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

S. Sinha-Roy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

The mineral beach sands of the southwest coast of India, particularly of Chavara and Manavalakurichi areas, contain 4-6% zircon. Although the general setting of the zircon-bearing sands in these two areas is similar, the zircon grain characteristics show some differences. The Manavalakurichi zircon is coarser than the Chavara zircon and in the former, the grain-size has in general a positive correlation with the aspect ratio, whilst in the latter, the correlation is not straightforward. Zoning is more prominent in the Chavara zircon than in the Manavalakurichi zircon. In contrast to the Chavara zircon, the Manavalakurichi zircon shows statistically significant preferred association of idiomorphic grains with microfractures. The level of inclusions (ilmenite, rutile, monazite, quartz, apatite and fluid), either in single phase or in polyphase, is higher in the Manavalakurichi zircon than in the Chavara zircon. The source rock (s) of these zircons may not have been much dissimilar.

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S. Sinha-Roy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

The Kerala basic dykes that cross-cut the crystalline rocks are quartz or olivinenormative tholeiites. In Trivandrum district. south Kerala, two of these dykes have yielded K-Ar whole-rock dates of 127±2 m.y. and 104±5m.y., whilst the third, 476± 11 m.y. The 104-127 m.y. group age dates the thermal response of the western continental margin distension, and it relates also to the early phase of the Deccan Trap volcanism. The 476 m.y. date is either obtained via crustal contamination or more likely, it represents an older and unrelated thermal event. This event has analog in South Africa, including Madagascar.

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S. Sinha-Roy ¹, T. Radhakrishna ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

A basic dyke swarm trending ENE-WSW traverses the Agali area of north-central Kerala. The geochemistry of these dykes suggests that they are transitional in character between continental and oceanic basalts. and are akin to one of the magmatic phases of the Deccan volcanics. They have been derived from a tholeiitic magma generated by partial melting of a heterogeneous mantle containing minor amounts of phlogopite and apatite. Fractionation of clinopyroxene. magnetite and plagioclase is indicated. The present study supports one of the earlier suggestions that these dykes are related to the rifting of the western continental margin of India.

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S. Sinha-Roy ¹, G. R. Ravindra Kumar ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 691 010, IN

Abstract

Fuchsite-bearing quartzite, is an important lithology of the high-grade supracrustal sequence where they constitute the extensions of Sargur Group of Karnataka in the Cannanore district, north Kerala. It occurs as interlayered bands with amphibolite, deformed gabbro and trondhjemite and this assemblage is suggested to represent a part of tectonised and metamorphosed oceanic chert and magmatic sequence.

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S. Sinha-Roy ¹, John Mathai ¹, Narayanaswamy ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum, IN

Abstract

Cordierite-bearing gneiss of the Quilon district, Kerala, occurs in a 8-10km wide zone within quartzo-feldspathic gneiss, being bordered on the north by a charnockite massif. These gneisses are deformed and are polymetamorphic in character. The cordierite-forming metamorphism is the second in metamorphic chronology. The reaction producing the bulk of cordierite was: biotite + sillimanite + quartz ⇌ cordierite + garnet + K-fetdspar + H₂O. The K_D (Mg-Fe) values of coexisting garnet-cordierite and garnet-biotite pairs indicate that the PT conditions of metamorphism (∼ 7 kb, ∼ 720°C) were those of granulite facies. Under these conditions cordierite was produced from Fe-Mg enriched melt restite, being trapped at the fringe of the charnockite massif.

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S. Sinha-Roy ¹, G. R. Ravindra Kumar ¹

Affiliations
1 Centre for Earth Science Studies, Trivandrum 695010, IN

Abstract

Pseudotachylyte veins occur sporadically within deformed granodiorite-tonalite body and mylonitic gneiss of possible Sargur-equivalent sequence in North Kerala. Pseudotachylyte-bearing zones constitute a part of the wide Bavali fault zone, a continuation of the Moyar shear zone. The second phase of deformation of probable Mid Late Proterozoic age shows a progression in strain regime from mylonite-forming ductile strain through moderate strain rate stage forming ultramylonite to a high strain rate stage forming pseudotachylyte. Pseudotachylyte developed via comminution and frictional melting at different stages within the fault zone. Although it appears to be in place of its origin; pseudotachylyte has not strictly attained 'minimum' melt composition.

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S. Sinha-Roy ¹, G. R. Ravindra kumar ¹

Affiliations
1 Centre for Earth Science Studies, P. B. 2235, Trivandrum 695 010, IN

Abstract

The Ezhimala pluton emplaced in possible Sargur-equivalent sequence in North Kerala is a differentiated suite showing a continuous compositional range from gabbro to granite through tonalite and trondhjemite. Geochemistry indicates that the suite is the product of diapiric emplacement of magma generated at shallow depth and of fractional crystallization in which the major participating phases were hornblende and plagioclase. Biotite and Fe-Ti oxides were minor fractionating minerals. There is an indication that this suite is one of the many igneous bodies of variable composition which are the products of important magmatic event in the Kerala region, in late Proterozoic and early Palaeozoic times.

