Articles written in Journal of Earth System Science

    • Insights into the P–T evolution path of TsoMorari eclogites of the north-western Himalayas: Constraints on the geodynamic evolution of the region

      Preeti Singh Ashima Saikia Naresh Chandra Pant Pramod Kumar Verma

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      The present study is on the Ultra High Pressure Metamorphic rocks of the Tso Morari Crystalline Complex of the northwestern Himalayas. Five different mineral associations representative of five stages of P–T (pressure–temperature) evolution of these rocks have been established based on metamorphic textures and mineral chemistry. The pre-UHP metamorphic association 1 of Na-Ca-amphibole + epidote ± paragonite ± rutile ± magnetite with T–P of ∼500° C and 10 kbar. This is followed by UHP metamorphic regime marked by association 2 and association 3. Association 2 (Fe< Mg< Ca-garnet + omphacite + coesite + phengite + rutile ± ilmenite) marks the peak metamorphic conditions of atleast 33 kbar and ∼750° C. Association 3 (Fe< Mg< Ca-garnet + Na-Ca amphibole + phengite ± paragonite ± calcite ± ilmenite ± titanite) yields a P–T condition of ∼28 kbar and 700°C. The post-UHP metamorphic regime is defined by associations 4 and 5. Association 4 (Fe< Ca< Mg-garnet + Ca-amphibole + plagioclase (An05) + biotite + epidote ± phengite yields a P–T estimate of ∼14 kbar and 800°C) and association 5 (Chlorite + plagioclase (An0.5) + quartz + phengite + Ca- amphibole ± epidote ± biotite ± rutile ± titanite ± ilmenite) yields a P–T value of ∼7 kbar and 350°C.

    • Interpolation study on ambient gamma levels in parts of Khasi Hills, Meghalaya (India): Preliminary findings for U exploration

      B M Kukreti G K Sharma Pramod Kumar Sandeep Hamilton

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      This paper discusses an experimental approach to examine uranium exploration avenue over the geologicallyextended parts of Mahadek basin in Meghalaya, amid some of the environmental constraints. Studycomprises periodic measurements of prevailing ambient gamma levels across 320 georeference points, inrelation to the major litho units of Mahadek basin, covering 673 line km of Khasi Hills. Acquired sampledata points were then analysed in geostatistical software (Surfer^{TM}) to develop analytical model of samplevariogram having bearing on the uranium exploration in the area. Study findings have given encouragingsurface indicators with mostly elevated gamma levels over the parts of West Khasi Hills. Delineatedgamma anomalous zones are lithologically well correlated including to that of existing uranium occurrencesin the basin. Identified anomalous zones over the parts of West Khasi Hills by this study work, aremainly associated with the Mahadek sandstone (Upper and Lower Mahadek) and Precambrian basementgranites. Lower Mahadek sandstone is host rock for uranium mineralisation in the basin. Initial findingssuggest with the closer spatial resolution (∼1 km) of sample data points, the approach adopted by thestudy work holds promising application in locating potential uranium exploration targets especially tothe extended and inaccessible parts of the basin.

    • Effect of irregularity on torsional surface waves in an initially stressed anisotropic porous layer sandwiched between homogeneous and non-homogeneous half-space

      Anup Saha Santimoy Kundu Shishir Gupta Pramod Kumar Vaishnav

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      The present paper is concerned with the propagation of torsional surface waves in an initially stressedanisotropic porous layer sandwiched between homogeneous and non-homogeneous half-space. We assumethe quadratic inhomogeneity in rigidity and density in the lower half-space and irregularity is taken inthe form of rectangle at the interface separating the layer from the lower half-space. The dispersionequation for torsional waves has been obtained in a closed form. Velocity equation is also obtained inthe absence of irregularity. The study reveals that the presence of irregularity, initial stress, porosity,inhomogeneity and anisotropy factor in the dispersion equation approves the significant effect of theseparameters in the propagation of torsional waves in porous medium. It has also been observed that fora uniform media, the velocity equation reduces to the classical result of Love wave.

    • Lithologic boundaries from gravity and magnetic anomalies over Proterozoic Dalma volcanics

      Pramod Kumar Yadav P K Adhikari Shalivahan Srivastava Ved P Maurya Anurag Tripathi Shailendra Singh Roshan K Singh Ashish K Bage

