Palaeo-Environmental Reconstruction of Pedogenic Calcretes in Sathankulam, Southern India: An Integrated Approach

Abstract

Calcrete deposits are identified as intermediary layers between soil outcrops and either Proterozoic hard metamorphic granite or Tertiary calcareous sandstone basement rocks in the Sathankulam region of the Thoothukudi district, Tamilnadu, India. Field observations delineate various forms of calcrete, including gravel, chalky, laminated, massive, and nodular structures. Photomicrographs of calcrete thin sections reveal features such as calcified macrocystis, calcified fungal hyphae, calcified microtubules, peloidal structures, 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 indicates a prevalence of micritic calcite with limited occurrences of clay minerals such as sepiolite, palygorskite, and sap grains of quartz, feldspar, hornblende, biotite, and gypsum. Geochemical studies on two calcrete profiles highlight major elements like CaO, Al₂O₃, MgO, Fe₂O₃, and minor/trace elements such as Ba, Nb, Zr, Y, Sr, Rb, Ga, Zn, Ni, Cu, and Cr, along with rare earth elements/chondrite values of La, Ce, Pr, Nd, Sm, Gd, and Dy. Statistical analyses, including multiple correlations, principal component analysis, and cluster analysis, elucidate the inter-relationships and affinities among the source elements of the two profiles. Stable isotope studies of δ¹³C and δ¹⁸O, specifically employing the Hudson diagenetic plot, suggest that the calcrete profile samples reflect a reconstruction of paleo-diagenetic environmental conditions in a meteoric cement and continental freshwater limestone depositional environment. Additionally, stable isotope analysis plotted on Horbury and Qing (2004) plot indicates a weaker monsoonal climate and a meteoric diagenetic environment in the studied area.