Journal of African Earth Sciences 99 (2014) 568–580

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Journal of African Earth Sciences

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Characterizing weathering intensity and trends of geological materialsin the Gilgel Gibe catchment, southwestern Ethiopia

http://dx.doi.org/10.1016/j.jafrearsci.2014.05.0121464-343X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +251 91 1407553; fax: +251 11 1239462.E-mail address: asfawossen.asrat@aau.edu.et (A. Asrat).

Alemayehu Regassa a,b, K. Van Daele a, P. De Paepe a, M. Dumon a, J. Deckers c, Asfawossen Asrat d,⇑,E. Van Ranst a

a Department of Geology and Soil Science (WE13), Ghent University, Krijgslaan 281/S8, B-9000 Gent, Belgiumb Department of Natural Resources Management, Jimma University College of Agriculture and Veterinary Medicine, Ethiopiac Department of Earth and Environmental Sciences, KU Leuven University, Celestijnenlaan 200E, B-3001 Heverlee, Belgiumd School of Earth Sciences, Addis Ababa University, P.O. Box 1176, Addis Ababa, Ethiopia

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 June 2013Received in revised form 13 May 2014Accepted 20 May 2014Available online 9 June 2014

Keywords:Gilgel Gibe catchmentVolcanic rocksParent materialWeathering trendEthiopia

Detailed geological and geochemical characterization is crucial to support soil studies in such geologicallyand topographically complex systems as the Gilgel Gibe catchment in southwestern Ethiopia. Field stud-ies, as well as mineralogical, petrological and geochemical analyses on selected rock samples and theirweathering products revealed that the catchment is dominantly underlain by rhyolites and trachytes,which occur as both lava flows and pyroclastic associations. Most of the lavas have a trachytic texture,while few others are massive or show spherulitic or perlitic texture. The rocks have a SiO2-content rang-ing from about 62 to 73 wt% (intermediate to felsic composition, on an anhydrous base) and a relativelyhigh Na2O + K2O content ranging from about 9 to 12 wt% (anhydrous base). The dominant phenocrystspresent in the rocks are plagioclase, sanidine and Fe–Ti oxide minerals. Alkali-rich amphiboles and quartzoccur in most of them, while hornblende, titanite and clinopyroxene are rare. The amount of phenocrystsvaries from less than 1 vol.% to about 30 vol.%. The pyroclastic associations are discontinuously scatteredwithin the study area. They all have a glassy matrix (vitrophyric texture) and are composed of a mixtureof lithics, crystals and glass. In comparison with the lava samples, the pyroclastic samples exhibit a morevariable chemistry. In contrast, the X-ray diffractograms of the pyroclastic deposits and the lavas showlittle difference. The Chemical Index of Alteration values for the studied samples vary from 53 to 99 indi-cating moderate to high intensity of weathering. Samples from lava flows have shown less degree ofweathering than samples of the pyroclastic associations.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Characterization of geological materials is crucial in soil genesisstudies, as parent material (i.e., petrographic, geochemical andmineralogical composition) has long been recognized as a funda-mental factor in soil formation (Dokuchaiev, 1879; Jenny, 1941;Wilson, 1975; Paton, 1978). Several studies have shown that par-ent material has a strong influence on chemical weathering rates,profile depth, clay content and cation exchange capacity (Vaselliet al., 1997; Venturelli et al., 1997; Dekayir and El-Maataoui,2001; Palumbo et al., 2000; Driese et al., 2003). Furthermore, thechemical and mineralogical properties of the rock–soil interfaceconstitute essential information useful for studying soil genesis.Weathering characterization of parent rock and hillslope deposits

is also important for the development of models linking tropicalweathering processes and the slope instability allowing better pre-diction and control of landslides (Aristizabal et al., 2005). Many ofthe engineering properties of a rock mass also result from changesin the chemical and physical properties produced by weathering(Gupta and Rao, 2001).

In the Gilgel Gibe catchment, in southwestern Ethiopia, onlyvery limited geological information is available from a few regionallevel studies. Notable among such studies encompassing thecurrent study area include the geological mapping and related geo-logical survey of the southwestern part of the country (Davidsonet al., 1976; Davidson and Mcgregor, 1976; Moore and Davidson,1978; Davidson and Rex, 1980) and the Omo-Gibe River BasinMaster Plan study by Ministry of Water Resources (Woodrofe,1996). Limited medium scale geological mapping of selected areashas also been conducted by the Geological Survey of Ethiopia inconnection with studies of coal, lignite and oil shale deposits in

A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580 569

the Delbi-Moye (Bae et al., 1989, 1991; Getahun et al., 1993;Wolela Ahmed, 1995) and Gojeb–Chida–Lala areas (Yirga andZewdineh, 1995), immediately south of the study area. Despitethe geographical proximity of these basins to the study area(located at a distance of about 50–80 kms), the deposits are con-fined to localized sedimentary basins that lack appreciable regionalextent.

A number of studies on soils and geological materials, such asinvestigation of the causes and spatial distribution of landslidesin the catchment and investigation of the sources of sedimentloads to the rivers, have been conducted in the Gilgel Gibe catch-ment with little background information about the detailed geol-ogy of the catchment (Van Daele, 2011). Our field observationsduring soil survey activities in the catchment provided importantground-truthing for understanding the distribution of the soilsover the landscapes. These revealed that the soil distribution inthe catchment is closely associated with the geological setup. Ver-tisols, Nitisols, Ferralsols and Planosols derived from various typesof parent materials indicate the complexity of the geology, and itsinfluence on soil genesis. In addition, all the rocks in the catchmentare characterized by deep weathering profiles. Detailed investiga-tion of the geological materials and their weathering productshas therefore been conducted in the Gilgel Gibe catchment withthe aim of elucidating their petrographical, mineralogical andgeochemical features, in order to characterize the rock-to-soilalteration processes and the intensity and trends of weathering.

2. Regional geological setting

The study area occurs within the Omo-Gibe River basin, thelargest basin in southwestern Ethiopia. The Omo-Gibe River basincomprises a Precambrian crystalline basement, covered by Tertiaryvolcanic rocks, with isolated exposures of Quaternary volcanicrocks, lacustrine deposits, and alluvial sediments (Davidson,1983). Close to 90% of the basin is underlain by Tertiary volcanicrocks. No Paleozoic and/or Mesozoic sedimentary successions areexposed in this basin.

