Earth Sciences

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Hydrogeology of the Borkena River Basin Hollo – Ethiopia
(Addis Ababa Universty, 1980-07) Tadesse, Ketema; Bonard, G. (PhD)
The Borkena Hiver Basin is located in Wollo governorate general. It is part of a major graben related to the Ethiopian Rift System, which extends north to Korbeta graben around Maichew and south upto Debre Berhan, and approximately stretches in a north-south direction. There are significant contrasts of topography in the basin where the structural control is very strong. The hydrology of the area varies in different parts of the basin. Consequently,the hydrogeology of the basin alse differs, such that there is a great potential of groundwater in the flat low lying alluvial sediments while a less signi fican t amount exists wi thin the volcanic rocks forming the uplifted borders of the graben. Several springs with discharges of less than 5 lilt/sec. occur in the area issued from the volcanic rocks. There are eleven water supplying drilled wells in the area, seven located in the alluvium aquifers and the remaining four on basaltic aquifers; Pump tests have been carried out in some of the wells which gave transmissibili ty coe fficient of 91+ to 28.1 m3 per day per meter. However, it was impossible to calculate the coefficient of storage, a significant parameter which measures the volume of water, the aquifer releases or takes into storage. Proper chemical analysis of both the ground and surface waters carried out in the area show low total dissolved solids. All of the samples in tho area have loss thun 1000 mg/l (1 mg/l = 1 ppm) dissolved solids, except for the hot spring waters. Few of the bacteriological tests of the waters in the area show low pathogenic organism though most of the water samples, (particularly the ground waters ) contain high ni tra te con ten t. The water balance for the area has been determined. The total annual rainfall calculated on the basis of the mean annual precipitation for the basin is 1846.41 x 106 m3 • The groundwater recharge for the area has been calculated based on the evapotranspiration data at Cheffa, where the water loss is maximum to approximate the minimum infiltration, and amounts to 185.11 x 10 6 m3 . This is 10.02% of the precipitation. The actual evapotranspiration in the area amounts to 872, 834.8 and 836.1 mm. for Cheffa, Dese and Kombolcha respectively. 218.5'5 x 106 m3 of water leaves the basin at the BOrkena River outlet as runoff and is equivalent to 11.83% of the precipitation •
Geothermal Study in Northwest Lake Abaya Area (Southern Ethiopian Rift)
(Addis Ababa Universty, 1980-07) Yirgu, GeZahegne
Geothermal energy, the cheapest amorlY ttl~ differeDt soutced of power, is thought to provide a solution for the rising demand of electricity in Ethiopia, because the Ethiopian Rift Valley and Afar area have been identified to have one of the greatest geothermal potential in the world. The Nvl Lake Abaya geothermal province, situated in the southern part of the Ethiopian Rift Valley near to its western man'in, is one of the target areas singlE'd out for detailed exploration purposes. The area:i.s characterized by high heat flo", indicated by the presence of a number of hot-"l'rings. fumaroles and a geysE'r. '-.- The volcano-tectonic evolution \·'hich started in early 1110cene, has been in progress throughout Plio-Pleistocene and Quaternary activity has been evidenced 'in the most active part of the Rift. 1'he volcanic products, represented by basal ts and rhyolitic products, vary from alkaline to mildly alkaline and the recent fissural b'lsaltic volcanism sho\'ls a transitional nature. The final silicic products', are represented by peralkaline rhyolites, probably commendites. The hydrogeological evidence suggests that quite abundant amount of \~ater is recharged from the surrounding highland areas in the north and N\'I into the reservoir system and that the hot springs are all of meteoric origin. Important circulation of \~ater at depth is evidenced from isotopic data. The chemical characteristics of the the,rmal manifestations suggest the presence of two groups of thermal waters \'lith different geochemical histories. A high temperature reservoir is represented by the first group while different degrees of mixing and dilution of the original
Variations in Soils and their Mineralogy in the Kulubi Area,Harar, Ethiopia
(Addis Ababa Universty, 1981-07) Wolde, Begashaw
The geology, geomorphology and soils of the Kulubi area, in eastern Ethiopia, are described. The succession of rocks in the region comprises Precambrian rocks consisting of gneisses, granites, and raigraatit - es overlain by a trachyte flow; Lower Sandstone Unit (Triassic to Lower Jurassic) consisting of a lov/er arenaceous and an upper calca-^ reous sub-units; Limestone Unit (Lower Jurassic to Middle-Cretaceous) consisting of sandy and fossiliferous carbonate rocks; Upper Sandstone Unit (Middle Cretaceous) consisting of a crossbedded, parallel-bedded, calcareous, and massive and laminated sub-units with Mesozoic volcanic intercalations; Tertiary basalts and Quaternary sediments., The three stages in the geomorphological evolution of the area include a Late Cretaceous-early Cenozoic peneplatation; Trappean tectonics and formation of basin-plains; and renewed erosion associated with the rift tectonics* The major soils in the region are Vertisols, Entisols, and Mollisols. The Vertisols occupy depressions in alluvial plains and scattered spots on Hie'uplands and are developed on basalt and alluvium. They are ddep, clayey soils with low chromas and values. Chemically, they are very slightly alkaline, very low to medium- .in organic matter, low in total nitrogen, and very low to low in phosphorous. These are the least productive soils.The Entisols occur on alluvium and on steep slopes of basalts, sandstones and gneisses. r They are dark grayish and reddish brown, shallow and discontinuous, loamy soils. They are slightly acid to very slightly alkaline, low to medium in organic matter and total nitrogen, and medium to high in phosphorous. Their occurrence on either actively eroding or flooding surfaces significantly reduces their agricultural productivity. The Idollisols are the dominant soils occupying flat to gentle slopes on all theparent materials excepting the basalt. Those are brownish-blaclc and reddish-brown, shallow to deep, clay, clay loam, and sandy clay loam soils. They are strongly acid to slightly alkaline. The organic matter and available phosphorous contents differ widely. They have a low to medium total nitrogen. These are the most productive soils. Comparison of these soils in terms of their textures, pHs, and organic-matter contents indicates that topography is the main soil-forming factor controlling the morphological, physical, and chemical properties of these soils.
