Konodonty ordowickie z otworu wiertniczego Jezioro Okrągłe

Maria Nehring




The present article contains an analysis of a conodont assemblage found to occur in the Ordovician deposits drilled by borehole Jezioro Okrągłe (NE Poland – the southern part of the Peri-Baltic syneclise). Here, the deposits are developed in a shallow-water facies, their thickness amounting to 913,7 m. The stratigraphy of these deposits has·been worked out, on lithologic and microfaunistic evidences, by Z. Modliński (1968). Conodonts are here well preserved. They occur in the Llanvirnian Llandeilo, Cararadocian and Ashgilian deposits, most frequenty in the Llanvirnian ones, however. The conodont assemblage encountered in the deposits of this stage is rich in both species and specimens (Fig. 1). The assemblage is represented by 42 species that belong to 26 genera. Most conodonts found in the Ordovician deposits pierced by bore hole Jezioro Okrągłe are characterized by a wide stratigraphical range. However, this assemblage, taken as a whole is typical rather of the Middle Ordovcian (Table 1). Interesting is here a marked similarity of the Orągłe Jezioro conodonts to the assemblage of conodonts described from the Middle Ordovician deposits in Sweden. A comparison of this assemblage with the conodont assemblages described from the Ordovician deposits in the areas of Wales demonstrates that though the species found here are common, the conodonts characteristic of the Welsh Middle Ordovician are absent at the Jezioro Okrągłe, and, except for a few cases, the common species represent here cosmopolitan forms only. The conodont assemblage folund in bore hole Jezioro Okrągłe contains species characteristic of the deposits of this period, known to occur in North Europe.
Simultaneously, however, we find here also species typical of the eastern regions of North America. A detailed analysis of the so far investigated conodont assemblages from the north-eastern areas of Poland proves a marked resemblance of the character of the deposits and of the fauna to the Ordovician deposits known from the areas of Sweden, Scotland, from the central region of Oslo, and the Baltic area of the Soviet Union.

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