• Volume 120, Issue 4

      August 2011,   pages  557-782

    • Modelling the response of an alluvial aquifer to anthropogenic and recharge stresses in the United States Southern Great Plains

      Joseph T Zume Aondover A Tarhule

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      This paper uses Visual MODFLOW to simulate potential impacts of anthropogenic pumping and recharge variability on an alluvial aquifer in semi-arid northwestern Oklahoma. Groundwater withdrawal from the aquifer is projected to increase by more than 50% (relative to 1990) by the year 2050. In contrast, climate projections indicate declining regional precipitation over the next several decades, creating a potential problem of demand and supply. The following scenarios were simulated: (1) projected groundwater withdrawal, (2) a severe drought, (3) a prolonged wet period, and (4) a human adjustment scenario, which assumes future improvements in water conservation measures. Results indicate that the combined impacts of anthropogenic pumping and droughts would create drawdown of greater than 12 m in the aquifer. Spatially, however, areas of severe drawdown will be localized around large-capacity well clusters. The worst impacts of both pumping and droughts will be on stream–aquifer interaction. For example, the projected aquifer pumpage would lead to a total streamflow loss of 40%, creating losing stream system regionally. Similarly, a severe drought would lead to a total streamflow loss of < 80%. A post-audit of the model was also carried out to evaluate model performance. By simulating various stress scenarios on the alluvial aquifer, this study provides important information for evaluating management options for alluvial aquifers.

    • Determining the optimum cell size of digital elevation model for hydrologic application

      Arabinda Sharma K N Tiwari P B S Bhadoria

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      Scale is one of the most important but unsolved issues in various scientific disciplines that deal with spatial data. The arbitrary choice of grid cell size for contour interpolated digital elevation models (DEM) is one of the major sources of uncertainty in the hydrologic modelling process. In this paper, an attempt was made to identify methods for determining an optimum cell size for a contour interpolated DEM in prior to hydrologic modelling. Twenty-meter interval contour lines were used to generate DEMs of five different resolutions, viz., 30, 45, 60, 75, and 90 m using TOPOGRID algorithm. The obtained DEMs were explored for their intrinsic quality using four different methods, i.e., sink analysis, fractal dimension of derived stream network, entropy measurement and semivariogram modelling. These methods were applied to determine the level artifacts (interpolation error) in DEM surface as well as derived stream network, spatial information content and spatial variability respectively. The results indicated that a 90 m cell size is sufficient to capture the terrain variability for subsequent hydrologic modelling in the study area. The significance of this research work is that it provides methods which DEM users can apply to select an appropriate DEM cell size in prior to detailed hydrologic modelling.

    • Modelling spatial anisotropy of gold concentration data using GIS-based interpolated maps and variogram analysis: Implications for structural control of mineralization

      Abani R Samal Raja R Sengupta Richard H Fifarek

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      Linear trends of anomalously high gold values in the Florida Canyon gold deposit, Nevada have been identified using a combination of contour maps of gold (Au) concentration developed with a geographic information system (GIS) and variogram maps created using a geostatistical analysis package. These linear trends are interpreted to represent major fault zones that exerted a prinicipal control on gold mineralization and therefore imparted a spatial anisotropy to gold concentrations.

      Oxidation state information such as oxide, sulfide or mixed was used initially to map and contour the lower limit of the oxidation zone. Linear trends on this surface suggest the location and trend of major structural elements in the deposit that guided late oxidizing fluids. Subsequently, four contour maps of gold concentrations in oxidized rocks were produced, each map representing 500 ft vertical intervals starting at 3500 ft above mean sea level (msl). Relatively high concentrations of Au that form linear trends on these maps suggest the presence of structural features, such as shear zones that controlled mineralization. Finally, to validate the observed trends, variogram maps of gold concentrations were derived through geostatistical analysis and the major axes of anisotropy were determined for each map.

      The results that emerge suggest linear trends of northeast, northwest and, less prominently, north– south orientations. The north–south and northeast trends match those of known and mapped major structures associated with the Florida Canyon deposit. However, the results imply a stronger control on mineralization by northwest-trending structures than previously recognized and the location of possible structures of all trends not previously mapped. They also serve to identify faults that controlled both early hydrothermal fluids and late oxidizing fluids since the gold distribution represents the time integrated effects of both fluid events.

      The linear trends derived by spatial analysis (contour maps, variogram maps) of geochemical data (i.e., gold concentration), combined with the results of the field observations prove to be advantageous in understanding the structural control of gold mineralization. Such spatial analyses of geochemical concentration data are particularly useful in the field of mineral exploration.

