Mineral chemistry of clays associated with the Late Cretaceous-early Palaeogene succession of the Um-Sohryngkew river section of Meghalaya, India: Palaeoenvironmental inferences and K/Pg transition

Abstract

A unique, continuous, shallow marine succession within the Langpar Formation in the Um-Sohryngkew river section of Meghalaya contains late Maastrichtian through early Danian planktonic foraminiferal biozones (CF4-P1a) and the K/Pg boundary (between CF1 and P0). To resolve compositional overlap [in the three-fold clay mineral based sub-divisions of the section (include 1–2 mm thick yellowish brown clay layer in biozone CF3)] and to understand paleoenvironmental conditions prevalent at the time of K/Pg transition, micro-structural and compositional studies were carried out. The paper discusses accurate compositional limits of solid solubility in smectite and illite. Clay morphological attributes distinctly vary with the changes in the biozonation. Major oxide data plots for the litho-units (samples JP1-16) over binary diagrams, show clustering of plots within the illite compositional field. Plots based on structural formulae and layer charges of illite and kaolinite rich clays (from CF4 to Pla biozones), show closeness with the clay data fields of Agost, Caravaca, Petriccio and El-Kef K/T boundary sections as their charge occupancies at tetrahedral (Zt), octahedral (Zo) and interlayer (Zi) sites are similar. Thermodynamic data plots over ternary [AR23+Si3O10(OH)2 − R23+SiO4O10(OH)2 − A3AlSi4O10(OH)2] diagram, clustered within the illite compositional fields. Majority of illites shows high K values. They represent occasionally higher Altet. and lower Aloct. layer charges. Calculated palaeotemperature values for illite (from the CF4 to Pla zones) vary from 68 – 232oC. Sudden rise in the temperature (>140oC) of illite formation noticed in the upper part of the biozone CF3 (sample JP-12) is comparable to the K/T boundary layer of Caravaca section. Wide variation in the humid tropical to arid-semiarid climatic and thermal (diagenetic to low grade metamorphic) conditions noticed across the succession is possiby linked with the contemporaneous Abor / Deccan volcanic activities (at the time of their deposition) as also reflected in their clay layer [Si (2.95–3.68)] and interlayer [K+Na (0.4–4.61)] charges.