Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Sumit K. Ghosh ¹, Rohtash Kumar ¹

Affiliations
1 Sedimentology Group, Wadia Institute of Himalayan Geology, P.O. Box 74, Dehradun 248001, IN

Abstract

The 1.6 km thick Neogene fluvial sandstone (Middle Siwalik; 9-5.23 Ma) in the Dehradun sub-basin (west central sector of Himalayan Foreland Basin) is characterised by distinct grey colour, and salt and pepper texture. They can be divided into poorly indurated (friable), Matrix-Rich Sandstone (MRS) and indurated (hard), Matrix-Poor Sandstone (MPS). The sandstone is classified as lithic arenite (Q₅₅F₅R₄₀; Mx₂; 50%), lithic graywacke (Q₆₁F₃R₃₆Mx₂₄ 29%) and sublithic arenite (Q₇₅F₅R₂₀ Mx₃; 21%). The MRS is fine- to very fine-grained lithic graywacke, and the MPS is rnediurn to fine-grained lithic to sublilhic arenite.

The framework composition of the sandstone points to its derivation from a provenance comprising sedimentary, low to medium grade metamorphic and igneous source rocks. Significant increase of metamorphic element above 750ml level (7.65 Ma) and particularly from 900m level (7.50 Ma) suggests a widespread erosion of the uplifted Himalayan gneiss or the Higher Himalayan Crystallines (HHC). The uplift of Higher Himalayan Crystalline was due to tectonic adjustment along the Main Central Thrust (MCT) between 7.65-7.50 Ma. Results of this study also suggest fluctuating semi-arid to semi-humid climate till 8.5 Ma. However, a distinct deviation from semi-arid to humid climatic phase is observed between 8.5 and 7.6 Ma.

Keywords

Sedimentology, Petrography, Himalayan Foreland Basin, Neogene, Sandstone, Siwalik, Dehradun.

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Sumit K. Ghosh ¹, B. K. Chatterjee ²

Affiliations
1 Wadia Institute of Himalayan Geology, 33, General Mahadev Singh Road, Dehra Dun 248001, IN
2 Department of Geology, Banaras Hmdu University, Varanasi 221 005, IN

Abstract

The early Proterozoic Kolhan sihciclastrc rocks of the Kolhan Group (c. 2100 Ma) exhibit extensive-development of thickly bedded sandstone, impersistent conglomerate and siltstone. Six major lithofacies were recognised, viz, granular lag (GLA), granular sandstone (GSD), sheet sandstone (SSD). parallel laminated sandstone (PLSD), rippled sandstone (RSD) and thinly laminated siltstone-sandstone (TLSD). The vertical and lateral arrangement of lithofacies with their general finmg upward sequence is interpreted to reflect superimposition of retrograding shoreline on the earlier, prograding humid alluvial fan-sandflat complex and can be comparable with the present day storm-induced sedimentation on the NW Atlantic shelf.

Keywords

Palaeoenvironment, Petrology (Sedimentary), Early Proterozoic, Kolhan, Keonjhar, Orissa.

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Sumit K. Ghosh ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehra Dun-248001, IN

Abstract

The Nagthat siliciclastics consist of sublithic-arenite and subarkose with subordinate proportion of quartz wacke,lithic graywacke, quartzarenite,lithic arenite and arkosic wacke. Triangular plot of lithic fragments, variable diagrams of quartz types and heavy mineral assemblage collectively suggest their derivation mainly from low to high grade metamorphic terrain with minor contribution from sedimentary and plutonic igneous rocks. This is further supported by the dominance of brown luminescing quartz and blue to non-luminescent feldspars. The palaeoflow analysis reveal three prominent modes NE. NW, ESE and suggests that the source areas for the Nagthat sediments was towards the SW. SE and WNW directions.

Taking all these points into consideration, it can be inferred that the major part of the detrital framework materials for the Nagthat siliciclastics were possibly supplied due to uplift and subsequent erosion of Aravalli-Delhi mountain chain and Bundelkhand granite-gneiss massif sometime during Aravalli and Delhi orogenic events.

Keywords

Sedimentology, Cathodoluminescence. Late Proterozoic, Nagthat Fm, Kumaun, Lesser Himalaya.

