Rhizolithic Calcrete in Teris, Southern Tamil Nadu: Origin and Paleoenvironmental Implications

Abstract The early-modern red coastal/or inland sands (“Teris”) in the coastal tract of southern Tamil Nadu show the infrequent occurrence of rhizolithic calcretes in the vadose zone. These are reddish brown coloured, cemented and cylindrical in form. No other types of rhizolith and/or calcrete have been recorded from here. Textural and mineralogical studies show that these rhizoliths contain notable amounts of acid insoluble detrital sands (essentially quartz and opaques) and mud, and are presumed to be derived from the host, i.e. teris. Petrographic studies reveal that these detritals are embedded in a micritic matrix and the carbonate cement shows rim, vein and blocky structures. Further, floating, alveolar and clotting textures are also noticed. Clay minerals in the rhizoliths include kaolinite as the singular component. Rhizoliths are presumed to be formed by surface weathering and pedogenic processes in the vadose zone. The pedogenesis of host teri sands might have released Ca2+ ions and were later deposited in the root cavities formed by the decay and complete removal of the tissues of roots. Another source of Ca2+ would be the calcareous sediments which underlies part of the teris. The absence of root anatomy is a characteristic feature of these rhizoliths. A sample of rhizolith collected at Sattankulam at a depth of 2.5 m gave a 14C age of 3680±110 years B.P. Hence, semi-arid conditions of the present day in the Teri tract of southern Tamil Nadu, should have set in at least as way back as 3680±110 yr. B.P.