Bangalore, India

Geological Survey of India

www.portal.gsi.gov.in/
Bangalore, India

The Geological Survey of India , established in 1851, is a government organization in India which is an office attached to the Ministry of Mines of Union Government of India for conducting geological surveys and studies. It is one of the oldest of such organizations in the world and the second oldest survey in the country. The GSI is the prime provider of basic earth science information to the government, industry and the general public, as well as responsive participant in international geoscientific fora. The vibrant steel, coal, metals, cement and power industries. Wikipedia.

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En Inde, après avoir obtenu les principales autorisations environnementales et techno-économiques en 2015, Velcan a poursuivi en 2016 le développement de ses projets hydroélectriques. Deux d'entre eux (totalisant 426 MW) sur les trois du portefeuille (totalisant 571 MW) entrent maintenant dans la phase des activités de pré-construction qui comprend notamment l'acquisition de terrains et les investigations techniques préalable à la construction. Ces deux projets sont parmi les projets hydro-électriques les plus avancés en Inde et le Ministère de l'Energie (Gouvernement Central de l'Inde) les a sélectionnés avec quelques autres pour faire partie d'un pool de projets dans l'Arunachal Pradesh qui bénéficient d'un soutien et d'un suivi particuliers afin d'accélérer leur développement. Leurs excellentes caractéristiques technico-économiques intrinsèques, leur tarif compétitif, leur accessibilité et la simplicité des structures, placent ces deux projets en tête de liste des rares investissements hydroélectriques privés actuellement en développement en Inde. En Indonésie, les incertitudes réglementaires liées aux conditions et aux tarifs des contrats  d'achat de l'électricité (PPA - Power Purchase Agreement) hydroélectrique n'ont pas encore été résolues, et la plupart des projets hydroélectriques en Indonésie sont par conséquent restés à l'arrêt. Le dernier décret du ministère de l'énergie et des ressources minérales (mars 2017) a débouché sur des tarifs plafonnés à 85% des coûts de production d'électricité de PT PLN (PLN - la société nationale d'électricité), selon la région, et applicable à tous les projets hydroélectriques indépendamment de leur capacité. La structure tarifaire aurait pu avoir un impact positif, mais compte tenu des coûts de production retenus, les tarifs hydroélectriques restent très peu attrayants en pratique pour les projets de moins de 10 MW. Cela a provoqué une forte contestation des professionnels et des investisseurs concernés. Les nombreuses réactions et discussions en cours n'ont pas encore abouti à un résultat favorable pour les projets hydroélectriques. Pour ces raisons, la situation du projet Sukarame (7 MW), dont nous avions dû suspendre la construction en mars 2016 en raison de l'absence de  PPA, n'a pas changé. Le projet Meureubo 2 (59 MW) fait face aux mêmes incertitudes administratives et n'a pas encore obtenu un PPA en raison de désaccords administratifs avec PLN concernant le processus de négociation du PPA. Le Groupe est activement engagé dans des discussions avec plusieurs partenaires potentiels pour tenter de trouver des solutions adéquates. Elles pourraient avoir une incidence financière positive sur les résultats financiers de 2017. Des études et investigations de pré-construction ont été conduites telles que les investigations géotechniques du sous-sol, les investigations hydrologiques et de sédimentation et les études topographiques détaillées des zones de structures des projets, comme l'exige l'Autorité centrale de l'électricité (CEA) ou en vue de la conception du design d'appel d'offres. Certaines de ces investigations de pré-construction sont terminées à ce jour. Les équipes ont également lancé la procédure d'acquisition de terres, qui doit être menée par le gouvernement de l'État avec le soutien de Velcan. La commission d'évaluation de l'impact social (Social Impact Assessment Unit) en charge des aspects sociaux a été sélectionnée et notifiée par le gouvernement local en janvier 2017 et devrait commencer l'enquête sociale et l'étude d'impact dans les prochaines semaines. Une fois l'acquisition terminée, le gouvernement local louera le terrain aux sociétés de projet de Velcan pour une période de 40 ans. Les projets de Velcan sont les premiers projets pour lesquels le gouvernement local mettra pleinement en oeuvre la nouvelle loi sur l'acquisition des terres (Land Acquisition Act, 2013). Le ministère de l'énergie, gouvernement central, (Ministry of Power) a sélectionnédans l'Arunachal Pradesh huit projets hydroélectriques prioritaires sur plus de 75 projets de taille comparable, pour lesquels une cellule spéciale de hauts représentants de différents ministères fournit un soutien, une coordination et un suivi afin de résoudre les blocages administratifs et d'accélérer le développement des projets. Heo et Tato-1 sont parmi ces 8 projets. Des efforts sont faits par le gouvernement pour assurer que la mise à niveau des routes d'accès existantes ne retardera pas le début de la construction de Heo et Tato-1. Enfin, le long processus de négociation pour l'amendement, la prolongation et la mise à jour des contrats de concession a atteint