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Himanshu K. Sachan ¹, Sumit K. Ghosh ¹

Affiliations
1 Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN

Abstract

The fluid inclusion study in quartz overgrowth in the siliciclastics of Neoproterozoic Nagthat Formation reveals that the quartz cementation took place in the range of 80°C to 130°C with a salinity range of 0.82 to 9.18 wt. % NaCl. The microthermometric data, combined with petrographic and geologic evidences pennitted to constrain the nature of cementing fluids and source of cement. The silica for quartz cementation was derived partly from both the internal source (quartz and feldspar dissolution) and external source (pore fluids). It can be further suggested that the quartz cementation in the lower stratigraphic level (near the Chandpur Formation) is mainly by marine water while in the upper stratigraphic level (near the Blaini Formation), meteoric water has played an important role for the same.

Keywords

Fluid Inclusion, Neoproterozoic, Nagthat Formation, Lesser Himalaya, Uttar Pradesh.

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S. J. Sangode ¹, Rohtash Kumar ¹, Sumit K. Ghosh ¹

Affiliations
1 Palaeomagnetics Laboratory, Wadia Institute of Himalayan Geology, Dehradun-248001, IN

Abstract

A 2375 m thick sequence consisting of sandstone-mudstone alterations was sampled for magnetic polarity stratigraphy in Somb Nadi and its tributary, Jamni-Khol in the Haripur area of Himachal Pradesh. 200 palaeomagnetic sampling sites were selected from 45 overbank intervals of fluvial cycles. Only fine-grained sediments were sampled to get the ideal results from stable single domain grains. Progressive thermal demagnetization was carried out at intervals of 25, 50 and/or 100°C. A considerably stable secondary remanant magnetization was removed above 450°C with the isolation of characteristic remnant magnetization component at 600°C. Corrected mean directions for the normal and the reversed sites are D = 349°, I= 28° and D = 168°, I= -28°, respectively. The Tatrot/Pinjor faunal event is recognised on the local magnetic polarity time scale at 2.6 Ma (1685 m level) and the Middle to Upper Siwalik transition (Dhok Pathan/Tatrot boundry) at 5.26 Ma (500 m level), approximately. Predominance of piedmont depositional system at around 3.2 Ma (1450 m level) suggests uplift and denudation of the Outer Leesser Himalaya during this period. First record of the Pre-Tertiary clast-dominated conglomerate is observed at 2.6 Ma and that of Tertiary clast-dominated conglomerate at 1.76 Ma. An average rate of sedimentation of 45 cm/1000 years was observed till 2.6 Ma. Further, it shows increasing and decreasing trend after 2.6 Ma with a maximum value of 54 cm/1000 years and a minimum of 8 cm/1000 years sediment accumulation.

Keywords

Magnetic Polarity Stratigraphy, Geophysics, Siwalik, Himachal Pradesh.

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Subhajit Sinha ¹, Sumit K. Ghosh ¹, Rohtash Kumar ¹, R. Islam ¹, P. Sanyal ², S. J. Sangode ³

Affiliations
1 Sedimentology Group, Wadia Institute of Himalayan Geology, Dehradun - 248 001, IN
2 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur - 721 302, IN
3 Department of Geology, University of Pune, Ganeshkhind, Pune - 411 007, IN

Abstract

The Neogene Siwahk fluvial succession (12 77-4 48 Ma) of the NW Himalayan foreland basin, east of Ravi River, are deposited under varied Tectono-Climatic conditions. Petrography and geochemistry of the sediments were carried out to understand the relative roles of tectonics and climate in this basin.

The sandstones are classified as sub-litharenites with subordinate arkosic-graywacke. The dental components particularly the rock fragments are consistent with the inferred source area presently disposed towards the north of the depocenter and suggest that the Higher, Lesser and Lower Tertiary formations supplied detntus since 12 77 Ma. The Th/Sc vs. Zr/Sc ratio indicates sediment recycling. The Cr and Ni, negative Eu anomaly, LREE enrichment and moderately flat HREE in the mudstones collectively suggest dominant contribution from felsic igneous rocks beside contribution from basic, sedimentary/metamorphic hithology. Near consistent nature of the dental modes and geochemical parameters through time suggest unvarying source since 12 77 Ma.The present study also infers that the source areas uplift owing to tectonic activity of Chad Thrust at least by 12 77 Ma with a perceptible activity of MBT after 8 Ma.

The 5¹⁸0 variations in soil carbonates reveals ongoing intense monsoon system since 12 77 Ma followed by a phase of aridity at 91 Ma. Presence of fresh and weathered feldspar, limestone, basic volcamcs and mica, both in humid and arid phase indicate rapid deposition and preservation. Despite the climatic turnover from humid to arid, distinctive changes in the framework mineral compositions over time is not discernable.

