JOUR.GEOL.SOC.INDIA, VOL.85, JAN. 2015 26 J. P. MOHAKUL AND S. P. BHUTIA Regional Structural Analysis and Reinterpretation in the Bonai-Keonjhar Belt, Singhbhum Craton: Implication for Revision of the Lithostratigraphic Succession J. P. MOHAKUL 1 and S. P. BHUTIA 2 1 PGRS Division, GSI Training Institute, Hyderabad - 500 068 2 Geological Survey of India, State Unit Odisha, Nayapalli, Bhubaneswar - Email: jpmohakul@gmail.com; spbhutia@rediffmail.com Abstract: The Mesoarchaean Iron Ore Group (IOG) of rocks in Bonai-Keonjhar belt is disposed in a horseshoe shaped synclinorium structure in the western part of the Singhbhum Craton. Morphology of this synclinorium is expressed by a near continuous western ridge and dissected eastern highlands with the broad Jamda-Koira valley at the centre. Detailed structural analysis in the northern part of the belt suggests that the synclinorium is in fact a series of NE-SW to NNE- SSW trending shallow plunging, second generation (D 2 ) syncline and anticline which have overturned, inclined, relatively tight geometry in the western part and upright, open to gentle in the eastern part. This structure is clearly manifested by synclinal ridges and anticlinal valleys. Evidence of an early deformational event (D 1 ) manifested by the presence of isoclinal folds on outcrop scale has been recorded. D 1 and D 2 are near co-axial. The last deformational event (D 3 ) has led to dome and basin interference pattern due to cross folding along NW-SE striking axial planes. The volcano-sedimentary package in the belt commenced with platformal package represented by basic volcanic-quartzite association. The Bonai range volcanic, Nuakot volcanic and the Jagannathpur volcanic occurring in the western, southern and eastern parts of the belt has been interpreted to be correlatable representing the basal volcanic suite. The linear body of basic volcanic occurring at the central part of the belt is found to be occupying the core part of a shallow plunging anticlinal axial trace and thus has also been equated with the basal volcanic suite. There is but only one major BIF horizon in the Bonai- Keonjhar belt which has been structurally repeated across the belt, BIF occurring as NNE-SSW trending synclinal ridges flanked on either side by the older phyllitic sequences. Keywords: Horseshoe synclinorium, Iron Ore Group, Bonai-Keonjhar Belt, Singhbhum craton.. synclinorium is defined by a considerably thick linear ridge of BIF whereas the shallow dipping eastern limb is manifested by a series of dissected highlands occupied by BIF. The intervening Jamda-Koira valley between these two limbs is dominantly occupied by a shale-phyllite sequence (Fig. 1B). The study of the geological cross section given by Jones (1934) indicates a central anticline flanked by synclines. Ghosh and Mukhopadhyay (2007) re-examined the regional structural disposition and opined that the synclinorium consists of a syncline-anticline fold pair with a western overturned syncline and an eastern asymmetric non-plunging anticline, which were later cross folded along an E-W axis. Major iron ore bodies are associated with the Cretaceous- Cenozoic laterite profiles developed over the BIF on both the limbs and the closure region of this synclinorium. The isolated bodies of iron ore associated with low lying linear INTRODUCTION The batholithic mass of Singhbhum Granite (SBG), effectively limited to its north by the Singhbhum Shear Zone (SSZ), is rimmed on its west by the Bonai-Keonjhar belt (Fig. 1A). This belt, well known as ‘Horse shoe synclinorium’, is a very low-grade supracrustal assemblage of Mesoarchaean Iron Ore Group (IOG, Sarkar and Saha, 1962, or Koira Group, Murthy and Acharya, 1975, Mukhopadhyay et al., 2008). On the basis of the disposition of a major horizon of Banded Iron Formation (BIF) the general structure of the belt was interpreted to be a NNE-SSW trending southerly closing asymmetrical synclinorium with overturned western limb (Jones, 1934; Dunn, 1940, Dunn and Dey, 1942), which has been later cross folded along E-W axis (Sarkar and Saha, 1962, 1963, 1977, 1983; Chatterjee and Mukherjee, 1981; Saha, 1994; Mukhopadhyay, 2001). The western limb of the JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.85, January 2015, pp.26-36 0016-7622/2015-85-1-26/$ 1.00 © GEOL. SOC. INDIA