un stade avancé et les équipes de Velcan espèrent terminer ce processus dès que possible. Pauk (145 MW) progresse à un rythme qui reflète les caractéristiques techniques spécifiques à ce projet et sa situation locale. Les principaux progrès réalisés l'ont été en vue de la validation, par la Commission Centrale de l'Eau (Central Water Commission) et par l'organisme en charge de la géologie de l'Inde (Geological Survey of India), du design de base et du plan d'investigations. Les conflits liés à la propriété des terres continuent de ralentir le développement de ce projet. VELCAN considère que, conformément au contrat de concession actuel, le règlement des conflits de propriété relève du gouvernement concédant. Une nouvelle règlementation hydroélectrique a été discutée au cours des mois précédents, au sein du gouvernement central, en vue de mettre en place des mesures d'incitations significatives pour le secteur, telles que des taux d'intérêt préférentiels, des prêts à long terme, des aménagements sur les amortissements, ainsi qu'une obligation d'achat d'énergie hydroélectrique a la charge des sociétés de distribution. Une telle politique rendrait les projets Heo et Tato-1 encore plus compétitifs et attractifs. En Indonésie, le Groupe cherche à trouver une solution aux blocages réglementaires auxquels fait face son portefeuille. Comme décrit plus haut, et compte tenu des incertitudes administratives, il est impossible pour Velcan de faire des prévisions fiables quant à la planification du développement ou aux perspectives dans ce pays, pour tous ses projets. C'est un problème qui existe à l'échelle du pays et tous les développeurs sont confrontés à des difficultés similaires. Le projet Bilah (62 MW) a été abandonné en 2016 suite aux résultats techniques négatifs. Sukarame (7 MW) est toujours suspendu à ce jour. Le projet Meureubo 2 (59MW), développé en un consortium avec PJB, une société étatique indonésienne détenue à 100% par la société nationale PT PLN, n'a pas encore progressé sur le processus conduisant à la signature du PPA comme explique plus haut. Le Groupe participe activement à des discussions avec plusieurs partenaires potentiels pour essayer de trouver une solution appropriée. Ces négociations pourraient avoir une incidence financière positive sur les résultats financiers de 2017. Redelong (18 MW) continuera à progresser lentement jusqu'à ce qu'il y ait plus de visibilité sur Sukarame et Meureubo 2. Comme il le fait depuis 2005, et plus activement depuis 2008, le groupe a continué de gérer sa trésorerie pour couvrir ses coûts opérationnels et réaliser des gains financiers. Le groupe a investi au fil des ans dans une gamme extrêmement diversifiée d'actifs financiers et notamment : obligations d'entreprises et d'État en monnaie locale ou en monnaie de réserve, actions cotées et non cotées (private equity), produits dérivés et en prêts directs a des entreprises. En 2016, les principaux investissements du groupe ont été des obligations d'entreprises. L'exposition a progressivement augmenté, passant de 29 millions d'euros au début de l'année à 46 millions d'euros à la fin de l'année. Le groupe est principalement exposé a des émetteurs de catégories BBB et BB (telles que définies par Standard's and Poor). Les maturités couvrent un large éventail. Bien que la majorité soit inférieure à 6 ans, certaines obligations ont des maturités de plus de 30 ans et quelques-unes sont perpétuelles. En 2016, le groupe a investi en majorité dans des obligations en US dollar d'émetteurs situés dans les marchés développés et dans les marchés émergents. À la fin de l'année, les principales lignes du Groupe sont les émetteurs suivants : JBS (Brésil), Ethias (Belgique), Atradius (Pays-Bas / Espagne), Eksportfinans (Norvège), Olam (Singapour), Zurich Insurance (Suisse), Bluescope Steel (Australie), Israël Electric Corp (Israël) et Areva (France). Ces placements obligataires ont été très performants en 2016. Le Groupe investit dans des obligations depuis 2008 et ces placements se sont révélés être un bon moyen d'améliorer les bénéfices du Groupe à ce jour. Le Groupe n'a subi qu'un seul défaut (OI / opérateur de télécommunications brésilien) depuis le début de ses investissements obligataires, qui a coûté 0,3 M EUR au groupe. Cette perte est très significativement inférieure aux gains supplémentaires que le groupe a tiré de l'investissement dans des obligations, par rapport au maintien de la trésorerie en liquidités. Velcan dispose également de filiales et de bureaux administratifs et financiers à Singapour, l'Ile Maurice et Dubaï. Le présent communiqué contient des informations prospectives relatives au potentiel des projets en cours et/ou des projets dont le développement a débuté. Ces informations constituent des objectifs rattachés à des projets et ne doivent en aucun cas être interprétées comme des prévisions directes ou indirectes de bénéfice. L'attention du lecteur est également attirée sur le fait que la réalisation de ces objectifs dépend de circonstances futures et qu'elle pourrait être affectée et/ou retardée par des risques, connus ou inconnus, des incertitudes et divers facteurs de toute nature, notamment liés à la conjoncture économique, commerciale ou réglementaire, dont la survenance serait susceptible d'avoir un effet négatif sur l'activité et les performances futures du groupe.


