Browsing by Author "Ayalew, Dereje (PhD)"

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    Application of Remote Sensing and GIS for Geological Investigation and Groundwater Potential Zone Identification, Southeastern Ethiopian Plateau, Bale Mountains and the Surrounding Areas
    (Addis Ababa University, 2005-07) Rango, Tewodros; Ayalew, Dereje (PhD)
    The application of remote sensing and GIS has found to be a quick and inexpensive technique in order to obtain the desired output efficiently. For the present study an attempt was made to map dykes, lithology and other thematic maps such as of drainage density, slope, elevation, lineament, rainfall, landcover and burrowing of rodents and then to integrate them in a GIS environment to get information about the occurrence of groundwater and used to select promising areas for further groundwater exploration. The present study was conducted on southeastern part of Ethiopia plateau, the Bale Mountains and the surrounding areas. Satellite image of Landsat ETM+ of all bands except the thermal bands were utilized for lithologic and geologic structures mapping. Topographic map at the scale of 1 :50,000 were used to generate elevation contour at the interval of 20m. Slope map were derived from TIN (Triangulated Irregular Network), which is derived from elevation contour map. Spatial distribution of drainage density was derived by using three softwares AutoCAD map 2000 engineering software, Arcview3 .2 and MapInfo professional 6.0. The burrowing of Rodents were mapped from field Knowledge and using 742(RGB) that shows areas of rodent burrowing activities. Secondary data of landcover, soi l were also utilized. Groundwater potentiality in the area has been assessed through the integration of the different thematic layers that contributes for the natural recharging of aquifer. The predicted groundwater potential zones were divided into 5 classes from very good up to poor. Color composite, ratio and PCA (Principal Component analysis) were made to interpret the lithology of the area. Due to vegetation cover and similarities of reflectance of different rock units it was difficult to separate them. The field knowledge and some petrographic analysis support the identification of the lithology. Key words: dykes, lithology, groundwater potential zone prediction, Remote Sensing and GIS
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    Assesment on the Hydraulic Properties of the Ethiopian Ashange Formations
    (Addis Ababa University, 2011-05) Getahun, Seid; Ayalew, Dereje (PhD)
    The Ethiopian Ashange formation is an extensive development of fissural and central type eruptions in Oligocene period (~ 30 mas) that produced a thick succession of the trap serious basalts that forms mainly the north western and southern Ethiopian plateau (see Fig.3). The Ashange basalt is thus considered to be the lower formation in the stratigraphy of Ethiopian volcanic plateau commonly injected by dolerite sills, acidic dykes and other intrusions. It covers an area and volume of about 85,000km2 and 66,300 m3 respectively contributing to 37 % of the overall volume of Ethiopian continental flood basalts. A total of 70 wells raw pumping test data are analyzed and used besides their respective lithological log to determine hydraulic property of Ashange formation. The dominant aquifer types recognized after analysis are consolidated, confined, double porosity and single plane vertical fractured aquifer systems. Ashange formation shows decreasing aquifer productivity with respect to increased drilled boreholes depth and age of formation. The established relationship between T and Sp.Ca data also shows R2 value 0.957 that shows Sp.Ca can be used as fair estimator of T in areas with only Sp.Ca data. A relatively less well loss coefficient is observed in the Ashange formation where the existing variation still indicates the existence of improper well site location, improper well design and construction factors and well yield deteriorations with time due to clogging, corrosion and incrustation activities of well screens. Comparison of transmissivity values of the Ethiopian Ashange formation with other continental flood basalts of the world shows a decreasing trend with respect to age which may be related to the gradual closing up of open spaces like fractures by secondary materials hindering free flow as well as storage of water .The productivity of Ashange formation can be generally categorized as moderate to high which is controlled by location and geomorphologic setup of the formation outcrop; hydrothermal processes and nature; extent, frequency and orientation of the associated structural features and nature and degree of weathering. This work can be considered as an effort to characterize the whole aquifer system of Ethiopia for a better understanding of the available ground water reserve and distribution for further use in developmental activities. Key words:-Ashange,;Flood basalts,; Aquifer;, Pumping test;Theoretical relations, Transmissivity; Specific capacity
