A multiproxy study of the Puhmu core section (Estonia, Upper Ordovician): consequences for stratigraphy and environmental interpretation

A multiproxy study of the Katian and Hirnantian in the Puhmu core in NE Estonia resulted in new data on chitinozoan and brachiopod biostratigraphy. Some mass occurrences of dasycladacean algae are tied to small early Katian ‘reefs’. Carbon isotope chemostratigraphy showed four earlier known excursions beginning with the early and middle Katian Rakvere and Saunja events, the late Katian Moe excursion and the most prominent Hirnantian excursion. These new data, combined with the distribution of ostracodes and lithostratigraphy, enabled us to apply five regional stages of the traditional Baltic chronostratigraphic framework. Discontinuity surfaces are numerous and diverse in the Puhmu section, especially at two levels, coinciding with the δ13C lows before the two most prominent isotope excursions. A special surface with extra deep cavities at the upper boundary of the Saunja Formation is interpreted as a karstification event likely analogous to the one reported from Sweden. The isotope data from the whole-rock and bioclast material were compared, providing very similar results. This may be connected with a very high content of calcite (more than 90%) in most of the Puhmu rocks and a weak impact diagenetic imprint as evidenced by a low colour alteration index of conodonts. The variability of δ18O values is larger than considered good, but the observed trends, even if not understood satisfactorily, suggest that these data might have some use in environmental interpretation.