Abstract
The stratigraphy of the Ordovician carbonates of Baltoscandia was initially based, during the 19th century, on the stratigraphical ranges of macrofossils, mainly trilobites, but other fossils (brachiopods, echinoderms and cephalopods) were also used. During the 20th century, their importance in biostratigraphy gradually decreased due to a greater reliance on microfossils, especially conodonts and chitinozoans, which enable accurate correlation of carbonate successions where graptolites are absent or very rare. New methods have further reduced the attraction of macrofossils for biostratigraphy, although they are useful tools in different fields of geology such as palaeobiogeography and palaeoecology. The revised data on species diversity and the stratigraphical distribution of articulated brachiopods with carbonate shells (rhynchonelliformeans) in the East Baltic are used here for the evaluation of their role and potential in the modern stratigraphy of the Ordovician System. The 106 stratigraphical units (mainly formations and members) belonging to 17 Ordovician and the lowermost Silurian regional stages are analysed based on the taxonomic composition of their brachiopod faunas comprising in total more than 400 species. The cluster analysis used in this stratigraphical experiment captures the major dissimilarities between and continuity of the regional subseries, stages and subregional units by the similarity of their brachiopod faunas. Bryozoans, another important group of benthic organisms in shallow-water facies, are analysed for comparison, providing a test for the correlations based on brachiopods.
Highlights
The Ordovician stratigraphical framework of Baltoscandia, including the East Baltic, was established during the 19th and beginning of the 20th centuries (Schmidt 1858, 1881, 1907; Törnquist 1883; Warburg 1910)
The cluster analysis is based on the distribution of more than 400 brachiopod species together with those identified under the open nomenclature in the Ordovician and lowermost Silurian of the East Baltic using the neighbour-joining algorithm and the Raup–Crick and Simpson similarity coefficients (Hammer & Harper 2006). These analyses enable us to visualize the similarity of stratigraphical units according to the presence and absence of brachiopod species or their generic composition and highlight how these fossils support the accepted stratigraphy and its correlation
In spite of the differences in faunal dynamics and the regional stratigraphy of the Ordovician sections in the East Baltic, the dendrograms based on the Raup–Crick and Simpson similarity coefficients contain clusters comprising the units of the subseries (Fig. 9A, A1)
Summary
The Ordovician stratigraphical framework of Baltoscandia, including the East Baltic, was established during the 19th and beginning of the 20th centuries (Schmidt 1858, 1881, 1907; Törnquist 1883; Warburg 1910). The current study deals with the East Baltic region and covers the Baltic States of Estonia, Latvia and Lithuania together with the northwestern part of Belarus This region represents the main areas of the Estonian and Lithuanian shelves separated by the Livonian Basin (Fig. 1). The stage-level data on brachiopod distribution in the southern East Baltic (text-table 5 in Ropot & Pushkin 1987) are not used here, because we have revised data for the Lithuanian part of the region (Table S2). The lithostratigraphical units (formations and members) identified in the Lithuanian Shelf (Ropot & Pushkin 1987; Paškevičius 1997) (Fig. 2), are ranked on the chart in stratigraphic order within regional stages without exact correlations with the formations and members in Estonia. In Belarus, the gap is much more extensive than in other parts of the East Baltic and the oldest Silurian is of Telychian age (Abushik et al 2007)
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