Sen B.,Geological Survey of India
Journal of the Geological Society of India | Year: 2017

Morphological evolution of classical pāhoehoe to transitional flow-type in a compound basalt flow occurring in the lower pile of Deccan traps is described here from the southern slope of Mesan Dongar in Manmad-Chandwad area, western part of Maharashtra plateau, India. The transitional flow lobes, largely identified as rubbly pāhoehoe-type, are characterized by a crust of breccias, with thickness ranging from 10 to 20 m. The breccia is generally a welded one and consists of mm to m-size, angular to sub-angular basaltic clasts that vary in vesiculation (vesicle size, shape and population). The crust, at times, is supported by an inner vesicular zone coherently developed above a persistent core that varies in thickness from 10 to 15 m. Base of these lobes is smooth, sparsely vesicular, without any basal breccia. Many of the transitional lobes of Mesan Dongar, however, shared a few common morphological traits with slabby pāhoehoe and this inherent uncertainty in classifying them as rubbly pāhoehoe has been greatly reduced using the semi-quantitative tool of Keszthelyi (2002). Moreover, it is also proposed that this morphological evolution of flow lobes in Mesan Dongar represents a NW-SE trending, southerly propagating and at least four kilometre long flow-front of pāhoehoe lava. Identification of flow fronts is important, even in this ~65 Ma old, dissected Deccan flood basalt province, as these can reveal vital information on flow dynamics, slope and cooling histories of individual flows/lobes. © 2017, Geological Society of India.


Wilson J.A.,University of Michigan | Mohabey D.M.,Geological Survey of India | Peters S.E.,University of Wisconsin - Madison | Head J.J.,University of Toronto
PLoS Biology | Year: 2010

Derived large-mouthed snakes (macrostomatans) possess numerous specializations in their skull and lower jaws that allow them to consume large vertebrate prey. In contrast, basal snakes lack these adaptations and feed primarily on small prey items. The sequence of osteological and behavioral modifications involved in the evolution of the macrostomatan condition has remained an open question because of disagreement about the origin and interrelationships of snakes, the paucity of well-preserved early snake fossils on many continental landmasses, and the lack of information about the feeding ecology of early snakes. We report on a partial skeleton of a new 3.5-m-long snake, Sanajeh indicus gen. et sp. nov., recovered from Upper Cretaceous rocks of western India. S. indicus was fossilized in association with a sauropod dinosaur egg clutch, coiled around an egg and adjacent to the remains of a ca. 0.5-m-long hatchling. Multiple snake-egg associations at the site strongly suggest that S. indicus frequented nesting grounds and preyed on hatchling sauropods. We interpret this pattern as ''ethofossil'' preservation of feeding behavior. S. indicus lacks specializations of modern egg-eaters and of macrostomatans, and skull and vertebral synapomorphies place it in an intermediate position in snake phylogeny. Sanajeh and its large-bodied madtsoiid sister taxa Yurlunggur camfieldensis and Wonambi naracoortensis from the Neogene of Australia show specializations for intraoral prey transport but lack the adaptations for wide gape that characterize living macrostomatan snakes. The Dholi Dungri fossils are the second definitive association between sauropod eggs and embryonic or hatchling remains. New fossils from western India provide direct evidence of feeding ecology in a Mesozoic snake and demonstrate predation risks for hatchling sauropod dinosaurs. Our results suggest that large body size and jaw mobility afforded some non-macrostomatan snakes a greater diversity of prey items than previously suspected on the basis of extant basal snakes. © 2010 Wilson et al.


