Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

R. T. Shukla ¹, B. L. Sharma ¹

Affiliations
1 Department of Geology, Indian School of Mines, Dhanbad, IN

Abstract

The paper is an attempt to study so ancient rocks as the Alwars from a sedimentary petrographic point of view.

The Alwar series of rocks at Debari near Udaipur (Rajasthan) comprise basal conglomerates, arkoses, grits, schists, quartzites, slates and phyllites in chronological sequence from bottom upwards. The Alwars rest unconformably over the Aravalli schists and phyllites, and are inverted in their stratigraphic sequence. The conglomerate consists of pebbles of granite, tourmaline, quartzite, vein quartz and fragments of phyllite and schist embeddl:d in a fine grained quartzo-felspathic matrix. The pebbles exhibit a wide variation in size and shape. A few pebbles are sheared to some extent. Shearing appears to be postdepositional in origin. Elongated shape of the pebbles seems to be inherited from the nature of weathering of the source rocks and post-depositional tectonic stretching.

Sedimentary structures like current and graded bedding are present in arkoses and quartzite. Statistical parameters of size distribution of these clastics, namely, the median, sorting coefficient, skewness and kurtosis are computed and interpreted.

Heavy grain residue obtained from the rocks under study contains zircon, rutile, ilmenite, kyanite, andalusite, sillimanite and fluorite. The source rocks of these sediments appear to be of mixed origin comprising orthoquartzites. low to high grade metamorphics and granite.

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B. L. Sharma ¹, G. V. Damle ²

Affiliations
1 Centre of Advanced Study in Geology, University of Saugar, Sagar, IN
2 Department of Geology, University of Udaipur, Udaipur, IN

Abstract

Palaeocurrent directions in sandstone around Jodhpur (Vindhyan or early Palaeozoic) were determined by measuring direction of cross-bedding, grain orientation, quartz 'c' axis orientation, and also with the help of asymmetrical ripples. A comparison was made of the results derived from these different methods. It was found that there is a fairly good agreement between the directions derived from the study of grain orientation, quartz 'c' axis orientation and cross-bedding. Usually asymmetrical ripples also agree and indicate the general direction of the palaeocurrent; hut occasionally asymmetrical ripples turn out to be climbing ripples and indicate a direction opposite to the true palaeocurrent direction. This paper attempts to correlate the results obtained by different methods and concludes that a fairly good correlation exists between the results obtained by different methods. The grain orientation studies indicate that there is an up-current imbrication of the elongate quartz grain a in general; but in the case of fore-set surface in the cross-bedded strata, they tend to be parallel to the fore-set bedding and therefore develop a down-current imbrication. It is also found that existence of climbing ripples may indicate the necessity of caution and should be used only to assess the depositional strike.

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N. K. Chauhan ¹, B. L. Sharma ¹, Sabah A. Mohemmad ¹

Affiliations
1 Department of Geology, M.L. Sukhadia University, Udaipur - 313 001, IN

Abstract

Detailed mapping and analyses of the structural elements from Gorimari revealed a complex structural geometry and deformation history of the rocks. Only two prominent planar structural elements have been observed i.e., the bedding schistosity (S₀S₁) and cleavage (S2). The earliest AF1 and the later AF2 folds on every scale are suggestive of reclined geometry. The strain analyses of AF1 and AF2 folds suggest their initiation by buckling and subsequent flattening resulting into class I C geometry. Some of the folds show a complex geometry which may be due to original layer irregularities or may be due to differential flattening because of the variation in thickness of single layer. The values of compressive apparent strain ratio varies from 0.1 to 1.0. Stratigraphically these rocks are equivalent to the Early Proterozoic rocks of Bagdunda. The grade of metamorphism is oflower amphibolite facies.

Keywords

Structural Geology, Aravalli Rocks, Proterozoic, Udaipur, Rajasthan.

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Harsh Bhu ¹, B. L. Sharma ¹

Affiliations
1 Department of Geology, Mohanlal Sukhadia University,Udaipur- 3 13 002, IN

Abstract

The paper reports the study of strain analysis on pebbles of a horizon of polymictic conglomerate belonging to the Debarn Formation of the Aravalli Super group. The variable shape geometry of pebbles observed in different parts of the conglomerate outcrops, which cover a long linear distance east of Udaipur, is indicative of heterogeneous superimposed stream on somewhat Non-Spherical initial bodies.

Micro textural studies indicated a variety of strain forms in constituent grains of both pebble and matrix along with attendant recovery through Sub-Gram formation and recrystallisation. Mica grams in the matrix have acted as a strainpartitioning mineral.

The lattice preferred orientation of c-axes in quartz is comparable to partially developed Type I pattern developed in regions of coaxial deformation. The pattern has been influenced by strain a d dynamic recrystallisation. The slip planes that operated in the deformation of quartz grains varied from basal slips to prism slips Basal slips were dominant at low temperature and low strain regions in quartz while prism slips at moderately higher temperature and moderate strain. The variation in slip planes can be accounted by in homogeneous nature of deformation possibly due to difference in critically resolved shear stresses in the conglomerate.

Keywords

Paleoproterozoic, Deformed Conglomerate, Strain Analysis, Quartz, Dynamic Recrystallisatlion, Deban Formation, Aravalli Supergroup, Rajasthan

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