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Paleozoic (Silurian and Devonian) cherts from Western Bulgaria occur in three tectonic units: Svoge, Lyubash-Golo Bardo and Morava. The SiO2 content in these rocks varies from 76.06% to 96.02%. The silica source and depositional settings of the cherts have been interpreted based on the received geochemical major element data. The calculated Si/(Si+Fe+Al+Ca) ratio and Al-Fe-Mn diagram plotting at non-hydrothermal field suggest that most of silica in the cherts is biogenic or both biogenic and terrigenous in origin. On the other hand, all studied siliceous samples plot into the field of the continental margin sediment environments on the Fe2O3/TiO2 vs. Al2O3/(Al2O3+Fe2O3) diagram. The calculated MnO/TiO2 ratio also indicates deposition in continental shelf or slope environments. [ABSTRACT FROM AUTHOR]

Geochemical studies of different Paleozoic intervals composed of bedded cherts, revealed in the western parts of the Srednogorie and Morava-Rhodope zones are presented: Saltar Fm; the lower and middle part of Katina Fm; the lowest part of Parchar Fm, and Tumba Fm. To determine whether the different bedded cherts intervals are hydrothermally affected, triangular diagrams of the A-F-Mn ratio were used. The low content of Fe compared to Al gives grounds to claim that the Pre-Varisian Paleozoic bedded cherts in Bulgaria are not associated with endogenous processes. [ABSTRACT FROM AUTHOR]

The Late Devonian (372-359 million AD) is a time of prolonged climate instability with catastrophic disturbances to global marine ecosystems at the Frasnian-Famennian (F/F) and Devonian-Carboniferous (D/C) boundaries. Global atmospheric CO2 content and climate change in the middle Paleozoic, especially in the Late Devonian, coincide with major events of extinction of organism groups. Anoxic intervals were documented within the Kellwasser event near the Frasnian/Famennian boundary (F/F), the Annulata event in Famennian and the Hangenberg event near the Famennian/Tournaisian boundary (D/C). [ABSTRACT FROM AUTHOR]