Pal T.,Geological Survey of India
Journal of the Geological Society | Year: 2011

The Andaman ophiolite occurs as thrust slices in the outer arc of the Andaman-Java infduction zone. This ophiolite preserves the mantle sequence, layered ultramafic-mafic rocks, intrusive and extrusive rocks. The mantle sequence is represented by serpentinized lherzolite and harzburgite, hosting dunite and chromitite pods. The low Cr-number (0.2-0.4), Cr-number-TiO 2 relation of the chromites, oxygen fugacity (fO 2) values (δlog fO 2(FMQ)=-1.90) and trace elements of mantle peridotites indicate a mid-ocean ridge basalt-suprainfduction-zone (MORB-SSZ) setting. The MORB mantle underwent a low degree of melting (c. 10-15%) and interacted with the infduction-zone melts. Melt-rock interaction of the peridotites in a suprainfduction zone is demonstrated by the replacement of pyroxene grains by olivine grains (Fo90), composition of chromites and oxygen fugacity (δlog fO 2(FMQ)=-1.90 to +2.16, where FMQ is the fayalite-magnetite-quartz buffer). The chromite composition of chromitite pods (Cr-number 0.72-0.75), fO 2 levels and trace elements for layered peridotites, and occurrence of the extrusive rocks as low-Ca boninite and island arc tholeiitic (IAT) basalt indicate interplay of both boninite and IAT melts for the Andaman ophiolite. The MORB mantle of the infducting Indian plate accreted into the mantle wedge and then melting of the accreted mantle produced boninite melt at the first stage and tholeiitic melts at the second stage. © The Geological Society of London.


Radhakrishna T.,Center for Earth Science Studies | Joseph M.,Geological Survey of India
Bulletin of the Geological Society of America | Year: 2012

New geochemical and paleomagnetic results are presented on two Late Cretaceous dikes of the 85-90 Ma leucogabbroic and doleritic dikes and the 65-70 Ma dolerites in Kerala, India. The dikes are rich in incompatible elements, have fractionated patterns with light rare-earth element enrichment and are akin geochemically to Cretaceous basalts on the east coast of Madagascar. The magmas were formed at garnet lherzolite depths above the Marion plume, constituting part of a large igneous province in Madagascar. In contrast, the 65-70 Ma dolerites are moderately depleted in incompatible elements, with almost flat, rare-earth element patterns and resemble the upper formations of the Deccan Traps and the tholeiitic dikes of the Seychelles. These dolerites were formed by melting of spinel lherzolite over the Reunion plume. Paleomagnetic data from the dikes and the other coeval igneous units from south India provide the 90 Ma pole (latitude: 24°; longitude: 293°; A95 = 5.9; N = 18 sites) for India. The 65-70 Ma dolerites possess both normal and reverse polarities, and the mean pole (latitude: 36°; longitude: 283°; A95 = 5.7°; N = 10 sites) compares well with the Deccan superpole. Paleolatitude estimates indicate ~5° southward migration for the Marion plume and a northward migration for the Reunion plume, in conformity with global mantle-circulation models; however, distinguishing migration of the Reunion plume from the effects of true polar wander is difficult. Furthermore, the geodynamic reconstructions extending the shear zones of southern Madagascar into south India are not tenable. © 2012 Geological Society of America.


Mukhopadhyay S.K.,Geological Survey of India
Journal of the Geological Society of India | Year: 2012

Guembelitria is an essential biotic component in the Late Cretaceous-Early Paleogene (K/Pg) marine successions to provide crucial information about the K/Pg boundary; however, it is not well studied in Indian subcontinent. Biostratigraphically well constrained K/Pg successions of Therriaghat and Mahadeo in the East Khasi Hills District, Meghalaya, India provided a scope to present a comprehensive account of the genus in the perspective of differences of opinion about its species. A total of six species including Guembelitria langparensis n. sp., are recognized and their stratigraphic distribution is recorded. A review of the taxonomic validity of the known species, an evaluation of the diverse concepts of Guembelitria cretacea Cushman, and semiquantitative analysis of the recovered species permitted their clustering into two morphogroups that had different ecology. Morphogroup I comprising Guembelitria cretacea, Guembelitria trifolia and Guembelitria langparensis n. sp., is characterized by flared and short spire tests that form a bioseries and thrived as surface floaters. Morphogroup II comprising Guembelitria irregularis, Guembelitria danica and Guembelitria sp a, possesses high spire, narrow tests that have morphological abnormalities and had preference for living in subsurface stressed environment. The occurrence of the species and accompanying other features like ratio of planktonic to benthic foraminifera (p/b), lithologic assemblage and incidence of phosphorite, are used to infer depositional environments and sea level changes during successive biozones of the sequence. The recognized Guembelitria events during the K/Pg transition are discussed with reference to world occurrences. © GEOL. SOC. INDIA.


