Mejo map sheet

The Mejo map sheet is located in the southeastern Ethiopian highland in the upstream courses of the Genale River. Comprehensive thematic maps, including basic geological map, soil map, hydrogeological map and geo-hazards risk map have been compiled based on field, remote sensing data. An accompanying detailed explanatory booklet on each of the thematic layer content and recommendation on preventive actions and strategy for geo-hazards risk identified are also prepared.

Abstract

The map sheet is exposed in the Genale River Gorges. The map sheet is built by metamorphic rocks of Precambrian age and partly covered by Cenozoic volcanics. The metamorphic rocks belong to the ~N-S trending, metavolcano-sedimentary Megado belt, which forms a northern part of the Adola belt. Metamorphic rocks are exposed in the southern and central parts of the Mejo map sheet, mainly along the rivers Genale and Gambelto (around the villages of Girja and Chebe). From the lithological point of view, this unit consists of biotite to muscovite-biotite paragneiss, graphitic schist, quartzite, biotite-muscovite phyllite, sericite schist, biotite orthogneiss, amphibolite, talc-tremolite to chlorite schist and serpentinite and metagabbro. The regional metamorphic foliation or compositional banding is mostly parallel to the contacts and lithological boundaries dip steeply to ~ W (WNW) or E ~. These basement rocks are overlain by two distinct volcanic units – pre-rift Nazret Formation (Pliocene to Eocene age) and syn-rift Dino Formation (Pleistocene to Pliocene age). The Nazret Formation includes the Amaro–Gamo basalts alternating with Shole ignimbrites followed by the Getra-Kele basalts. This sequence is exposed mainly in the northern and eastern part of the map sheet. The syn-rift Dino Formation is represented by Pleistocene Hantate ignimbrites cropping out as the erosional relics in the valleys of Gambelto, Logita and Genale rivers. The lithological boundary with the basement rocks as well as the planar fabrics in the overall volcanic and volcanoclastic sequence are mostly horizontal, The geological map representing all identified lithologies and structural relations is produced at a scale of 1: 50,000, which served as the base for the thematic maps.

Soil type classification was compiled based on the geological map, field survey, satellite image interpretation and digital elevation models. Four soil catena units have been identified as reference groups: Cambisols, Nitisols-Acrisols, Regosols-Cambisols, and Fluvisols in the respective order from the top of the escarpment to the rift floor. The soil classes have been identified based on chemical and physical properties.

Hydrogeological assessment has been made following the regional morphology, local geological and water point’s inventory from the field inventory and published secondary data. Surface hydrological characteristics and groundwater dynamics have been addressed to develop water resource assessment in the region. The total water resources of the area were assessed to be as high as 434 million m³/year. The total volume of renewable groundwater resources of active aquifers in the area has been assessed to be 169 Mm³/year. The quality of water resources available is also investigated, showing that the dominant hydrochemical types of groundwater in the study area are mainly of a calcium-bicarbonate type in composition, while also magnesium and sodium bicarbonate types are identified. Distribution of the aquifers along with the hydro-chemical compositions is combined in the produced hydrogeological map at a scale of 1: 50,000 of the Mejo map sheet in this study.

Engineering geological assessment of the area indicates regions of rock mass ranging in strength classes as high, medium and low. These correspond to the variation in lithological units, which on average show distinctly variable intact strength and variable susceptibility to weathering based on the field documentation and physiographic areal disposition. Prominent and sharp morphological regions stand out as generally resistant and stronger rock mass units, contrary to the rock mass units in subdued and smoothly undulating morphology. Genetic associations of the soil in the area are also distinguished as residual lateritic soil, colluvium slope wash deposits and alluvial cones.

As an important endogenous process with a potential risk, it is found that the Mejo area has low seismicity as well as recent eruption history apart from potential distant earthquakes and volcanic eruptions. Due to the morphological set up of the area, both aggradation and suffusion-related hydrological hazards appear to be important, affecting farmlands as well as infrastructure. In the hazard maps, all the potential risk areas concerning such exogenous hazards are shown. In the scarps of deep valleys, an increasing threat of reactivation of mass wasting processes occurs in the form of landslides, debris avalanche and rock falls. Due to expansive agriculture and deforestation, the problem of surface erosion and land degradation increases.