Facies variation in a half graben tectonic model

case study from Kolhan Basin, Jharkhand-Orissa

Abstract

The pear shaped Kolhan Group in the studied sub-basins of Chaibasa–Noamundi and Chamakpur-Keonjhar 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. This fining upward sequence alongwith  the vertical and lateral facies variation in the Kolhan implies superimposition of retrograding shorelines on an earlier prograding alluvial fan sand complex. The variations in the size parameters are indicative of changes in the water depth and the velocity at the time of sediment deposition. 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 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. 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.

Keywords: half-graben, fan-delta lacustrine,braided-ephemeral