Evidence of K/PB Transition Linked Crystallographic Defects in the Smectites Associated with the Jhilmili Intertrappean Bed, Central India

Abstract

Discovery of ~13 m thick, Jhilmili intertrappean bed attained significance as it contains brackish water ostrocods and planktonic foraminifers. Previous studies revealed montmorillonite>montmorillonite/chlorite mixed layer>palygorskite abundance. However, rare information is available on layer-wise K/PB transition linked cell parametric and interatomic changes in the montmorillonite (C2/m space group). To understand these changes, layer-wise clay separates were analyzed by X-ray diffraction method and subjected to Rietveld refinement. Obtained cell parameters and interatomic distances show notable changes in a- and b- axes. In JM-3 layer, expansion of Mg-O, Fe-Al, Fe-O and Al-O bonds led to substantial loss of Mg2+, Fe2+ and Al3+ in the cell structure. Significant bond length variation associated with the JM-3 layer suggested incidence of high temperature (150–250°C) conditions. Owing to reinforcement of Al, Fe, Mg ions, Al-O, Fe-Ca, Mg-O, Al-Ca bonds held-up tightly, thus, resulted in the lattice shortening. Moreover, volcanism related acid rains were also accountable for lattice distortion and causative factor for the expansion and the contraction of b- and c- axes, respectively. Almost similar cell parametric and inter-atomic changes were also observed in the successive JM-4 layer representing terminal phase of the event. Thus, abrupt degeneration in the montmorillonite cell parameters noticed in greenish grey, clayey JM-3 layer which continued over a protracted period in the succeeding ostrocods and planktonic foraminifers bearing yellowish brown JM-4 layer is linked to K/PB transition event, most likely Deccan volcanism induced environmental changes, including global warming, acid rains and pCO2 rise in the atmosphere.