Organotemplate silica deposition in Neoproterozoic deep-marine environments: evidence from the Penganga Group, Adilabad, India

Silica in bedded cherts interstratified with manganese carbonates in a deep‐water carbonate ramp succession of the Neoproterozoic Penganga Group, India, displays film‐like, spherical to rod‐shaped, and tubular branching microstructures. The microstructures resemble mineralized extracellular polysaccharides, biofilms and bacterial morphologies. The microstructures suggest silicification by nucleation of silica on organic‐templates or indirectly as sorbed species accumulating on organic templates. Given that similar microstructures have also been documented in Archean cherts it is proposed that organotemplates might have been an important sink for the deposition of silica in Precambrian deep‐water marine environments.     

Free vibration of semisubmersibles: A parametric study

Like any other engineering structure, the dynamic aspects of semisubmersible offshore platforms require serious consideration. The free vibrations of the semisubmersible structures have been investigated in the present work, in which the effect of the variation of the length, draft and hull spacings on the natural frequencies and mode shapes has been studied and the nature of the variations and their reasons have been discussed.     

Application of visual,statistical and artificial neural network methods in the differentiation of water from the exploited aquifers in Kuwait

The ability of artificial neural network to differentiate water samples from the two aquifers of Kuwait on the basis of their major ion chemistry has been demonstrated. The major ion concentration distribution in the groundwater of the Kuwait Group and the Dammam Formation aquifers of Kuwait appears very similar. Cross-plots, supported by the discriminant function analysis of the data, however, suggest that there are some subtle differences in the overall composition of the water from the two aquifers that make it possible to differentiate the water from the two aquifers in almost 80% of the cases. An artificial neural network improved the differentiation capability to 90% of the cases. It is also possible to estimate the fraction of Kuwait Group water in the flow stream of dually completed wells with the help of an artificial neural network developed for this purpose. Electronic Publication     

Comparison of source rock geochemistry of selected rocks from the Schei Point group and Ringnes formation, Sverdrup basin, arctic Canada

Organic-rich from the Schei Point group (middle to late Triassic in age) and the Ringnes formation (late Jurassic) from the Sverdrup basin of the Canadian arctic archipelago have been geochemically evaluated for source rock characterization. Most samples from the Schei Point group are organic-rich (> 2% TOC and are considered as immature to mature oil-prone source rocks [kerogen types I, I–II (IIA) and II (IIA)]. These kerogen types contain abundant AOM1, AOM2 and alginite (Tasmanales, Nostocopsis, Leiosphaeridia, acritarch and dinoflagellate) with variable amounts of vitrinite, inertinite and exinite. Samples from the Ringnes formation contain dominant vitrinite and inertinite with partially oxidized AOM2, alginite and exinite forming mostly immature to mature condensate- and gas-prone source rocks [kerogen type II–III (IIB), III and a few II (IIA)]. Schei Point samples contain higher bitumen extract, saturate hydrocarbons and saturate/ aromatic ratio than the Ringnes samples. Triterpane and sterane (dominant C₃₀) distribution patterns and stable carbon isotope of bitumen and kerogen suggest that the analyzed samples from the Schei Point group are at the onset of oil generation and contain a mixture of sapropelic (algal) and minor terrestrial humic organic matter. Sterane carbon number distributions in the Ringnes formation also suggest a mixed algal and terrestrial organic matter type. There are some variations in hopane carbon number distributions, but these are apparently a function of thermal maturity rather than significant genetic differences among samples. Pyrolysis-gas chromatography/mass spectrometry of the two samples with similar maturity shows that the Schei Point sample generates three times more pyrolyzate than the Ringnes sample. Both samples have a dominant aliphatic character, although the Ringnes sample contains phenol and an aromaticity that is higher than that of the Schei Point sample.     

Petrology,geochemistry and geochronology of the Chilka Lake igneous complex,Orissa state,India

The Chilka Lake igneous complex of Orissa, the largest known anortosite massif of the Indian Shield, occurs in a catazonal environment of high-grade metamorphics of the Eastern Ghats Precambrian Orogenic Province. The syntectonic massif consists of the anorthositic Balugaon dome, leuconoritic Rambha lobe and quartz-mangeritic Kallikota cover. A completely gradational suite comprising anorthosite-leuconorite-norite-minor jotunite (the anorthositic suite) constitutes most of the complex. The subordinate of suite of acid rocks spatially associated with this is of a broad quartz-mangeritic lithology with minor granitic rocks (the acidic suite). Geochemical evolution of the complex in the sequence anorthosite-leuconorite-norite-jotunite-acidic rocks shows moderate iron enrichment in the noritic-jotunitic stage and is marked by an overall decrease in Al₂O₃, CaO, MgO, Ni/Co, Sr/Ba, K/Rb and increase in SiO₂, K₂O, V/Ni, K/Ba and Rb/Sr. Such progressive variation in geochemical parameters appears (i) essentially gradual and frequently overlapping in rock members of the intergradational anorthositic suite and (ii) rather abrupt across transition zones between the anorthositic suite and the acidic suite due to near absence of intervening intermediate lithologies. RbSr whole rock isochron studies indicate that the complex was emplaced ca. 1400 Ma ago. The initial ⁸⁷Sr/⁶⁸Sr (0.70661) implies limited hybridisation of the parent magma prior to emplacement. A critical appraisal of all the available evidence suggests that (i) the anorthositic suite of rocks form a perfectly consanguinous and comagmatic assemblage and (ii) the spatially associated acidic suite emerged through a convergence of magmatic and metasomatic processes (the latter brought about by contact anatexis of the host rocks). The complex as well as the host metamorphics are intruded by an atectonic suite of noritic dykes emplaced ca 850 Ma ago.     

Evidence for the thrust emplacement of the ‘Lesser Himalaya’ Chur granite,Himachal Pradesh

Numerous peraluminous and porphyritic granitic bodies and augen gneisses of granitic compositions occur in the nappe sequences of the Lower Himalaya. They are Proterozoic-to-lower Paleozoic in age and have been grouped into the ‘Lesser Himalaya granite belt’. The mode of emplacement and tectonic significance of these granites are as yet uncertain but they are generally considered to be sheet-like intrusions into the surrounding rocks. The small and isolated granite body (the Chur granite) that crops out around the Chur peak in the Himachal Himalaya is one of the more famous of these granites. Several lines of evidence have been adduced to show that the Chur granite has a thrust (the Chur thrust) contact with the underlying metasedimentary sequence (locally called the Jutogh Group). The Chur granite with restricted occurrence at the highest topographic and structural levels represents an erosional remnant of a much larger sub-horizontal thrust sheet. The contact relations between the country rocks and many of the other granite and granitic augen gneisses in the Lesser Himalaya belt are apparently similar to that of the Chur granite suggesting that at least some of them may also represent thrust sheets.     

Disturbances produced by variable body forces in a layer of finite thickness resting on a rigid foundation

Summary The disturbances produced by time-dependent torsional body forces in a layer of elastic material lying on a rigid foundation have been discussed in this paper. Solutions have been obtained first for a general type of body force, and the solutions for an impulsive cylindrical source and an impulsive torsional line source of finite length have been deduced. It has been found that for any point in the layer, there is no disturbance up to a certain instant, after which there is a sudden displacement, which gradually falls off and tends to zero as the timet increases.