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Kristalina Stoykova, Daria Ivanova:
Upper Jurassic-Lower Cretaceous calcareous nannofossil and calcareous dinocyst occurrences and their relationship to sea-level changes: an example from the W Balkan Mountains, Bulgaria
(Talk)

This common study of calcareous nannofossil and calcareous dinocyst occurrences of the Upper Jurassic-Lower Cretaceous pelagic carbonate sections of the W Balkan Mountains was undertaken to directly correlate biostratigraphic zones, defined on each of the calcareous microplankton groups (Lakova et al., 1999). Another main objective of the joint study is to reveal a possible relationship between calcareous microplankton distributions and eustatic changes of sea-level.

Calcareous nannofossils and calcareous dinocysts were studied in two complete pelagic sections, excellently exposed in the W Balkan Mountains. Lithologically, the sections are represented by nodular limestones, micritic limestones, and alternations of micritic limestones, marly limestones and marls. As a result, five nannofossil zones and 10 calcareous dinocysts zones, covering the Late Kimmeridgian to Late Valanginian time-interval were recognised. In addition, a number of bioevents in this interval were detected and directly correlated (Lakova et al., 1999).

Quantitative analyses were performed on the representatives of the two microplankton groups, in order to evaluate the species diversity and relative abundance in the sections studied. All data are displayed in the graphs, and the graphs are compared (Figure 1). Analysis of the graphs has shown that all of them mark general trends of long-term sea-level changes (2nd order). For example, a relatively high stand of sea-level in the mid-Tithonian, followed by a gradual drop around the Jurassic/Cretaceous boundary, and a relatively high stand in the mid-Berriasian and fall in sea-level in the Early Valanginian are clearly documented on the graphs. Short-term (3rd order) eustatic cycles also can be recognised (some additional investigation is needed).

We suppose that maximal values of species diversity and abundance of the two microplankton groups generally reflect the maximum rise of sea-level in the long-term cycles and/or maximum flooding surfaces in the short-term cycles.

Reference

Lakova, I., Stoykova, K. & Ivanova, D. 1999. Calpionellid, nannofossil and calcareous dinocyst bioevents and integrated biochronology of the Tithonian to Valanginian in the Western Balkanides, Bulgaria. Geol. Carpathica, 50(2): 1-18.

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