Evaluation of Deformational and Depositional Controls on Pebble Characters - A Study of Bisrampur Conglomerate, Singhbhum, Bihar

The conglomerate horizon near Bisrampur in Singhbhum, Bihar, on the flank of Archaean (Singhbhum granite) platfonn, manifests a highly mature pebble assemblage in an argillaceous to arkosic matrix. The triaxial pebbles in conglomerate show readily recognizable down dip preferred orientation of longest axes and a common graving lineation on pebble surfaces. Strain analyses indicate a feeble relative dominance of flat type pebbles and a possible non-random orientation pattern of the primary deposited nonspherical pebbles. Calculation of finite strain also suggests that the axial parameters of pebbles cannot be related to a single phase of deformation. Thus, there may be two factors involved responsible for triaxiality of pebbles, one prior to deposition and a later modification by feeble deformation. Although spatially associated to a strongly tectonised domain to the north, the conglomerate largely retains the primary depositional character controlled by mass flow on the basin flank. This behaviour may be resolved by protective character of the channel filling on continental slope.