    • Efficient two-dimensional magnetotellurics modelling using implicitly restarted Lanczos method

      Krishna Kumar Pravin K Gupta Sri Niwas

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      This paper presents an efficient algorithm, FDA2DMT (Free Decay Analysis for 2D Magnetotellurics (MT)), based on eigenmode approach to solve the relevant partial differential equation, for forward computation of two-dimensional (2D) responses. The main advantage of this approach lies in the fact that only a small subset of eigenvalues and corresponding eigenvectors are required for satisfactory results. This small subset (pre-specified number) of eigenmodes are obtained using shift and invert implementation of Implicitly Restarted Lanczos Method (IRLM). It has been established by experimentation that only 15–20% smallest eigenvalue and corresponding eigenvectors are sufficient to secure the acceptable accuracy. Once the single frequency response is computed using eigenmode approach, the responses for subsequent frequencies can be obtained in negligible time. Experiment design results for validation of FDA2DMT are presented by considering two synthetic models from COMMEMI report, Brewitt-Taylor and Weaver (1976) model and a field data based model from Garhwal Himalaya.

    • Development of the negative gravity anomaly of the 85°E Ridge, northeastern Indian Ocean – A process oriented modelling approach

      K M Sreejith M Radhakrishna K S Krishna T J Majumdar

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      The 85°E Ridge extends from the Mahanadi Basin, off northeastern margin of India to the Afanasy Nikitin Seamount in the Central Indian Basin. The ridge is associated with two contrasting gravity anomalies: negative anomaly over the north part (up to 5°N latitude), where the ridge structure is buried under thick Bengal Fan sediments and positive anomaly over the south part, where the structure is intermittently exposed above the seafloor. Ship-borne gravity and seismic reflection data are modelled using process oriented method and this suggest that the 85°E Ridge was emplaced on approximately 10–15 km thick elastic plate (Te) and in an off-ridge tectonic setting. We simulated gravity anomalies for different crust-sediment structural configurations of the ridge that were existing at three geological ages, such as Late Cretaceous, Early Miocene and Present. The study shows that the gravity anomaly of the ridge in the north has changed through time from its inception to present. During the Late Cretaceous the ridge was associated with a significant positive anomaly with a compensation generated by a broad flexure of the Moho boundary. By Early Miocene the ridge was approximately covered by the postcollision sediments and led to alteration of the initial gravity anomaly to a small positive anomaly. At present, the ridge is buried by approximately 3 km thick Bengal Fan sediments on its crestal region and about 8 km thick pre- and post-collision sediments on the flanks. This geological setting had changed physical properties of the sediments and led to alter the minor positive gravity anomaly of Early Miocene to the distinct negative gravity anomaly.

    • Characteristics of pegmatoidal granite exposed near Bayalan, Ajmer district, Rajasthan

      Nilanjan Dasgupta Taritwan Pal Joydeep Sen Tamoghno Ghosh

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      The study involves the characterization of pegmatoidal granite, southeast of Beawar, Ajmer district, Rajasthan. Earlier researchers had described this granite as part of the BGC, basement to the Bhim Group of the Delhi Super Group rocks. However, the present study indicates that it is younger than the rocks of Bhim Group of South Delhi Fold Belt, into which it is intrusive. The intrusion is structurally controlled and the outcrop pattern is phacolithic. The granite had intruded post-D2 deformation of the Delhi orogeny along the axial planes of D2 folds. The intrusion has also resulted in the formation of a contact aureole about the calc gneisses.

    • Mafic rocks from Erinpura gneiss terrane in the Sirohi region: Possible ocean-floor remnants in the foreland of the Delhi Fold Belt, NW India

      M K Pandit H De Wall H Daxberger J Just M Bestmann K K Sharma

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      A small isolated mafic body occurs to the south of Sirohi near village Daba within the Neoproterozoic Erinpura Granite in the southern sector of the Proterozoic Delhi Fold Belt in NW India. This mafic body occurs close to a 100 m wide NE–SW trending shear zone (Daba Shear Zone) which overprints the felsic rock fabrics. Further south, a small mafic body near village Kui was also sampled which forms the southern limit of the Phulad Ophiolite Suite which is a 300 km long major NE–SW trending lineament, described as Western Margin Fault. Some of the lithological components of the Daba mafic body show locally preserved magmatic fabric but completely transformed mineralogies under lower amphibolites facies metamorphic conditions where two-stage deformation has been inferred. Magnetic fabric analysis underlines a general correspondence of structural elements in both felsic and mafic lithologies. Binary correlations of Zr with other high field strength elements underline fractionation as the main process in the evolution of Daba and Kui rocks. Geochemical characteristics indicate subalkaline tholeiitic basalt affinity for these mafic rocks. The trace element characteristics, such as enriched LIL elements, high Th, absence of negative Nb anomalies and depletion in compatible elements in Daba samples suggest an enriched mantle source and lower degree of melting. The trace and rare earth element characteristics for Kui (Th anomaly, Nb–Ta trough and less spiked patterns, flat REE trends) indicate derivation from a refractory mantle source affected by fluids derived from subduction. Distinct differences in trace and REE characteristics between Daba and Kui can be interpreted in terms of different stages of ophiolite development.

