The Pirgu Regional Stage (Upper Ordovician) in the East Baltic: lithostratigraphy, biozonation and correlation

The Upper Ordovician Pirgu Regional Stage of the East Baltic is represented by numerous formations, which characterize the lithological variability of the rocks and are used in the geological mapping of the region. Onshore-offshore drill core profiles from northern Estonia to southern Lithuania provide data on the distribution and succession of the formations. The formations described are arranged into four groups, each comprised of units with similar and characteristic lithological composition and depositional facies. Chitinozoan biozonation and bentonite (K-bentonite) beds are used to correlate selected sections and in the facies analysis of the described formations. Special attention is paid to the age and interpretation of the Jonstorp Formation in the East Baltic and Scandinavia. Correlations of the Pirgu Stage across the East Baltic are presented, and the use of the last occurrence of the chitinozoan Acanthochitina barbata Eisenack as a marker for the lower boundary of the stage is discussed.