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    Geochemistry of lower sandstone in Blue Nile Gorge Mesozoic sedimentary sequences: Implication for provenance composition and paleoclimate
    (Addis Ababa Universty, 2011-06) Bekele, Barsisa; Ayalew, Dereje (PhD)
    Mineralogy, major, traces, and rare earth elements geochemistry of Adigrat Sandstones Formation from the Blue Nile Basin (BNB), NW Ethiopian plateau have been analyzed to determine their provenance composition, paleoweathering and paleoclimaice scenarios. Samples of this sandstone are slightly variable in composition. Mineralogically the framework grains are quartz (Q), feldspar (F) and lithic fragments (L) and on QFL diagram; most o the samples are plotted in subarkose and lithic subarkose fields. Geochemically, Adigrat sandstones are classified as arkose, subarkose, litharenite, and sublitharenite. The CIA, PIA and CIW values for this sandstone and the A–CN–K diagram agreeably suggest that source rocks experienced intense chemical weathering under hot and humid tropical to sub-tropical climate. Mineralogical evidence suggests that plutonic and metamorphic rocks were major contributing source rocks. Perhaps, multi-cyclic processes reworked the sediment. Relatively immobile elements have been selected for provenance studies. Ratios and plots of relatively immobile elements consistently suggest that the Adigrat Sandstone is derived from compositionally mixed source between mafic and intermediate igneous rocks. Discriminant function diagram constructed from major oxides, the Chondrite normalized REE plot, Cr/V vs. Y/Ni ratios, La vs. Th plot and Th-Hf-Co ternary diagram all suggest that the Adigrat Sandstone is derived from compositionally mixed source. Significant contributions of intermediate supply are also confirmed by Al2O3/TiO2 ratios and lower Eu/Eu* (0.552) values than that of PAAS (0.65). Provenance composition and degree of sorting between Adigrat Sandstone in BNB vs. Mekele Basin (MB) have been compared. The result shows that sediment of the former were highly sorted and compositionally derived from mafic and intermediate input, where as that of the later was mainly derived from felsic rocks. Keywords: Adigrat sandstones, provenance, paleoweathering, paleoclimate and Discriminant function diagram
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    Geology and Geochemistry of Guna Volcanic Massif Northwestern Ethiopian plateau
    (Addis Ababa Universty, 2006-07) Mekonnen, Adise; Ayalew, Dereje (PhD)
    Guna volcanic massif is found in Northwestern Ethiopia plateau. It has an average basal diameter of about 40 km and a height of 1553 m with elevation of 4135m at the peak. It covers an estimated area of about 760 m2. A recent work of kieffer et al., 2004, provided an Ar/Ar age of 10.7 ma. The general objective of this thesis research work is to study the geology and the petrogenetic processes responsible to produce the rock suites of the volcanic massif. The methodologies employed to achieve the objectives includes fieldwork for mapping and sampling, petrographic investigations, electro microprobe analysis on major phenocryst phases of phonolte lavas, major and trace element geochemical analysis by ICPMS method, analysis and interpretation of data using different computer soft wares. Geologically the massif overlying the flood basalt mainly consists of rhyolite lava flow, pyroclastic flow deposit and phonolite lava flow from bottom to top. The rhyolites occur as layered lava flows, glassy with columnar jointing and huge domes. In thin section they have vitrophyric, perlithic and porphyrithic textures with essential mineral composition of alkali fieldspars, plagioclase and quartz. The pyroclastic flow deposit includes ash, tuffs, ignimbrite and trachyte flows. The phonolite occurs as thick viscous lava flow. They show porphrithic and trachythic textures with phenocrysts of alkali feldspar, feldesphathoids (nepheline, leucite and nosean) and alkali pyroxenes. The lavas are rich in silica and alkalis. Major element variation plots displays features agreeable with fractionation of major phenocryst phases, Fe-Ti oxides and apatites. The chondrite normalized Rare earth element (REE) patterns shows enriched light rare earth element LREE, strong negative Eu anomaly and fairly flat heavy rare earth (HREE) pattern that indicates fractionation processes. In the Primitive mantle normalized profiles (spider diagrams) Ba , Sr, Ti and P shows strong negative anomaly which verifies fractionation. In most samples, Nb shows a remarkable positive anomaly. The lavas have low Rb/Nb (0.31-1.31) and La/Nb (0.34-1.03) ratios high La/Lu ratios that increase from rhyolite through trachyte to phonolite. The field observations, petrography, mineral chemistry and whole rock geo chemical data of this work indicates that Guna is not a basaltic shield volcano; instead it is a huge felsic centre mainly consisting of pyroclastic flow deposits, rhyolite, phonolite and trachyte lava flows. The lavas are co genetic and derived from mantle origin basaltic magma through low-pressure extensive fractional crystallization and limited crustal contamination