    • Pb–Pb zircon ages of Archaean metasediments and gneisses from the Dharwar craton, southern India: Implications for the antiquity of the eastern Dharwar craton

      B Maibam J N Goswami R Srinivasan

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      $^{207}Pb–^{206}Pb$ ages of zircons in samples of metasediments as well as ortho- and para-gneisses from both the western and the eastern parts of the Dharwar craton have been determined using an ion microprobe. Detrital zircons in metasedimentary rocks from both yielded ages ranging from 3.2 to 3.5 Ga. Zircons from orthogneisses from the two parts also yielded similar ages. Imprints of younger events have been discerned in the ages of overgrowths on older zircon cores in samples collected throughout the craton. Our data show that the evolution of the southwestern part of eastern Dharwar craton involved a significant amount of older crust (< 3.0 Ga). This would suggest that crust formation in both the western and eastern parts of the Dharwar craton took place over similar time interval starting in the Mesoarchaean at ca. 3.5 Ga and continuing until 2.5 Ga. Our data coupled with geological features and geodynamic setting of the Dharwar craton tend to suggest that the eastern Dharwar craton and the western Dharwar craton formed part of a single terrane.

    • Palynological correlation of coal-bearing horizons in Gundala area, Godavari Graben, India

      Neerja Jha Neha Aggarwal

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      The distributional pattern of various palynotaxa in the coal bearing sediments recovered from six borecores (MLG-23, MLG-24, MLG-28, SGK-2, SGK-3 and SGK-4) in Gundala area of Godavari Graben has suggested occurrence of five distinct palynoassemblages: Palynoassemblage-A marked in borecore MLG-23 shows dominance of monosaccates chiefly Parasaccites along with Plicatipollenites, Caheniasaccites, Divarisaccus, and Potonieisporites, represents Talchir palynoflora; Palynoassemblage-B characterized by dominance of radial monosaccates chiefly Parasaccites along with trilete taxa Callumispora in borecores MLG-23 and MLG-24, corresponds to Lower Karharbari palynoflora; Palynoassemblage-C distinguished in borecores MLG-24, MLG-28, SGK-2, SGK-3 and SGK-4 is marked by dominance of radial monosaccates mainly Parasaccites along with nonstriate disaccate Scheuringipollenites, represents Upper Karharbari palynoflora; Palynoassemblage-D in borecores MLG-23, MLG-24 and MLG-28 demarcated by dominance of nonstriate disaccates chiefly Scheuringipollenites, Ibisporites along with sub-dominance of few striate disaccates, viz., Faunipollenites, Striatopodocarpites, Crescentipollenites and Striatites signifies Barakar palynoflora; the youngest, Palynoassemblage-E identified in borecores MLG-24 and MLG-28 shows dominance of striate disaccates, viz., Striatopodocarpites and Faunipollenites in conjunction with Strotersporites, Crescentipollenites, Hamiapollenites, Corisaccites, Weylandites and Falcisporites. This palynoassemblage also shows the appearance of some stratigraphically significant palynomorphs, viz., Lunatisporites, Lundbladispora, Playfordiaspora, Klausipollenites, Kamthisaccites, Guttulapollenites and Crustaesporites symbolizing Late Permian Raniganj palynoflora. Almost a complete palynological succession from Talchir to Raniganj has been demarcated in Lower Gondwana succession of Gundala area.