Davidson (1983) broadly divided the Tertiary volcanic rocks insouthwestern Ethiopia into pre-rift and post-rift successions. Thepre-rift succession comprises the oldest main sequence (49–35 Ma) and the younger flows (32–21 Ma). The main sequence(basalt and rhyolite flows) overlies the crystalline basement withbasal red sandstone in between, the latter deposited on a peneplainduring early Tertiary. The younger flows either lie above the oldestvolcanic sequence or rest directly on the crystalline basement. Thepost-rift sequence (19 Ma to present) comprises the mid Mioceneflood basalts, which lie unconformably on tilted pre-rift flows,overlying silicic lavas and pyroclastic rocks, phonolite and alkalitrachyte flows with minor intrusions, and Quaternary volcanics.Felsic volcanic rocks occupy a dominant portion of the Tertiary vol-canic succession within the study area (Fig. 1). A thick successionof felsic volcanic flows, pyroclastic rocks and subordinate interca-lated basalt flows dominate the upper part of the pre-rift volcanicsuccession while rhyolitic and trachytic flows, and associatedignimbrites are the dominant post-rift succession in the study area.

Some studies, using K–Ar age determinations and field relationshave divided the volcanic rocks of southwestern Ethiopia into threeFormations: Omo basalts (40–25 Ma), Jima volcanics (37–11 Ma)and Wollega basalts (15–7 Ma) (Merla et al., 1979; Berhe et al.,1987; Tsegaye, 1997). The Jima volcanics, the dominant successionin the study area, form a thick succession of basalts and felsic rockswith basalts dominating the lower part. Two units (Jima basaltsand Jima rhyolites), which show a conformable relationship, wereidentified. The Jima volcanics often rest on the Precambrian base-ment, the unconformity being marked by basal residual sandstone.

Silicic volcanic centres, such as Tambero, Wagabeta and Amberi-cho, which straddle the boundary between the Omo-Gibe andthe Rift valley drainage systems, are comprised of interlayered,per-alkaline ignimbrites and trachytes. The age of these volcanoesrange from 4.5 to 3.0 Ma (Kazmin et al., 1980). Kazmin et al. (1980)also reported the occurrence of recent eruptive centers and associ-ated flows in the basin in four main areas: the Korath range, westof Omo River; the Tepi basalt shield, west of Jima; the WumbaHayk basalt, south of Chida town and the Woliso basalt. Thoughage of eruption of these centers has not been radiometrically deter-mined, Kazmin et al. (1980) suggested late Pleistocene to Holoceneages based on field associations.

3. Location and environmental setting of the study area

The study area, Gilgel Gibe catchment, is located between7�2207200 and 7�3408400N latitude and 37�2100500 and 37�2808000Elongitude, on the southwestern highlands of Ethiopia (Fig. 2). Thecatchment covers an area of about 4225 km2. The altitude in thecatchment varies between 1096 and 3259 m a s l. It is generallycharacterized by warm and humid sub-tropical climate. Meanannual rainfall is ca. 2000 mm, about 60% of the rainfall occurringin the rainy season, lasting from May to September. Mean annualair temperature is ca. 19 �C (National Meteorological Agency ofEthiopia, 2009).

The catchment area comprises topographic features rangingfrom rugged and hilly terrains to flat plains. The physiography ofthe catchment is generally controlled by the tectono-volcanicsystems, which have been active during Middle Tertiary in thewestern and southwestern parts of Ethiopia. On a smaller scale,such factors as denudation caused by rivers, and amelioration ofrelief caused by deposition, play some role. The North–South andEast–West trending perennial and intermittent streams are con-trolled by the master drainage system of Gibe River. Gibe Riverarises at an elevation of 2000 m a s l. and flows to the southeastalong a fault controlled valley until it turns to the south along amajor North–South running fault trace, draining the deep gorgeof the Omo River.

The main land use type in the study area is agricultural crop-ping, mainly wheat, teff, barley, faba bean, sorghum and maize.The farmers also keep certain plots as grazing land next to the cropfields. The major soils of the study area include typical wet tropicalsoils such as Nitisols which are dominant in the more hillyareas while Planosols and Vertisols form the dominant soil typesin the lower lying, level areas in the catchment (Van Ranst et al.,2011).

4. Methods

4.1. Field sampling

Reconnaissance survey along all available access roads allowedappraising the geological setting of the catchment and to selectrepresentative transects and exposures for detailed observationand sampling. Eleven suitable outcrops were selected from variousparts of the catchment (Fig. 2). The outcrops were described andphotographically documented. A total of 46 samples consisting of26 fresh rocks, 15 soft rocks and 5 soil samples were collectedfor petrographic, geochemical and mineralogical analyses. Whenavailable, a fresh rock with a weathering rim was sampled; in othercases, fresh and weathered rock samples were collected from thesame exposure. On sites where rock–soil interface was clearlyexposed, multiple samples were taken along a profile going fromfresh rock to saprolite and eventually soil material. The sampleswere documented in the field following systematic numbering.

Fig. 1. Simplified geological map of the Gilgel Gibe catchment (adapted from the Geological map of the Omo-Gibe river basin, Woodrofe, 1996).

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Table 1 provides locations and brief description of the collectedsamples. The relevant petrological and field associations of thesamples were also noted and photographed.

4.2. Petrographic, mineralogical and geochemical analyses

All the petrographic, mineralogical, and geochemical analyseswere conducted in the laboratory of Soil Science at Ghent Univer-sity, Belgium following standard methods (Van Reeuwijk, 1993). Atotal of 39 thin sections were prepared for petrographic studies, 33from hard rock and 6 from soft rock samples. The thin sectionswere studied under polarizing microscope, microphotographswere taken, and appropriately described following Pichler andSchmitt-Riegraf (1997) and Mackenzie et al. (1991).

For mineralogical studies, rock samples were broken in jawcrusher until a grain size of about 1 cm (coarse sample) wasobtained. These coarse samples were further pulverized to powdersamples with an agate mill. The sample was placed into an agatebeaker together with 6 agate balls and clenched in the mill. Miner-alogical compositions of the materials were determined by X-RayDiffraction (XRD) of the whole powder samples. Analyses wereperformed by using a Philips X’PERT SYSTEM with a PW 3710

based diffractometer equipped with a Cu tube anode, a secondarygraphite beam monochromator, a proportional xenon filled detec-tor, and a 35 position multiple sample changer for randomly ori-ented powders. Mineral proportions were evaluated based on thesurface areas observed between peaks. The presence of clay miner-als was screened qualitatively by checking for the presence ofbroad peaks in the low angle region (between 3� and 8�2h) of thediffractogram and looking for 020-reflections (around 20�2h).