Relation Ship Sbetween Geology, Mor Phology and Soil Associations of Bichena Area ( S. E. GOJAM )
(Addis Ababa Universty, 1981-10) Ayalew, Teklewold
The study investigates the extent of influence of the geolog- ical history , essentially parent rock * topography and time , on soil formation and distribution in Bichena , Southeastern Gojjam * Lithostratigraphy of the Mesozoic sedimentary sequence and Cenozoic volcanics is described * An attempt has been made to reconstruct the morphologic evolution of the area * Four soil mapping units in close relationship with the parent material and geomorphology have been differentiated: Vertisol, Mollisol, Alfisol and Entisol Orders 0 The soils are found to assume different states when the above considered soil forming factors are changed * The vertisol development from aphanitic basalts is determined by the evolution of the structural plateau geomorphic surface through lateral planation, which has allowed for two cycles of pedogenesis* The Mollisols have developed from porphyritic plagio- clase-Olivine basalt* These indicate that soil parent materials with similar mineralogy give way to soil varieties * A lateral variat- ion in some soil properties of the Mollisol order is found to be strongly explained in terms of the slope gradient * The study shows that the Plio-Pleistocene neotectonic uplift and subsequent develop- ment of a deep canyon including morphogenetic threshold slopes has resulted in lack of sufficient time for pedogenic horizons to develop in Alfisols and Entisols *
Stratigraphy and Carbonate Microfacies of the Hirna Mesozoic Sequence
(Addis Ababa Universty, 1981-12) Atnafu, Balemwal; Lentini, F. (Professor)
The study are apresent sex posuresr a n g i n g i n a ge from the Precam bria n ( b a s e m e n t ) t h r o u g h M e s o z o i c (s e d i m e n t s) t o t h e T e r t i a r y ( v o l c a n i cs) w it h a l l u v i a l s e d i m e n t s o f r e c e n t a g e a l s o p r e s e n t. T h e d i f f e r e n t l i t h o l o g i c a l u n i t s h a v e b e e n m a p p e d a s w e l l a s des c r i b e d base d o n fi e l d o b s e r v a t i o n s. T h e M e s o z o ic s t ra t i gr a p hy o f H i r n a ar e a h a s b e e n s t u d i e d w i t h p a r t i c u l a r r e f e r e n c e t o t h e c a r b o n a t e l ith o l o g y. T h ree m ai n c ar b o na t e m i c r of a c i e s ha v e b e e n i d e n t i f ied a s b e i n g r e p r es e n t a t iv e major c o n s t i t u e n t s o f t h e s e c a r b on a t e s. T h e s e m i c r o fa c i e s a re d i st r i b u t e d i n a c y c l i c m a n n e r w i t h t i me. T h e c y c l ic i t y h a s b e e n i n t e r p r e t ed a s s h ow i ng s ma l l ch a n g e s o f se a l e ve l i n a g ene r a l l y s h a l l o win g u p s eq u en c e. B i o s t r a t i g r a p h i c a n d pa l e o g e o gr a p h i c co n si d e r a t i o n s s h o w t h a t t he L o w e r S a n d s t o n e U n i t ( T ri a s s i c.- M i d d l e J u r a s si c) i s c or r e l at i v e w i t h t he regionally i d en t i f i e d A d i g r a t. S a nd s t o n e s. T h e c a r b o n a te s o n t h e o t h e r h a n d c a n b e r ef e rr e d i n p a r t t o t h e H a m a n l e i s e q u e n c e (?B a t- h o n i a n- Lower Ox f o r d ia n a g e i n H ir n a a r e a ) a n d i n p a r t ' t o t h e U a r a n d a b s e q u e n c e ( K i m m e r i d gi a n a g e i n Hir n a ar e a). T h e U p p e r S a n d st o n e U n i t i s a p a r t o f t h e. Y e s o m m a s e q u e n c e a n d i s o f C r e t a c e ou s a g e i n H i r n a a r e a. Tectonically , Hirna area is only lightly deformed , with a SW - NE fault trend dominating the structures. Other trends , however , do exist with not so clear a relationship with the dominant, trend. Gentle dips of local significance are shown by some of the Phanerozoic lithologies.
Alleonathy in some Ethiopian Grasses.
