Upper Carnian to Rhaetian (Upper Triassic) conodont and carbon-isotope stratigraphy of the Csővár borehole succession, Hungary

Abstract

A total of 125 samples from core from the Csővár borehole (north-central Hungary) were investigated for conodont bio- and carbon-isotope chemostratigraphy. This 620 m thick continuous pelagic carbonate succession comprises strata from the upper Carnian to upper Rhaetian, including both, the Carnian-Norian and the Norian-Rhaetian stage boundaries. The mass occurrence of the Metapolygnathus communisti conodont group followed by the occurrence of Primatella gulloae mark the base of the Norian in the lowermost part of the section, which allows good correlation with the Pizzo Mondello section (Sicily, Italy). The lower Norian conodonts of the core belong predominantly to the genus Ancyrogondolella. Subdivision of the middle Norian interval is hampered by the lack of fully adult specimens. The upper Norian shows a greater diversity than the rest of the Norian, and the rich assemblages give an insight into the evolutionary transition from Misikella hernsteini to Mi. posthernsteini (sensu stricto). The first occurrence of Mi. ultima in the core changes the previous views of the thickness of the upper Rhaetian in the Csővár area and emphasizes the necessity of re-calibration of the isotope stratigraphic results of previous studies. The carbon-isotope results show distinct fluctuations and trends. Some of them are similar to literature data, others are different. It can be stated from the new data that a ∼ 2 ‰ long-term δ13C background decline throughout the Norian is most probably global in extent. Negative carbon-isotope excursions occur in Norian substages: (upper) Alaunian, lower Sevatian, middle Sevatian and upper Sevatian. Distinct positive δ13C excursions are present across the Alaunian-Sevatian boundary and the Norian-Rhaetian boundary, and the climax of the latter is located in the lower part of the posthernsteini Zone. This last isotope event is particularly important because at the Global Stratotype Section and Point (GSSP) candidate for the base of the Rhaetian stage at Pignola (Southern Apennines, Italy) a distinct negative δ13C excursion has been reported for this stratigraphic level.