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      Dalma volcanics (DVs) has intruded the older Singhbhum Group of Metapelites. Despite DVs being rich in mineralisation, its boundaries are not clearly demarcated. Gravity and magnetic surveys have been attempted for mapping the boundaries in DVs. These surveys were made in the northern fringeof the DVs over an area of ∼0.70 km2 along 13 parallel lines at 50 m spacing. The data was acquired at ∼25 m spacing. The surveys were taken for determination of lithological boundaries, depths and nature of causative source using Euler depth solutions and radially averaged power spectrum (RAPS).Residual anomaly maps of gravity and magnetic intensity show the same trend as that of Bouguer gravity anomaly and total magnetic intensity anomaly map indicating towards shallow sources. The magnetic map in general follows the same pattern as that of gravity anomaly maps. The map shows coincident highgravity and magnetic anomalies. These anomalies together with resistivity signatures confirm that the northern fringe of DVs hosts volcanogenic massive sulphide settings. The Euler depth solution delineated the lateral boundaries and nature of the source. It seems that the source is of spherical nature lying withina depth range of 25–40 m. The obtained lithological (vertical) units from RAPS are between Lower DVs, Upper DVs and Singhbhum Group Metapelites at depths of ∼15, ∼25 and ∼40 m, respectively. The metallogeny is associated with the Upper DVs and the corresponding delineated lithological (vertical) unit is indicative of the top of the ore body. Good agreement is observed with the geological successionfrom the drilling data and resistivity data. The findings suggest that the northern fringe of DVs could be a preferred target for drilling.

    • Audio-magnetotelluric investigation of Bakreswar Geothermal Province, Eastern India

      Anurag Tripathi Shalivahan S S Ashish K Bage Shailendra Singh Pramod Kumar Yadav

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      Bakreswar Geothermal Province (BGP) lies over the Archaean gneisses and schists formations. The heat flow and geothermal gradient of BGP are $\sim$230 mW/m$^{2}$ and 95$^{\circ}$C/km, respectively. The Bakreswar hot spring (BHS) (23$^{\circ}$52$^\prime$30$^{\prime\prime}$N; 87$^{\circ}$22$^{\prime}$30$^{\prime\prime}$E) is located over NNW–SSE trending fault. It lies in the eastern flank of BGP and is connected by Sone–Narmada–Tapti (SONATA) and deep-seated ONGC fault systems. Earlier studies indicated that the geothermal fluid flowed towards the same path as that of deep ONGC fault. In view of this, 24 audio-magnetotelluric soundings in the frequency range 10 kHz–10 Hz were conducted at one end of the deep ONGC fault, which is about 20 km northwest of BHS. The data analysis indicated that the data are internally consistent. Analysis of dimensionality and strike are indicative of complex conductivity structures. The 3D inversion of full impedance was performed to obtain a conductivity model of the study area and resulted in root mean square (RMS) for sites close to unity. The results have been discussed by extracting cross-section along 3 to 7 profiles (four of the profiles are perpendicular to strike, whereas three profiles were oblique to the strike direction and are perpendicular to east–west trending ONGC faults). Several low conducting zones are observed in the profiles drawn parallel and perpendicular to the strike. These common conductors show consistent model features but modified resistivity and dimension. Depth slices from 200 to 1400 m suggest that the conductors in the northern part of the study area are deeper than the conductors in the south. The study area is dominated by several shallow conductors which coincide with several fractures present in the study area. The conductors are surrounded by a resistive feature that can act as an impermeable bed and probably restricts the flow of water within the zone. The four major conductors along the oblique profiles (profiles perpendicular to deep ONGC fault) coincide with deep ONGC fault, through which probably the geothermal fluid flows.

    • Earthquake-induced soft sediment deformation (SSD) structures from the Bilara limestone formation, Marwar basin, India

      Partha Pratim Chakraborty Rajesh Sharma Pramod Kumar

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      The Neoproterozoic Bilara limestone Formation of the Marwar Group, Rajasthan, India exposes metres-thick layers of soft sediment deformation (SSD) structures at different stratigraphic levels which could be traced over hundreds of metres on the outcrop scale. The SSD structures include disharmonic folds, low-angle thrusts, distorted laminae, fluidisation pipes, slump and load structures, homogeneities, diapirs, etc. Whereas SSD structures suggesting tensional stress, viz., intrastriatal graben, fluidisation, slump, etc. dominate in the lower part of the Bilara succession, features implicating compression, viz., folds, low-angle thrust are prevalent in the uppermost part. Since SSD structures are mostly confined within the algal laminites, we interpret that enhanced micritic fluid pressure below early cemented algal carbonate played a major role in laminae deformation. Depending on the degree of lithification and pore-water pressure, deformation features formed either plastically or led to diapiric injection at enhanced pore water pressure. Separated by near-horizontal underformed strata, the SSD layers, traceable over hundreds of metres, are interpreted as products of seismic shacking. Considering the time frame of the Marwar basin, i.e., the Precambrian–Cambrian transition, the SSD horizons present within the Bilara succession may hold the potential for the correlation with SSD structures reported from the time-correlative stratigraphic successions present in erstwhile adjoining tectonic terrains, e.g., China, Siberia, etc.