(Addis Ababa University, 1982-06) Medhanie, Gebrehiwot; B. G. Egziabher, Dr . Tewolde
I nvestigation fo r possible chemi cal interactions among some of the common grass species that graw in and around Addis Ababa
Shallow Resistivity Invistigation in the Filowoha Fault, Addis Ababa
(Addis Ababa Universty, 1985-12) Selassie, Haile; Ferri, M.(PhD)
The effectiveness of the electrical resistivity methods has been studied using various configurations over the Filwoha Faul t. It has been opserved that the del inea tionof the faul t was possible under differential weathering. Geoelectric sections across the fault has been prepared on the basis of Schlumberger sounding results. In profiling three diffllrent electrode configurations namely Wenner, two-electrode and half-Schlumberger, were used along profile AA' and Wenner only in Profile BB'. On,the basis of data obtained from profiles AA' and BB' it was possible to delineate the strike and extent of the fault \d thin Addis Ababa. Furthermore, the interpretation of the resul ts in AA' su!!/!ests that the two-electrode configuration posses a certain diagnostic feature which heln in mapping latteral contact of this type. The age of the fault is es timated to be between 5. O~la and 6.4 M.yrs. and the down thrown side is to the south.
Neotectonics of Nazret-Dera Area
(Addis Ababa Universty, 1990-06) Damte, Alula; Torotrici, L. (Professor)
The Nazret=DGra area forms a segment of the northern part of the Main Ethiopian Rift. The region is character-' ized by a complicated morphology which is a result of several episodes of volcanism and tectonism. The volcanic rock units outcropping in the area are principally bimodal in nature and it is possible to sub~ divide them into 6 groups according to their spatial distribution. The groups include; the stratoid Eastern Margin and Dera-Nazrot group mainly formed by a series of flood basalts inter layered with ignimbrites and Rhyolites; Keleta group, representing the oldest rift floor product and constituted by Ash flows and Ignimbrities; Boku and Gedemsa group,'which are rosults of central type volcanic activity evidenced by preserved caldera of Gedemsa and destructed caldera of Boku, giving rise to Pantellerites, Rhyolites and pumice fall deposits and Melkasa group which is constituted by basaltic spatter concs and associated 'AA' type lavn f1.ow. The nge of the rocks vary from 1.7 my to 0.06 my. The large volume of acidic product as compared to the basaltic one and the lack of transitional rock units suggest that there is a pnrtial melting of the lower crust probably induced by intrusion of large bodies of basaltic magma From Tectonic POitit of view, tho area is chAracterized by right stepping, "8n echelon" halt grabens, grabens and faults which affect evan the youngest rock outcropping in the region, The geometry of these structures i,D. an overstepping towards the right suggest a left lateral motion along this part of the rifto The theory is substantiated by meso-scale structural data which has been collected from, small faults with slicklines, extensional fractures and joints which show that the area is undergoing an eastnortheast, west-southwest extensiono Thus strike-slip system and oblique extension is the major mode of deformation for this part of the Rift System and possibly may extend across the whole length of the Main Ethiopian Rift
The Gravity Field and Crustal Strecture of the Main Ethiopian Rift
(Addis Ababa University, 1992-01-05) Alemu, Abera; Sjoberg, Lars E (Profossor); Asfaw, Laike M (PhD)
All Existing Data (1500 taken by the the author and 800 reprocessed older data )in the Main Ethiopia Rift and the adjacent plateaus (between latitudes 4045’ N and 12000’ N)were reduce to Bougure and free-air values The resulting values are compiled in to 5 mGal and 10 mGal contour interval Bouger anomaly and free-air anomal maps respectively. The accuracy of the Bouger anomaly at each gravity station is estimated to Type equation here. ±2.9 mGal. The Bouguer anomaly map reveals at :P the Western and Eastern Plateaus are associated with gravity minima(e.g ≤265mGal near Asasa on the Eastern plateau and near fiche on the Waster plateaus),and those gravity minima are located in area od ,aximum elevation. Within the middle of the rift floor,a trend of gravity maximum(median highs) whose locations generally coincide with th location of various Quaternary volcanic centers runs parallel to the main escarpments. Between 7 30'N and 8 45'N,trend of elongated gravity maxima corresponding to marginal grabbers appear to run along appear to run along the whole length of thr east side of he the rift floor. Along the rift axis gravity values general increase north and south latitude of Awasa (7 N).This increase is frome-195 mGal in the Awasa district to -65 mGal in the Gewane area when going northward and to -135 mGal in the konso area south of Lake Chamo when going southward along the rift axis. This trend of gravity maxima coincides with the locations of maximum crustal thinning along the youngest structural deformation of the wonji Fault Belt Six Profiles were extracted from the Bouger anomaly map through the major plate tectonic system(the western platean ,the Eastern platea the Main Ethiopia Rift and southern Afar)of interest in the area. Two-dimational gravity models;i.e infinite in strike extent, were computed along these profile. The objective was to obtain modale whose gravity effect possessed the same qualitative feature and approximately the same anomaly shapes and magnitudes as the observed profiles. The densities used to compute the models were constrained by the results of the deep seismic refraction sounding profiles of the region and the adjacent area the results of these calculation are that: the crust in the plateaus is underlinain by a normal mantle. It is 35 to 41 km thick beneath both the Western and the Eastern plateaus and occurs at the locations of gravity minima in the region surveyed. Within the main Ethiopian Rift, the curst thins from about 30km in the south to23 km in the north. The observe things is some 7 to 8km and is cased by an upward progression of low density mantle material (anomalous mantl) that intrudes itself into the lower curst. The Quantity of the anomalous mantle material decreases frame north to south beneath the rift It