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    Landslide Assessmeent and Hazard Zonation in Mersa & Wurgessa, North Wollo, Ethiopia
    (Addis Ababa Universty, 2011-06) Ibrahim, Jemal; Ayalew, Dereje (PhD)
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    Origin Of opal Hosted in Volcanic Rocks At Koste – Amba and Gift Mariyam, North Shoa (Central – Eastern Ethiopia)
    (Addis Ababa Universty, 2006-07) Ali, Seid; Ayalew, Dereje (PhD)
    Gemstone- quality opal in North Shoa /koste - Amba & Gift – Maryam localities is found in small amount. The opal – bearing rhyolitic glass is found situated at the topmost part of the Ethiopian flood basalt. It is sandwiched between an underlying rhyolite and an overlying volcanic lava composed of some stacks of superimposed basalt flows at the immediate top / just next to the weathered mantle of the rhyolitic glass. The main objective of the present study is to examine the origin of volcanic – hosted opal in Koste – Amba & Gift – Mariyam /North – Shoa, Amara - Region using petrographic description, major & trace element analysis, as well as x –ray diffraction(for samples collected from surficially altered part of the opal bearing rhyolitic glass/volcanic glass). Besides, it has been intended to establish the geological and geochemical conditions responsible for opal mineralization. The deposit is associated with surfically altered rhyolitic glass, i.e. this lithologic unit has the same mineralogical composition with the rhyolite. A cliff forming rhyolite characterized by alternating vesicular subunit with finely laminated subunit on the same lithology underlies the opal – bearing rhyolitic glass. The size of the vesicles in this underlying rhyolite increases from bottom to top. However, these vesicles are not filled with any secondary minerals like opal. Generally, opal accommodating lithologic unit of the study area is the rhyolitic glass/ volcanic glass, so that the mineralization seems to be highly influenced by lithology and other physico-chemical factors. Features related to opal, like gemmology, genesis (formation environments, associated minerals and rocks) are elaborated based on cumulative global findings Using results of major element analysis and TAS classification diagram, fresh rock samples are plotted in the field of basalt and rhyolite. Therefore, the host & parent rock of amorphous silica (SiO2 . nH2O) seems to be the rhyolitic glass /volcanic glass that presently accommodate opal. Samples have been taken from mantle material as well as from the fresh rhyolitic glass. According to the petrographic description, both fresh and mantle samples of the rhyolitic glass are found to have Sanidine and pyroxene main primary rock forming silicate minerals as well as zeolite. Pyroxene is found in trace amounts in the mantle material of the rhyolitIc glass but it is about 4% in the fresh rhyolitic glass. Besides both fresh & mantled rhyolitic glass contains high amount of volcanic glass and lesser amounts of opaque minerals & rock fragments. X-ray diffraction analysis that has been done on samples from the highly altered top part of the rhyolitic glass shows clay minerals (montmorillonite and kaolinite/ halloysite), zeolite minerals (Philippsite & mordenite), as well as silica (quartz & cristobalite). Deep surface weathering and a lack of detailed site investigations may obscure the difference between surface weathering process and post – volcanic hydrothermal activity. In the presence of this fact, (a) the formation of montmorillonite possibly from simultaneous surface weathering of sanidine and pyroxene that are contained in the rhyolitic glass or from the devitrification of volcanic glass fragments(shards) accompanied by the release of surplus silica to surrounding waters for the creation of opal mineralization in the study area ; (b) the occurrence of zeolite minerals(mordenite & philippsite) usually in earth surface weathering environments(with some exceptions where philippsite can occurs in saline - lake deposits, calcareous deep – sea sediments, hot – spring deposits) characterized by percolating ground water in acidic tephra; (c) the existence of opal – empted vesicular rhyolite and successive lava flows that underlies and overlies respectively the opal bearing horizon; all these together may exhibit that silica – rich waters derived from surface weathering processes can be the origin of opal in Koste – Amba & Gift – Mariyam localities. Besides, recommendation as what is to be done regarding further research works on the deposit is forwarded.
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    Petrogenesis of Basalts of Ghibe River Gorge, South-Western, Ethiopia.
