Ashish Sarkar

Basal Tuffs and Contemporaneity of the Chattisgarh and Khariar Basins Based on New Dates and Geochemistry

Time bracketing of thousands-of-meters-thick, unmetamorphosed, fossil-free sedimentary successions is a long-standing challenge in Indian Proterozoic stratigraphy. Limited geochronologic ages from either the basal or the upper part of the successions prevent workers from constraining the precise age interval for individual basin fill, leaving aside the issue of correlatability between different basin sequences. This article deals with silicic tuff units from the basal part of two areally separated Proterozoic sedimentary successions of central India, the Singhora Group of the Chattisgarh Supergroup and the Khariar Group. The Singhora tuff is enriched in large ion lithophile (Rb, Ba, Th, etc.) elements and light rare earth elements and in a discrimination diagram falls within the volcanic arc granite field. U-Th-Pb electron probe microanalyzer geochronology of monazite and zircon grains from the Singhora tuff revealed several age data clusters, namely, ∼2500, ∼2100, ∼1800, ∼1500, and ∼1000 Ma. In contrast, the age data from the Khariar tuff fall within a narrow range of ∼1455 Ma. To isolate the depositional/crystallization age of the Singhora tuff from inherited provenance ages, we also dated monazite and zircon grains from stratigraphic units overlying and underlying the Singhora tuff. The ∼1500-Ma age is proposed as the depositional/crystallization age for the Singhora tuff, considering its most frequent and exclusive presence within the tuffaceous unit. Comparable ages from the tuffaceous units at the basal part of areally separated basins, namely, Singhora (∼1500 Ma) and Khariar (1455 ± 47 Ma), suggest nearly synchronous initiation of the two basins. Considering the recently reported U-Pb SHRIMP zircon age of 1631 ± 5 Ma from the basal part of the Vindhyan Supergroup, the largest Proterozoic sedimentary basin of peninsular India, we presume a crustal-scale event in early Mesoproterozoic time (1600–1500 Ma) on the Indian continent.

Oxygen and carbon stable isotope studies onGloborotalia menardii from Pleistocene DSDP Cores in northern Indian Ocean and their paleoclimatic and paleoceanographic implications

Stable isotope ratios of oxygen (δ18O) and carbon (δ13C) in tests ofGloborotalia menardii from samples at 25 cm intervals of top 900 cm cores, representing different thicknesses of the Pleistocene, from DSDP Sites 219, 220 and 241 in the northern Indian Ocean have been measured. Based on the δ18O stratigraphy, glacial and interglacial phases during the Pleistocene have been recognized, which are in good agreement with the standard Quaternary planktonic foraminiferal/climatic zones i.e., Ericson zones at these sites, based onG. menardii abundances. The GIA (glacial interglacial amplitude) at Sites 241, 219 and 220 are of the order of 1·2, 1·4 and 1·9‰ respectively. The last glacial and interglacial maxima (18 ka BP and 125 ka BP respectively) could be identified in DSDP Cores 241, and 219 with some precision. ‘Isotopic ages’ could be assigned to the different levels of these core sections based on the correlation of δ18O record from these sites with the SPECMAP record (Imbrieet al 1984). Changes in sediment accumulation rates at different levels of the Pleistocene have been worked out on the basis of changes in oxygen isotopic ratio.Oscillations in δ13C stratigraphy at Site 241 indicated southwest monsoon induced increase in upwelling and productivity during warmer periods. At Sites 219 and 220, variations in the δ13C record were due to the mixing of bottom water.