is about 22 km thick and 150 km wide in the north, whilst in it central and southern parts, it is 15 to 20 km think over a 100 km wide zone To determine the degree to which the crust attained isostic balance, the following methods were employed. (i)Computation of mass distributions along the seismically controlled2-D gravity models for 20,30,40,50 and and 60 km depth levels; (ii)computation of an isostatic Moho-depth map according to the Airy-Heiskanen model of compensation. The result of these calculation are that: with reference to the mass distributions, the curves show that the crust is isostically balanced at the 30 km depth level, north of latitude 90N.South of this latitude, isostatic equilibrium is first reached between the 40 and 60 km depth levels. Isostic equilibrium north of latitude 90N is achieved though the contribution of the anomalous upper mantle lying at shallow depths. The normal mantl. As regards the isostatic Moho-depth map, there is a significant correlation between the Bougure anomaly and the calculated Moha-depth maps. The steep gravity gradient which marks the transition from the plateau area to the rift is also reflected in the Moho-depth map. As in the the2-D crustal density models, the Moho-depth map reveals that the crust beneath the rift thins from south to north. The magnitude of the axial positive gravity anomalies observed on the compiled map increase in magnitude north and south of latitude 70N. The compiled gravity maps, isotatic Moho-depth map and Geoid undulation map are found to be sensitive indicators of the tectonic and geologic structures of the region . A two dimensional correlation analysis made between the different parameters considered in this study (Bouguer anomaly, free-air anomaly, free-air anomaly, elevation and Moho-depth) shows that there are significant correlations between some of them.
Hydrogeology of Debre Zeit Area
(Addis Ababa Universty, 1992-06) Alemayehu, Tamiru; Vernier, A , (Professor)
The investigated area is located east of Addis Ababa on the western border of the main Ethiopian rift valley * Being part of the Ethiopian rift system , it is characterized by Plio-Quaternary volcanism which gave rise to trachytic- domes, rhyolitic flows , and rhyolitic ignimbrites in the highly elevated part of the area while olivine basaltic flows and surge deposits in the lower elevated plain area . The maximum point in the area is represented by Mt . Yerer with an elevation of 3100 m above sea level. The plain area has an average elevation of 1900 m above sea level . The yearly minimum possible rainfall average in the area is 859.9 mm , the actual and potential evapotranspiration is 606 , 2 and 857.1 mm respectivly and the runoff that leaves the basinis 25 . 568 x 10d m3 «of water. From these determinations, the amount of infiltrated water in the area is <5 3 calculated and the value is 54.39 x 10 m . The hydraulic parameters , transmissivity and hydraulic conductivity are determined based on the pumping test information obtained for few wells , The transmissivity and permeability of the rocks found in the central part “3 2 \ _3 of the area is 0 ,9x10 m /s and 4.0-5x 10 cm/s respectively .8 ,8 x 10 m and the latter is 1.5 x 10 cm/s. But the storage coefficient is not calculated because of the absence of observation wells in which the effect of further drowdown is observed , Because of the lack of bore wells in the northern part, of the area , a thorough study of the actual variation of groundwater level was not possilble. But based on the available static level data , for the central part of the area , appropriate groundwater level contour lines are constructed The chemical analyses result- showed that most waters are calcium -magnesium-bicarbonate type .
Tectonics of the Precambrian Rocks of the Negash Area, Tigrai Region, Northern Ethiopia
(Addis Ababa Universty, 1993-01) Kidane, Tesfaye; Boccaletti, M ,(Professor)
The Wukro area is constituted by weakly metamorphosed largely Precambrian age volcano-sedimentary rock units of Tsaliet Group , Tembien Group , Didikama Formation , Hatheos Formation and pebbly slate which form part of the Upper Complex of the Precambrian basement . The younger Palaeozoic-Mesozoic sedimentary cover consisting of the Enticho Sandstone , Edagga Arbi Glacials and Adigrat Sandstone unconformably overlies the Precambrian units . Detailed structural analyses at macro , meso and microscale have revealed three generation of ductile deformational events ( Dj , D2 , & Dj ) and late faulting . The first phase of deformation Dj is characterized by transposition phenomena which has obliterated the original stratigraphy and produced rootless intrafolial foldings . The intensity of transposition is greatest on the rocks of Tembien Group and Didikima Formation and is least on the rocks of Hatheos formation and Pebbly slate . Associated with the Dj deformation is the N -S trending Sj foliation largely defined by slaty cleavage . The second phase of deformation Dj is characterized by the. development, of a (10 * - 20 ‘) — > (170 * -195‘) plunging overturned major folds , which has folded the structural elements of the Dj generation . It developed two synforms and two antiforms , namely North Western Synform , Western Antiform , Central Synform and Eastern Antiform from west to east . The planar fabric element S2 produced by this generation is nearly subparallel to S « along the limbs of the P2 folds and is nearly perpendicular to Sj along tne hinge zones of the D2 folds . The third phase of deformation Dyis the last deformation event in the study area . Throughout the area its effect is a development of ^ndulations on the previous N-S trending composite ( Sp S( S2 ) surfaces . Its effect is greatest in the North-Western synform , where the D2 axial plane appears to have a curvilinear ’' trace . Following this event, is an intrusion of small granite and dykes into the rocks of the Tembien group and Didikama formation . Metamorphism in the area is largely synchronous with the Dj and D2 deformations . Metamorphic conditions during those deformations were restricted to low gree. nschist facies . Following peneplanation of the basement and deposition of Palaeozoic - Mesozoic sediments , numerous normal faults affected the basement and the younger sedimentary cover succession .
