Solid Earth Sciences (Sep 2020)
Cretaceous/Palaeogene boundary transition induced lattice defects in illite and kaolinite associated with the Um-Sohryngkew river section, Meghalaya, India
Abstract
Cetaceous-Palaeogene Boundary (KPB) transition records restricted to a unique, continuous, late Maastrichtian-early Danian, shallow marine Um Sohryngkew river succession (bearing planktonic foraminifers' CF4 to P1a biozones) have not been studied in detail. Earlier studies sub-divided it into lower, middle and upper parts, defined by the dominance of illite> illite/smectite; illite>kaolinite> montmorillonite and kaolinite > montmorillonite assemblages, respectively. However, limited studies were carried out on the KPB transition events, particularly for those reproduced in the illite (C2/m) and kaolinite (P1) lattice structures. Thus, illite and kaolinite cell parameters and interatomic distances have been studied to understand KPB transition events. For this purpose, layer-wise clay (0.2–2 μm) separates were analyzed by X-ray diffraction method and subjected to Rietveld refinement. Obtained data show substantial expansion in the c-axis of the kaolinite grains associated with the shaly marlite (P0 biozone) layer. Also, b- and c- axes of the kaolinite associated with the yellowish brown (CF3 biozone) layer indicate expansion and contraction, respectively. Moreover, Al–O bond in the kaolinite reflects contraction. Obtained cell parametric data and interatomic distances when plotted across the stratigraphic succession show notable contraction along b- and c- axes of the illite associated with the yellowish brown and shaly marlite layers of the CF3 and P0 biozones, respectively. The illite containing 26 atoms, linked to 38 and 34 bonds present in the intensely weathered lower and middle parts of the CF4 biozone, respectively. The illite in the moderately weathered upper part of the CF4 – CF1 biozones bearing 32 atoms and 44 bonds, represent Si–K and Al–K bond distortions together with the loss of Si4+ and K+. The coprolite-bearing shaly marlite layer (between the CF1 and P0 biozones) shows drastic changes in the illite structure, containing 48 atoms and 44 bonds, characterized by K–O, Si–K and Al–O bond contractions along with the retention of K+ and Al3+. Moreover, Al–O, Si–K, Al–K and Al–Al bonds connected strongly due to reinforcement of Al ions. A very low degree of alteration is noticed in case of preceding yellowish brown (CF3 biozone) layer. Owing to the weakening of the Al–O and Si–O bonds in the intensely weathered illite lattices of this layer show distortion as well as Al3+ and Si4+ losses, suggest incidence of thermal high. Sudden degeneration of illite lattices noticed in the shaly marlite layer is also marked with the planktonic foraminifers’ extinction, signifying biostratigraphic boundary. Thus, contraction in the c- axis as well as notable decrease in the bond lengths of illite (in this layer) is linked to KPB transition which includes major tectonic events, sea level changes and global-climatic changes caused environmental stress in the sea shelf. The substantial distortion in the illite lattices of the biozone CF2 corresponds to the most vigorous second phase of the Deccan volcanic eruption that occurred during 65.4–65.2 Ma.
