CARBON ISOTOPE ANALYSIS (DELTA 13C) OF EARLY TO MIDDLE DEVONIAN CARBONATES FROM THE PRAGUE SYNCLINE, THE CARNIC ALPS, AND THE MONTAGNE NOIRE (CZECH REPUBLIC, AUSTRIA, AND FRANCE)

Introduction: Several sections in the Prague Syncline, the Carnic Alps and the Montage Noire were sampled for delta¹³C analysis of whole rock carbonates and organic carbon (TOC). The Silurian/Devonian boundary is defined in the Klonk Section of the Prague Syncline which is also the type area for the Lochkovian and Pragian stages. Most of the analyzed Czech carbonates were deposited well below the wave base. The sections of the Carnic Alps represent the transition from shallow water reefs in the W to a basin in the E. A deepening upward Megacycle interfere with the carbonate sedimentation in the Montage Noire.

Results: The delta¹³C curves exhibit similar trends in all sections. A positive excursion with values up to + 6 ‰ marks the Silurian/Devonian boundary. The late Lochkovian is characterized by lighter carbon isotopes in the range of 0 to +1 ‰. Two broad positive excursions are observed in Pragian carbonates. Zlichovian isotope ratios are again in the range of 0 to +1 ‰ delta¹³C. A positive shift of 2 ‰ occurs at the Zlichovian/Dalejan transition in the Prague syncline. It is only in the range of 1 ‰ in the Cellonetta section. The Zlichovian is probably partly eroded or represented by conglomerates in the Oberbuchach II section. At the Eifelian/Givetian transition a positive excursion is indicated by scattered delta¹³C data.

Conclusions: The delta¹³C isotope curve is correlated to the sea level and to (bio-)events. A high sea level at the Silurian/Devonian transition obviously favored deposition of organic carbon resulting in the large positive excursion. Also the other (black-) shale events (Daleje and Kacak) resulted in positive carbon isotope shifts. Further investigations have to prove whether these are global isotope events. The double excursion during the Pragian is not recorded in black shale events in Europe. The relative sharp drop at the base of the Zlichovian coincides with a regression resulting in the formation of breccias (Chapel Limestone in the Prag syncline and erosion and breccias in the Oberbuchach II section).