12 Nov Paleo-Climate Labels of Late Quaternary, Southern Tamil Nadu.
Paleo-Climate Labels of Late Quaternary, Southern Tamil Nadu.
Thrivikramj.K.P. and Joseph, S
Formerly, University of Kerala, Dept. of Geology, Kariavattom Campus, 695581
Univ. of Kerala, Environmental Sciences, Kariavattom Campus, 695581
The planet earth, since the very early days of its history, had gone through dramatic climate shifts (like repetitive glaciations, Humid-temperate spells and periods of aridity) of widely varying amplitudes. The labels of past climate or climate shift are invariably or often hidden in the rocks, especially in fossil assemblages, mineral associations, sedimentary facies associations, geochemical attributes etc.. Further older the rock formation, harder it is for the labels to come by, which is primarily due to multiple deformations and variations in the chemistry of pervading fluids through the rock-intergranular- pores. .
The Teris of southern Tamil Nadu, a fairly contiguous and distinctly prominent sedimentary deposit, coloured in sharp or well defined shades of red, is noticed in the Tirunelveli Dist. (Table 1). Colour of the sand sheet ranges from yellowish red (5YR 4.5/6) to dark reddish brown (2.5 YR ¾) and to dark red (10R 3/6) Teris are broadly grouped into the ITD or Inland Teris Deposits (with poorly sorted, medium to coarse sand) occurring adjacent to the eastern side of the Western Ghats (area =~33.0 km2) and CTD (moderately sorted, medium sand) or coastal teri deposits (area=387.0 km2) positioned adjacent to modern or ancient shorelines of the Tirunelveli Dist.
This thick sheet sand of variable thickness especially in the CTD has been moulded into large barchans, barchanoids and transverse-subaerial-dunes, which in turn are covered by bedforms like ripples of different morphologies, suggesting the current occurrence of a dynamic surface layer of wind driven sand. This “sea of red sand” is unique to India, in that that there is no other comparable occurrence anywhere else in the rest of the country.
The Teri sand sheet rests unconformably either on variably weathered and/or eroded Precambrian basement (e.g..Kulathur) or on a calcareous sandstone (e.g., at Melmandi) or on a fossiliferous limestone bed (like at Meyyur and Menjanapuram). Inspite of the considerable thickness of the sand body (10.0 m at Sattankulam and 12.0 m at Ovari), the primary depositional structures are difficult to discern in the sections – perhaps due to shift in mass balance during pedogenesis.
The ITD has a large content of feldspar, in contrast with the CTDs wherein large presence of opaques and subordinate feldspar are noticed. Clay fraction in both ITD and CTD is not only autochthonous but display larger bulk of kaolinite and subordinate content of illite. Hematite and goethite dominate the mineralogy of cutan.
The kaolinisation (a humid climate label) and illitisation (a semi-arid climate label) processes that the teri sand underwent did not over lap instead the former preceded the latter. The scarce or intensely corroded garnet grains, altered pyriboles, opaques and feldspar in the teri sediment unequivocally point to intrastratal origin of cutan.
The ubiquitous occurrence of rhizoliths of calcrete in the teris of Syyarpuram and Sattankulam, and calcretisation of the basement crystallines rimming the teri sheet sand clearly indicate the continuation of the semi-aridity that promoted ilitisation of earlier formed kaolinite through the modern times. The 14C date of 3680+-110 yr B.P. of calcrete, is a robust timeline indicating transition from the humid to semi-arid climate in southern Tamil Nadu.
Further, in future i.e., by 2050, the extent of semi-arid zone in Tamil Nadu and rest of India are bound expand and enlarge further forced by the climate change phenomenon. Therefore, the Tamil Nadu state along with the other Indian states will have to brace itself to face up the challenges of the impending climate change by harnessing the tools and services of science and technology , and possibly a a new and appropriate life style..