Petrography and Major Element Geochemistry of Late Maastrichtian- Early Palaeocene Sediments of Tiruchirapalli, Tamil Nadu - Palaeoweathering and Provenance Implications

Petrographic and geochemical studies of the Late Maastrichtian-Early Palaeocene sediments of Kallamedu and Niniyur Formations of Tiruchirapalli have brought out significant results on palaeoweathering and source area characteristics. In the bivariate plot of SiO₂ vs Al₂0₃+K₂0+Na₂0₃ all the clastic samples fall in the arid field indicating that the arid climatic condition prevailed in the source region during the deposition of these sediments. In the Al₂0₃ vs Na₂0 plot, the clastic samples fall between low grade Archaean shale and post-Archaean shale suggesting that the detrital components have been derived from low to moderately weathered terrain. The Chemical Index or Alteration of clastic sediments range from 40 to 69 with lower values for sandstones (40 to 57) than for silty shale and shale (61 to 69) and this indicates that the source rocks have undergone low to moderate chemical weathering. In QFL diagram, most of the sandstones fall in the field of cratonic interior basin which indicate that these sediments were deposited in the stable craton. SiO₂/Al₂0₃ ratios are high observed for sandstone, silty shale and shale, with a range starting from 3.15 to 8.75. The higher ratios of SiO₂/Al₂O₃ in clastic sediments indicate a largely felsic rock provenance. Major element data indicate an overall taphrogeosynclinal continental rift-related depositional basin.