The lower boundary of the Haljala Regional Stage (Sandbian, Upper Ordovician) in Estonia.

Multiproxy correlation is used to address the boundary of the Kukruse and Haljala regional stages across the Estonian part of the Baltoscandian Ordovician Palaeobasin. New biostratigraphic information on conodonts, chitinozoans and ostracods from the Peetri outcrop, northwestern Estonia, integrated with micropalaeontological data from five core sections all over Estonia, allows justification of the position of the stage boundary and correlation between conodont and chitinozoan biozones. In the Peetri outcrop a marked stratigraphic hiatus was recognized at the Kukruse-Haljala boundary - the Baltoniodus gerdae conodont Subzone and the Armoricochitina granulifera, Angochitina curvata, Lagenochitina dalbyensis and Belonechitina hirsuta chitinozoan zones are missing in the section. The lowermost part of the Tatruse Formation in central Estonia and the Adze Formation in southern Estonia are older than previously thought, and the results show a principal advantage of the multiproxy method over singlegroup studies. The lower boundary of the Haljala Regional Stage is tied to the interval with gaps that are common all over the stratotype region. [ABSTRACT FROM AUTHOR]