The petrographic and geochemical data reveals that the near similar source area, relatively rapid transport, moderate chemical weathering and sediment recycling controlled the composition of the sediments of the HFB in response to tectonics and climate.

Keywords

Climate, Provenance, Tectonics, Himalayan Foreland Basin, Siwahk, Kangra.

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Rohtash Kumar ¹, Sumit K. Ghosh ¹, S. J. Sangode ¹, V. C. Thakur ¹

Affiliations
1 Wadia Institute of Himalayan Geology P. 0. Box 74, Dehra Dun - 248 001, Uttaranchal, IN

Abstract

Neogene basin fills of the Middle and Upper Siwalik Subgroup in the central sector of Indian foreland basin record changes in fluvial architecture, dispersal pattern and provenance in response to deformation and uplift of the frontal Himalaya. The 2.4 km thick Neogene Siwalik succession in the Subathu sub-basin exposed in the vicinity of Nahan Thyst (intra-foreland thrust or IFT) shows fine-to medium-grained grey sandstones of multi-storey, sheet type deposited by transverse trunk rivers. These sandstones are gradually replaced by fine- grained buff sandstone of ribbon type deposited by relatively small piedmont rivers. A significant change in sediment dispersal from SE to SW is observed at 5.5 Ma for grey sandstone and to SE around 4.8 Ma for buff sandstone. Both the sandstones are lithic arenites with dominance of sedimentary rock fragments in the buff sandstone. The filling pattern and occurrence of softsediment deformation structures, particularly between 4.8 and 2.6 Ma, indicate tectonically controlled re-organisation of drainage pattern and gradual change in source area from Lesser Himalayan to Sub-Himalayan regions. These changes after 4.8 Ma are mainly governed by intra-foreland thrust activity, which resulted in partitioning of foreland basin. Further, a significant overlap of Tertiary clast-bearing conglomerate with that of pre-Tertiary clast-bearing conglomerate at around 1.77 Ma shows a major reactivation along IFT.

Keywords

Foreland Basin, Himachal Pradesh, Subathu Sub-Basin, Intra-Foreland Thrust, Magnetostratigraphy, Fluvial Architecture.

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R. Islam ¹, Sumit K. Ghosh ¹, Himanshu K. Sachan ¹

Affiliations
1 Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road, Dehra Dun - 248 001, IN

Abstract

The shallow marine Nagthat siliciclastics of the Jaunsar Group show extensive development of medium to thickly bedded arenites and wackes, and represent the terminal Proterozoic siliciclastic episodes of the Lesser Himalaya. Chemical variation between arenites and wackes of Nagthat siliciclastics is related to the elemental distribution within the different mineralogical phases. Inverse correlation of most of the major oxides and trace elements (Ni, Cr, Co and Nb) with SiO₂, indicate quartz dilution effect. The majority of arenites show the common presence of plagioclase, K-feldspar and clay minerals, but lack ferromagnesium minerals and therefore indicate minimal decomposition of felspar and greater decomposition of ferromagnesian minerals. This phenomenon is well documented by the positive correlation of Fe₂O₂^(t) Ni, CO, K₂O, Rb, Sr, Th, Nb and Y with Al₂O₃ hereby suggesting that elemental distributions are mainly controlled by clay minerals. A moderate correlation exists between Al₂O₃ and La and Yb suggesting that REEs are similarly controlled in part by clays or micas. The plots of Nagthat siliciclastics in A-CN-K diagram, along with their enriched K₂O contents suggest variable degree of post-depositional K-metasomatism. Presence of higher amounts of monocrystalline quartz than the polycrystalline quartz, characteristic major and trace element contents, and enriched LREE pattern with negative Eu anomaly indicate a source of granite or granitic gneiss. High Eu/Eu ratio, low Ni concentration and low La/Co, Th/Co and Cr/Th ratios corroborate the acidic source. Granite or granitic gneiss source signature, along with indicative palaeocurrent directions, suggest that the detritus for Nagthat siliciclastics may have been derived from the southerly direction where Banded Gneissic Complex (BGC) of Aravalli-Bundelkhand craton is widely developed.

Keywords

Neoproterozoic, Nagthat Siliciclastics, Source Rock, Geochemistry, Banded Gneissic Complex (BGC), Kumaun Lesser Himalaya.

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