Major elements were analyzed by an Inductively CoupledPlasma Atomic Emission Spectroscopy (ICP-AES) on pulps after0.2 g of rock-powder was fused with 2 g of lithium metaborateand then dissolved in 100 ml, filtered through a pre-washed 10%HNO3. Loss on ignition (LOI) represents the weight loss followingheating to 1000 �C.

5. Results

5.1. Petrography

On the basis of detailed field and petrographic observations, thestudied rock samples can be grouped into three broad categories:lavas, pyroclastic deposits and a hydrothermally altered rock.

Fig. 2. Map of the Gilgel Gibe catchment with locations of the studied sections and samples.

A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580 571

5.1.1. Lava flowsMost of the studied samples from the felsic lavas exhibit dis-

tinct trachytic textures. Total phenocryst abundance is highly var-iable ranging between 1% and 30% by volume. The dominantphenocrysts are plagioclases and sanidine. Most phenocrysts showeuhedral to subhedral forms with rare anhedral varieties. Thegroundmass ranges from aphanitic to glassy. The feldspar pheno-crysts are clustered in some samples yielding a glomeroporphyritictexture (Fig. 3A). Moreover, plagioclase shows typical polysynthet-ic twinning, and sector and patchy zoning in most studied samples.Some plagioclases show sieve textures containing abundant, small,interconnected, box-shaped glass inclusions, giving the crystal aspongy or porous appearance (Fig. 3B). Sanidine phenocrysts(Fig. 3C) with typical carlsbad twinning are also common, indicat-ing the coexistence of plagioclase and sanidine phenocrysts.Amphibole microlites (2–25 vol%) are found in most samples, withhornblende rarely enclosed in feldspar phenocrysts. Many samplescontain subhedral to anhedral opaque minerals (possibly magne-tite or titanomagnetite, based on their crystal shape) varyingbetween 5 and 20 vol.%, occurring as microphenocrysts and micro-lites and as inclusions in macrophenocrysts and microphenocrystsof feldspars. Titanite microphenocrysts (62 vol.%) occur in fewsamples as euhedral to subhedral crystals with high birefringence.

Pale brown, isotropic glass with some opaque inclusions occurs insignificant amounts (up to 10 vol%) in some samples. Clinopyrox-ene microlites occur only in a single sample. One rhyolite samplecontains a glassy groundmass with a typical vitrophyric texture,which often shows devitrification. A large part of the sample con-sists of shrinkage induced spherical fractures (perlitic cracks;Fig. 3D) and spherulites. Some broken subhedral to anhedralmacro- and microphenocrysts of feldspar (sanidine and plagio-clase; 50 vol.%), clinopyroxene (30 vol.%) and quartz (20 vol.%) arealso observed. In this sample, an intimate micrographic inter-growth of quartz and alkali feldspar (Fig. 3E) is also presentwhereby the quartz appears as isolated wedges and rods in thefeldspar.

Two rhyolite samples collected at Marawa quarry show a vitro-phyric texture with a fine-grained matrix of brownish glass withphenocrysts. The glass is mostly converted into spherulites of radi-ally arranged fibres of quartz or low-cristobalite and alkali feldspar(Fig. 3F and G). Three rhyolites from Busase Elen have no distinctflow texture, though they show a similar mineralogical associationto those with flow textures. The groundmass consists of feldspar(plagioclase and sanidine), Fe–Ti oxides and clinopyroxenemicrolites. The feldspar microlites (30–85 vol.%) are subhedral toanhedral and most of them are lath-shaped.

Table 1Sample locations and general description of collected samples.

Outcrop site Sample # X Y Z (m) Field name Material property

Dora 1-1 37301775E 0860994N 1759 Ignimbrite Hard rock, slightly weathered1-2 37301775E 0860994N 1759 Ignimbrite Hard rock, slightly weathered

Near Manisa stream 2-1 37301791E 0862052N 1732 Rhyolite Soft rock, strongly weathered2-2 37301791E 0862052N 1732 Rhyolite Hard rock, weathered

Dibu Diba 3-1 37265549E 0827586N 2118 Rhyolite Hard rock, slightly weathered

Geshe Volcano 4-1 37263055E 0825134N 2909 Trachye Soft rock4-2 37263055E 0825134N 2909 Rhyolite Soft rock4-3 37263055E 0825134N 2909 Trachyte Hard rock, weathered4-4 37263055E 0825134N 2909 Rhyolite Hard rock, slightly weathered4-5 37263055E 0825134N 2909 Umbrisol Soil material4-6 37263055E 0825134N 2909 Umbrisol Soil material4-7 37263055E 0825134N 2909 Rhyolite Hard rock, slightly weathered4-8 37263055E 0825134N 2909 Rhyolite Hard rock, slightly weathered

Bulbul 5-1 37289244E 0853060N 1720 Rhyolite Hard rock, weathered5-2 37289244E 0853060N 1720 Rhyolite Hard rock, slightly weathered5-3 37289244E 0853060N 1720 Rhyolite Hard rock, weathered5-4 37289244E 0853060N 1720 Rhyolite Hard rock, slightly weathered5-5 37289244E 0853060N 1720 Rhyolite Hard rock, slightly weathered5-6 37289244E 0853060N 1720 Trachyte Hard rock, slightly weathered

Goro Sibilu 6-1 37306324E 0847836N 1745 Welded tuff Soft rock6-2 37306324E 0847836N 1745 Welded tuff Soft material6-3 37306324E 0847836N 1745 Welded tuff Hard rock, strongly weathered6-4 37306324E 0847836N 1745 Welded tuff Soft rock6-5 37306324E 0847836N 1745 Welded tuff Soft rock

Marawa 7-1 37269650E 0851151N 1849 Rhyolite Soft rock7-2 37269650E 0851151N 1849 Nitisol Soil material7-3 37269650E 0851151N 1849 Rhyolite Soft rock7-4 37269650E 0851151N 1849 Nitisol Soil material7-5 37269650E 0851151N 1849 Rhyolite Hard rock, weathered7-6 37269650E 0851151N 1849 Rhyolite Hard rock, weathered7-7 37269650E 0851151N 1849 Rhyolite Hard rock, weathered7-8 37269650E 0851151N 1849 Tuff Soft rock7-9 37269650E 0851151N 1849 Tuff Soft rock7-10 37269650E 0851151N 1849 Trachyte Hard rock, weathered7-11 37269650E 0851151N 1849 Trachyte Hard rock, weathered

