Slope-controlled seismic deformation and tectonic framework of deposition: Koldaha Shale, India

Abstract

Penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the Meso–Proterozoic Koldaha Shale, Vindhyan Supergroup, central India. Their consistent bipolar orientation reflects the master control of basinal tectonics. Boudinage, small-scale conjugate faults and bidirectional joint planes indicate east-northeast–west-southwest basinal extension. Detailed study of a 27-km strike-parallel stretch between Chorhat and Shikarganj against a broader perspective reveals existence of multiple NW–SE-elongated subbasins. Stratigraphic and sedimentologic criteria point to inheritance and subsequent sustenance of steep western and gentler eastern flanks of the sub-basins. It is also indicated that the perennial supply of sediment took place from the east, while the western slope contributed only during tectonic seiches. The relatively large-scale faults are correlatable with the steeper and the smaller deformation structures to the gentler flank. The NNW-oriented half graben structures that were reactivated time and again, is elicited.Previous geophysical studies inferred roughly E–W-elongated rift valley in the granitic basement and the present observations indicate that the rifting continued at least up to the time of Koldaha Shale deposition. The half graben could then be guided by cross-faults resulted from dextral shear. It is imperative then, the basin-interior sedimentation and deformation patterns were more closely controlled by the cross-faults, rather than by the basin-margin faults. Only a set of slump folds in a 10-m-thick interval indicates that the northern margin of the main basin was relatively steeper. This is further corroborated by concentration of large fans in mineralogically immature sediments and southerly paleocurrent at the northern fringe of the basin. In contrast, the Vindhyan sediments commonly belong to orthoquartzite–carbonate association. A regional northerly slope of the Vindhyan basin floor towards the relatively steeper flank provides an explanation for the general northwesterly paleocurrent, even though the eastern flank of the sub-basins sloped WSW. This integrated study of the physical aspects of the sedimentary pile against the background information of basement tectonics recognize rifting with a dextral shear component as the overriding factor controlling the early part of evolution of the Vindhyan basin.