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

Abstract

Cross-folding in Precambrian rocks of India affects the earlier folds to varying degrees and has been classified into (a) cross-fold axes at right angles to earlier folds, (b) at an angle to earlier folds, (c) parallel to earlier folds, and (d) with reversals in plunge relations resulting in culminations and depressions. En echelon folding in Precambrian rocks of India is of simple type producing a zig-zag pattern in the interference of minor folds on the axial trend of major folds. Elliptical pattern of en echelon folding is uncommon in these rocks.

Various types and degree of cross-folds and en echelon folds are described drawing illustrations from (i) intensely folded and metamorphosed charnockitic regions of Madras, DharwaI' schist belts of Mysore, and Sausar-Gangpur-Iron-ore Series rocks of Madhya Pradesh, Bombay, Orissa, and Bihar, and also from (ii) less intensely folded and metamorphosed Cuddapah-Kurnool formations of Andhra. The plunge of cross-folds and en echelon folds have consistent relation to the differential shear couple.

Metalliferous ore deposits of both syngenetic and epigenetic origin in India show remarkable localization in regions of cross-folding and en echelon folding. The controls of these folds in localizing various types of ore deposits are described, including the plunge of folds and its relation to the pitch of ore bodies. Illustrations are drawn from (i) bedded and enriched iron and manganese-ore deposits of Bonai-Keonjhar, Bailadila, Mysore, Madhya Pradesh, Bombay and Andhra, (ii) magmatic chromite deposits of Mysore, (iii) gold-quartz veins of Kolar, Ramagiri and Gadag, (iv) copper deposits of Singhbhum, Kurnool and Guntur, and (v) pyrite deposits of Chitaldrug. The recognition and application of cross-fold and en echelon fold structures to evaluate structural control of ore deposits have great value in future mining exploration and development in India.

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The Cuddapah Basin in Andhra Pradesh is a remarkable tectonic and orogenic belt of the unfossiliferous Precambrian rocks of Peninsular India. The paper reviews the stratigraphy and structure of the rocks of the Cuddapah System and Kurnool Series as originally proposed by King and gives the author's conclusions on the detailed structure and tectonics of the Basin. A north-plunging low-amplitude asymmetrical synclinorium having a gently dipping western limb and intensely folded, overfolded and thrusted eastern limb is the major structural element. This is further complicated by a major domal upwarp across the middle of the basin marked by monocjinal flexures of the Kurnool strata in Gani Kalawa area; culminations and depressions at Jammalamadugu, Giddaluru and Srisailam plateau; Eshwarakupam dome of the lower Cuddapahs in the northern part of the Nallamalai hills; and granite domes of Vinukonda area in the north-east extremity.

The relation of the structural elements to the localization of mineral deposits in the Cuddapah Basin is discussed and the necessity for their precise interpretation for exploring the depth extension of the ore mineralization is stressed. The author recommends further detailed structural mapping, especially in the eastern half of the basin, for solving the unsolved problems in the Cuddapah stratigraphy and structure.

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