Carbon and oxygen isotopic composition of the Middle Miocene Badenian gypsum-associated limestones of West Ukraine

Abstract

The Middle Miocene Badenian basin of the Carpathian Foredeep is characterized by complex sedimentary and diagenetic carbonate-evaporite transitions. Six locations have been selected to evaluate the controls on the carbon and oxygen isotopic composition of the Badenian gypsum-associated limestones of the Tyras Formation in West Ukraine. At three locations marine limestones overlie the gypsum, at one location (Anadoly) the gypsum-associ-ated limestones are polygenic, and at two locations (Pyshchatyntsi and Lozyna) gypsum deposits are lacking. The studied limestones have originated as primary, mostly peloidal carbonates as well as secondary carbonates formed by  hypogene  sulphate  calcitization.  They  show  a  wide  range  of δ¹³C  (from  from  -0.9‰  to  -39.8‰)  and δ¹⁸O values (from 0.9‰ to -12.2‰). The Badenian limestones formed in marine environments (either as deposits ac-cumulated at the bottom of the sea or forming the infillings of solution cavities within gypsum) have less negative δ¹⁸O values compared to predominantly diagenetic formations. Wide ranges and usually very negative δ¹³C values and low δ¹⁸O values of those limestones indicate that they suffered important meteoric diagenesis as supported by common sparitic fabrics. In addition, a large range of δ¹³C values even in the group of samples characterized by less-negative δ18O values shows that bacterial sulphate reduction and methane oxidation were active processes in the pore fluids of the Tyras Formation. Very low carbon isotopic compositions (δ¹³C values from -22 to -40‰) of some sparitic limestones in the studied sections indicate the occurrence of oxidized methane within the diagenetic environment. Accordingly, the isotopic signatures of the studied limestones are a combination of both primary and secondary processes, the latter having a primordial importance. The common occurrence of similar negative δ¹³C and δ¹⁸O values in evaporite-related carbonates in other Miocene evaporite basins suggest that extensive dissolu-tion-reprecipitation in diagenetic or vadose-phreatic environments was common in evaporite-related carbonates.