Palaeo-environmental reconstruction of pedogenic calcretes in Sathankulam, Southern India: an integrated approach

Abstract

Abstract A widespread and thick calcrete level occurs as an intervening deposit between soil surface and Proterozoic hard metamorphic granite (or) Tertiary calcareous sandstone basement rocks of Sathankulam region, Thoothukudi district of Tamilnadu, India. Field observation reveals the nature of calcrete deposits as gravel, chalky, laminated, massive, and nodular forms. Photomicrographs of calcrete thin sections reveal calcified macrocystis, calcified fungal hyphae, calcified microtubules, peloidal, calloforms with clay clast rimmed by micritic calcite, sesquioxide preservation and micritic calcite preservation in septarian nodules. X-ray diffraction analysis of calcrete profile samples reveals predominance of micritic calcite and limited occurrence of clay minerals of sepiolite, palygorskite, and other grains of quartz, feldspar, hornblende, biotite, and gypsum. Geochemical studies of two calcrete profile samples show the predominance of major elements of CaO, Al2O3, MgO, Fe2O3, minor/trace elements of Ba, Nb, Zr, Y, Sr, Rb, Ga, Zn, Ni, Cu, and Cr and rare earth elements/chondrite values of La, Ce, Pr, Nd, Sm, Gd, and Dy. Statistical evaluation of all major, minor/trace, and rare earth elements through multiple correlations, principal component analysis, and cluster analysis establish the inter-relationship and affinities between the parameters of the source elements of the two profiles. Stable isotope study of δ13C and δ18O of Hudson diagenitic plot study reveals that the two calcrete profile samples represent the reconstruction of palaeo-diagentic environmental condition as meteoric cement and continental freshwater limestone depositional environment. Further, stable isotope analysis of the area plotted on Julian Andrews and Keith and Weiber standard plot represents the weaker monsoonal climate and meteoric-diagenetic environment respectively.