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S. Sinha-Roy ¹

Affiliations
1 A 4, Vijaipath, Jaipur 302004, IN

Abstract

Ignimbrite and ash-flow tuff's have been identified within the graywacke unit of the Daling turbidite sequence of the Sikkim Lesser Himalaya. These, together with basic flows constitute the basal part of Proterozoic Daling Formation. Geochemistry of these rocks indicates that the tuffs are derived from calc-alkaline volcanism while the basic flows have MORB characters. It is concluded that within the Daling ocean (ProtoTethys or Palaeo-Tethys) graywacke was deposited as a trench prism, the tuffs were the products of island-arc explosive volcanism, and the basic flows, of spreading in the back-arc basin. This island arc system disappeared due to tectonic activity which deformed (the first recognisable deformation) the Daling rocks. This event probably represents the Caledonian orogeny in the Himalayas.

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S. Sinha-Roy ¹, M. Mohanty ¹, G. Malhotra ¹, V. P. Sharma ¹, D. W. Joshi ¹

Affiliations
1 Geological Survey of India, 15-16 Jhalana Dungri, Jaipur 302 004, IN

Abstract

The Proterozoic sequences of South-Central Rajasthan are contained in the Lower to Middle Proterozoic Aravalli and Middle to Upper Proterozoic Delhi fold belts. The stratigraphic relations within these fold belts as well as the basement-cover relations are beset with several problems. The conglomerate horizons have been studied to address some of these problems. Based on the conglomerate horizons, representing erosional unconformities, the Aravalli Supergroup has been sub-divided into three groups and the Delhi Supergroup of the South Delhi fold belt into two groups with constituent formations. These horizons are also useful in identifying three stages of basin evolution for the Aravalli fold belt. The Middle Aravalli Conglomerate is the most prominent representing mid-Aravalli hiatus, and corresponding to the Debari Transgressive Stage which followed the Delwara Rifting Stage in the Lower Aravalli in a system of ensialic distensional tectonics where triple-junctions and pull-apart basins are formed. The Upper Aravalli has been the Jharol Ocean Opening Stage, the oceanic/transitional crust having been consumed at a median ultramafic suture line during the formation of the Aravalli fold belt. The South Delhi fold belt has also multi-stage development history with the Barr Conglomerate representing the base of the youngest Delhi sequence.

Keywords

Proterozoic, Conglomerate, Aravalli Fold Belt, Delhi Fold Belt, Rajasthan, Basin Evolution.

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S. Sinha-Roy ¹

Affiliations
1 Birla Institute of Scientific Research, Statue Circle, Jaipur - 302 001, IN

Abstract

Stylolites are the product of pressure solution mechanism that plays an important role during diagenesis and Hthification of sedimentary rocks. There are several problems related to geometry, structure and origin of stylolites, some of which have been addressed in this paper.

From a study of the stylolites developed in the limestone of the Nimbahera Formation of the Semri Group of the Lower Vindhyan sequence in Chittorgarh district, SE. Rajasthan, different stages of formation of differentiated stylolites, leading to their variable geometry and microstructures, have been identified. The stylolite geometry changes from an initial planar type through conical type to columnar and wavy types with progressive pressure solution in domains of microtextural instability caused by the non-uniform concentration and random distribution of relatively insoluble opaque and clay mineral grains in layers and clumps, and by an uneven distribution of rock pore-maxima. In the planar type of stylolites the fluid taking the soluble phases (CaCO₃, SiO₂) out of the pressure solution seam, flows transverse to the local maximum compression (a,) direction and parallel to the planar stylolite seam. At a later stage when textural instability that is imparted by and the blockade to fluid-flow that is caused by the concentration of insoluble clay and opaque mineral grains in the stylolite plane lead to a loss of permeability whereby the fluid-flow gets partitioned at the planar stylolite seam. In that situation while the fluid-flow parallel to stylolite seam is reduced, a part of the fluid changes flow polarity, finds new avenues and the flow takes place at a high angle to the planar stylolite seam. The new fluid-flow regime, facilitated by conjugate shear microfractures and microslides at stylolite sides, and enhanced pressure solution due to stress focussing at stylolite head, drives the stylolite head away from the stylolite seam plane. This increases the amplitude of the stylolite cones and columns, causing their displacement parallel to the local maximum compression (a,) direction, given by the stylolite facing direction, away from the stylolite seam plane. This causes the initially planar stylolite seam to differentiate into cones and columns.

At the late-or end-stage of diagenesis when pressure solution diminishes the insoluble residues of opaque and clay minerals forming a mush within the stylolite seam shrink and collapse, giving rise to the secondary compositional layering, and to the kinks and microfaults in the layers. The major part of the saturated fluid film, enveloping the stylolite at that instant, is unable to flow and escape, and progressive crystallisation of quartz first and calcite later takes place at the margins of the stylolite seam. Dimensionally oriented calcite fibres grow in dilation zones from this fluid film under instantaneous extensional regime, caused by volume loss of the seam material and shrinkage-controlled progressive displacement of the stylolite seam-walls, parallel to the stylolite facing direction, to give rise to antitaxiat fibrous bands around the stylolite seam.