Bala Wajo 8-1 37297607E 0856662N 1715 Welded tuff Hard rock, slightly weathered8-2 37297607E 0856662N 1715 Welded tuff Soft rock8-3 37297607E 0856662N 1715 Welded tuff Hard rock, slightly weathered

Sajo Adami 9-1 37304364E 0850971N 1730 Basalt Hard rock, slightly weathered

Busasse Elen 10-1 37276376E 0831213N 1887 Nitisol Soil material10-2 37276376E 0831213N 1887 Trachyte Hard rock, slightly weathered10-3 37276376E 0831213N 1887 Rhyolite Hard rock, slightly weathered10-4 37276376E 0831213N 1887 Trachyte Hard rock, slightly weathered10-5 37276376E 0831213N 1887 Rhyolite Hard rock, slightly weathered

Kawa 11-1 37285841E 0834188N 1771 Rhyolite Soft rock11-2 37285841E 0834188N 1771 Rhyolite Soft rock, strongly weathered11-3 37285841E 0834188N 1771 Rhyolite Soft rock

572 A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580

A rhyolite sample from Marawa quarry (sample 7-6) is differentfrom most lava flows as it shows clear perlitic cracks at macro-scale and micrographic texture (fine-grained intergrowth of quartzand alkali feldspar) at microscopic scale (Fig. 3I). Two other rhyo-lite samples from the same quarry (samples 7-5 and 7-7) are themost felsic samples of all and both exhibit clear spherulitic texture,suggesting an increase in temperature and/or hydration aftercooling of the original glassy rock, allowing it to recrystallize(Davis and McPhie, 1996; Fowler et al., 2002). These spheruliticas well as perlitic textures could possibly suggest widespreadhydrothermal events in the study area.

5.1.2. Pyroclastic rocksThe pyroclastic rocks have a glassy matrix with vitrophyric tex-

ture, and most are either tuffs or lapilli-tuffs. The groundmass con-sists mostly of glass with some devitrification features andweathering to clay minerals. Feldspars, quartz and amphibolesand rarely olivine and clinopyroxene, and Fe–Ti oxides are themost common primary mineral constituents of these rocks. Broken

crystals and angular lithic fragments are found in most samples(Fig. 3H) while pumice fragments occur in some samples. The sam-ples are classified as lithic-crystal tuff (pumice tuff) (Schmid,1981), containing up to 30 vol.% of pumice fragments of variablesizes, the glassy matrices often converted into variously coloredclay minerals. The matrix has dominantly a cryptocrystalline tex-ture (Fig. 3J) due to consolidated ash. Devitrification featuresincluding spherulites are found in some samples. Pumice frag-ments (Fig. 3K) occur as pale yellow glass fragments of differentsizes. Many of these pumice fragments are slightly weathered toclay minerals, which appear in the cavities of the pumice, makingthe rocks denser. Most visible lithic fragments are rather small(about 1 mm) (Fig. 3L), except in one sample where large lithicsconstitute about half of the sample volume. The ignimbrite sam-ples are glassy welded tuffs where the glass fragments show devit-rification features including spherulites. Crystals are embedded inthe glass matrix and sometimes in elongated glass fragments. Thefeldspar crystals (1.5–4.0 vol.%) are subhedral to anhedral. Bothsanidine and plagioclase are present as large crystallites. Opaque

Fig. 3. Some petrographic features of the bedrocks: A. Sample 4-8 (xpl): glomerocryst consisting of feldspar minerals; B. Sample 4-1 (xpl): plagioclase with a sieve texture; C.Sample 4-2 (xpl): sanidine phenocryst with typical calsbad twinning; D. Sample 7-6 (ppl): perlitic cracks; E. Sample 7-6 (xpl): micrographic intergrowth; F. Sample 7-7 (xpl):spherlutic texture; G. Sample 2-2 (xpl): plagioclase with a microcline-like texture in the centre; H. Sample 7-11 (ppl): broken crystals, which are typical of pyroclasticdeposits; I. Sample 7-6 (xpl): ophimottled texture; J. Sample 2-1 (xpl): cryptocrystalline texture with low birefringence minerals; K. Sample 7-10 (xpl): pumice fragments; L.Sample 7-10 (ppl): lithic fragments.

A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580 573

minerals are present in the groundmass and consist of subhedral toanhedral small crystallites.

One hydrothermally altered sample collected near the Manisastream is composed almost exclusively of kaolinite and quartz,while feldspars are absent. It has a light yellow, cryptocrystallinematrix embedding anhedral quartz (Fig. 3H). The quartz crystalsin this sample are broken, corroded and exhibit perforatedtextures.

Fig. 4. Classification of the ‘fresh’ rocks in a TAS diagram (total alkali contentagainst silica content; Le Maitre et al., 2002).

5.2. Mineralogy

Mineralogical composition of rock samples was determined byXRD analysis of the total samples and clay fractions (Figs. 5 and6). X-Ray Diffraction patterns for samples 4-3A&B are shown inFig. 5. Except for four samples (the hydrothermally altered, andthree pyroclastic samples from the same location) all samplesshow high feldspar content. Most lava and pyroclastic samplescontain quartz, with the pyroclastics containing more quartz.Clinopyroxene was observed in most of the analyzed samples.Among the clay minerals, smectite is found in almost all rocksfollowed by kaolinite. Mica, chlorite, and vermiculite are not com-mon except in few samples. A broad peak in the 20–35�2h range

indicating the presence of glass is found in both the lava and pyro-clastic deposits. The hydrothermally altered rock consists only ofquartz, kaolinite and small amounts of smectite.

Fig. 5. Total sample (<2 mm) XRD-patterns of samples 4-3A and B (Fsp = feldspar, Qz = quartz).

Fig. 6. XRD-patterns for the clay fraction of sample 7-11 (Sme = Smectite;Kln = Kaolinite, Crs = Cristobalite).

574 A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580

5.3. Geochemistry

The major elements analysis (Table 2) indicates that SiO2 valuesrange from 41.7% to 81.0% (anhydrous basis). The lava flow sam-ples show little variation of major elements content, except abasaltic rock (sample 9-1 from a Sajo Adami outcrop) and to a les-ser degree a trachyte sample (sample 10-4 from Busase Elenquarry), which show significantly higher CaO content than theother samples. Moreover, a rhyolite rock from Marawa (sample7-6) has lower Na2O and K2O contents and higher LOI values,which could be an indication of a higher degree of alteration.