Mineralogical and Geochemical Studies on the Sakaro Primary Gold Deposit (Sidamo, Southern Ethiopia): Zonation in Ore Bodies and Host Rocks
(Addis Ababa Universty, 1993-06) Girma Mesfin; A.Sliouniaev Anatoli (PhD)
The Sakaro deposit is a vein-type fault-hosted gold deposit in the Adola area, southern Ethiopia. It is composed of quartz veins up to 9.3 m wide which occupy fault planes along foliation and petrographic contacts in precambrian low-grade metamorphic rocks. The veins are gold-rich and sulfidepoor, forming a mineralized zone of 760 m in strike length and up to at least 150 m in vertical extent. Veins Nos. 1 and 2 are the richest shoots with an estimated total ore reserves of 760,800 tons capable of producing 2,551 kg of gold at an average grade of 3.4 gft and 5.7 gft respectively. Geochemical and mineralogical studies of the ores and the host rocks reveal that the Sakaro deposit might include tungsten other than gold as a potential exploration target. Tungsten minerqlization is the earliest to be formed. Tungsten content increases up to >1% in the ore veins and in the wall rocks (mainly in amphibolites proximal to the veins), forming a distinct zone that assumes W-As-Cu, a geochemical zone or wolframite-scheelite-iron sulfides zone, a mineralogical zone. Gold is mostly associated with Ag and Pb, showing asymmetrical dispersion pattern that starts in the hanging wall side of the vein and decays towards the foot wall rocks. Its tight relation with Ag and Pb is expressed by forming a Au-Ag-Pb zone though it is also related at a lesser degree "dth other elements especially with Cu. Outward from this zone, Ag-Pbcu, W-As-Cu, and Fe-Co-Ni-Mo-Cu follow in respective distances from the ore veins toward enclosing rocks. Gold distribution in the are veins is controlled by thickness variation of the quartz fillings in both strike and dip directions, poor in swells and rich in constrictions. In the plane of the main vein, five are columns are identified with marked contrast in gold mineralization: (1) obliquely running I EW dipping gold rich zone I (2) northerly trending, vertically dipping gold rich zone, (3) intermediate low gold zone, (4) oxidation zone, and (5) surficial weathering zone. The first two zones are ore shoots representing the fronts of gold depositing mineralizing fluids. Gold contents follow a systematic pattern of more or less regular course within and away from these fronts which otherwise are perceived as very irregular and erratic distributions. loloreover, statistical treatment on the metal content shows that gold distributions in populations separated according to the rock types and environments obey lognormal law, with mean values highest in ore veins, less in amphibolites, and least in mica schists. studies on the vein development reveal three distinct types of hypogene mineralization from veins margins to the center, of marcasite + chalcopyrite in the peripheral parts, galena + chalcopyrite + marcasite in the following portion, and sphalerite + high galena + chalcopyrite in the central part. Gold is present in all of the above associations, however, it is highly enriched in sphalerite-high galena-chalcopyrite zone, followed by hanging wall side marcasite-chalcopyrite zone, and least in foot wall side marcasitechalcopyrite zone. The gold enrichment in sphalerite-high galena-chalcopyrite zone indicates the close association of Au, Ag, Pb, Cu and Zn. The selective enrichment of gold in the hanging ''1all side (marcasite-chalcopyrite) zone is most likely due to intense shear remobilization effects. The veins display a crude ascending mineral sequence, from depth to the surface, of pyrrhotite, gold + galena + sphalerite + chalcopyrite + iron sulfides (productive zone) I and marcasite. A descending sequence in the zone of weathering ShO"'S that Fe-hydroxides and Fe-disulfides grade downward where gold is enriched "lith secondary minerals of Pb, Cu, Zn. In the enclosing rocks, wolframite + scheelite + Fe-sulfides form a zone adjacent to the are veins and marcasite + pyrrhotite + chalcopyrite in distal areas. These zones are in agreement with geochemical zones W-As-Cu and Fe-Co-Ni-Mo-Cu respectively.Disulfidization process of iron monosulfides increases in the upper levels of the deposit alld is more intense at the vein margins especially in the hanging wall side of the ore veins due to shearing which is also responsible for recrystallization of quartz at the vein selvages. The vein mineralogy shows as the vein development is the result of multiple episodes of vein opening and filling characterized by wolframitescheelite- quartz, sulfide-gold-quartz and disulfide-carbonate stages. statistical parameters of distribution of the components, ore geochemistry, peculiarities of horizontal (along the strike and across the thickness) I vertical and three dimensional geochemical and mineralogical zonation of the mineralized rocks and the general sequence of formation of minerals in ores of the saka·ro deposit considered and proved in the thesis are the features of its genetic model. These features are also of practical importance for are blending and further improvements of are dressing technological schemes. They are of value and use in prospecting for similar mineral deposits in the region and their successful exploration.