Keywords

Stylolite, Pressure Solution, Microstructures, Fluid-Flow Partitioning, Antitaxial Fibrous Band, Vindhyan Limestones, Rajasthan.

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S. Sinha-Roy ¹

Affiliations
1 Birla Institute of Scientific Research, Statue Circle, Jaipur 302 001, IN

Abstract

Longitudinal river profiles can be quantified by normalizing the elevation and the distance along rivers. Several parameters such as profile shape (concavity), gradient fluctuations (knickpoints), river grade and valley incision have been derived from longitudinal river profile. These quantified parameters and their interrelations are useful in comparing different drainage basins. These also help draw inferences on neotectonism. These parameters, either individually or in combination, substitute some hydrological factors such as river capacity and competence, and provide clues to the morphotectonic evolution of drainage basins.

Ten tributaries (5^th, 4^th and 3^rd order) whose cumulative catchment area is ca. 82% of the 6^th order Banas drainage basin, the largest in Rajasthan, have been studied. Their longitudinal profile parameters have been interpreted, especially in respect of neotectonic activities. It is concluded that the Aravalli hill range and the surrounding areas in the SW segment of the basin are being currently uplifted, while the NE block is generally subsiding. Within these major neotectonic blocks, there are several sub-Blocks that have differentially moved nearly vertically to give rise to horst-Graben structures bounded by active faults, generally trending N-S and NE-SW. Some of these faults represent reactivated old dislocation zones. These conclusions have been substantiated by neotectonically controlled geomorphic features. Strike-Slip faulting in the NE neotectonic block and differential vertical movement of the neotectonic sub-Blocks have played an important role in forming the structural depressions and grabens. One such structural depression is the Sambhar Lake that is likely to have formed as a pull apart basin where neotectonic uplift of the sub-Blocks caused stream capture and ponding. Several faults in the area are seismically and thermally active.

Keywords

Banas Drainage Basin, Longitudinal River Profile, Neotectonics, Pull-Apart Structures, Aravalli Uplift, Sambhar Lake, Rajasthan.

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S. Sinha-Roy ¹, G. Malhotra ¹

Affiliations
1 Geological Survey of India, A-4, Vijay Path, Jaipur 302004, IN

Abstract

In the Jahazpur belt of SE Rajasthan, the Proterozoic Jahazpur cover sequence of dolomite, BIF, orthoquartzite and tuffaceous carbonaceous phyllite unconformably overlies the Archaean basement comprising granite-greenstone sequences of the Hindolis and the Mangalwar Complex. The Jahazpur rocks developed in ensialic aborted rifts. Structures of four deformations in the basement rocks and of three deformations in the cover rocks have been recognised, the first deformation of the basement rocks being pre-Jahazpur in timing. The first and the second regional folds in the Jahazpur sequence have produced type-2 interference pattern. The original orientation of the first fold was almost E-W. The second fold, trending NE-SW, reoriented the first deformation structures, and controlled the linear trend of the belt. The unconformity between the cover and the basement has been modified by thrusting.

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S. Sinha-Roy ¹

Affiliations
1 Birla Institute of Scientific Research Statue Circle Jaipur - 302 001, IN

Abstract

No Abstract.

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S. Sinha-Roy ¹

Affiliations
1 Birla Institute of Scientific Research, Statue Circle, Jaipur 302001, IN

Abstract

The hypsometry (area-elevation analysis) of drainage basins is a useful method to decipher the stages of landform evolution and assess the influence of geologic and tectonic factors on topography. Various parameters of hypsornetric curve have been identified and defined here, and their implications discussed. An empirical relation involving the mean elevation and the hypsometric integral has, been derived to determine the relative tectonic uplift of the subcatchments.

The hypsometry of the 6^th order Banas drainage basin in Rajasthan, and its nine 5^th and 4^th order subcatchments have been analysed. In the Mashi and Morel subcatchments, catchment widening is accompanied by significant channel networking and high erosion, leading to mass accumulation and elevation increase at the lower reaches. Other subcatchments show 2D stream branching without significant valley incision and downstream mass accretion. The subcatchment aspect ratio has influenced the hypsometric curve shape. The landform age of the subcatchments appears to increase in the following order: Dhil, Dai, Kothari, Chandrabhaga, Mashi, Morel, Berach, Khari and Galwa.The hypsometry of the studied subcatchments distinguishes two landform development trends. Two landform evolution models, one based on discharge-dominant fluvial process and the other on slope-dominant diffusive process, have been derived.

Tectonic uplift deduced from hypsometry is relatively greater in the SW part of the Banas drainage basin than in the N and NW parts. This is because of differential vertical movement of several tectonic sub-blocks, bounded by reactivated old dislocation zones and active faults. Tectonic uplift seems to have either defeated or matched the denudation in the Aravalli hill range, and the Banas catchment landform has approached a stage of steady-state equilibrium.

Keywords

Hypsometric Parameters, Landform Evolution Model, Banas Drainage Basin, Aravalli Uplift, Rajasthan.

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