High-resolution profile of stable isotopes and iridium across a K/T boundary section from Koshak Hill, Mangyshlak, Kazakhstan

Oxygen and carbon isotopes were measured to a high depth resolution in a nannoplankton carbonate sequence spanning the Cretaceous-Tertiary boundary at Koshak Hill, Mangyshlak Peninsula, Kazakhstan. The boundary is characterized by the presence of a ∼ 1 cm thick clay layer having a sharp peak in iridium concentration with a maximum value of 3.7 ng g^-1. The δ¹⁸O data reveal rapid (∼10³ years) excursion of sea-surface temperature at the boundary where an initial cold pulse is followed by a persistent warm period. The δ¹³C data, in contrast, indicate only a gradual change in productivity across the boundary. The observations suggest a biogeochemical scenario for the boundary event pertaining to shallow epicontinental seas.     

Carbon and oxygen isotopic variations in peritidal stromatolite cycles, Paleoproterozoic Kajrahat Limestone, Vindhyan basin of central India

Variations of carbon and oxygen isotopic ratios in response to cyclical sea level fluctuations have been documented from a Paleoproterozoic peritidal stromatolite succession. The upper division of the Kajrahat Limestone, Vindhyan Supergroup of central India consists of several shallowing upward stromatolite cycles identified by regular and systematic changes in stromatolite size. Normally, larger stromatolites are followed upward in the succession by smaller stromatolites and microbial laminites that occupy the top of the cycle. Desiccation cracks are found in all the facies indicating subaerial exposure. We investigated the stable isotope compositional variations across nine complete stromatolite cycles showing frequent subaerial emergence. Carbon and oxygen isotopic values of the limestones, in general, are comparable to contemporary marine values available from earlier studies but show regular depletion in response to shallowing of the water level. The δ¹³C and δ¹⁸O values of the limestones vary within an individual stromatolite cycle; depleted values characterize the topmost part of the cycles. The isotope pattern is explained by micritic carbonate deposition in different sub environments of the shallow marine domain having different salinity and variable duration of exposure. These variations also probably caused the observed scatter in δ¹³C and δ¹⁸O values of supratidal microbial laminites.     

Facies architecture of asymmetrical branching distributary channels: Cretaceous Ferron Sandstone,Utah, USA

Distributary channel systems are an important component of deltaic systems, but details of their branching pattern, stream‐order, internal variability and relation with adjacent levée, bay and bayhead delta are rather poorly documented in ancient examples. Photomosaic and measured sections collected along a gooseneck‐shaped canyon in southern Utah allow direct mapping of the branching pattern of an ancient distributary system. The main channel belt is ca 250 m wide and narrows to ca 200 m downstream of the branching point. A subordinate channel belt, ca 80 m wide, branches off of the main channel, forming a distinctly asymmetrical branching pattern. Water discharge in the main channel is estimated to be 85 to 170 m³ sec^?1. Comparison with palaeodischarge estimates of trunk rivers mapped in previous studies suggests that the branching documented in this study probably is a fourth‐order split. The distributary channels are characterized by a U‐shaped geometry filled with medium‐grained, cross‐bedded sandstone, and are dominated by lateral accretion, suggesting limited lateral migration and moderate sinuosity. Tidally influenced facies and limited trace fossils indicate direct marine influence. The distributary channels erode into adjacent levée and underlying heterolithic bay‐fill deposits, and the marine influence suggests that they were deposited on a lower delta plain, rather than on a non‐marine floodplain. The subordinate channel fed a bayhead delta, suggesting that it was formed by a partial avulsion, rather than bifurcation around a mouth bar, as is more characteristic of terminal distributary channels. Channel‐floor drapes, bar‐accretion drapes and abandoned channel fills within the sandstone channel belts represent the most important heterogeneity from the perspective of reservoir characterization.     

On short period signals obtained from explosions within the earth

Summary Propagation of dilational waves in radial directions from a spherical cavity in a medium with velocity varying periodically with distance has been considered. It has been shown that when the wave-length of the propagating wave is twice the wave-length of variation of the velocity in the medium the displacement even at a large distance is very great.     