Bandopadhyay P.C.,Geological Survey of India
Journal of Asian Earth Sciences | Year: 2012

Turbidites composed of sandstone-shale alternations on the Kalipur-Shibpur coast, North Andaman Island, classified under the Palaeocene-Eocene Mithakhari Group in several recent papers are identified as and compared with the type section of the Oligocene Andaman Flysch exposed at Corbyn's Cove, South Andaman Island. The Kalipur turbidites were interpreted as the inner fan and the latter as the distal mid fan facies of a forearc submarine fan. The turbidites of these two locations are separated by ~250. km. Detailed studies concerning the identification and comparison, have however, revealed significant differences, and the turbidite outcrops are not continuous between these two localities.Turbidites at Kalipur-Shibpur and adjacent areas are part of a melange terrane, normally gritty and coarse grained, massive to locally graded bedded, calcareous, intercalated/interstratified with conglomerates and reefoidal limestones and show a framework composition varying from volcanolithic to lithic-poor arkosic sandstones, deposited in several isolated basins, fed by transverse supply of detritus from an accreted and uplifted ophiolite and arc massif. The Kalipur-Shibpur coast exposes marginally deformed, sandstone-dominated turbidites (coherent units) containing abundant ichnotraces, late Palaeocene foraminifera, and rip-up shale clasts, indicating deposition in shallow water accretionary slope basins. South of Kalipur, the Ramnagar coast exposes mud-rich turbidites showing accretion-related deformation. These deformed turbidites are interpreted as offscraped trench deposits. Further south, on the Rampur coast, turbidites intercalated with reefoidal limestones containing late Palaeocene foraminifera indicate deposition on the upper slope or on top of the accretionary slope basin. In contrast classical Bouma sequence-bearing sandstone-shale turbidites at Corbyn's Cove, are part of a continuous outcrop belt of siliciclastic turbidites, lack fossils, carbonate facies and conglomerates, and consist of compositionally uniform greywackes, deposited in an open deep sea fan, fed by axially transported detritus derived from the continental blocks of western Burma.These key differences together with previous mapping and stratigraphic studies confirm the incorrect identification of Andaman Flysch in North Andaman Island in recent papers. The turbidites of these two locations were neither produced by the same sediment gravity flows nor deposited in the same forearc fan during the Oligocene, instead, they were derived from different palaeographic domains, deposited in different tectonic and sedimentary environments and also at different times. This provides new insights into the Paleogene turbidite deposition in this part of Western Sunda Arc. © 2011 Elsevier Ltd.


The area adjoining the western part of Archaean Nellore schist belt and the eastern margin of the Proterozoic Cuddapah basin in south Peninsular India is marked by emplacement of a number of granite plutons of Proterozoic age, intermittently extending over a stretch of 350 km from Vinukonda in the north to Sri Kalahasti in the south. Vinukonda, Darsi, Podili and Anumalakonda plutons are intensely deformed particularly along the margins, while development of crude deformational fabric is noticed in Kanigiri, Rapur and Kayyuru-Vendodu plutons. Petrographically majority of these granites vary from alkali feldspar granite to granite with the exception of Rapur granite which varies from granite to granodiorite. Geochemically they exhibit calc-alkaline trend and in A/NK-A/CNK plot they are positioned at the juncture of peraluminous- metaluminous-peralkaline field. Characteristically, majority of these granites are fluorite bearing. Biotite mineral chemistry suggests high FeOT contents (31.68 to 34.69 %) and very low MgO contents (0.49 to 2.41 %). Geochemically, these are charecterised by high SiO2 (69 to 74.5 %), Na2O+K2O (8.19 to 10.11%), Zr (280-660ppm), Y (70-340 ppm), Rb content (180-370 ppm) and high REE contents (except Eu); and low CaO (0.01 to 1.99), MgO (0.01 to 0.92%) and Sr (10 ppm to 85 ppm) contents. Rare earth element studies reveal a general enrichment of LREE, pronounced negative Eu anomaly; flat and depleted HREE. Enriched LILE and HFSE contents; presence of fluorite and interstitial biotite indicate that these granites are crystallized from a fluorine saturated magma derived from enriched crustal source. The field setup, distinct mineralogy and chemical characteristics suggest that these granite plutons are emplaced along a major tectonic zone i.e. terrane boundary shear zone (TBSZ) in a late-orogenic to anorogenic tectonic setup, close to the vicinity of a collision boundary zone; western margin of NSB and eastern margin of Nallamalai Fold Belt (NFB). The Proterozoic granite magmatism reported in the present studies represents a significant event of Precambrian crustal growth at the juncture of two tectonically contrasting terranes i.e. the Archaean Nellore schist belt and the Proterozoic Cuddapah basin in eastern Dharwar craton. © 2013 Geological Society of India.


