme............................................................30 Fig. 4-1. Landsat image of the Arba Minch Map sheet. False colour image with the bands 432 displayed as RGB composite (upper) is showing vegetation in red colour. Results of the principal component analysis (PCA) displayed as RGB composite of PCA1, PCA2 and PCA3 (lower)....................34 Fig. 4-2. Morphotectonic analysis of the Arba Minch map sheet. Morphotectonic linear indices are displayed on the colour digital elevation model (Aster DEM) combined with a shaded relief map.................................36 Fig. 5-1. Alternating layers of lava flows and pyroclastic rocks in the quarry on the northern edge of Arba Minch (DE076)....................................38 Fig. 5-2. Reddish paleosol below the basalt lava flow, a road cut between Arba Minch and Lante (DE056)..................................................38 Fig. 5-3. Quartz and chalcedony filled amygdale in a weathered basalt in the quarry on the northern edge of Arba Minch (DE076)....................................38 Fig. 5-4. Microphotograph of the intersertal texture of the Amaro–Gamo basalts (DE056), crossed polarized light (XPL) image. .38 Fig. 5-5. Classification diagrams for Ca–Fe–Mg pyroxenes (a) and feldspars (b): Amaro-Gamo