Delineation of Rajmahal basaltic flows,inter-trappeans and associated coal layers using Audio-magneto-telluric method

The extensive volcanic activity of Rajmahal occurred during early Cretaceous (~117 Ma). Potentially perspective Gondawana sediments containing coal seam is partially covered by these basalts. Basalts as well as coal are already known to be of immense economic importance over Rajmahal traps. The present study mainly deals with delineation of basaltic lava flows and associated coal layers using the Audio-magneto-telluric (AMT) study in the south central region of Rajmahal traps near Suri, West Bengal, India. Initially, the field situations have been simulated using forward model with top alluvium cover followed by basaltic lava flows, a wedge shaped inter-trappean sedimentary formation with second basaltic lava flows and basement at the bottom. The model was constrained using available litholog. The model study reveals the presence of moderately resistive basalts, relatively conductive inter-trappeans and the basement, except the top alluvium layer and thin inter-trappeans. Subsequently, threeAMT sounding have been carried out over Cholaguria, Hatgacha and Pachami, Suri, West Bengal. The field AMT study reveals three basaltic lava flows with varying thickness from ~40m to ~350m and composite thickness of ~620m to ~640m, which match well with the published borehole litholog. The inter-trappeans have been identified at 500m to 620m, 310m to 400m and 500m to 640m depths over Cholaguria, Hatgacha and Pachami, respectively. It is interesting to mention that the identified inter-trappeans are inferred to be coal/shaly-coal which are matching very well with the published borehole lithologs.     

Origin of loess-like silt in Northern Jakutia,USSR

Sediments of different lithological composition of a common feature — a relict system of thick syngenetic ice wedges — are called ice complexes. Three types of ice complex are distinguished in N Jakutia, with different topographic roles. They formed in the second part of the Upper Pleistocene. Author found that it is not the matter of transportation that accounts for the formation of the main features of their composition, but the specifics of lithogenesis first of all determined by the conditions of cryogenic weathering.     

Untersuchungen mit der Elektronen-Mikrosonde an zonargebauten Mineralen der Pyromorphitgruppe

Zusammenfassung Natürliche Glieder der Pyromorphitgruppe wurden mit einer Elektronen-Mikrosonde untersucht. Bei allen untersuchten Proben tritt gegenseitige isomorphe Ersetzung des Phosphors, Arsens oder Vanadins auf. Die fast reinen Endglieder zeigen dabei einen ziemlich gleichmäßigen, geringfügigen Einbau der Nebenkomponente unabhängig vom optischen Zonarbau, während für die Mischglieder starke zonare Schwankungen in der chemischen Zusammensetzung in Abhängigkeit von den optisch erkennbaren Zonen typisch sind.In einem Falle wird zonar Blei isomorph durch Kalzium ersetzt. Der Chlorgehalt scheint von keiner der übrigen Komponenten abhängig zu sein und unterliegt nur z.T. zonaren Schwankungen. Zonarbau durch Farbänderungen und Einbau von Fremdpartikeln, fast nur bei den Mischgliedern zu finden, bedingt zumeist entsprechende zonare chemische Veränderungen der Phosphor-, Arsen- oder Vanadingehalte. Zonarbau infolge unterschiedlicher optischer Orientierung kann sowohl ohne chemische Zonen vorkommen (bei den Endgliedern), als auch mit gleichlaufenden Veränderungen der Komponenten bei den Mischgliedern.Zonarbau als Folge unterschiedlicher optischer Orientierung, Felderteilung und Zweiachsigkeit haben nach den vorliegenden Untersuchungen nicht isomorphen Ersatz als Ursache; Veränderungen der physikalischen Bedingungen während oder Sekundärvorgänge nach Beendigung des Wachstums der Kristalle könnten eine Rolle gespielt haben. In Frage kommt auch eine Abweichung von der hexagonalen Symmetrie; eine kürzlich von U. Keppler (Karlsruhe) durchgeführte Einkristall-Untersuchung einer Mimetesit-Zone ergab für diese eine monokline Symmetrie (im Druck).Das Auftreten von diffusen Reflexen oder Doppellinien in den Röntgendiagrammen der Mischglieder kann zwanglos durch die gegenläufigen Schwankungen der Phosphor-, Arsen- oder Vanadin-Gehalte gedeutet werden.

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Electron microprobe investigation of members of the pyromorphite group with zoned structure

Natural members of the pyromorphite group, pyromorphite Pb₅(PO₄)₃Cl, mimetite Pb₅(AsO₄)₃Cl, kampylite Pb₅(PO₄, AsO₄)₃Cl, and vanadinite Pb₅(VO₄)₃Cl were investigated by the electron microprobe method. Mutual isomorphic replacement of P, V and As was observed even in the purest available end members occurring in nature. The frequently appearing zoned structure, observed in thin sections, may or may not coincide with corresponding chemical changes with respect to the V, As or P contents, mainly for the intermediate members, while for the rather pure end members nearly no connection was found between optical zones and chemical composition (also as to the division of basal sections into six triangular areas).

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Den Herren Prof. Dr. F. Laves, Zürich, und Prof. Dr. H. Wondratschek, Karlsruhe, danken wir für die Ermöglichung dieser Arbeit und für das fördernde Interesse. Der Deutschen Forschungsgemeinschaft danken wir für die finanzielle Unterstützung dieser Arbeit durch eine Beihilfe.     

The structural and textural features of felsic volcanics of the Sarbai Formation (Southern Urals)

The results of a microscope study of samples of felsic volcanic rocks from the upper horizons of the Sarbai Formation (O₂–S₁ sb) in the Southern Urals that compose the formation are presented. A detailed study of the samples and an evaluation of the results of a silicate analysis of these rocks showed their volcanogenic genesis. The rocks have been classified as volcanic glass that underwent several stages of transformation. Three newly-formed structural and textural types of mineralization were defined: spherulitic (felsophyric), axiolitic, and felsitic.     

A FAST FOURIER TRANSFORM METHOD FOR RAPID COMPUTATION OF GRAVITY AND MAGNETIC ANOMALIES DUE TO ARBITRARY BODIES*

The spectrum of a magnetic or a gravity anomaly due to a body of a given shape with either homogeneous magnetization or uniform density distribution can be expressed as a product of the Fourier transforms of the source geometry and the Green's function. The transform of the source geometry for any irregularly-shaped body can be accurately determined by representing the body as closely as possible by a number of prismatic bodies. The Green's function is not dependent upon the source geometry. So the analytical expression for its transform remains the same for all causative bodies. It is, therefore, not difficult to obtain the spectrum of an anomaly by multiplying the transform of the source geometry by that of the Green's function. Then the inverse of this spectrum, which yields the anomaly in the space domain, is calculated by using the Fast Fourier Transform algorithm. Many examples show the reliability and accuracy of the method for calculating potential field anomalies.