Two almost entirely spherulitic rhyolite rocks from Marawa quarry(samples 7-5 and 7-7) have markedly higher SiO2 contents. Threesoft, rather weathered rhyolite rock samples from Kawaa (samples11-1, 11-2 and 11-3) have high MgO contents, about three timeshigher compared to the other samples. Two rhyolite rock samplesfrom Bulbul (samples 5-1 and 5-2) show very low MgO contents.

The pyroclastic samples on the other hand show significantchemical variation. Most samples have been altered and have highLOI values, and low Na2O and K2O contents. A less weatheredignimbrite rock sample from Dora (sample 1-1) has a high Fe2O3

content.A sample of Nitisol soil has much higher FeO� and Al2O3 con-

tents. Two other soil samples (umbrisols) from the top of the Geshevolcano (samples 4-5 and 4-6) and a Nitisol soil sample fromBusase Elen (sample 10-1) have lower Na2O and K2O contentsand higher LOI values. Some major elements (e.g., K2O, CaO, FeO�

and MgO) show slight correlation with SiO2 concentrations;however, no real trends are observed.

Samples with LOI values less than 5% have been classified onTotal Alkalis – SiO2 (TAS) diagram of Le Maitre et al. (2002)(Fig. 4). The rocks have SiO2 contents ranging from 62.5 to73 wt% (anhydrous base) and fall within the intermediate (SiO2:65–52 wt%) to felsic (SiO2 > 65 wt%) composition fields, and rhyo-lite is the most common followed by trachyte/trachydacite.

Table 3 presents the calculated CIPW-norm of the sixteen freshrock samples. According to the norm, in all samples, feldspar min-erals (plagioclase and orthoclase) are the dominant group, fol-lowed by quartz. Plagioclase consists almost completely of albiteand negligible amount of anorthite. Hypersthene, ilmenite, apatiteand zircon also occur in all rocks. Small amounts of corundum,diopside, aegerine, sodium metasilicate and magnetite are presentin some of the samples.

6. Discussion

6.1. Lithological, mineralogical and geochemical variation and fieldrelationships

Most of the lava samples show porphyritic and trachytictextures. The petrogenetic history of the rocks in the study areacannot be determined based on the few samples analyzed, whichshow little compositional variation. However, the petrographic

Table 2Total Major element analyses. Oxides are expressed as weight percentages, P: pyroclastic deposit, L: lava flow, O: other.

Oxide 1-1A P 1-1B P 1-2A P 1-2B P 2-1 O 2-2A L 2-2B L 3-1 O 4-1 L 4-2 L 4-3A L 4-3B L 4-4A L 4-4B L 4-5 L

SiO2 66.03 65.74 70.82 72.24 68.09 69.35 61.07 32.16 64.59 67.88 65.50 63.40 69.93 68.99 51.09Al2O3 10.03 10.04 10.84 11.32 20.06 14.15 14.93 22.96 18.24 16.26 17.31 16.26 14.42 14.72 11.98FeO� 9.75 9.70 4.99 3.65 1.42 3.66 8.27 18.48 2.70 2.60 2.36 5.37 2.57 2.95 4.30MnO 0.33 0.33 0.14 0.15 0.04 0.05 0.05 0.41 0.03 0.05 0.03 0.04 1.20 0.09 0.08MgO 0.27 0.27 0.27 0.24 0.09 0.07 0.13 0.40 0.04 0.05 0.03 0.05 0.05 0.06 0.28CaO 0.41 0.41 0.31 0.30 0.14 0.28 0.42 0.19 0.11 0.11 0.46 0.50 0.11 0.11 0.32Na2O 4.06 4.04 4.71 4.50 0.02 5.36 3.53 0.02 5.10 5.70 7.16 6.44 6.70 6.48 2.57K2O 4.08 4.07 4.41 4.58 0.04 4.83 3.95 0.02 3.76 4.13 5.13 4.29 4.06 3.81 2.17TiO2 0.53 0.53 0.57 0.60 0.21 0.80 1.11 2.74 0.22 0.16 0.49 0.47 0.13 0.15 0.43P2O5 0.14 0.14 0.05 0.08 0.03 0.12 0.20 0.24 0.04 0.06 0.05 0.12 0.02 0.03 0.20LOI 4.51 4.13 2.19 2.37 9.28 1.15 5.52 22.05 4.71 2.75 1.94 3.50 1.48 2.66 27.13

Total 100.14 99.39 99.31 100.03 99.41 99.80 99.19 99.65 99.54 99.75 100.46 100.44 100.66 100.05 100.56

Oxide 4-6 L 4-7A L 4-7B L 4-8A L 4-8B L 5-1 P 5-2 L 5-3A P 5-3B P 5-5 L 5-6 L 5-7 L 6-1 P 6-2 P 6-3 P 6-4 P

SiO2 53.69 70.69 68.50 67.16 68.21 70.54 67.56 69.77 69.83 69.16 63.45 68.79 64.07 48.35 45.79 56.47Al2O3 13.89 14.81 16.07 15.84 14.94 12.96 13.29 13.24 11.37 13.83 11.76 13.69 11.96 20.88 15.11 15.43FeO� 4.41 2.06 1.84 2.76 5.02 2.08 4.51 3.32 5.93 2.85 9.05 3.07 7.70 11.98 18.32 11.42MnO 0.07 0.05 0.04 0.09 0.08 0.23 0.36 0.02 0.05 0.09 0.80 0.02 0.23 0.15 2.85 0.12MgO 0.23 0.02 0.03 0.04 0.03 0.01 0.04 0.01 0.01 0.02 0.08 0.05 0.21 0.43 0.20 0.24CaO 0.17 0.08 0.13 0.10 0.10 0.07 0.16 0.13 0.11 0.14 0.33 0.18 0.36 0.17 0.08 0.26Na2O 2.87 7.14 6.99 6.60 6.09 5.07 5.70 5.44 4.54 5.94 5.34 5.60 1.41 0.41 0.28 1.11K2O 2.36 4.31 4.24 4.00 3.84 4.55 5.07 5.00 4.24 5.30 4.66 5.17 3.46 1.32 0.78 1.79TiO2 0.42 0.10 0.13 0.13 0.10 0.62 0.69 0.68 0.59 0.72 0.64 0.70 0.43 1.30 1.08 1.17P2O5 0.17 0.07 0.09 0.02 0.03 0.03 0.03 0.03 0.03 0.03 0.02 0.02 0.05 0.07 0.06 0.03LOI 22.14 0.56 1.49 2.48 1.68 3.48 1.87 1.59 2.71 1.18 3.93 1.98 9.49 14.69 14.84 11.60

