Calpionellid zonation at the Jurassic/Cretaceous boundary within the Vigla Limestone Formation (Ionian Zone, western Greece) and carbon isotope analysis

The calpionellid standard zones, Crassicollaria, Calpionella andCalpionellopsis , identified in the Tithonian–Berriasian carbonate sequence at Paramythia (Ionian Zone, western Greece), have been subdivided into eight subzones: the Tithonian Crassicollaria Zone into the Crassicollaria intermedia and Crassicollaria brevis Subzones, the early Berriasian Calpionella Zone into theCalpionella alpina , Remaniella and Calpionella elliptica Subzones, and the late BerriasianCalpionellopsis Zone into the Calpionellopsis simplex, Calpionellopsis oblonga andPraecalpionellites murgeanui Subzones. On the basis of this zonation the Jurassic/Cretaceous boundary (Tithonian/Berriasian boundary) is equated to the lower boundary of the Calpionella alpina Subzone. The carbon-stable isotope measurements performed on bulk samples from Paramythia indicated a decrease in δ13C values from the Tithonian to the Berriasian owing to (a) a decrease in transfer of organic matter into the sea caused by the lowering in Calcite Compensation Depth, (b) the presence of a deeper-water environment in the Ionian Basin and (c) a fall in the sea level and increased oxygenation of the water column. These changes, which favoured the deposition of carbonates, led to a decrease in burial rate of the organic matter and to the fall in the carbon-stable isotope values.

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