General relativistic spectra of accretion discs around rapidly rotating neutron stars: effect of light bending

We present computed spectra, as seen by a distant observer, from the accretion disc around a rapidly rotating neutron star. Our calculations are carried out in a fully general relativistic framework, with an exact treatment of rotation. We take into account the Doppler shift, gravitational redshift and light-bending effects in order to compute the observed spectrum. We find that light bending significantly modifies the high-energy part of the spectrum. Computed spectra for slowly rotating neutron stars are also presented. These results would be important for modelling the observed X-ray spectra of low-mass X-ray binaries containing fast-spinning neutron stars.     

An assessment of IS codal provisions for the design of low rise steel moment frames through incremental dynamic analysis

The current Indian Standard (IS) code for seismic design of structures (IS 1893:2002) specifies the use of time history analysis for structures with height greater than 40m. However, for structures less than 40m it recommends the concept of equivalent static analysis. This study attempts to investigate the adequacy of the current design code when it comes to the actual evaluation of structures shorter than 40 m subjected to seismic loading using dynamic analysis as opposed to the code specified static analysis. Incremental dynamic analysis, which subjects a structure to a progressively increasing series of intensity measures, has been adopted here for the purpose. Three 2D moment resisting steel structures under the 1991 Uttarkashi and the 2001 Bhuj earthquakes (both of which predate the current IS1893) have been studied—a single storeyed portal frame, a 2 storey 3 bay frame and a 3 storey 2 bay frame. While it can be argued that two records are never enough for any generalization, and that only a full probabilistic analysis can determine if the limiting collapse prevention probability has been exceeded for these structures, the IS code in both cases does significantly under predict the seismic demands on the structures. At the same time, and perhaps why the codal provisions usually work, the structural capacities are in most cases underestimated as well. These suggest that a thorough study is in order and that there is scope for rationalization in the IS codal provisions.     

High-pressure silicate liquidus data: Retrieval of partial molar enthalpy and partial molar entropy of mixing,a test of the regular solution formulation,and comments on theP-T modelling of ascending magmas

The activity of a silicate liquid component in a melt at an elevated liquidus temperature and pressure may be expressed analytically in terms of the 1-bar liquidus temperature activity and functions of the partial molar volume and partial molar enthalpy of mixing. Alternatively, the activity of the elevated (i.e. higherP-T) liquidus may be expressed in terms of the difference of heat content, heat capacity, entropy and volume of the component in the crystalline form and in the melt. Equating these two expressions, the partial molar enthalpy of mixing and there-from the partial molar entropy of mixing may be determined, provided the liquidus temperatures of the phase in question at both 1 bar and higher pressure and at a constant melt composition are known. Several such retrievals for CaMgSi₂O₆, Mg₂SiO₄, NaAlSi₃O₈, and TiO₂ from experimental phase equilibrium data are presented. It is argued that as the partial molar enthalpy of mixing generally has large values, the regular solution formulation on the basis of a constant function of the activity coefficient would lead to erroneousP-T estimates for ascending magmas.     

Bending–buckling interaction as a failure mechanism of piles in liquefiable soils

This paper investigates the importance of bending–buckling interaction in seismic design of piles in liquefiable soils using numerical techniques. A pseudo-static analysis has been performed using a well documented case history, where the pile–soil interaction is modelled as a beam on nonlinear winkler foundation (BNWF). Six possible analytical methods, three force based and three displacement based, are performed in which the pile is subjected to both lateral and axial load. Three out of six analysis cases did not predict the failure of the piles when analysed only for bending (i.e., lateral loads only). The buckling analysis showed that the pile was also safe against pure buckling during full liquefaction. Further, two out of those three cases which did not predict failure in bending were reanalysed for bending–buckling interaction (i.e., lateral and axial loads acting simultaneously). These combined analyses showed a more realistic behaviour of pile response and did predict the pile failure. Hence, it can be concluded that if a pile is designed for bending and buckling criteria separately and safe for these individual design criteria, it may fail due to their combined effect.