Regional Structural Analysis and Reinterpretation in the Bonai-Keonjhar Belt, Singhbhum Craton: Implication for Revision of the Lithostratigraphic Succession

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 NNESSW trending shallow plunging, second generation (D2) 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 (D1) manifested by the presence of isoclinal folds on outcrop scale has been recorded. D1 and D2 are near co-axial. The last deformational event (D3) 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.