Total 100.42 99.89 99.56 99.22 100.11 99.63 99.28 99.23 99.42 99.27 100.06 99.26 99.37 99.74 99.39 99.64

Oxide 6-5 P 7-1 P 7-2 P 7-3 P 7-4 P 7-5A L 7-5B L 7-6 L 7-7A L 7-7B L 7-8 P 7-9 P 7-10 P 7-11 P 8-1 P

SiO2 64.05 61.35 48.93 52.45 53.45 72.67 73.57 55.40 72.89 72.64 52.09 61.04 73.50 55.82 58.45Al2O3 12.65 17.09 19.01 17.72 20.68 11.81 12.21 16.38 12.24 11.93 20.15 13.98 8.81 14.96 15.96FeO� 6.20 5.01 9.96 7.66 9.31 2.96 2.59 5.91 3.35 3.66 7.27 8.34 3.65 7.18 3.81MnO 0.11 0.16 0.20 0.06 0.22 0.01 0.02 0.05 0.01 0.01 0.04 0.09 0.04 0.03 0.10MgO 0.23 0.35 0.22 0.13 0.29 0.05 0.04 0.81 0.08 0.06 0.42 0.81 0.32 0.58 0.35CaO 0.47 0.40 0.11 0.04 0.04 0.09 0.08 1.24 0.14 0.12 0.64 0.62 0.89 0.98 0.73Na2O 1.10 1.37 0.86 0.51 0.59 2.34 2.51 0.53 3.09 2.92 1.29 0.49 0.79 0.69 1.01K2O 3.06 2.21 1.38 0.75 1.08 6.89 6.89 0.96 5.59 5.72 2.86 3.05 2.47 0.54 2.59TiO2 0.41 0.47 0.73 0.60 0.88 0.42 0.43 0.42 0.44 0.45 0.41 0.47 0.21 0.60 0.42P2O5 0.03 0.01 0.04 0.04 0.05 0.02 0.02 0.08 0.02 0.02 0.06 0.01 0.01 0.01 0.02LOI 10.98 11.78 17.75 19.39 12.98 2.22 1.98 17.67 2.37 2.57 14.68 11.22 9.26 18.20 16.82

Total 99.28 100.18 99.19 99.33 99.58 99.48 100.35 99.44 100.21 100.10 99.90 100.13 99.95 99.58 100.27

Oxide 8-2 P 8-3 O 9-1 L 10-1 L 10-2 L 10-3A L 10-3B L 10-4A L 10-4B L 10-5A L 10-5B L 11-1 P 11-2 P 11-3 P

SiO2 49.08 36.81 62.45 49.88 63.37 67.36 66.83 66.46 58.10 67.25 67.53 43.64 40.55 45.22Al2O3 18.25 11.52 14.62 20.80 18.06 14.56 14.44 14.50 16.80 14.68 14.45 17.87 16.63 19.59FeO� 6.10 27.09 5.80 10.57 4.41 3.08 3.90 3.01 3.77 3.24 3.36 12.46 13.16 8.29MnO 0.25 0.23 0.13 0.76 0.19 0.03 0.03 0.07 0.06 0.04 0.05 0.10 0.30 0.08MgO 0.57 0.47 0.34 0.27 0.18 0.17 0.18 0.22 0.31 0.22 0.22 1.29 1.13 1.48CaO 0.87 0.79 1.98 0.07 0.22 0.67 0.60 1.21 1.31 0.59 0.59 1.10 0.99 1.01Na2O 0.70 0.64 4.38 0.84 2.87 5.24 4.93 3.90 4.35 5.15 5.05 0.03 0.04 0.03K2O 1.07 0.75 4.57 1.45 3.50 4.72 4.64 5.24 3.20 4.74 4.67 0.25 0.33 0.23TiO2 1.43 1.56 0.96 1.54 1.04 1.04 1.07 1.02 1.23 1.08 1.05 1.84 1.40 1.94P2O5 0.05 0.69 0.27 0.09 0.08 0.16 0.11 0.18 0.12 0.04 0.04 0.10 0.15 0.06LOI 21.66 19.37 4.55 12.76 6.78 2.55 3.10 3.63 9.94 2.50 2.59 21.06 24.48 21.31

Total 100.03 99.92 100.06 99.03 100.70 99.58 99.84 99.44 99.20 99.53 99.61 99.73 99.16 99.26

A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580 575

features (presence of albite and perlite phenocrysts set in alkalirich matrix and glass, with little mafic minerals) and major ele-ment geochemical trends (enrichment in alkalis and silica) suggestformation of these silicic volcanic rocks from an evolved stage ofcrystal fractionation. Moreover, the Aluminum Saturation Index(ASI: molar ratio of Al2O3 to CaO + Na2O + K2O) values for the‘fresh’ rock samples (Table 4) are greater than 1, indicating a pos-sible assimilation of country rocks by the fractionating magma.

The outcrop at Dora contains ignimbrites (ash flow deposits),with many pumice fragments, some crystals but rare lithic frag-ments. The pumice fragments show an inverse grading suggestingthe deposit being situated close to the top of an ignimbrite flow(Sparks et al., 1973).

Petrographic and XRD analyses showed that the ash material ofthe Manisa outcrop consists almost entirely of quartz and kaolinite,with some small amounts of smectite, suggesting the rock has beenhydrothermally altered, metasomatizing the feldspar-rich materialto kaolinite and some smectites, but leaving quartz intact. Tworhyolitic samples (samples 7-5 and 7-7) from Marawa also showtraces of hydrothermal alteration, suggesting the possibility ofwidespread hydrothermal activity throughout the catchment.

The profile on top of Geshe Mountain is located at a highposition in the landscape and receives no colluvial deposition.Therefore, the soil profile likely represents a young weatheringprofile, largely undisturbed by down-slope movement. Samplestaken from this mountain are all trachytic lavas but have different

Table 3Calculated CIPW-norm of the ‘fresh’ rock samples.