A systematic account of micro-textures and a few compositional profiles of plagioclase from high-alumina basaltic aa lava erupted during the year 1994-1995, from Barren Island Volcano, NE India ocean, are presented for the first time. The identified micro-textures can be grouped into two categories: (i) Growth related textures in the form of coarse/fine-sieve morphology, fine-scale oscillatory zoning and resorption surfaces resulted when the equilibrium at the crystal-melt interface was fluctuated due to change in temperature or H2O or pressure or composition of the crystallizing melt; and (ii) morphological texture, like glomerocryst, synneusis, swallow-tailed crystal, microlite and broken crystals, formed by the influence of dynamic behavior of the crystallizing magma (convection, turbulence, degassing, etc.). Each micro-texture has developed in a specific magmatic environment, accordingly, a first order magma plumbing model and crystallization dynamics are envisaged for the studied lava unit. Magma generated has undergone extensive fractional crystallization of An-rich plagioclase in stable magmatic environment at a deeper depth. Subsequently they ascend to a shallow chamber where the newly brought crystals and pre-existing crystals have undergone dynamic crystallization via dissolution-regrowth processes in a convective self-mixing environment. Such repeated recharge-recycling processes have produced various populations of plagioclase with different micro-textural stratigraphy in the studied lava unit. Intermittent degassing and eruption related decompression have also played a major role in the final stage of crystallization dynamics. © 2013, China University of Geosciences (Beijing) and Peking University.


Purkait B.,Geological Survey of India
Earth Surface Processes and Landforms | Year: 2010

The grain-size distribution of aeolian dune sands in the Thar Desert, India was analyzed and compared with three model distributions - log-normal, log-hyberbolic and log-skew-Laplace - to determine the best-fit statistical model. In total, 51 samples were collected along a single transect over a transverse dune, of which 15 were from the stoss side, 12 from the crest and 24 from the lee side. Samples were collected during a calm period in the afternoon of a winter's day. It was observed that of these 51 samples, 33 fit best to a log-hyperbolic distribution, 14 fit best to a normal distribution and only four fit best to a Laplace distribution. However, it was further observed that of 24 samples from the lee side, 13 fit best to a normal distribution, eight fit best to a hyperbolic distribution, and three fit best to a Laplace distribution. Of 12 samples from the crest of the dune, 11 fit best to the log-hyperbolic distribution, only one to the Laplace distribution but none to a normal distribution. Of 15 samples from the stoss side of the dune, only one sample best-fits a normal distribution, 14 fit best to a log-hyperbolic distribution, and none best fit to a Laplace distribution. During sample collection a calm period prevailed and there was no dusty wind. It was therefore assumed that in the initial stage a mixture of coarse, medium and fine sands was laid down on the stoss side of the dune. As wind speeds increased and saltation started, the coarser fractions were segregated and lagged behind on the stoss slope. In the final stage when the remaining intermediate and finer fractions reached the dune crest, the finer fractions were winnowed away to suspension from the crest of the dune. As a result, a narrow range of intermediate sized sediments was deposited by rolling down the lee side to explain the development of log-normality. In such a situation, both the coarser and finer fractions, to which the skewed distributions can be attributed, are separated from the initial mixture of coarse, intermediate and fine fractions. Hence the main criteria for the development of a normal distribution is the lack of skewed fractions and the concentration of the narrow, intermediate size fractions in the final grain size distribution. This is also corroborated with the index of symmetry, which is a measure of the difference between the angle of two slopes of the hyperbolic distribution as represented by the coarser and finer fractions. © 2010 John Wiley & Sons, Ltd.

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