Chitinozoan dynamics and biostratigraphy in the Vao Formation (Darriwilian) of the Uuga Cliff, Pakri Peninsula, NW Estonia

摘要

The distribution of chitinozoans in the Vao Formation (Lasnamagi and Uhaku regional stages, Darriwilian) of the Uuga Cliff, Pakri Peninsula, NW Estonia, was investigated from 62 samples. Chitinozoans are very common and assemblages are diverse, with a total of 36 species and up to 170 specimens per gram of rock. The assemblage is dominated by Belonechitina, Desmochitina, Cyathochitina, and Euconochitina. The relative and absolute frequency of particular taxa displays regular and possibly cyclic patterns, which are not directly reflected in lithology or geochemistry. Abrupt changes in chitinozoan abundance, coinciding with some of the discontinuity surfaces, suggest the presence of stratigraphical gaps. The diversity of chitinozoans is the highest in the lower part of the Vao Formation (Rebala Member), where up to 20 species were identified in one sample and the standing species diversity exceeded 25. Within the studied interval at least 11 biostratigraphical horizons can be distinguished, including the subzonal boundaries within the Laufeldochitina striata Zone. The disappearance of abundant Belonechitina pellifera can be used for tracing the lower boundary of the Uhaku Stage.