Late Cretaceous mafic dykes in the Dharwar craton

Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (D _m =329°,I _m =−55°) and the corresponding palaeopole coordinates (λ _p = 34°S,L _p =108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between 69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events.     

Hydroclimatic fluctuations of the Upper Narmada catchment and its association with break-monsoon days over India

In this paper, hydroclimatic fluctuations of the Upper Narmada catchment (upto Narmadasagar damsite) have been studied by examining the time series (1901–80) of (i) 1-to 10-day annual extreme rainfall; (ii) seasonal total rainfall between May and October; (iii) the precipitation concentration index (PCI); (iv) a modified version of PCI(MPCI); and (v) parameters of the periods contributing specified percentages of rainfall to annual total. Most of these parameters followed the normal distribution and did not show any significant long-term trend. However, some dominant long period oscillations have been noticed in extreme rainfall, seasonal rainfall, PCI and MPCI series. Influence of break-monsoon days over India during July and August on the rainfall activities of the Upper Narmada catchment has also been investigated and salient findings discussed.     

Radioelement geochemistry of alkali granites of the Kerala region,south-west India

Th, U and K abundances in four alkali granites of the Kerala region, south-west India, are presented. The plutons show high radioelement levels, correlatable with those of alkali granites in other regions. The nature of variation is consistent with the correlation of Th and U with accessory phases like sphene, zircon, allanite, apatite and monazite. A geochronologic correlation is also observed between the alkali granites and the Th-bearing beach placers of the region. The petrogenetic features of the alkali plutons, their taphrogenic association, Pan-African affiliation and high Th/U levels suggest that the alkali plutons are favourable locales for radioelement exploration.     

Genesis of certain bauxite profiles from India

An attempt is made to reveal a geochemical compatibility with paleoenvironment to understand the genesis of bauxite deposits that are derived from two different parent rocks. For this purpose two in situ bauxite profiles, one developed over basalt and another over sandstone, have been taken. Gibbsite is the major mineral in all these bauxites with minor occurrences of goethite, hematite, anatase, kaolinite, etc. From the study it is evident that the paleoenvironment and geochemical processes are vital factors that could lead to the formation of bauxites. The significance of this is fully discussed.     

Lateritisation of gabbro-granophyre rock units of the Ezhimala Complex of north Kerala, India

Laterites are ubiquitous in Kerala and are developed over different rock types. The laterite profiles developed over gabbro, granophyre of the Ezhimala Complex are discussed here.

The principal mineral assemblages of the laterites are kaolinite, gibbsite, goethite, hematite and quartz with minor amounts of zircon. Amorphous ferric hydroxide, Al-hydroxide and Al-silicate are also present. Samples from various units of the weathering profile have been analysed for major and trace elements. Major-element data show a steady depletion of Si and enrichment of sesquioxides and TiO₂ during weathering. It is also observed that the more the content of the mobile elements (Si, Mg, Ca, K, Na) in the parent rock, the greater is its depletion during lateritisation. The trace elements Co, Cr, V, Cu and Ni show an enrichment while Zn and Ba show depletion over both the rock types. Pb shows almost a consistent behaviour.     

Weathering potential index for rocks based on density and porosity measurements

Rock samples belonging to ten lithological types under different stages of weathering, were collected from different stratigraphical horizons at Bhagalpur. Their densities and porosities were determined experimentally and the data obtained were fitted empirically in a linear equation for each lithological type. The slopes of the curves, which were negative in each case, showed that the increase in porosity for the same decrease in density were in the order, white sandstone > ferruginous sandstone > white claystone > porphyritic gneiss > quartzite > pegmatite > amphibolite ≅ biotite gnejss > basalt ≅ dolerite. A new weathering potential index based on the density-porosity data was proposed and the values for a specific stage of weathering for all the lithological types studied fall within the same range.     

Electron delocalization and magnetic behavior in a single crystal of ilvaite,a mixed valence iron silicate

Ilvaite, Ca(Fe²⁺, Fe³⁺)Fe²⁺Si₂O₈(OH), a black mixed valence iron silicate shows considerable Fe²⁺?Fe³⁺ electron delocalization above 400 K, reminiscent of magnetite. A crystallographic phase transition from orthorhombic (Pnam) to monoclinic (P2 ₁/a) symmetry takes place on cooling at 343 K induced by electron ordering. In both phases, Fe²⁺ and Fe³⁺ occur in double octahedral chains parallel to the c axis. The thermal characteristics of the magnetic susceptibilities and their anisotropies in different crystallographic planes have been measured in the temperature range 400?21 K. Below 343±1K, a continuous rotation of the molar susceptibility K _∥ in the ab plane down to 90±2 K is observed, where the symmetry of the magnetic ellipsoid remains unchanged. X _a, X _b and X _c increase abruptly below 123±0.5 K, although antiferromagnetic ordering of Fe²⁺ and Fe³⁺ spins on A sites was suggested in previous Mössbauer and neutron powder diffraction studies. In addition, 1/X _a shows an antiferromagnetic maximum at 50±3 K, whereas 1/X _b and 1/X _c at first increase sharply below 123 K, followed by antiferromagnetic curvatures in the lowest temperature region. This behavior is consistent with the antiferromagnetic ordering of Fe²⁺ spins in the B sites. The observed magnetic phenomena suggest charge delocatization effects between adjacent Fe²⁺(A)-Fe³⁺(A) pairs not only along c, but also along a and b directions. The negative sign of the molar anisotropy (K _∥-K_⊥) suggests a singlet ground State ⁵A₁ for the Fe²⁺ ions, in agreement with previous Mössbauer studies.     

Flood inundation mapping in a part of Mahanadi river basin

Man has had to live with floods since the very inception of his existence. Fury of flood causes heavy damages to both life and property. Accurate flood inundation data is one of the essential requirements for effective management of flood problem. Remote Sensing methods are well suited for acquiring flood inundation data because of synoptic, repetitive coverage of the satellite data. This paper briefs on an attempt to map flood inundated areas in a part of Mahanadi river basin using remote sensing data.