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-associated
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 accumulated
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 dissolution-
reprecipitation in diagenetic or vadose-phreatic environments was common in evaporite-related carbonates.