QUARRIED CALCRETE CAPPINGS: A WIDESPREAD LEGACY OF LATE CENOZOIC CLIMATE CHANGE ACROSS THE PRECAMBRIAN BASEMENT OF SEMI-ARID PENINSULAR INDIA

Accumulations of calcium carbonate in weathering profiles are widespread in semi-arid lands where annual rainfall ranges between 600 and 400 mm/yr. Based on this understanding they have been used as paleoclimatic and paleoecological indicators. With a rainfall of 350 to 750 mm/yr, the land surface of South India located in the rainshadow of the Western Ghats exhibits a large number of sites where diverse forms and fabrics of calcrete occur almost exclusively on Precambrian silicate bedrock. These economic grade calcrete deposits involve employment in quarrying, transport, off-site production of lime and construction material for houses and dry stone walls.

We have mapped the distribution of calcareous and calcified soils across South India based on state-wise soil maps and on a study engaged in classifiying, U-series dating and isotopically tracing the most massive occurrences of calcrete in the region. Depending on geomorphic setting we find that the quarried calcretes are either pedogenic or phreatic with complex petrographic histories of surface reworking. Their ages vary from early/mid-Pleistocene to Recent, and depending on geographical position with respect to paleomonsoon wind patterns they sometimes contain aeolian dust input with marine strontium signatures. Such diversity and wide distribution reflects the stability, in the rainshadow of the SW monsoon, of semi-arid Quaternary conditions oscillating around a rainfall average similar to that of the present but involving phases of fabric reworking.

Occurrences in the semi-arid zone of fossil Cenozoic laterite capped by calcrete further indicate that (1) the parent material of the calcrete is only occasionally the bedrock itself; (2) humid climates, which created the conditions for laterite development, preceded the drier conditions without any detected reversals to lateritizing conditions during the Quaternary; and (3) as a consequence of lasting climate change laterite has been eclipsed by calcrete as the chief economic surface deposit, although it is still a key resource for the production of free-stone bricks in the humid areas seaward of the Western Ghats. Quarrying industries of Quaternary calcrete and Cenozoic laterite place South India in a unique position amongst the many other laterite- and calcrete-capped regions of the Tropics.