Magnetostratigraphy and rock magnetic studies on the Cretaceous-Paleogene transition strata along the Um Sohryngkew River, Therriaghat, Meghalaya, India

A combination of magnetic polarity and rock magnetic analysis on the well-documented section of the Um Sohryngkew River (USR) in the south Shillong Plateau, NE India, produced a sharp reversal marking the C29r-C29n geomagnetic polarity transition at approximately 65.688 Ma. The rock magnetic studies indicate ferrimagnetic dominant mineralogy with abundance of SSD grains, with an anomalous peak in susceptibility coinciding with Ir-rich limonitic layer. The magnetic reversal occurs precisely 61 m above the Ir-rich distinct in situ limonitic layer, indicating that the C29r-C29n geomagnetic reversal post-dates the widely accepted Ir-anomaly based K-Pg boundary by approximately 355 Ka. Furthermore, the rock magnetic studies indicate its frequency dependence coinciding with the Ir-rich limonitic layer suggesting a possible dust/aerosol source, while akaganéite is reported from the interval approximately 1 m below peak susceptibility, indicating signature of Deccan volcanism. This study infers the completeness of the USR section with a high rate of sedimentation of approximately 17 cm/ka among the marine K-Pg boundary sections in the world.