A comparison of different magnetic methods for determining the relative grain size of magnetite in natural materials: Some results from lake sediments

A new rapid method for identifying relative grain size variations in magnetic involves the parameter anhysteretic susceptibility (χ_ARM, i.e. specific ARM obtained in a 1 Oe steady field), which is particularly sensitive to the single domain (SD) and small pseudo-single domain (PSD) grains of the finer magnetite fraction. A second parameter, low-field susceptibility (χ), is relatively more sensitive to the coarser magnetite fraction (larger PSD and smaller multidomain (MD) grains). We can then obtain a measure of the ratio of coarse- to fine-grain magnetite for large numbers of samples by plotting χ_ARversusχ. A simple idealized model based on sized magnetite samples is proposed to explain the use of the χ_ARMversusχ plot for detecting relative grain-size changes in the magnetic content of natural materials. The sediments of three lakes that contain magnetite or a similar magnetic carrier and have a wide range of values of χ_ARM and χ are used to test the model.The model is used to interpret the magnetic variations observed, and the interpretations are supported by high-field hysteresis measurements of the same sediments. The combination of the high-field hysteresis method of Day et al. [1] and the χ_ARM vs. χ method is a powerful technique allowing the rapid identification of both the relative grain size and domain state for large numbers of samples containing magnetite. The χ_ARMvs.χ method should be used as an intial means of identifying distinct groups of samples.The high-field hysteresis method should then be applied to a few representative samples from each group to confirm the initial interpretation.     

Distortion of planar structures around rigid spherical bodies

Theoretical and experimental results show that determination of sense of rigid rotation from drag-patterns of foliation around a rigid spherical inclusion (such as a porphyroblast of garnet) is not as simple as was previously thought. An asymmetric drag-pattern can develop even when the bulk deformation is non-rotational, provided a pre-existing foliation was initially at an angle to the principal axes of strain.In simple shear, the drag-pattern of a pre-existing foliation around the rigid inclusion may belong to any one of the following four types:

1. (1) asymmetric pattern, with the same sense of drag all over the contact.

2. (2) more or less symmetrical pattern, with different senses of drag at different parts of the contact.

3. (3) asymmetrical pattern, with different senses of drag.

4. (4) pattern with inward bowing of foliation (i.e. convex toward the rigid inclusion).

The geometry of the distorted foliation will depend on the amount of simple shear and the initial angle between the pre-existing foliation and the direction of simple shear.If the development of schistosity and the subsequent development of drag around a rigid porphyroblast are results of the same continuous simple-shear movement, a wellpreserved drag-pattern may be utilized to confirm that the schistosity initially developed perpendicular to the maximum finite compressive strain, provided the total deformation is not very large.     

Microbial mat-induced sedimentary structures in siliciclastic sediments: Examples from the 1.6 Ga Chorhat Sandstone, Vindhyan Supergroup, M.P., India

This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-old Chorhat Sandstone of the Semri Group — the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet. The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands.     

Reconstruction of paleovegetation and paleoclimate in the Early and Middle Eocene, Hainan Island, China

The Early–Middle Eocene palynoflora and paleoclimate of Changchang Basin, Hainan Island, South China, is described in the present paper and is compared with that of the Middle–Late Eocene, Hunchun City, Jilin Province, North China. The nearest living relatives (NLRs) of the recovered palynotaxa suggest a subtropical evergreen or deciduous broad-leaved forest at the center of the basin but a temperate evergreen or deciduous broad-leaved forest and needle-leaved forest growing in the peripheral part of the basin. Based on the climatic preferences of the NLRs, the climate in the Changchang Basin during the Early–Middle Eocene was warm and humid subtropical with a mean annual temperature of 14.2–19.8°C, a mean temperature of the warmest month of 22.5–29.1°C, a mean temperature of the coldest month of 1.7–11.9°C, a difference of temperature between coldest and warmest months of 12.1–24.6°C, a mean annual precipitation of 784.7–1,113.3 mm, a mean maximum monthly precipitation of 141.5–268.1 mm and a mean minimum monthly precipitation of 6.9–14.1 mm. A comparison of the palynoflora and paleoclimate between the Changchang Basin and Hunchun City, suggests essentially a similar climate in South and North China during Eocene time in contrast to the oceanic tropical climate in South China and cool dry temperate climate in North China as at present.     

Microbial mat structures in profile: The Neoproterozoic Sonia Sandstone,Rajasthan, India

Ubiquitous microorganisms, especially cyanobacteria preferably grow on the sediment surface thereby producing microbial mats. In the absence of grazers and bioturbators, microbial mat is a unique feature of the Proterozoic. Most of the papers so far published described a wide variety of bed surface microbial mat structures with rare illustrations from sections perpendicular to bedding. Nonetheless, bed surface exposures are relatively rare in rock records. This limitation of bed surface exposures in rock records suggest that a study of microbial mats in bed-across sections is needed. The 60 m thick coastal marine interval of the Sonia Sandstone Formation is bounded between two terrestrial intervals, a transgressive lag at the base and an unconformity at the top, and has been chosen for exploration of microbial mat structures in bed-across sections. A wide variety of microbial mat-induced structures in bed-across sections are preserved within the coastal interval of the Sonia Sandstone. Though many of these structures are similar in some aspects with bed surface structures, some of those presented here are new. The palaeogeographic range of these microbial structures extends from supralittoral to neritic. Diagenetic alterations of microbial mats produce pyrite and those zones are suitable for the preservation of microbial remains. SEM and EDAX analyses show fossil preservation of filamentous microbial remains that confirm the presence of microbial mats within the coastal interval of the Sonia Sandstone. Effects of proliferation of microbial mats in the siliciclastic depositional setting are numerous. The mat-cover on sediment surfaces hinders reworking and/or erosion of the sediments thereby increases the net sedimentation rate. Successive deposition and preservation of thick microbial mat layer under reducing environments should have a great potential for hydrocarbon production and preservation and therefore these Proterozoic formations could be a target for exploration.     

