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.
Conodonts are an important biostratigraphic tool for many Phanerozoic stages. Along with graptolites, they define all global Ordovician Stage boundaries. Within the Upper Ordovician interval, a known species of Amorphognathus tvaerensis (Bergström) is present in both Sandbian and Katian stratotype sections. Study of changes in the succession of A. tvaerensis revealed that elements in the upper part of its range differ morphologically quite distinctly from those in its lower part. Here, they are described as a new conodont species, A. viirae sp. nov. This new species is recognized in several Estonian and Swedish sections, with apparent occurrence also in Mójcza Quarry, Holy Cross Mountains, Poland and Black Knob Ridge, Oklahoma, USA. Detailed analysis of early Amorphognathus elements from Estonian and Swedish sections revealed the absence of A. inaequalis (Rhodes) in both regions, although a conodont subzone based on this species was identified earlier by some authors. Both the absence of A. inaequalis (Rhodes) and recognition of the new species A. viirae sp. nov. resulted in the revision of the conodont zonation, and a new version of it is proposed for the Sandbian Stage in the Atlantic Realm. The new zonation includes (from below) Pygodus anserinus, Baltoniodus variabilis, A. tvaerensis, B. gerdae, A. viirae and B. alobatus Conodont zones.
One of the key objectives of the studies proposed by the Ordovician Subcommission is the improvement of regional stratigraphy for further advancements in global correlation. The results of this work can be found in numerous updates and reviews published in the recent Geological Society, London, Special Publications 'A Global Synthesis of the Ordovician System' . Several of these papers refer to the Baltic Ordovician conodont biozones. While different schemes share many common features, their correlation with stages and biozones differ in detail. Considering the recent developments in the studies of the Sandbian stratigraphy in the Baltoscandian region, it is possible to complement the current conodont biozonation.Pygodus anserinus, Amorphognathus tvaerensis and A. superbus conodont zones are recognized in the Sandbian Stage in Baltica. The lower boundary of the stage is located within the Pygodus anserinus Zone, and the main part of the stage corresponds to the Amorphognathus tvaerensis Zone. In Scandinavia, the upper boundary of the stage correlates with a level within the A. superbus Zone or lies in a conodont-poor interval in the eastern Baltic region. The lower boundary of the A. inaequalis Subzone is tentatively correlated with the base of the Sandbian in Scandinavia but is located in the uppermost Darriwilian in the eastern Baltic region. In both areas, the A. tvaerensis Zone is subdivided into the Baltoniodus variabilis, B. gerdae and B. alobatus conodont subzones.The A. inaequalis Subzone -in some cases indicated as a zone -has been included in the regional stratigraphic charts for more than a decade. However, so far, A. inaequalis (Rhodes) has been reliably identified and also illustrated only from Avalonia, i.e. outside the palaeocontinent Baltica. A recent restudy of collections from the Fjäcka main section and the Smedsby Gård drillcore (both from Sweden), as well as from several Estonian sections, did not prove the occurrence of A. inaequalis in these areas.The A. tvaerensis Zone comprises almost the whole Sandbian, both in Scandinavia and the eastern Baltic areas. During this long age, the morphology of the P and M elements of A. tvaerensis (Bergström) gradually changed, and elements of distinct morphology appeared and were assigned to a new species, Amorphognathus viirae Paiste, Männik et Meidla, 2022, in the upper part of the range of the species. In succession, A. viirae appears in the upper part of the B. gerdae Subzone. Cur rently, A. viirae has been identified in numerous Estonian sections, as well as in the Fjäcka main section and the Smedsby Gård drillcore in Sweden. Based on published figures, it occurs evidently in the Mójcza Formation of the Holy Cross Mountains (Poland) and the Black Knob Ridge section in Oklahoma (USA), in the GSSP for the base of the Katian Stage.A. inaequalis has also been reported and an eponymous zone identified in two other sections located on the palaeocontinent Baltica, in the Bliudziai-150 drillcore (Lithuania) and the Kovel-1 drillcore (Ukraine). During the res...
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