    (Addis Ababa University, 2015-06-06) Dessalegn, Firawalin; Ayalew, Dereje (PhD)
    Ghibe River gorge is located in Southwestern Ethiopia, in the central part of Omo-Ghibe River Basin. The main objective of this study was to constrain the petrogenesis of Ghibe River gorge basalts using the geochemical signature of major and trace elements and thereby to fill the gap of geochemical data and geochemical process that control the Ghibe River gorge basalts evolution. The ICP-OES and ICP-MS geochemical methods were used for major and trace element analysis, respectively. The study area generally comprises of dominantly basalt, and some trachyte and tuff, with aphyric to sparsely porphyritic texture. Classification of the basalts on TAS variation diagram of LeBas et al., (1986) Ghibe River gorge samples fall in the basalt field and considered to be alkaline in composition. The Ghibe gorge basalts have low content of MgO (3.35-6.00 wt %) and compatible trace elements (Ni, Cr and Co), and positive correlations of Al2O3 and CaO with MgO, suggest that fractionation of olivine, plagioclase and pyroxene. Given that the high ratio of incompatible elements (Ce/Pb, Nb/U), which are not affected by fractional crystallization and low ratio of Nb/Pb and Nb/U, Ghibe River gorge basalts are not affected by crustal contamination, rather they indicate their derivation from lithospheric mantle. The positive anomaly of Ba in the primitive mantle normalized multi-element variation diagram shows the presence of amphibole which is stable in lithospheric mantle. Therefore, this signifies their derivation from lithospheric mantle. The flat HREE pattern together with (Gd/Y)n = 1.02-1.14 suggests that Ghibe River gorge basalts were derived from spinel peridotite, with no significant contamination by crustal rocks.
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    Petrogenesis of Flood Basalts From Hirna Area, South Eastern Ethiopian Plateau
    (Addis Ababa University, 2013-07-07) Getaw, Andualem; Ayalew, Dereje (PhD)
    This study presents new petrography, major and trace element data from Hirna basalts, southeastern Ethiopian plateau which directly overlies the Cretaceous Upper sandstone for a maximum thickness of 540m. The study exhibit the presence of variation in geochemical distribution over the area. Major and trace element variation mostly controlled by source region heterogeneity and fractionation history of the primary magma. Generally silica saturated and undersaturated basalts have been recognized. Silica undersaturated basalts characterized by high Fe2O3 (17.34-17.54 wt%), and low SiO2 (34.89-35.02 wt %) and show enrichment in highly incompatible trace elements Sr (806-1198ppm), Zr (323-422), Nb (111-142). Silica saturated basalts on the other hand exhibit SiO2 (45.45-49.04 wt %) MgO varies between 4.55-6.16wt% and comparatively the concentration of highly incompatible trace element in silica saturated basalts are low Sr (594-72 ppm), Zr (145-171ppm), Nb (17-19ppm). The incompatible trace element ratios between the two basalts form two distinctive groups. Silica saturated basalts exhibit La/Y <1, La/Lu 65-70, La/Yb 9.5-10.61, La/Nb >1 and silica undersaturated basalts show La/Y 2.96-3.12, La/Lu 322-323, La/Yb 44-45, La/Nb <1. This incompatible trace element ratio variation and the crossing pattern on the Chondritic normalized REE (rare earth element) plot between the two groups shows source region heterogeneity. Enrichment of LREE (light rare earth elements) and depletion of HREE (heavy rare earth element) have been observed for both basalts. This signature is indicative of melting in the presence of residual garnet in the source region. The highly compatible trace element concentration of the area is very limited (Ni<45ppm and Cr<60ppm) and have low MgO/ (MgO+Fe2O3) ratio (0.26-0.33) it is a characteristics of high involved magma spectrum. The basaltic successions of the area‟s major and trace element data petrogenetic model suggests fractionation of Olivine, pyroxene, Fe-Ti oxide and ±plagioclase are the major factors responsible for the modification of the primary magma.
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    Tectonics of the Western Margin of the Northern Main Ethiopian Rift
    (Addis Ababa,University, 2003-06) Tesfaye, Ashenafi; Ayalew, Dereje (PhD)
    Afar is junction for the red- Sea, gulf of aden and main ethiopian rifts the timing of Initiation of in the other sectors of the ethiopian rifts is well Understood. However. there is a poor understanding of the onset of rifing in the northern Main ethiopian rift the objective of the present study is to establish chronology of Rifing and volcanism in the northern main ethiopian rift. And correlate this with the other sectors of the rift to the south afar. Red- sea and aden rifts.