Rhythmic shelf storm beds: Proterozoic Kaimur Formation, India

Abstract

The shallow-marine, turbidite-like storm beds of the Proterozoic Kaimur Formation exhibit intrabed rhythmic vertical variations in sedimentary structure and texture attributable to different patterns of velocity pulsations within an event of storm current deposition. The alternate repetition of parallel-laminated (S b) and ripple cross-laminated (S c) divisions (S bcbc…) within individual storm beds has been interpreted to represent periodic fluctuation in flow intensity beyond the stability limits of ripple and upper flat-bed during sedimentation. Sandstone beds characterised by successively occurring thin, internal mud partings indicate pulsing discontinuous flows with alternate periods of slack water within an event of storm current deposition. Successive mud drapes occurring within the ripple cross-laminated divisions (S c) of some of the storm beds also represent pulsing discontinuous flows. Palaeoccurrent analyses of these storm beds of the Kaimur Formation reveal a northwesterly current mode against an E—W trending shoreline. These storm beds thus may be interpreted to have deposited from geostrophic storm currents that were pulsating in nature with a periodicity of the order of tens of hours similar to that observed on present-day storm-dominated shelves.