Heavy Mineral Placer Sand Deposits of Kontiagarh Area,Ganjam District,Orissa, India

The Kontiagarh placer deposit in the Ganjam district, Orissa, India extends in northeast direction having a width of 700–1000 m. A total of 187 samples were collected meterwise from 55 bore holes in a grid pattern from beach, frontal, intermediate and back dunes covering an area of approximately 1 km². Light minerals decrease in size from the beach to the back dunes, whereas the size distribution of heavy minerals in the beach and dunes is more or less uniform. The average heavy mineral content in the beach and dunes vary from 9.38% to 24.20%. The heavy minerals are ilmenite, garnet, sillimanite, rutile, monazite, and zircon with trace amounts of magnetite, hornblende, diopside, sphene, tourmaline, and epidote. Heavy minerals are mostly less than 350 µm in size, with a peak distribution in the range between 180 and 125 µm. Ilmenite shows exsolution intergrowth with hematite. Mineral chemistry of ilmenite, hematite, leucoxene, magnetite, monazite and sillimanite are examined by EPMA. Leucoxene is lower in Fe and higher in Ti, Al, Cr and V than ilmenite. The litho‐units of the Precambrian Eastern Ghats Mobile Belt, comprising primarily khondalite, charnockite, calc‐silicate granulite and gneiss, are the source of heavy minerals for this deposit. The bulk sample has 7.30% ilmenite, 5.24% sillimanite, 9.16% garnet, 0.18% rutile, 0.14% monazite, 0.06% zircon and 0.52% other heavy minerals. The deposit has good potential for economic exploitation of ilmenite, rutile, sillimanite, monazite, zircon and garnet.     

Dynamic analysis of column-supported hyperboloidal shells

The dynamic response of hyperboloidal shells on discrete column supports is studied using a curved rotational shell finite element. In this finite element formulation, the displacement field over each element domain is approximated by polynomial functions in which the coefficients of the linear terms correspond to the nodal values of the displacements and the higher order terms vanish at the nodal circles. The stiffness and mass matrices associated with the equations of motion are derived from Hamilton's variational principle and include the effects of transverse shearing deformation and rotatory inertia. Since the formulation, as such, involves a great many degrees of freedom because of the use of higher order displacement functions, the kinematic condensation technique is employed to reduce the order of the dynamic problem The dynamic analysis indicates the importance of realistically modelling the base region of the shell. Studies on a prototype tower indicates that the base flexibility reduces the natural frequencies of the shell and increases the displacements near the base. The magnitude of this reduction, which could be significant, depends primarily on the tangential stiffnesses of the supporting columns and is hardly affected by the thickened ring beam at the base.     

Correlation of magnetic properties with Δ(Sr) of igneous rocks from the northeast Pacific and Gulf of California

The magnetic properties of basalts and diabases recovered under Legs 63 and 64 of the Deep Sea Drilling Project are summarized. They are first correlated with the measured grain sizes of the opaque minerals and the overall alteration states of the rocks, the latter measured by conventional chemical parameters such as water content, total volatiles content, and Fe₂O₃/FeO ratio. It is shown, however, that the decrease in ⁸⁷Sr/⁸⁶Sr upon leaching (Δ(^87/86Sr)) is perhaps the best quantifier of the overall alteration state of the whole rocks as well as of the degree of low-temperature oxidation suffered by the titanomagnetites. It is well correlated with Curie-point variations caused by changes in the Fe³⁺/Fe²⁺ ratio.     

Epeiric Sea Formation on Neoproterozoic Supercontinent Break-up: A Distinctive Signature in Coastal Storm Bed Amalgamation

Literature review underlines uncertainty in the configuration of the Neoproterozoic supercontinent, (with Rodinia and Palaeopangaea reconstructions enjoying wide support), that stems primarily from inadequate palaeomagnetic data. Nonetheless, breakup of this supercontinent at ca. 0.65 Ga was conducive for epeiric sea formation globally. In the Vindhyan basin, India, a carbonate depositing sea developed over a fluvial-aeolian plain, at approximately 0.6 Ga. The top part of the Vindhyan Supergroup, the Upper Bhander Sandstone, was, however, able to prograde because of a decline in the rate of relative sea level rise. Within this general setting, temporal increases in this rate caused storm deposition at the coastline, largely in a supralittoral setting. Bizarre amalgamation of these storm beds without erosion likely owes its origin to severe curtailment of the velocity of the downwelling flow on the very gentle, muddy coastal slopes, and is thought to be a hallmark of deposition in an open epeiric setting. The storm domination in the Bhander embayment shelf is compatible with the Palaeopangaea supercontinental configuration.