Geochemistry of the Dalmiapuram Formation of the Uttatur Group (Early Cretaceous), Cauvery basin, southeastern India: Implications on provenance and paleo-redox conditions
Keywords: rare earth elements, provenance, paleo-redox conditions, Dalmiapuram Formation, Cauvery basin, southeastern India
AbstractMajor, trace and rare earth elements (REE) study was carried out on the Lower Cretaceous Dalmiapuram Formation in southeastern India in order to understand the geochemical variations among various litho-units. Coral algal limestone (CAL) shows high content of CaO (53 ± 0.5, n=4) than the bedded limestone (BL) (42 ± 2, n=4) and gray shales (GS) (19 ± 6, n=3). The limestones are depleted in most of the trace elements when compared with the Post-Archean Australian Shale (PAAS). Likewise, gray shales are also depleted in many trace elements (Co, V, Rb, Ba, Zr, Y, Nb, Hf and Th), whereas few elements (Ni, Cr, Sr, Pb and U) show similar concentrations with respect to PAAS. The observed large variations in ΣREE contents among CAL (18 ± 8, n=4), BL (59 ± 17, n=4) and GS (157 ± 49, n=3) are mainly due to the amount of terrigenous matter present in them. The Eu and Ce anomalies were calculated from the PAAS-normalized values. The limestones (CAL and BL) and gray shales show small variations in Eu anomalies (Eu/Eu*: 1.23 ± 0.10, n=4, 1.16 ± 0.10, n=4 and 1.17 ± 0.10, n=3, respectively). The observed positive Eu anomalies in the limestones and gray shales may be due to the presence of plagioclase feldspar. The CAL shows a negative Ce anomaly (Ce/Ce*: 0.90 ± 0.06, n=4) whereas BL shows a positive Ce anomaly (Ce/Ce*: 1.05 ± 0.14, n=4; except KI15) and gray shales show no Ce anomalies (Ce/Ce*: 0.98 ± 0.02, n=3). Variations in Ce anomalies in these limestones may be due to the mixing between sediment components and a seawater end member. The characteristics of non-seawater-like REE patterns, elevated REE concentrations, high LaN/YbN ratios and low Y/Ho ratios, suggest that the observed variations in ΣREE contents are mainly controlled by the amount of detrital sediments in the limestones of the Dalmiapuram Formation. The REE patterns and La/Sc, La/Co, Th/Co, Th/Cr, Cr/Th, and Th/Sc ratios suggest that the terrigenous materials present in the Dalmiapuram Formation were mainly derived from intermediate to felsic rocks. The limestones (CAL and BL) show positive values of Mn* and low V/(V+Ni) ratios (≤0.50) suggesting that these limestones have been deposited under oxic condition whereas gray shales show negative Mn* values and high V/(V+Ni) ratios (≥ 0.50) which support that these shales were deposited under reducing conditions.