Micromorphology and major element geochemistry of calcretes in the Thoppukulam mine section, Sathankulam region, Southern Tamil Nadu, India: implications on depositional environment

The origin of carbonate sediments in continental region is subjected to numerous environmental controls. The calcrete is impure limestones which are formed within the regolith part either by evapotranspiration of alkaline-rich groundwater or by pedogenic leaching of surface water under arid and semi-arid climatic condition. Widespread deposits of calcrete profile occur between the Tertiary calcareous sandstone basement and Recent-Sub-Recent top red teri soil deposit located at Thoppukulam village, Southern Tamilnadu, India. The general sequential stratigraphic successions which were observed in and around the study area are given. The field observation proves the undulating occurrence of sub-surface calcrete profile which is in the form of dome (Sivalingam)-shaped structure below the teri soil. The undulation characteristics of calcrete below teri soil represent the differential evapotranspiration process of groundwater derived from the Tertiary calcareous sandstone aquifer. The macroscopic characters of calcrete appear as rhizome concretion, gravel, massive, oolitic, laminated, and chalky forms. The micromorphology of calcrete reveals calcified peloid, colloform, lensoidal, and rimming features with the clasts of quartz and clay, microtubules, calcified microcystis, calcified fungal hyphae, septarian nodule, and dendritic impregnation of deep brown sesquioxide mineral. The major element geochemistry of calcrete profile indicates CaO, SiO2, Al2O3, and Fe2O3 are in an elevated concentration above 1% than the other oxides, such as Na2O, K2O, MnO, MgO, TiO2, and P2O5. The major element geochemistry of two calcrete profile sections illustrates the geochemical implications and depositional environment.