    • Facies architecture and spatio-temporal depositional variability in the Pliocene Sandhan fluvial system, Kutch Basin, India


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      The glaciation-induced base-level fall and basin physiography straightforward controls fluvial style in any coastal setting. An alternate climatic influence including warm and cold (glacial) conditions may result in marine onlap with interception by unconformity formation and fluvial incursion in a coastal stratigraphic record. Unconformably overlying the marine sediments, the fluvial sandstone in the upper part of Plio- cene Sandhan Formation is studied herein for documentation of architectural element and variability in the fluvial sedimentation motif, if any, in space-time framework. The identified architectural elements include channel (CH), gravel bar and bedforms (GB), sandy bedforms (SB), downstream accretion (DA), sediment gravity Cow (SG), compound bar (CB) overbank fines (OF) and paleosol (P). From dominance of coarse-grained, granular pebbly sandstone with SB, GB, SG and DA elements, and incidence of 6–12 m thick fining-upward cycles, it is inferred that the Sandhan fluvial system was of Donjek-type braided in character. Only at the Nagmati River section in the south-east, a change in fluvial character is recorded as Platte-type. A role of basin physiography involving median-high across the depositional profile is iden-tified from (i) decrease in thickness of fluvial deposit from north-west to south-southeast, and (ii) reduction in topographic-gradient in the south-southeast. Considering the Pliocene time frame, the fluvial incursion onto the marine system within Sandhan depositional history is identified as a result of base-level fall under influence of global glaciation.

    • Depositional architecture of sub-aqueous part of a tide-dominated delta and its palaeogeographic implications: Laisong Formation (Barail Group), Indo-Myanmar Ranges, western Manipur


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      In western Manipur, India, a ${\sim}$765 m thick dominantly fine-grained succession of the Late Eocene–Early Oligocene Laisong Formation, constituted of siltstone-silty-shale heterolithic units at its lower part and thickly bedded sandstones in the upper part, allowed documentation of subaqueous part of a tidal delta.The abundant incidence of features including lenticular, wavy bedding, starved ripple trains, syn-sedimentary deformation, reactivation and erosional surfaces, double-mud drapes, tangential bottom setcontact, rip-up mud clasts bear tell-tale evidence in favour of tidal modulations. Furthermore, a prominent thickening- and coarsening-up progradational facies stacking motif is correlated as signature for tide-dominated delta. From process-based facies and facies succession analysis, five different subaqueous environments of delta were delineated which include prodelta, terminal distributary channel, distal delta front, proximal delta front sheet and proximal delta front lobe in order of stratigraphic superposition. The river-fed sediments were extensively reworked by accentuated tidal currents in an embayed coastline, developed along a narrow, elongated ocean basin bordered by the Indian plate on its west and Burmese micro-plate in the east. A local-scale subsidence and sea-level rise is inferred as trigger for the Laisong tidal delta development in the backdrop of its Late Eocene–Early Oligocene time frame that otherwise witnessed large-scale growth of east Antarctic ice sheet and regional scale fall in sea-level.

    • Mineral chemistry of giant plagioclase basalt: Petrogenetic inferences


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      Earlier, whole-rock data assisted in constraining magmatic processes accountable for the evolution of Giant Plagioclase Basalt (GPB), although plagioclase chemistry data-based implications are rare and critical, while combining with bulk-rock chemistry to understand magmatic processes regionally. For this reason, giant and small-size plagioclase grains were analyzed using Electron Probe Microanalysis (EPMA). Core to margin of the Kalsubai Subgroup GPB grains show significant variation ranging from An$_{99}$Ab$_1$–An$_{94}$Ab$_6$ to An$_{65}$Ab$_{35}$–An$_5$Ab$_{95}$, respectively. Nucleation of An-rich core initiated in the midlithospheric level at elevated temperatures, varying between 1490 and 1510°C, but successive An-poor intermediate and marginal layers crystallized at low temperatures between 1200–1360°C and 910–930°C that also in shallower depth, respectively. Moreover, supercharged feldspar-rich magma persisted throughout the Kalsubai Subgroup lavas, as noticed by the absence of zoned plagioclase. Almost similar processes were followed, but in a subdued manner at 1220–1300°C temperature in the Kevlari–Kalan area that lies ${\sim}$900 km apart in the eastern Deccan Volcanic Province (DVP). Presence of plume-fed magma chamber is also justified by the size and thickness of the Kalsubai Subgroup lavas. Uninterrupted recharge of magma source was continued as Ab/An value remains constant throughout the Kalsubai Subgroup lava sequence. Further, residence period of 3.2–17.4 ka is assigned to GPB of the Kalsubai Subgroup. Owing to high eruption rate, younger formations of the main DVP do not contain GPB, but Kevlari Kalan GPB contains labradorite (An$_{60}$Ab$_{40}$–An$_{50}$Ab$_{50}$) rich plagioclase, crystallized during storage of the magma for a shorter interval comparing to Kalsubai Subgroup GPB lavas.


      $\bullet$ Giant plagioclase contains Ca- rich (An99) core and Ca-poor (An65) rim

      $\bullet$ Three crystallization episodes assigned to Kalsubai GPB and one to Kevlari Kalan GPB

      $\bullet$ Owing to high eruption rate, younger western DVP is devoid of GPB.

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