Normative minerals 2-2A 4-1 4-2 4-3 4-4 4-7 4-8 5-2 5-5 5-6 5-7 7-5 7-7 9-1 10-4 10-5

Quartz 17.7 19.7 17.6 2.5 14.9 15.6 11.8 16.0 17.3 12.4 17.6 31.6 31.9 13.1 20.8 16.7Plagioclase 46.3 45.9 49.9 61.4 52.0 52.5 58.1 41.2 42.5 35.9 42.8 20.6 27.2 45.8 39.7 47.9Albite 46.0 45.5 49.8 61.4 52.0 52.5 57.7 41.2 42.5 35.9 42.8 20.3 26.7 39.1 34.7 45.1Anorthite 0.3 0.6 0.2 0.0 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.3 0.5 6.7 5.0 2.8Orthoclase 28.9 23.4 25.2 30.8 24.2 25.7 24.4 30.7 31.9 28.7 31.4 41.9 33.8 28.3 32.3 28.9Corundum 0.0 6.0 2.4 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.4 0.9 0.0 0.6 0.0Diopside 0.4 0.0 0.0 2.0 0.6 0.0 0.0 0.6 0.0 1.4 0.7 0.0 0.0 1.7 0.0 0.0Hypersthene 3.9 3.8 3.7 1.7 5.8 3.2 4.1 6.4 3.2 14.4 3.5 3.7 4.3 7.1 3.4 3.5Acmite/Aegerine 0.0 0.0 0.0 0.1 1.2 1.0 0.0 2.2 1.4 4.5 1.5 0.0 0.0 0.0 0.0 0.0Na2SiO3 0.0 0.0 0.0 0.0 0.9 1.7 0.0 1.4 1.7 1.4 1.0 0.0 0.0 0.0 0.0 0.0Ilmenite 1.5 0.4 0.3 1.0 0.3 0.2 0.3 1.4 1.4 1.3 1.4 0.8 0.9 1.9 2.0 2.1Magnetite 0.9 0.7 0.6 0.5 0.0 0.0 0.7 0.0 0.0 0.0 0.0 0.7 0.8 1.6 0.8 0.8Apatite 0.3 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.7 0.4 0.1Zircon 0.2 0.2 0.2 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.1 0.2 0.2

Total 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.1 100.2 100.1 100.2

Table 4ASI-index for the ‘fresh’ rock samples.

Sample ASI

2-2A 1.354-1 2.034-2 1.644-3A 1.364-4A 1.334-7A 1.284-8A 1.485-2 1.225-5 1.225-6 1.145-7 1.257-5A 1.277-7A 1.399-1 1.3410-4A 1.4010-4B 1.40

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weathering states. These samples contain feldspar crystals withglassy inclusions at the edge, skeletal alkali amphiboles and glass.

A rhyolite rock sample from Bulbul quarry (sample 5-3)contains lithic fragments larger than 5 cm. These are much largerthan the lithic fragments found in other samples. The welded tuffsamples from the Goro Sibilu quarry on the other hand are all wellsorted, except for the larger-sized pumice fragments, which have amuch lower bulk density than the crystals.

At Bala Wasjoa, petroplinthite layer is found. Plinthite com-monly forms in soils saturated with water for some time duringthe year. After exposure to the surface, plinthite can form petropl-inthite when dried and wetted repeatedly.

In general, the rocks in the study area can be grouped into twomajor types: trachytic and rhyolitic lava flows and their weather-ing products, and felsic pyroclastic deposits and their weatheringproducts. In addition, there are a few anomalous samples (e.g.,hydrothermally altered rock and petroplinthite). The pyroclasticdeposits cover a wider area with significant implication to theweathering and soil formation processes, as they serve as the com-mon protoliths of the soils in the area.

There is little difference between the bulk mineralogy of thelavas and pyroclastic deposits. However, the pyroclastic rocks aremore weathered compared to the lavas and consequently, moreclay minerals (mostly kaolinite and smectite) can be detected.The adapted CIPW-norm for highly weathered soils also exhibitssignificant amounts of kaolinite. Smectite is not part of this norm,as this mineral does not represent an ultimate weathering state,while kaolinite does. Due to its ability to swell and shrink, smectitewill migrate more easily and will eventually be depleted in more

weathered, well-drained horizons. In addition, a number ofprocesses have been identified that can transform smectitesinto kaolinites (e.g., by incongruent weathering or dissolution–recrystallization reactions). This explains the occurrence ofkaolinite-rich soils in higher, better drained landscape positions,while smectite is more likely to occur in lower, poorly drainedlandscape positions as is the case in the study area (Velde andMeunier, 2008). This suggests that most of the (weathered)samples still represent an early stage of weathering.

6.2. Weathering intensity and trends

The properties and formation of soils are determined to a greatextent by the rate and nature of chemical weathering at the parentmaterial – regolith interface. Although the chemical composition ofsoils is for the larger part inherited from parent materials, thedegree of the mobility of the individual elements determines theirabundance in the soils.

Comparison of the total elemental analysis of 10 lava sampleswith those of their weathering rinds (sub-samples A and B fromsamples 2-2, 4-3, 4-4, 4-7, 4-8, 7-5, 10-3, 10-4, 10-5, respectively;Table 2), shows that the overall changes in composition at this veryearly stage of weathering is very limited. These rinds wereselected based on their brownish discoloration compared to thecenter of the sample, indicating the possible effect of weatheringin inducing some mineralogical changes (e.g., release of iron fromsilicate lattices). However, the weathering has not been ableto actually remove these elements, and change the chemistrysignificantly.

Thus it is more useful to compare the entire data set using aweathering index that only requires the total elemental analysisof a single sample regardless of its weathering state. For example,when using the Chemical Index of Alteration (CIA = Al2O3/(Al2O3 + Na2O + K2O + CaO#) * 100) as developed by Nesbitt andYoung (1982) and the CIA division scale as proposed by Lambe(1996), a pattern can be observed (Fig. 7). While the majority ofthe lava samples are classified as ‘discolored by weathering to freshrock’, the bulk of the pyroclastic samples are classified as‘weathered material’ to ‘residual soil’ (Table 5 and Fig. 7). In asimilar way, scatter plots of Na2O, K2O, FeO� and TiO2 over Al2O3

contents were made (Fig. 8), clearly showing that the mobileelements like sodium and potassium are lost more easily whilethe relatively immobile elements like iron and titanium are accu-mulating faster in the pyroclastic samples compared to the lavaflow samples. Assuming the rocks have a similar age of exposureto weathering, this correlation between weathering state and rocktype can be interpreted as caused by a difference in porosity and

Fig. 7. A-CN-K and ACNK-FM diagrams (Nesbitt and Young, 1989) with indication of the weathering index CIA; the CIA scale is divided into the simplified typical weatheringprofile described by the Geological Society of London (Lambe, 1996). Arrow indicates the general weathering trend.

