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Major, trace and rare earth elements (REE) composition of sandstones from the Upper Cretaceous Sillakkudi Formation, Ariyalur Group, Cauvery Basin were studied to decipher their weathering and provenance history. Texturally, these sandstones are immature, poorly sorted and grain supported. Abundance of feldspars especially, plagioclase indicates rapid deposition of sediments from a nearby source rocks. Using the geochemical classification diagram the Sillakkudi sandstones are classified as fe-sand, quartz arenite, litharenite, sub-litharenite, sub-arkose, arkose, and wacke types, which is also supported by the petrography study. The transition trace elements like Co, Ni, and V are higher in the Sillakkudi sandstones than upper continental crust (UCC) values. However, the Sillakkudi sandstones are lower in Cr (mean ~21) content than average UCC value (~ 35). The poor correlation between Cr and Ni (r = 0.08, number of samples n = 20) imply that these sandstones were derived from felsic source rocks. Similarly, the Eu/Eu * (0.35-1.73), La/ Sc (1.93-9.36), Th/Sc (0.41-6.57), Th/Co (0.14-5.01), Th/Cr (0.23-2.94), and Cr/Th (0.34-4.28) ratios support a felsic source for the Sillakkudi sandstones. The significant enrichment of Zr, Hf, and Th in fe-sand, sub-arkose and litharenite could be related to the presence of heavy minerals, especially zircon. However, the zircon geochemistry did not affect the REE distribution and its patterns in the Sillakkudi sandstones. The Chondrite normalized REE patterns of Sillakkudi sandstones are characterized by relatively flat HREE (Gd/Yb CN = ~ 0.73-2.41; subscript CN refers to chondrite normalized value), enriched LREE (La/Sm CN = ~ 3.39-5.82) and negative Eu anomaly (mean value Eu/Eu * = 0.80). The Gd/Yb CN ratios (~0.73-2.50) are less than 2.5, which suggest that these Sillakkudi sandstones were derived from the less HREE depleted source rocks. The comparison of REE patterns and its Eu anomalies to the source rocks reveals that the Sillakkudi sandstones received a major contribution of sediments from Dharwar craton.

Major, trace, REE, and stable isotopic composition of Maastrichtian limestones of Cauvery Basin were studied to decipher their depositional and post-depositional environment. The major oxides and trace elements concentration of these limestones were normalized using Post-Archean Australian Shale (PAAS) values. The concentration of CaO is having a significant enrichment, and MgO, K 2 O, Co, Ni, Cu, Zn, Pb, Ba, U, and Th are moderately depleted, whereas Sr and Na 2 O contents are relatively similar to the PAAS values. The PAAS-normalized REE ? Y pattern of Maastrichtian limestones display relatively uniform pattern among the compositionally different limestones: (1) enriched LREE (average Nd SN /Yb SN = 5.03 ± 0.92); (2) positive Ce anomaly (average 1.79 ± 0.16); (3) negative Gd SN /Gd* anomaly (average 0.52 ± 0.02); (4) low Y/Ho ratio (average Y/Ho = 0.14 ± 0.01). The average RREE contents in ferruginous, arenaceous, and biostromal limestones are 65, 64, and 52 ppm, respectively, which indicate that the mineralogical variation plays a major role in incorporation of RREE. The Kallankurichchi limestones have d 13 C values range from -3.2 to -1.1% PDB , and the d 18 O values range between -7.8 and -5.5% PDB . The negative d 13 C and d 18 O values and a moderate co-variation between d 13 C and d 18 O (r 2 = 0.35) indicate that the alteration of original isotopic composition is due to diagenesis. The positive Ce/Ce* confirms the clastic input into the system, and negative Mn* values and Th/U ratios (*0.29-1.4) in these limestones indicate their association with precipitation of carbonates in reducing (anoxic-dysoxic: Th/U B 2) conditions.