STABLE-ISOTOPE STUDY OF CARBONATE-CEMENTED ROCKS FROM THE POBITITE KAMANI AREA, NORTH-EASTERN BULGARIA

The Precipitation of carbonate cements in the Pobitite Kamani area (Lo wer Eocene) began during early diagenesis of sediments. There is evide nce, however, that calcite is still forming today. The negative deltaC -13 values to -29.2 parts per thousand suggest that the carbonate form ed during degradation of C-12-enriched organic matter (perhaps partly from oxidation of methane). The deltaO-18 values of -0.9 to -1.6 parts per thousand reflect the marine origin of the early diagenetic carbon ate cements. Most of the carbonates, however, formed during late diage nesis (at approximately 1 300 m burial depth) and/or recently (after u plift) from percolating groundwaters. These carbonates have an isotopi c composition characteristic of carbonates which precipitated from met eoric waters under normal sedimentary temperatures in isotopic equilib rium with C-12-enriched soil carbon dioxide.