Biostratygrafia piętra podlaskiego (górny sylur) w polskim akwenie Bałtyku na podstawie małżoraczków

Abstract

Na podstawie szczegółowej analizy zasięgów stratygraficznych małżoraczków występujących w osadach syluru polskiej strefy akwenu południowego Bałtyku ustalono, że najwyższa część tego systemu odpowiada piętru podlaskiemu - najmłodszemu ogniwu syluru w Polsce. W osadach tych udokumentowano ekwiwalenty wszystkich uprzednio wydzielonych w piętrze podlaskim poziomów małżoraczkowo-trylobitowych. Wyróżniono ponadto poziom Neobeyrichia regnans - najnizszą część dolnego podlasia. Przeprowadzono korelację badanych osadów z równowiekowymi osadami syluru z lądowej części syneklizy perybałtyckiej (obszar Zatoki Puckiej i wyniesienia Łeby), Litwy, Łotwy i Estonii oraz Gotlandu.  STRATIGRAPHY OF PODLASIE STAGE (UPPER SILURIAN) IN THE POLISHPART OF THE SOUTHERN BALTIC SEA ON THE BASIS OF OSTRACODES Results of biostratigraphic investigations on the Silurian sediments from boreholes localised in the polish part of the southern Baltic Sea are presented. Eight drillings have been carried out by the "Petrobaltic Company". Taking into account the stratigraphic ranges of ostracodes it has been stated that these sediments correspond to the regional Podlasie stage which is the youngest member of the Silurian in Poland. It embraces sediments younger than the Ludlovian and older than the Gedinnian. The term "postludlow" is also used in Poland to denote the age of these deposits (L. Teller, 1969; B. Żbikowska, 1973).The sediments of the Podlasie Stage preserved under the southern Baltic Sea floor are analogous to those ones known from land in the Polish part of the Peribaltic syneclise. Those are clayey-marly sediments with inliers and lenses of limestone. They contain abundant ostracodes assemblage and tentaculites, crinoids, brachiopods, gastropods and remnants of trilobites. Thickness of the Podlasie sediments under the southern Baltic is from 200 up to 830 m. Large differences in thickness result from the fact that in many localities only the sediments classified to the Lower Podlasie Stage are preserved whereas the Upper Podlasie has been completely or partly eroded. In some cases also the sediments correlated with the upper part of the Lower Podlasie have been eroded as well. Presence of 5 previously estabilished biostratigraphic horizons (E. Tomczykowa, E. Witwicka, 1972, 1974) have been documented namely: Neobeyrichia incerta-Acastella prima, Frostiella pliculata-Acaste dayina, Nodibeyrichia tuberculata (Lower Podlasie Stage), Kloedenia wilckensiana, Nodibeyrichia gedanensis (Upper Podlasie Stage). In the Puck Bay the lowermost part of the Lower Podlasie embraces the Neobeyrichia incerta-Acastella prima horizon and the so called "transitory" beds described by H. Tomczyk (1968). In the beds (above the Monograptus formosus Zone) ostracodes are not found. As it comes from studies of the Upper Silurian sediments under the Baltic Sea floor at least the upper part of the "transitory beds" does not contain ostracodes and these sediments have been classified to the Neobeyrichia regnans Zone (Fig. 1). The ostracode assemblage is very close to that one characterising the Hemsiella hemsiensis Zone (B. Żbikowska, 1973) that has been distinguished in the Łeba Elevation. Primarily it included also the older sediments which contain fossil assemblage of the Neobeyrichia incerta-Acastella prima Zone. The Hemsiella hemsiensis Zone corresponds partly to the Monograptus formosus Zone, hence it corresponds to the upper part of the Lower Postludlow according to L. Teller (1969). In the material from the southern Baltic Sea floor Hemsiella hemsiensis is sporadic whereas Neobeyrichia regnans is much more frequent. Thus it has been taken as index taxon for the lowermost Podlasie Stage. There exists, however, a possibility that Neobeyrichiaregnans Zone is a regional counterpart of Hemsiella hemsiensis Zone or makes only the lower subzone within Neobeyrichia incerta-Acastella prima Zone.The ostracod assemblage occurring in the sediments of the Podlasie Stage in the Polish part of the southern Baltic is very close to that known from coeval sediments of the western part of the PeribalticSyneclise. A comparison with the assemblages of the Soviet Baltic countries shows that the ostracode assemblages of the Postludlow (= Podlasie) described from Latvia (L. Gailite, 1967), Lithuania (A.Pranskevicius, 1972) and Estonia (L. Sarv, 1968, 1970) are more diversified than the Polish one. Correlation of the particular biostratigraphic horizons can be based upon the occurrence of species of the Craspedobolbinidae Martinsson (Hemsiella hemsiensis Martinsson), H. loensis Martinsson, H. dalmaniana (Jones), Macrypsilon salterianum (Jones) and Beyrichiidae Matthew (Neobeyrichia regnans Martinsson, N. incerta Gailite, Nodibeyrichia tuberculata (Kloden), Frostiella pliculata Martinsson and some representatives of Aechminidae Swartz, Primitiopsidae Swartz and Thlipsuridae Jones. The assemblage of ostracods from the Podlasie Stage is very similar to that of the Beyrichia limestone (Beyrichienkalk) known from and from erractic boulders, northern Germany and northern Poland.Only lowermost part of the Podlasie Stage (Neobeyrichia regnans and N. incerta-Acastella primaZones) may be correlated with the Upper Silurian sediments preserved in the Island of Gotland (Hamra and Sundre Beds on the basis of occurrence of Neobeyrichia regnans Martinsson.