Deformation structures in a large slump horizon, Paleoproterozoic Vempalle Formation, Cuddapah Basin, southern India

Occurrence of slump folds and associated faults generated by soft-sediment deformation from the Paleoproterozoic Vempalle Formation, southwestern Cuddapah basin, India, is being reported here for the first time. The slump horizon is preserved within a more or less undeformed shallow to deep-water carbonate unit in the Cuddapah basin exposed near Parnapalle village, Andhra Pradesh, India. The stratigraphic framework includes the Gulcheru Quartzite, lowermost unit of the Cuddapah basin succession, deposited in an alluvial to shallow marine shelf environment, and the overlying Vempalle Formation, representing a ramp-type stromatolitic carbonate platform. The synsedimentary sliding along a steepened ramp is evidenced by northeast-verging kink-like folds with wavelength up to 400 m and an overprinting set of thrusts with ramp-flat geometry, fault-cored folds or small break-thrusts showing top-to-southwest displacement, and smaller congruent folds. From the isolated occurrence within a generally undeformed succession, association of structures, and the stratigraphic context, we suggest soft-sediment deformation at the toe of a large (kilometer-scale) slump, with the prevalent bedding-parallel anisotropy exploited for common flexural slip and ramp-flat geometry. In addition, the bedding-parallel slickensides in the Gulcheru Quartzite immediately below the Vempalle Formation indicate a top-to-east-northeast, normal sense of slip representing extensional slip at the slump head. Northeast-verging large folds in the Parnapalle slump horizon possibly represent structures formed during the translation phase of slumping. As the large northeast propagating slump was halted, the backthrust-like structures and associated folds developed at the slump toe.