Water Balance and Effect of Irrigated Agriculture on Groundwater Quality in the Wonji Area / Ethiopian Rift Valley
(Addis Ababa Universty, 1993-06) Dechasa, Teshome
Wonji basin covers a total area of 357 Km2 and has a population of 290,703 out of which 50,000 are sugar estate farm workers. The main source of domestic water in the area is groundwater. Awash river passes in the area dividing in to two equal parts whose water is mainly used for cattle drinking and industrial purposes. Meteorological and hydrological data were used to establish and calculate water balance in the Wonji basin. Groundwater recharge was estimated to be 557.75 x 106m3 per year. Different methods some with many parameters and others with few were checked side by side to obtain more realistic actual evapotranspiration. Their average gave 702 mm mean annual actual evapotranspiratin. The mean annual rainfall is 785 mm in the area. Based on joint data along with mathematical treatment and pumping test results, lithological units have been characterised into different permeability groups. From the results of water chemistlY and compositional diagrams, 5 groundwater systems with some interconnections were identified. Direction of regional groundwater flow of the basin is from west to east. Two horizons of cold water of deep aquifers occur at an average depth of 120 and 170 in the region. Using temperature of thermal springs, depth of geothermal water circulation has been calculated to be 242 m below the surface. This has been also confirmed by the drilling data of borehole 26. However this depth could vary up to 400-500 III under Gadamsa caldera rim, Boku ridge and Koka escarpment high lands. The estimated groundwater demand in the basin ranges up to 4.7xI06m3 , which is Illuch less than yearly recharge. Fluoride concentration showed zonal variability from low lying parts of the basin to the high lands of the study area corresponding to compositional variation of the rocks and thermal activity. Concentration of total dissolved solids in the groundwater, exhibited positive relation with irrigated sugar plantation. Pollution from inorganic fertiliser (NO,) is not however, observed under sugar cane farm. This is suggested to be due to high groundwater inflow dilution from Koka reservoir. Subsurface water in the region is generally of good quality for drinking purposes. But some dug wells and hot springs are of non promising quality for irrigation.
Volcanology, Petrology and Geochemi Stry of Gedemsa Volcano
(Addis Ababa Universty, 1994-01) Ayalew, Dereje; Yirgu, Gezahegn
The Gedemsa volcano lies on the floor of the northern sector of the Main Ethiopian Rift. It is characterized by a polygenic caldera resulting from large pyroclastic eruptions. K/Ar dating performed by previous studies indicates an age of 0.8 to 0.1 Ma for the exposed volcanic products. Volcanologic and stratigraphic studies allowed recognition of several phases of activity during the evolution of Gedemsa. The lowest exposed products are represented by acidic lavas, which are covered by thick plinian fall pumice deposits. This are followed by an ignimbrite deposit and by intra-caldera lava flows and interbedded pyroclastic products. The caldera, is clearly a composite structure resulting from several collapses which occurred after plinian and ignimbritic eruptions. A separate stage of volcanic activity connected to the Wonji Fa ult System (basaltic volcanism) formed surge deposits and numerous basaltic cinder cones and lavas, both within and outside the caldera depression. The volcanic products from Gedemsa volcano are petrologically and geochemically diverse. Alkaline and peralkaline silicic lavas and pyroclastics (trachytes and rhyolites) are by far the most abundant products. The mafic rocks are only represented by the mafic inclusions occurring within some of the post-caldera products. The younger riftrelated activity is, instead, represented by Na-transitional basalt. On the whole, the rocks occurring in the area have a very marked bimodal distribution, a situation which is typical of almost all the young volcanism of the Ethiopian Rift Valley. Major and trace element variations of peralkaline silicic volcanic rocks from Gedemsa volcano support an origin by crystal/liquid fractionation starting from mafic parental liquids, with separation of olivine, plagioclase, clinopyroxene and opaques during the early to intermediate stages and of alkalifeldspar and a few mafic phases (alkali amphiboles and pyroxenes) during the late stages of evolution. These processes generated strong enrichments in incompatible trace elements and depletion in compatible elements in the acidic magmas. Consequently, rhyolites display extremely high values of Zr, Y, Rb and F and low to very low abundances of ferromagnesian trace elements, Sr and Ba. Some rhyolites, however, have low Na and fluorine, most probably due to interaction with groundwaters. Such a process may represent an explanattion of the high F contents in the groundwaters of the Wonji area and of other zones inside the rift. Although, crystal fractionation best fits the geochemical variations, field and petrographic observations indicate that mixing processes were also active during the magma evolution.
