SLOPE-CONTROLLED SEISMIC DEFORMATION AND TECTONIC FRAMEWORK OF DEPOSITION - KOLDAHA SHALE, INDIA

Penecontemporaneous deformations with remarkable lateral continuity at selected stratigraphic levels record frequent seismicity in the Meso- Proterozoic Koldaha Shale, Vindhyan Supergroup, central India. Their c onsistent bipolar orientation reflects the master control of basinal t ectonics. Boudinage, small-scale conjugate faults and bidirectional jo int planes indicate east-northeast-west-southwest basinal extension. D etailed study of a 27-km strike-parallel stretch between Chorhat and S hikarganj against a broader perspective reveals existence of multiple NW-SE-elongated subbasins. Stratigraphic and sedimentologic criteria p oint to inheritance and subsequent sustenance of steep western and gen tler eastern flanks of the sub-basins. It is also indicated that the p erennial supply of sediment took place from the east, while the wester n slope contributed only during tectonic seiches. The relatively large -scale faults are correlatable with the steeper and the smaller deform ation structures to the gentler flank. The NNW-oriented half graben st ructures that were reactivated time and again, is elicited. Previous g eophysical studies inferred roughly E-W-elongated rift valley in the g ranitic basement and the present observations indicate that the riftin g continued at least up to the time of Koldaha Shale deposition. The h alf graben could then be guided by cross-faults resulted from dextral shear. It is imperative then, the basin-interior sedimentation and def ormation patterns were more closely controlled by the cross-faults, ra ther than by the basin-margin faults. Only a set of slump folds in a 1 0-m-thick interval indicates that the northern margin of the main basi n was relatively steeper. This is further corroborated by concentratio n of large fans in mineralogically immature sediments and southerly pa leocurrent at the northern fringe of the basin. In contrast, the Vindh yan sediments commonly belong to orthoquartzite-carbonate association. A regional northerly slope of the Vindhyan basin floor towards the re latively steeper flank provides an explanation for the general northwe sterly paleocurrent, even though the eastern flank of the sub-basins s loped WSW. This integrated study of the physical aspects of the sedime ntary pile against the background information of basement tectonics re cognize rifting with a dextral shear component as the overriding facto r controlling the early part of evolution of the Vindhyan basin.