VOLUME 51 (1)/April 2022

Coral whole-rock geochemistry and in situ LA-ICP-MS analyses of coral skeletons were performed on late Eocene–early Oligocene coral reef buildups from the Eastern Rhodope–Thrace region of Bulgaria and Greece. Coral reefs are locally associated with voluminous Oligocene volcanism in the region. The reefs are subdivided into (i) eruption-associated reefs (Krumovgrad); (ii) pre-eruption reefs (Ivaylovgrad); and (iii) Metaxades-Didymotycho reefs from field relations, trace element and rare-earth element (REE) abundances. Coral assemblages are dominated by Cladocora sp., which is accompanied by Porites sp., Colpophyllia sp., Favites sp. and Leptoseris sp. Eruption-associated reefs are characterized by their higher REE content than the lower in all REE contents of pre-eruption reefs showing negative Ce anomaly, and Metaxades-Didymotycho reefs that have lower middle-heavy REE contents compared to previous groups. Trace element and REE geochemistry of the coral skeletons indicates volcanic contribution to seawater, mostly evident in the eruption associated reefs, and contribution from terrestrial input in the site of coral buildup deposition. Contribution from a different source of prior diagenetic nature, along with subsequent diagenetic modification, is inferred. The increase in REE+Y (ΣREY) from pre-eruption to eruption-associated reefs is well correlated with elevated amounts of terrigenous elements like Al and Fe.

Geological settings are of major importance in selecting appropriate environment for disposal of high-level radioactive waste from nuclear power plants. Clay rocks are preferred in a number of countries as a suitable host rocks due to their insulating properties and ability to self- seal cracks and other disturbances. In Bulgaria, on the basis of multi-criterion comparative analysis, several prospective sites have been selected for further consideration. One of them is the Varbitsa site located in the Fore-Balkan Unit. The marls of the Sumer Formation are the host rock. Similar clay host rocks for high-level radioactive waste disposal have been extensively studied in France, Switzerland and Belgium. In this context, the experience gained in these countries is particularly valuable for the further development of the geological disposal in Bulgaria. The current paper presents a comparison between the geological settings of the Varbitsa site (NW Bulgaria) and the Bure site (SE France), whose argillites are similar to the Bulgarian marls. The outcomes of this comparison would be useful for planning and conducting future research activities, as well as for the public acceptance of the geological disposal.