Sulfide. Mineralization in the Lega Dembi Primary Gold Deposit, Sidamo, Southern Ethiopia
(Addis Ababa Universty, 1994-06) Getaneh, Worash; Tadesse, Solomon (PhD)
Sulfides in the LDPGD are found in small quantity. The deposit is a lowsulfide- gold quartz deposit which specifically belongs to the pyrite-arsenopyrite-galena assemblage type of "Volcanic precious metals tellurides association" which is typical of volcanic to subvolcanic settings. It is hosted in volcano sedimentary succession metamorphosed to greenschist to lower amphibolite facies. It spatially coincides with the nearly N-S trending shear zone restricted to its hanging wall. Types and details of features of the enclosing rocks are ·discussed. Structures dominating in the area and their possible relation to the mineralizing process are described. The ore zone and ore accommodating structures (fractures, veins, lenses, stringers and stockworks) are localized to the shear zone and generally trends parallel to major and minor structures in the area. The mineralization seems to be highly influenced by structures than lithology and other physico-chemical factors. Peculiarities related to the ore zone and ore bodies are elaborated. Extensive metasomatic alterations are developed in the enclosing rocks. Their types, disposition and relations to the mineralized zone and mineralization process are explained. Different ore minerals sulfides, sulfosalts, tellurides, tungestate and gold are identified. Their structure, texture, parageneses and zonal arrangement (especially the vertical one) are treated oased on field and laboratory (using polished sections) studies. Since sulfides are scarcely encountered, the treatment of geochemical data of sulfide forming metals was critical. This study was employed to systematically explain the nature of distribution of the sulfides. The most important sulfide forming elements (Pb, Zn, Cu, Ag, As, Sb, Mo, Co, Ni), their distribution (which can reasonably be explained by the presence or absence of their sulfides or sulfides of other metals which can host them) in relation to different lithologies and environments, their association in different environments and lithologies and their zonal arrangement along dip, strike and across the strike of the ore zone are explained. The distribution of the elements in relation to the distribution of the sulfides, the significance of each element as a tracer or best associate of gold are given. TIle details about spatial distribution and character of gold and sulfide rich zones are treated in this work. The mineralization is believed to be the result of regional metamorphism furnishing hydrothermal fluids which resulted in extensive metasomatic alterations and subsequent emigration, remobilization and redeposition of metals in the form of their sulfides. The reprecipitation was facilitated by the formation of low poT regime within and around dilatant zones created by movements along the eastern shear zone. Recommendations as what is to be done concerning further studies on the deposit and possible prospecting guides for gold in Adola area are suggested.
Hydrogeology of Jimma Area
(Addis Ababa Universty, 1994-06) Tadesse, Nata; Billi, Paolo
The studied area is located 335 Km. southwest of Addis Ababa in the southwestern Ethiopian plateau. Over half of the map-area is underlain by thick basalt flows inter layered with tuffs and volcanoclastic sediments. The Jimma Volcanics, which is mainly composed of massive rhyolite in thick flows alternating with trachytes, tuffs, ignimbrites and subordinate • • basalts, underlain small area in the eastern and northeastern part of the basin. Based on the hydrometeorologic data collected for the last 38 years an attempt ha~ been made to calculate the hydrological parameters like precipitatior, evapotranspiration, and runoff. The annual mean rain~All in the basin is 1476.89 mm. The values of the annual ,lotential e,vaportanspirtion and actual e,v apotranspiration are 837.51 mm and 822.29 mm respectively. The only runoff that leaves the studied area comes through Boye river and is 90.77 million cubic meter of water. The amount of water which is actually available to recharge the groundwater circulation within the hydrogeological basin is 51.14 million cubic meter. The hydrogeological character of the outcropping lithotypes was discussed with particular reference to infiltration and their water bearing capacity. The hydrogeology of the area varies in different parts of the basin. There is a great potential of groundwater in the valleys which is partly filled with alluvial sediments while a less significant amount exist within the volcanic rocks forming the uplifted borders of the valleys. Determination of the hydraulic parameters like transmissivity and permeability have been carried out. Due to the absence of observation well nearby a discharging wells it was impossible to calculate the coeff icient of storage, a significant ~arameter which measures the volume of water the aquifer releases~~ take into storage. For basaltic aquifers a transmissivity value of 18.39 m2/day and a permeability value of 3.31 x 10-4 cm/sec. have been calculated. A transmissivity value of 1.89 m2 /day. and a permeability value of 1.62 x 10-5 cm/sec. have been co.mputed for both alluvial and rhyolitic aquifers. Because of the lack of bore w~lls in the southwest, south, and southeast lower most part of the basin a thorough study of the actual variation of groundwater level was not possible. For the other part of the basin, appropriate groundwater level contour lines was constructed on the basis of the available static level data. Proper chemical analysis of both the groundwaters and surfaces water carried out in the basin show low total dissolved solids. All of the samples have less than 1000 mg/l dissolved solids. The analysed samples also compared with the acceptable standard for human, agricultural, and industrial use
Hydrogeology of Nazret
(Addis Ababa Universty, 1994-06) Ameneshoa, Woldu; Yirgu, Gezahegn (PhD)
The study area is found in the southern part of the northern sector of the Main Ethiopian Rift some 97 km southeast of the capital. It covers a surface area of about 370 sq.km. Elevation varies from below 1600 m.a.s.l. to more than 1970 m.a.s.l .. More than 70% of the area represents low lying flat land with slop~ of.1 to 10%. The area is covered by various igneous rocks which are products of volcanism_that spanned from Pliocene to Recent, and also by Quaternary sediments. The volcanics include; ignimbrites, basaltic flows, rhyolitic flows and domes, unwelded tuffs, pumice and ash deposits. Lacustrine sediments, alluvium and colluvium represent the non-volcanic deposits in the area. Mean annual rainfall of 822.53 mm and mean annual temperature of 21'C characterize the study area. According to water balance study over the small sub-catchment at the centre of the area, generally about 49% of the mean annual rainfall is lost through evapotranspiration. While about 20% is available for runoff and the remaining about 30% is accounted for infiltration to the ground. The main aquifers in the area are coarse grained lacustrine and alluvial sediments, pyroclastic rocks (cinders, volcanic sand and pumice), and weathered or fractured volcanic rocks mainly basalts and also somtimes ignimbiites From the pumping test data, 'analyes permeabities and transmissivities of rocks have been foundto vary: from O. 50 to 78.50 mid and 14.73 to 1355.20 m2/d respectively specific capacity also ranges from 0.26 to 9.13 mJ/hr/m. High permeability values are observed for the aquifers in the southern part of the area. The pumping test analyses also shol1ed that there is a recharge from AI1ash river to the aquifers at Melka Hida well field. Depth to groundwater level ranges from fel" meters in the southern part of the area to more than 170 meter in its northern part. From the groundwater contour, it can be observed that the general groundwater flow is to the south in the deeper aquifers. In the shallow aquifer zone (that is , southern part of the study area) groundwater flows towards Awash river in locally different directions from both sides of the river. The chemical analyses result showed that the ~Taters of the area are mainly bicarbonate type varying from sodiumbicarbonate to calcium-bicarbonate ones. The analyses also showed that there is high fluoride concentration above the acceptable limit for drinking in the groundwaters. Potential places for groundwater in the area are restr icted to its southern part. The remaining part is of limi ted potential not only due to 1011 permeabili ties of aquifers but also due to large depth to groundwater level. Finally, it seems that conjunctive use of groundwater and surface water from Awash river is the only solution to meet future water demand of the town and of its surrounding.
