Evolution of Palaeo-Proterozoic Kolhan as a Half Graben

The Kolhan Group is preserved in the sub-basins of ChaibasaNoamundi and Chamakpur-Keonjhar and is usually represented by a sequence of clastic (+carbonate) association alongwith development of thin and discontinuous patches of basal conglomerates draped by sandstone beds. Six lithofacies have been observed in the area. The IOG-fault marks the western& #039, distal& #039, margin of the Kolhan basin showing evidence of passive subsidence subsequent to the initial rifting stage. The basin is thought to evolve as a half-graben under the influence of an extensional stress regime. This assumption of a tectonic setting for the NE-SW trending Kolhan basin can be related to the basin opening as a consequence of E-W extensional stress system that prevailed during the development of the Newer Dolerite dyke. The Paleoproterozoic age of the Kolhan basin is based on the consideration of the conformable stress pattern responsible both for the basin opening, and on the development of the conjugate fracture system along which the Newer Dolerite dykes intruded the Singhbhum Archaean craton. The half-graben development and fault growth evolve differently through time and produce different basin-filling patterns. In the initial stage the basin evolution can be explained by detachment type half-graben filling model that incorporates a basin-bounding fault soling into a sub-horizontal detachment fault. Two types of genetic sequences reflecting variations in the generated accommodation space have been recognized within the sub-basins of Chamakpur-Keonjhar and Chaibasa-Noamundi. The lower sequence in Chamakpur-Keonjhar is characterised by shallow braided river deposits that lack repetitive facies patterns and were deposited during a period of the slower rate of fault growth and generated accommodation space. An upward increase in the generated accommodation space is recorded by sheet sandstones encased in sand-streaked siltstones representing ephemeral flood deposits. During the fault growth stage the Kolhan basin grew both wider and longer through time as the basin-bounding faults lengthen and displacement accumulated as evident in the sub-basin of Chaibasa Noamundi. Younger strata consistently pinch out against older syn-rift strata rather than pre-rift rocks in the later fault-growth stage. The basin fill thus commonly forms a fanning wedge during fluvial sedimentation, whereas lacustrine strata tend to pinch out against older syn-rift strata. The fluvial strata progressively onlap the hanging wall block, whereas the lacustrine strata pinch out against older fluvial strata at the centre of the basin but onlap along the lateral edges. The transition from fluvial to lacustrine deposition and hanging wall onlap relationships are thoroughly observed in the sub-basins of Kolhans. The pronounced variations in thickness of the fan delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on fan-delta evolution. A strong asymmetry in vertical basin architecture and the linearity in the outcrop pattern of the preserved sedimentary sequence are presumed to have developed in an elongated trough during the initial basinal rifting stage, while the later stage is marked by the progressive overlaps and coalesce of the facies built-up. The basin axis controlled the progradation direction which was likely driven by climatically induced sediment influx, a eustatic fall, or both.