Studies in Ordovician geology: the Baltoscandian region

Integration of chitinozoan and conodont biozones is based on detailed collections from the Bliudziai-150 well, southern Lithuania that penetrated upper Darriwilian – lower Sandbian sedimentary carbonate rocks. The strata accumulated at the southeastern side of the Livonian Tongue and represent the transition from shallow shelf facies of the Lithuanian shallow-water shelf facies belt to the deeper-water shelf facies belt. Systematically collected and closely spaced samples of the carbonates from the Bliudziai-150 core yielded both chitinozoans and conodonts in abundance. The chitinozoans are assigned to the Laufeldochitina striata, Laufeldochitina stentor, Angochitina curvata-Armoricochitina granulifera, Lagenochitina dalbyensis, Belonechitina hirsuta and Spinachitina cervicornis chitinozoan biozones and the conodonts to the Pygodus serra Zone (with three subzones), Pygodus anserinus Zone (with two subzones) and the three subzones of the Amorphognathus tvaerensis Zone. The fossils are clearly of Baltoscandian affinity and the integrated results support existing knowledge concerning the biostratigraphy of the two groups in Baltoscandia. Thus, the studied interval is confidently assigned to the Lasnamägi, Uhaku, Kukruse and Haljala regional stages of the East Baltic regional chronostratigraphical scheme, corresponding to the global upper Darriwilian – lower Sandbian stages (Middle to Upper Ordovician). International correlation of the chitinozoan and conodont succession is presented and the position of the base of the international Sandbian Stage is placed at 1380.00 m in the Bl-150 core. A horizon marked by multiple discontinuity surfaces represents the base of Kegel depositional sequence and coincides with the appearance of Armoricochitina granulifera, Angochitina curvata, Lagenochitina dalbyensis and Baltoniodus gerdae in the Bl-150 core.