Chemico-mineralogical attributes of clays from the late Cretaceous–early Palaeogene succession of the Um Sohryngkew river section of Meghalaya, India: Palaeoenvironmental inferences and the K/Pg boundary

A continuous shallow marine 10 m thick succession within the Langpar Formation in the Um Sohryngkew river section of Meghalaya, containing late Maastrichtian through early Danian planktonic foraminiferal zones – CF4, CF3, CF2, CF1, P0, Pα and P1a and the K/Pg boundary (between CF1 and P0) that makes unique of its kind. The section has been re-studied and sampled for clay mineralogy to understand the palaeoenvironmental conditions prevalent in the region and to assess the K/Pg transition. The relative abundances of the clay mineral phases permitted a threefold sub-division of the studied section with a illite, illite/ smectite dominated lower part, illite, kaolinite and abundance of montmorillonite dominated middle part and kaolinite–montmorillonite dominated upper part. Enriched HREEs in the lower part of the succession suggest variations in the pH of alteration solutions. Most of the samples show positive cerium (δCe) and europium (δEu) anomalies, the former reflecting oxidizing conditions at the time of clay formation. Illite dominated clays present a positive Eu anomaly, formed at relatively higher temperatures than the clays with less illite and without Eu anomalies, whereas clays occurring in the lower and upper parts exhibit a prominent negative Eu anomaly. Shifts in the redox condition found in this section are more or less similar to the foraminiferal changes and Au, Pt, Pd anomalies. Clay mineralogical attributes and REE patterns, comparable to those of the known K/Pg boundaries, appeared within the CF3 and CF2 zones in the Um Sohryngkew river section. The sample at the boundary between CF3 and CF2 is marked by a negative Ce anomaly, high La/Yb and TOC values, suggesting that sea level rise during the upper part of CF3 was caused by tectonism rather than warming. The similar characteristics of clay minerals and REE patterns, attributed to the initiation of tectonic events during the CF3 zone, indicate environmental changes that affected the shelf area and the provenance of these sediments.