    • Study of fossil wood from the Middle–Late Miocene sediments of Dhemaji and Lakhimpur districts of Assam, India and its palaeoecological and palaeophyto-geographical implications

      R C Mehrotra S K Bera S K Basumatary G Srivastava

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      In order to reconstruct the palaeoclimate, a number of fossil wood pieces were collected and investigated from two new fossil localities situated in the Dhemaji and Lakhimpur districts of Assam. They belong to the Tipam Group considered to be of Middle–Late Miocene in age and show affinities with Gluta (Anacardiaceae), Bischofia (Euphorbiaceae), Bauhinia, Cynometra, Copaifera-Detarium-Sindora, Millettia-Pongamia, and Afzelia-Intsia (Fabaceae). The flora also records a new species of Bauhinia named Bauhinia miocenica sp. nov. The assemblage indicates a warm and humid climate in the region during the deposition of the sediments. The occurrence of some southeast Asian elements in the fossil flora indicates that an exchange of floral elements took place between India and southeast Asia during the Miocene.

    • Carbon isotopic composition of fossil leaves from the Early Cretaceous sediments of western India

      S Chakraborty B N Jana S K Bhattacharya I Robertson

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      Stable carbon isotope analysis of fossil leaves from the Bhuj Formation, western India was carried out to infer the prevailing environmental conditions. Compression fossil leaves such as Pachypteris indica, Otozamite kachchhensis, Brachyphyllum royii and Dictyozamites sp. were recovered from three sedimentary successions of the Bhuj Formation, Early Cretaceous in age. A chronology was established based on faunal assemblage and palyno-stratigraphy and further constrained by carbon isotope stratigraphy. The three sampling sites were the Karawadi river bank near Dharesi; the Chawad river bank near Mathal; and the Pur river section near Trambau village in Gujarat. The Dharesi sample was also analyzed to investigate intra-leaf 𝛿13C variability. The mean 𝛿13C of the leaf was $−24.6$ ± 0.4‰ which implied negligible systematic change along the leaf axis. The Mathal sample was fragmented in nature and showed considerable variation in carbon isotopic composition. The Trambau sample considered to be the oldest, dating to the middle of Aptian (ca. 116 Ma), shows the most depleted value in 𝛿13C among all of them. The overall 𝛿13C trend ranging from mid Aptian (ca. 116 Ma) to early Albian (ca. 110 Ma) shows a progressive increase in 𝛿13C from −26.8 to −20.5‰. Based on these measurements the carbon isotopic composition of atmospheric carbon dioxide of the Aptian–Albian period is estimated to be between −7.4 and −1.7‰. The ratio of the partial pressure of carbon dioxide in leaf to that of the ambient atmosphere calculated based on a model is estimated to be similar to that of the modern plants. This indicates that the Early-Cretaceous plants adapted to the prevailing high carbon dioxide regime by increasing their photosynthetic uptake.

    • Tree-ring variation in teak (Tectona grandis L.) from Allapalli, Maharashtra in relation to moisture and Palmer Drought Severity Index, India

      Somaru Ram H P Borgaonkar A A Munot A B Sikder

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      We developed a ring-width chronology of teak (Tectona grandis L.) from a moisture stressed area in Maharashtra, India. Bootstrapped correlation analysis indicated that moisture index (MI) and Palmer Drought Severity Index (PDSI) showed better performance rather than same year rainfall over the region. Tree-ring variations were most correlated positively with PDSI during different seasons compared with MI. Significant strong positive correlation with MI, and negative association with temperature and potential evapotranspiration (PET) were found during previous and current year post-monsoon (ON). This study shows that the moisture availability during the post-monsoon of the previous year has a significant role in the development of annual growth rings. The reconstructed previous year post-monsoon (−ON) moisture index for the period 1866–1996 indicates 3.5 and 29.3 years periodicities.

    • Discontinuity surfaces and event stratigraphy of Okha Shell Limestone Member: Implications for Holocene sea level changes, western India

      Uday Bhonde Bhawanisingh G Desai

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      The Okha Shell Limestone Member of Chaya Formation is the coarse grained, shell rich deposit commonly recognized as the beach rocks. It has been age bracketed between Late Pleistocene and Holocene. Late Quaternary sea level changes have been studied with beach rocks along the Saurashtra coastal region. The present study has been carried out in the Okhamandal area of the Saurashtra peninsula especially on the Okha Shell Limestone Member as exposed at various locations along the coast from north to south. Temporal and spatial correlations of the observations have revealed three events in the Okha Shell Limestone Member of Chaya Formation that are correlated laterally. The events show depositional breaks represented by discontinuity surfaces, the taphofacies varieties and ichnological variations. The present study in the context of available geochrnological data of the region suggests a prominent depositional break representing low sea level stand (regression) during an Early Holocene during the deposition of Okha Shell Limestone Member.