Table 5Samples ordered per category according to the CIA content.

Rock discolored by weathering of fresh rockLava 2-2A&B, 4-1, 4-2, 4-3A&B, 4-4A&B, 4-7A&B, 4-8A&B, 5-2, 5-5,

5-6, 5-7, 7-5A&B, 7-7A&B, 9-1, 10-3A&B, 10-4A&B, 10-5A&BPyroclastic 1-1A&B, 1-2A&B, 5-1, 5-3A&B

Weathered/disintegrated materialLava 4-5, 4-6, 10-2Pyroclastic 6-1, 6-4, 6-5, 7-1, 7-8, 7-9, 7-10, 8-1

Residual soilLava 10-1Pyroclastic 6-2, 6-3, 7-2, 7-3, 7-4, 7-6, 7-11, 8-2, 11-1, 11-2, 11-3Other 2-1, 3-1, 8-3

A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580 577

permeability between lava flows and pyroclastic rocks (Che et al.,2012). In the A-CN-K diagram, which implicitly contains the CIAindex, the more weathered samples plot more towards the A apex,while the ‘fresh’ rocks plot closer to the CN-K ‘feldspar’ baseline.The A-CNK-FM diagram also considers iron and magnesium, and

Fig. 8. Major element-Al2O3 variation diagrams (anhydrous basi

these two elements are lost significantly less compared to sodiumand potassium, possibly the result of iron accumulating as oxidesand hydroxides. As is to be expected, the analyzed soil samples plotas more ‘weathered’ material in these diagrams. The only excep-tions are the soil samples taken at the top of Geshe volcano (sam-ples 4-5 and 4-6), which plot in the middle of these diagrams,indicating the limited weathering the soil has undergone.

Loss On Ignition (LOI), a parameter first proposed by Sueokaet al. (1985) as a good indicator of the degree of weathering, wasused to subdivide the entire dataset in 4 broad categories: (i)unweathered (LOI < 2.5), (ii) intermediate (2.5 < LOI < 8), (iii)weathered (8 < LOI < 16) and (iv) highly weathered (LOI > 16) sam-ples. Using these subdivisions, log-log isocon plots (Fig. 9) weremade using the average composition of each class and consideringAl2O3 and TiO2 as the immobile oxides to draw the isocon line(Grant, 2005). Elements above the isocon line are relativelyenriched or accumulated, while elements below the isocon lineare relatively depleted. These plots clearly show an increasing lossof potassium and sodium with increasing LOI values. Furthermore,SiO2 slowly leaches as weathering progresses. The other elements

s); arrows indicate general trends as weathering increases.

Fig. 9. An isocon diagram comparing fresh rock samples with the altered ones. The isocon line is indicated in dark grey and error bars indicate standard deviation.

Fig. 10. Depth profiles: Gorosibilu, Geshe mountain and Busase Elen plotted with total elements in wt% on a logarithmic scale.

578 A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580

are present in too low concentrations to make any reliableconclusions about their relative losses or gains, especially consid-ering the spread on the measurements.

Outcrops from three sites (Goro Sibilu, Busase Elen and Geshe)were suited to make depth profile diagrams as the samples weretaken from different depths extending from the top of the regolithto the parent material underneath (Fig. 10). The depths were plot-ted against element contents (in wt%) on a logarithmic scale. Ingeneral, the profiles exhibit decreasing SiO2, K2O, Na2O contents,in good agreement with the isocon plots for the whole data set.The profiles exhibit similar trends in the Goro Sibilu and BusaseElen profile, while the Geshe profile shows some differences: theSiO2 and Al2O3 remain nearly constant while the CaO contentsdecrease with depth. The distinctly different geomorphic settingbetween Goro Sibilu and Busase Elen on the one hand and Geshe

volcano on the other hand, and an age difference are the mostlikely reasons for the subtle difference in the behavior of the ele-ments within the profile depth. Goro Sibilu and Busase Elen arelocated in a relatively gently sloping topography which is condu-cive for rock weathering and leaching of the elements as comparedto the steep and elevated topography of the Geshe volcano whereerosion is a more dominant process than vertical mobility of ele-ments. In addition to this, the physicochemical and petrographiccharacteristics of the bedrock at these sites are quite different:the profile at Geshe volcano has developed on a remnant lava plugwhile the other two developed on more easily weatherable pyro-clastic materials.

The various plots show that weathering of alkali-rich minerals,mostly feldspars, followed by that of pyroxenes and amphiboles, isthe primary weathering pathway. This explains the observed losses

A. Regassa et al. / Journal of African Earth Sciences 99 (2014) 568–580 579

in potassium and sodium and the associated accumulation of iron.It was also possible to observe an important difference in weather-ing rate between rock types. The differences in the originalchemical composition, mineralogy and rock fabric of the parentmaterials significantly influence the leaching rates of elementsduring weathering and conversely the soil formation rate in thiscatchment.

7. Conclusions

The Gilgel Gibe catchment is underlain by felsic lava flows andpyroclastic deposits of generally trachytic and rhyolitic composi-tion with limited compositional variation, and with conspicuoustrachytic as well as typical glassy, porphyritic to aphanitic textures.Belts of melt inclusions, skeletal alkali amphiboles and albite over-growth rims, presence of resorption, sieve-textured plagioclasesand patchy zoning suggest a complex petrogenetic process (frac-tional crystallization with significant crustal assimilation) for theorigin of the rocks. Many pyroclastic rocks (vitric, lithic and crystaltuffs) with similar composition to the lava flows are present scat-tered over the study area. The Chemical Index of Alteration valuesof the investigated samples indicate that the geological materialsin the Gilgel Gibe catchment have undergone a moderate to highintensity of weathering, mainly resulting in a loss of alkali ele-ments. Considerable difference was also observed in the degreeof weathering between samples from lava flows and pyroclasticdeposits, the latter being more susceptible to weathering. Ourstudy showed that the leaching rates of elements during weather-ing and formation of soils are significantly influenced by the chem-ical composition, mineralogy and fabric of the parent materials.

Acknowledgements

All the chemical analysis work was carried out at the Laboratoryof Soil Science, Ghent University. The thin section analysis andlogistical support for the fieldwork (in the form of provision ofvehicle and field equipment) is partly funded by the VLIR JU IUC.Kim Van Daele acknowledges the VLIR UOS for the scholarship,which enabled her to conduct her M.Sc. thesis in the same area.The anonymous reviewers are acknowledged for their critical com-ments, which were helpful to improve the manuscript.

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