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|Title:||Deep sea drilling project site 219 on the Laccadive ridge|
|Authors:||Siddiquie, H. N.|
Rao, CH. M.
Murty, P. S. N.
Reddy, C. V. G.
|Abstract:||Deep Sea Drilling Project Site 219 on the Laccadive Ridge penetrates Pleistocene to upper Paleocene sequence. A re-examination of Unit 5 (upper Paleocene) indicates that the Unit consists of rhyolitic tuffs and limestones. Fragments of tuff and broken grains of feldspar in the overlying limestones suggest that explosive volcanic activity continued in nearby areas till about lower Eocene. Acidic tuffs of similar age also occur on the West Coast of India in the Deccan Traps. Lower parts of Unit V are characterized by low percentages of calcite and quartz and the rocks in this unit contain high percentages of montmorillonite and plagioclase with minor amounts of clinoptilolite. The overlying biogenous sediments of Units IV, III, II, and I are largely marked by high calcite content. Heavy mineral assemblage represents 2 different components: a terrigenous component containing hornblende, garnet, zircon, monazite, kyanite, sillimanite, epidote, etc., and an authigenic component containing glauconite, anatase, collophane, etc. The authigenic assemblage marks the dominantly volcanic sediments and rocks of Unit V and lower part of Unit IV whereas the terrigenous assemblage is associated with the overlying biogenous sediments. The chemistry of the sediments indicates higher concentrations of iron (max: 8.06%), titanium (max: 1.43%) and manganese (400-1800 ppm) in the lower parts of Unit V. Mineralogical and geochemical data also support the dominantly volcanic nature of lower and middle parts of Unit 5. Major lithological and mineralogical differences in the sediments of the Site are related to the tectonics and sedimentary history of the Laccadive Ridge itself. The culmination of the volcanic activity followed by the subsidence of the ridge to about 2000 m depth marks the change from the volcanic rocks and volcanogenous sedimentation to biogenous sedimentation. When the ridge started subsiding in lower Eocene, shelf edge conditions prevailed briefly at the site and this led to the formation of phosphorite and manganese rich sediments possibly from upwelled waters. The eruption of Deccan Traps was followed by extensive faulting on the Western Continental margin of India which formed the Cambay Laccadive graben. The Deccan Trap and the associated volcanics from the basement in this area on which 1.5-5 km thick sediments are deposited from Paleocene onwards. Faulting of the West Coast of India and tertiary uplift of the Western ghats created a slope of the South Indian Peninsula to the coast which deprived the Canibay - Laccadive graben of large quantities of terrigenous sediments from the East particularly in the Central and Southern areas and favoured deposition of biogenous and chemogenous (carbonate) sediments in this area till recent times. The Laccadive ridge therefore would have exercised considerable influence in separating distinct sedimentary facies in this region.|
|ISSN:||0975-1033 (Online); 0379-5136 (Print)|
|Appears in Collections:||IJMS Vol.05(1) [June 1976]|
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