Stratigraphy of the Ordovician and Silurian boundary deposits in northeastern Poland and their correlation with the stratotype sections in Estonia

The carbonate-silicoclastic sequence at the Ordovician and Silurian boundary in the Widowo IG 1 (depth 700.0-730.0 m) and Prabuty IG 1 (depth 3348.0-3372.0 m) boreholes contains a diversified and abundant Chitinozoa assemblage. A total of 57 taxa were identified (48 of them to the species and subspecies rank) which document 6 standard zones of the regional Baltoscandian scale. These are the following zones and subzones: in the Ordovician -(fungiformis) = spinifera Zone, including angusta and reticulifera subzones, bergstroemi; in the Silurian - longicollis, margaritana, proboscifera and mamilla zones. The chitinozoan zones of margaritana and proboscifera from the Widowo IG 1 borehole (depth 706.0-708.3 m) could not be identified. It was also impossible to distinguish the acuminata and banwyensis zones due to very strong condensation of the succession. The determination of extent and boundaries as well as the position of the zones and subzones within the section enables precise correlation of the Ordovician and Silurian carbonate-silicoclastic sequence from the Widowo IG 1 and Prabuty IG 1 boreholes with the isochronous sections in the adjacent areas of Poland (Kętrzyn IG 1, Proniewicze IG 1, and Klewno 1 boreholes), Estonia (Rapla 1, Tartu (453), Taga-Roostoja (25A), Ruhnu, Mehikoorma (42I) and Viljandi boreholes) and western Volhynia of Ukraine (Kowel 1 borehole - no 5415).