Depositional and post-depositional setting of Maastrichtian limestone, Ariyalur Group, Cauvery Basin, South India: a geochemical appraisal

• Published in: Carbonates and evaporites 2011-07, Vol.26 (2), p.127-147
• Main Authors: Nagendra, R., Nagarajan, R., Bakkiaraj, D., Armstrong-Altrin, John S.
• Format: Article
• Language: English
• Subjects: Anoxia; Carbonates; Chemical composition; Composition; Copper; Cretaceous; Diagenesis; Earth and Environmental Science; Earth Sciences; Gadolinium; Geology; Isotopes; Limestone; Manganese; Mineral Resources; Mineralogy; Nickel; Original Article; Oxides; Ratios; Sedimentary rocks; Shale; Trace elements; Zinc
• ISSN: 0891-2556; 1878-5212
• DOI: 10.1007/s13146-010-0041-2

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 ΣREE 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 ΣREE. The Kallankurichchi limestones have δ 13 C values range from −3.2 to −1.1‰ PDB , and the δ 18 O values range between −7.8 and −5.5‰ PDB . The negative δ 13 C and δ 18 O values and a moderate co-variation between δ 13 C and δ 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 ≤ 2) conditions.