Carbon and Oxygen Isotope Ratios in Rona Limestone, Romania

Abstract

Carbon and Oxygen Isotope Ratios in Rona Limestone, Romania. The carbon and oxygen isotopic compositions of limestones provide criteria for the evaluation of the depositional environment. For Jurassic and younger samples, the best discrimination between marine and freshwater limestones is given by Z parameter, calculated as a linear correlation between δ 13 C and δ 18 O (‰ PDB). Rona Limestone (Upper Paleocene-Lower Eocene), outcropping on a small area in NW Transylvania (Meseş area) is a local lacustrine facies. There, it divides Jibou Formation into the Lower Red Member and the Upper Variegated Member, respectively. Recently, a sequence containing a marine nannoplankton assemblage was identified in the base of Rona deposits. The main goal of our study was to characterize, based on the isotopic record, the primary environment of formation of the deposit, as well as that in which some diagenetic processes (the formation of dolomite and of green clay around the siliceous chert nodules) took place. Ten samples representing limestones, dolomitic limestone, marls and the green carbonate-rich clay were studied from petrographical and mineralogical points of view, and the carbon and oxygen isotopic ratios from the carbonate (calcite) component were measured. In conclusion, it was found that the procedure of extraction of CO 2 we used enabled the discrimination between the isotopic prints of calcite vs. dolomite. This pleads for considering our results as a primary isotopic pattern in the bulk rock. The oxygen and carbon isotope data indicate a freshwater depositional environment with Z<120. The δ 13 C mean value (-4.96 ‰ PDB) is, generally, representative for freshwater carbonates of the Tertiary period. The same environment characterized also the formation of carbonates within the green clay.