Nature and composition of interbedded marine basaltic pumice in the ∼52–50 Ma Vastan lignite sequence,western India: Implication for Early Eocene MORB volcanism offshore Arabian Sea

The recognition of pyroclasts preserved in sedimentary environments far from its source is uncommon. We here describe occurrences of several centimetres-thick discontinuous basaltic pumice lenses occurring within the Early Eocene Vastan lignite mine sedimentary sequence, western India at two different levels – one at ～5 m and the other at 10 m above a biostratigraphically constrained 52 Ma old marker level postdating the Deccan Volcanism. These sections have received global attention as they record mammalian and plant radiations. We infer the repetitive occurrence of pumice have been sourced from a ～52–50 Ma MORB related to sea-floor spreading in the western Arabian Sea, most plausibly along the Carlsberg Ridge. Pyroclasts have skeletal plagioclase with horsetail morphologies ± pyroxene ± Fe–Ti oxide euhedral crystals, and typically comprise of circular polymodal (radii ≤10 to ≥30 μm), non-coalescing microvesicles (>40–60%). The pumice have undergone considerable syngenetic alteration during oceanic transport and post-burial digenesis, and are a composite mixture of Fe–Mn-rich clay and hydrated altered basaltic glass (palagonite). The Fe–Mn-rich clay is extremely low in SiO ₂, Al ₂ O ₃, TiO ₂, MgO, alkalies and REE, but very high in Fe ₂ O ₃, MnO, P, Ba, Sr contents, and palagonitization involved significant loss of SiO ₂, Al ₂ O ₃, MgO and variable gain in Fe ₂ O ₃, TiO ₂, Ni, V, Zr, Zn and REE. Bubble initiation to growth in the ascending basaltic magma (liquidus ～1200–1250 ^°C) may have occured in ～3 hr. Short-distance transport, non-connected vesicles, deposition in inner shelf to more confined lagoonal condition in the Early Eocene and quick burial helped preservation of the pumice in Vastan. Early Eocene Arabian Sea volcanism thus might have been an additional source to marginal sediments along the passive margin of western India.     

SEPARATION OF TWO-DIMENSIONAL RESIDUAL GRAVITY ANOMALIES USING LATERAL DENSITY VARIATION *

A set of parabolae is defined to approximate the observed gravity profile for specifying the lateral density variation in a two dimensional causative body. This variation separates the “biased” as well as “unbiased” residual anomaly structure. The method may be directly incorporated into existing numerical and graphical techniques of interpretation for subsequent delineation of structural configuration. The simple relations derived by inspection of the observed gravity profile permit universal application of the technique and in particular for the determination of the size and shape of sub-shelf structures. The suitability of the method in various problems of two dimensional interpretation has been indicated through an illustration of the Godavari Basin (India).     

EXPERIMENTAL STUDY ON BED SHEAR STRESS AND SEDIMENT TRANSPORT IN A MEANDERING LABORATORY CHANNEL

1 INTRODUCTION River erosion is a complex phenomenon. The rate of bank retreat is determined by flow, bed topography, sediment transport, bank properties, and water quality. Prediction of future river planform changes and the knowledge of river erosion and river meandering are required for land use planning in alluvial river valleys and determining locations for bridges and hydraulic structures. The control of riverbank erosion requires prediction of flow and bed features in a meanderin…     

Thermal perturbation during charnockitization and granulite facies metamorphism in southern India

Abstract We have deduced the steady-state lithospheric geotherm at c. 1 Ga in the south Indian shield area using the available data on the concentration of radioactive elements, and the P-T conditions of Proterozoic mantle xenoliths in the south Indian kimberlites as constraints. The geotherm was adjusted back to 2.5 Ga by keeping the surface temperature constant and calculating the temperature change at the top of convecting upper mantle. The reduced or mantle heat flux, which was treated as an adjustable parameter, was 20.9–21.3 mW/m² at 1–2.5 Ga. Comparison of the calculated steady-state geotherm with the available P-T data of the Archaean (c. 2.5 Ga) charnockites and granulites from southern India suggests that the granulite facies metamorphism in this region had resulted from a major thermal perturbation, which was c. 400° C at 25 km. Seismic tomographic and gravity data essentially preclude any significant magma underplating of the granulitic crust in southern India. Previous workers have suggested that the formation of charnockites in this region was associated with copious CO₂ influx from a deep-seated source, possibly the mantle. In this work, we have evaluated both the transient and steady-state thermal effects of the heat convected by CO₂ outgassing from upper mantle. It is shown that the thermobarometric array of charnockites and granulites can be produced by the convective perturbation of the steady-state geotherm, and that a flux of CO₂ of ±90 mol/m² yr (corresponding to Darcy velocity of ±0.30 cm/yr) for a period of ±30 Ma was needed to produce the required perturbation. This is c. 150 times the average CO₂ flux through the tectonically active area of the Earth's crust at the present time. There is, however, an uncertainty of a factor of 3 in this value. Seismic tomographic and gravity data independently suggest thickening of the crust beneath the granulite terrane compared with the adjacent Dharwar craton. This suggests thermal perturbation due to overthrusting as a major potential cause for the granulite facies metamorphism in south India. Overthrusting of a 30–35-km-thick thrust block was needed to produce the required thermal effect. The estimated thickness of the original crust from geobarometric and seismic tomographic data south of the orthopyroxene isograd or ‘transition zone’is compatible with the emplacement of a thrust block of this magnitude. However, the latter fails to match the estimated pre-uplift crustal thickness at the transition zone, if it is assumed that the crust has not thinned by non-erosional processes since the Archaean. Thus, we propose a combination of overthrusting and CO₂ fluxing from a deep-seated source as the cause for the formation of charnockites in this zone. The required focusing of CO₂ in this case is c. 40% of that estimated in the model where CO₂ fluxing was considered to be the sole reason for thermal perturbation. This combined thrusting—CO₂ fluxing model also helps explain the development of patchy charnockites in the transition zone from amphibolite facies rocks.     

Well storage effect during pumping tests in an aquifer of low permeability

Abstract

Well storage effect is generally considered while interpreting pumping test data from large diameter wells. However, in an aquifer of low permeability, the well storage is found to be significant during pumping tests conducted on bore wells. The interpretation of such data gives ambiguous results unless well storage effect is taken into account. A field example is presented to illustrate the difficulty in interpretation of the pumping test data. In order to take into account the well storage effect, a finite difference approach of interpreting pumping test data is suggested.