    • The ion–aerosol interactions from the ion mobility and aerosol particle size distribution measurements on January 17 and February 18, 2005 at Maitri, Antarctica – A case study

      Devendraa Siingh Vimlesh Pant A K Kamra

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      A case study for the ion–aerosol interactions is presented from the simultaneous measurements of mobility spectra of atmospheric ions in the mobility range of 2.29 to 2.98 × 10−4 cm2 V−1 s−1 (diameter range 0.41–109 nm) and of size distribution of atmospheric aerosol particles in the size ranges of 4.4–700 nm and 500–20,000 nm diameters made at Maitri (70° 45′ 52′′S, 11° 44′ 2.7′′E; 130 m above mean sea level), Antarctica, on two days January 17 and February 18, 2005, with contrasting meteorological conditions. In contrast to January 17, on February 18, winds were stronger from the morning to noon and lower from the noon to evening, atmospheric pressure was lower, cloudiness was more, the land surface remained snow-covered after a blizzard on February 16 and 17 and the airmass over Maitri, descended from an altitude of ∼3 km after an excursion over ocean. On these days mobility spectra showed two modes, corresponding to intermediate ions and light large ions and an indication of additional one/two maxima for small/cluster ions and heavy large ions. The small ions generated by cosmic rays, and the nucleation mode particles generated probably by photochemical reactions grew in size by condensation of volatile trace gases on them and produced the cluster and intermediate ion modes and the Aitken particle mode in ion/particle spectra. Particles in the size range of 9–26 nm have been estimated to grow at the rate of 1.9 nm h−1 on February 18, 2005. Both, ions and aerosol particles show bimodal size distributions in the 16–107 nm size range, and comparison of the two size distributions suggests the formation of multiple charged ions. Attachment of small ions to particles in this bimodal distribution of Aitken particles together with the formation of multiple charged ions are proposed to result in the light and heavy large ion modes. Growth of the nucleation mode particles on February 18, 2005 is associated with the passage of the airmass over ocean. In contrast, though the ion size distributions were not much different, the aerosol size distributions did not show a dominant peak for the formation and growth of nucleation mode particles on January 17. More measurements are needed before the conclusion of this case study is generalized.

    • Analysis on MM5 predictions at Sriharikota during northeast monsoon 2008

      D Gayatri Vani S Rambabu M Rajasekhar G V Rama B V Apparao A K Ghosh

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      The Indian northeast monsoon is inherently chaotic in nature as the rainfall realised in the peninsular India depends substantially on the formation and movement of low-pressure systems in central and southwest Bay of Bengal and on the convective activity which is mainly due to the moist north-easterlies from Bay of Bengal. The objective of this study is to analyse the performance of the PSU-NCAR Mesoscale Model Version 5 (MM5), for northeast monsoon 2008 that includes tropical cyclones – Rashmi, Khai-Muk and Nisha and convective events over Sriharikota region, the rocket launch centre. The impact of objective analysis system using radiosonde observations, surface observations and Kalpana-1 satellite derived Atmospheric Motion Wind Vectors (AMV) is also studied. The performance of the model is analysed by comparing the predicted parameters like mean sea level pressure (MSLP), intensity, track and rainfall with the observations. The results show that the model simulations could capture MSLP and intensity of all the cyclones reasonably well. The dependence of the movement of the system on the environmental flow is clearly observed in all the three cases. The vector displacement error and percentage of improvement is calculated to study the impact of objective data analysis on the movement and intensity of the cyclone.

    • Meso-scale atmospheric events promote phytoplankton blooms in the coastal Bay of Bengal

      K Maneesha V V S S Sarma N P C Reddy Y Sadhuram T V Ramana Murty V V Sarma M Dileep Kumar

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      The Bay of Bengal is considered to be a low productive region compared to the Arabian Sea based on conventional seasonal observations. Such seasonal observations are not representative of a calendar year since the conventional approach might miss episodic high productive events associated with extreme atmospheric processes. We examined here the influence of extreme atmospheric events, such as heavy rainfall and cyclone Sidr, on phytoplankton biomass in the western Bay of Bengal using both in situ time-series observations and satellite derived Chlorophyll 𝑎 (Chl 𝑎) and sea surface temperature (SST). Supply of nutrients through the runoff driven by episodic heavy rainfall (234 mm) on 4–5 October 2007 caused an increase in Chl 𝑎 concentration by four times than the previous in the coastal Bay was observed within two weeks. Similar increase in Chl 𝑎, by 3 to 10 times, was observed on the right side of the cyclone Sidr track in the central Bay of Bengal after the cyclone Sidr. These two episodic events caused phytoplankton blooms in the western Bay of Bengal which enhanced ∼40% of fishery production during October–December 2007 compared to that in the same period in 2006.

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