Geological Society of India

Pseudotachylytes occur associated with mylonite and ultramylonite in the Mahanadi shear zone (MSZ) in the Eastern Ghats Mobile Belt (EGMB). The MSZ is about 200 km long curvilinear high strain zone trending WNW-ESE in its eastern part that splays out in the west. In Kantilo-Ganian segment of MSZ in northern EGMB, an interbanded sequence of granulite facies lithoassemblage has undergone ductile shearing. Kinematic studies of mylonite and ultramylonite indicate MSZ to be a NE-dipping, extensional type ductile shear zone. Non-coaxial metamorphic growth of garnet and presence of truncated sillimanite-fish in ultramylonite suggest high temperature regime during shearing. Pseudotachylytes in MSZ occur as millimetre thick layers to decimetre thick zones containing fragments of mylonite, ultramylonite and lithic clasts. Pseudotachylyte generation veins are mostly sub-parallel to C-planes and the injection veins cross-cut at high angle to these. The presence of an isotropic glassy matrix, injection features, corroded grains and dendritic microlites can be evidences for the existence of a melt phase. The composition of pseudotachylyte matrix (by EPMA) indicates silica deficiency with higher normative hypersthene, plagioclase and lower quartz compared with average whole rock composition for host. Absence of overprinting of mylonitic fabric on pseudotachylytes indicates their formation by brittle failure without passing through a plastic deformation and thus a two stage development for mylonite-ultramylonite and pseudotachylyte generation is suggested.

Cuddapah basin is known for hosting unconformity proximal uranium deposits viz., Lambapur, Peddagattu, Chitirial and Koppunuru along the northern margin of the basin. It is well known that these deposits are mostly associated with basement granitoids in Srisailam Sub-basin, and with cover sediments in Palnad subbasin where basement topography and fault/fracture system influence the fluid flow causing basement alteration and ore deposition. Geological setup, surface manifestation of uranium anomalies and association of the hydro-uranium anomalies near Durgi area in southern part of the Palnad sub-basin, have prompted detail investigation by geophysical methods to probe greater depths. Controlled Source Audio Magneto Telluric (CSAMT) survey conducted over five decades of frequency (0.1-9600 Hz) delineated the various lithounits of Kurnool and Nallamalai Groups along with their thicknesses as there exist an appreciable resistivity contrast. Interpretation of CSAMT sounding data are constrained by resistivity logs and litholog data obtained from the boreholes drilled within the basin indicated three to four layered structure. Sub-surface 2-D and 3-D geo-electrical models are simulated by stitching 1-D layered inverted resistivity earth models. Stitched 1-D inverted resistivity sections revealed the unconformity between the Kurnool Group and Nallamalai Group along with basement undulations. The faults/fractures delineated from the CSAMT data corroborated well with the results of gravity data acquired over the same area. Simulated 3-D voxel resistivity model helped in visualising the faults/fractures, their depth extent, thickness of the Banganapalle quartzite and basement configuration. Integrated interpretation of CSAMT, gravity and borehole data facilitated in delineating the unconformity and the structural features favourable for uranium mineralisation in deeper parts of the Palnad sub-basin.

The Mainpat plateau (N22°41′00″ to 22°55′ 00″ and E 83°08′00″ to 83°25′00″) is a table-land, rising >400 m above foothills, consisting of Archaean rocks (granite-gneisses, phyllite etc), Gondwana formations and Deccan basalt. Archaean rocks occur at the foot hills of Mainpat and Gondwana formations are exposed in the western escarpment of Mainpat plateau in turn overlain by the basaltic rock of Deccan Traps at the top. Barakar Formation contains coal seams from few cm to ~9 m are exposed near Chitaghutri, Kodwari, Tirkela, Tekta etc. villages of Lakhanpur tehsil of Surguja district, covered in Survey of India, Toposheet 64 N/1. Coal present in the area contains high volatile matters and high ash in the different seams in different localities. Analysis shows that coal of this area ranges between C and E grade with useful heat value varying from 3461 to 5223 K Cal/kg. This recently discovered area is located between Hasdo–Arand and Mand-Raigarh coalfields. Coal bearing horizon seems to be persistent, beneath the Deccan Traps.

Abstract For designing of pavements, California Bearing Ratio (CBR) value is an important parameter which is used to determine the strength of the subgrade soils. However, it is to be mentioned that, CBR test is tedious and laborious. Thus, in the present paper an attempt has been made to develop relationships between CBR and various soil index properties such as specific gravity (G), coefficient of uniformity (Cu), coefficient of curvature (Cc), liquid limit (LL), plastic limit (PL), plasticity index (PI), optimum moisture content (OMC) and maximum dry density (MDD) for alluvial soil in West Bengal, India. Empirical relationships have been proposed for both soaked and un-soaked CBR values as a function of these soil index properties by Genetic Expression Programing (GEP). Further, the same index properties have been used to predict CBR values by artificial neural network (ANN) and krigging method. The results clearly reveals that the GEP and ANN and krigging methods can be successfully used for predicting both the soaked and un-soaked CBR values by using the index properties of soil. Moreover, the developed relationships have been compared with the past available relationships. Furthermore, a multi objective optimization has been carried out for getting maximum CBR values.

Micro and macrofossil assemblage have been recovered and documented from the lignite beds of the Vastan lignite mines, Cambay Basin, Gujarat. These are extracted after dissolving amber pieces inclusive of pollen spores, epiphyllous fruiting fungal bodies, algal elements, insect fossils with some unidentified biotic groups. Of these, the major dominant flora belongs to angiosperm origin, in accompaniment to pteridophyte spores and fungal elements. Recorded palynotaxa viz. Proxapertites, Spinizonocolpites, Neocouperipollis and Acanthotricolpites are significant elements for tropical humid vegetation. Rich terrestrial amber palynomorphs and palynodebris suggest proximity source vegetation. Characteristic biotic interactions within the dense evergreen tropical rain forest environment is suggestive of mutual association among groups during the Early Eocene and share close affinities with those of Kutch and Barmer, Bikaner and Nagaur basins.