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Depositional History of the Chhattisgarh Basin, Central India Constraints from New SHRIMP Zircon Ages [查看] M.E.Bickford、AbhijitBasu、SarbaniPatranabis-Deb、PratapC.Dhang、JuergenSchieber

The Indian Shield includes the Singhbhum, Bastar, and East and West Dharwar cratons. Proterozoic sedimentary basins formed on these cratons have preserved rocks with a range of degrees of metamorphism and deformation. In the Chhattisgarh Basin, within the Bastar Craton, the ca. 2200–2500-m-thick Chhattisgarh Supergroup has been preserved in nearly pristine condition. Previous work has shown that the Sukhda Tuff, located about 2200 m from the base of the section, was formed ca. 1007 Ma. New U-Pb SHRIMP age determinations show that the Singhora Tuff, located about 100 m above the base of the basin, is not older than 1405±9 Ma. Thus, most of the Chhattisgarh Supergroup was deposited between 1400 and 1000 Ma. Age data for detrital zircons from sandstones show that,regardless of their stratigraphic position, there is a unimodal age peak near 2500 Ma, the typical age of adjacent granitic and rhyolitic basement rocks, indicating that these constituted the principal provenance of the sediments in the Chhattisgarh Basin. However, near the top of the succession, the Sarnadih Sandstone and a volcaniclastic sandstone near Sukhda Village, show a wide range of ages with peaks from ca. 1000 through 2680 Ma. The ca. 1000 Ma detrital zircons were probably derived from igneous sources similar to the Sukhda Tuff, but the zircons with other ages indicate a different source. Age data from rock units in the Central Indian Tectonic Zone to the north of the basin match this age spectrum better than any to the south of the basin and are consistent with a change in provenance direction to a northerly source late in the basin-filling cycle.

Depositional History of the Chhattisgarh Basin, Central India Constraints from New SHRIMP Zircon Ages A Reply [查看] M.E.Bickford、AbhijitBasu、SarbaniPatranabis-Deb、PratapC.Dhang、JuergenSchieber

The main issues in the discussion of our article(Bickford et al. 2011b) by Chakraborty et al. (2011)appear to be the stratigraphic position of the Singhora tuff and our interpretations of SHRIMP U-Pb zircon data. We address these issues but will not discuss stratigraphic classification, sedimentology,and plate tectonic considerations that are not relevant to the theme of our article.

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ∼500-Ma Adjustment in Indian Proterozoic Stratigraphy A Reply [查看] S.Patranabis-Deb、M.E.Bickford、B.Hill、A.K.Chaudhuri、A.Basu

SHRIMP Ages of Zircon in the Uppermost Tuff in Chattisgarh Basin in Central India Require ~500-Ma Adjustment in Indian Proterozoic Stratigraphy [查看] SarbaniPatranabis-Deb、M.E.Bickford、BarbaraHill、AsruK.Chaudhuri、AbhijitBasu

The Chattisgarh Basin of east central India and many unmetamorphosed Proterozoic sedimentary basins of Peninsular India have been considered mostly Neoproterozoic (1000–545 Ma) in age. A newly recognized succession of rhyolitic ignimbrite, ash beds, and volcaniclastic sandstones near the top of the ∼2.2-km-thick sedimentary fill of the Chattisgarh Basin is a chronostratigraphic marker. Euhedral igneous zircons from these units give U-Pb SHRIMP ages of 990–1020 Ma, indicating that the basin fill beneath this marker horizon is pre-Neoproterozoic. On the basis of newly reported zircon ages of 16315 Ma from the basal part of the Vindhyan Basin and accepting the consensus that all virtually undeformed and unmetamorphosed craton-interior Proterozoic sedimentary basins in peninsular India are approximately coeval, we conclude that these basins are approximately Mesoproterozoic (1600–1000 Ma) in age. The reassigned age for these rocks (1650 to 900 or possibly ∼1000 Ma), up to 500 Ma in variance with the current notion (∼1100 to ∼518 Ma; Naqvi 2005), calls for a thorough rethinking of contemporary models concerning tectonics,sedimentation, and other geological activity that affected the Indian shield in the Proterozoic Era.