Application of Remote Sensing in . Lithological Mapping and Reconstruction of the Stratigraphy of North - western Mekele Area, Ethiopia.
(Addis Ababa Universty, 1995-05) Temesgen, Berhanu; Russo, Antonio (PhD)
The area under study is located within the Mekele out lier. It comprises Mesozoic sedimentary rocks unconformably overlain by trap volcanics. Landsat-TM images have been processed and geologically analysed in combination with revision of the previous works and present field survey for lithological mapping and reconstruction of the stratigraphy of the study area. In remote sensing the sensor and the obj ect to be sensed are located remotely apart. The link is due to electromagnatic (EM) waves. Thus in image processing and interpretation the knowledge of how EM radiation is generated, propagated, and modified when it interacts with materials is the main key. Materials produce a diagnostic spectral signature in response to the EM interaction. Generally, spectral signatures are related to the nature of the corresponding rock, which help to identi fy a material through image processing. Image processing includes statistical and graphical evaluation, ratioing, contrast enhancement, principal and canonical component analyses, combination of selected bands, texture and vegetation index analyses, classification, and visual interpretation of the images. Structures and other lineaments, lithological boundaries, and drainage patterns are traced directly from the screen during image interpretation. Ten mappable units are recognized both by digital image processing and visual interpretation. These are from youngest to oldest: Dolorite, Basalt, Amba Aradam Formation, Agula Shale, five separately mappable Antalo Limestone units, and Adigrat Sandstone. From sedimentary point of view, Adigrat Sandstone is fluviatile in origin, while the Antalo Limestone and Agula Shale are related with transgression and regression of the sea during Jurassic time. Amba Aradam Formation, on the contrary, lying unconformably on Agula Shale, is probably due to the withdral of the sea caused by an uplift of cratonic area related to the separation of America and Africa plates during the Cretaceous time. The final thematic images are geometrically rectified using selected ground control points to refer them with other maps. Digitally processed images expose fairly well the main lithologic units with some limitation.
Hydrological and Hydrogeological Study on the Kulfo River Basin
(Addis Ababa Universty, 1996-06) Masresha, Paulos; Jeschofnig, Peter (Professor)
The study area, the Kulfo River basin, is located 500 km south of Addis Ababa. It is bounded on the basis of the surface divide of the Kulfo River. The area is composed of part of the rift floor and part of the western escarpment. More than 85% of the area is underlain by Oligocene basalt and the lowland areas by alluvium and Quaternary basalt. Using thrE;!e meteorological stations, the weighted mean annual rainfall of 1232 mm was computed by using the Thiessen Polygon method. The Thornthwaite method was applied to calculate the potential and actual evapotranspiration of the area, which are 971 mm and 940 mm, respectively. From the above data, the surplus water was obtained, which is 291 mm. The mean annual runoff depth of the Kulfo river is 361 mm which is less than the calculated surplus water. The reason might be that there is an inflow of groundwater from an adjacent basin. Concerning the aquifer characterstics of the different rock units, the alluvium is more productive than the other rock units. One representative well from AWn was taken to calcualte the hydraulic parameters of this unit. Using the discharge data from Arbaminch Springs, the general hydrological conditions of the springs were computed. This includes the computation of the recession constants and the recession curve for the springs. The hydraulic volume which feeds the springs instant by instant were also computed. The study of the chemistry of the springs was based on data from WSSA's Arbaminch water supply project. This study confirms that the quality of the water is suitable for drinking and agricultural purposes according to international standards. However, the presence of E. coli bacteria indicates that the water needs to be chlorinated on a regular basis.
The Gravity Field Southern Ethiopian Rift Systems Between 3.30 to 7.150 Nlatitude and 360 to 420 Elongitude
(Addis Ababa Universty, 1997-05) Asrat, Estifanos; Alemu, Abera (PhD)
1800 gravity data covering the research area are redilrc t·) f

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