Uwagi o biostratygrafii osadów ordowiku wschodniej części obniżenia podlaskiego

Zdzisław Modliński


Przedstawiono wyniki badań makrofauny, głównie trylobitów i ramienionogów, stwierdzonej w osadach od arenigu po aszgil we wschodniej części obniżenia podlaskiego. Uściślono schemat rozpoziomowania stratygraficznego osadów węglanowo-marglistych ordowiku oraz przedstawiono propozycję poziomów biostratygraficznych na podstawie trylobitów.






In the region mentioned, at the end of the sixties and in the seventies, the post-Tremadoc sediments of the Ordovician have been investigated by several tens of the new, full-cored borehole profiles. The fauna identified (Table 1-3; Plate 1-VIII) helped to the more detailed distinguishing of horizons and allowed to form the more exact correlation with the formations of the same age in other areas of the East-European Platform. The Lower Ordovician glauconitites of the eastern part of the Podlasie Depression are limited in principle to the graptolite horizons Didymograptus balticus and Phyllograptus densus of the same age. The dolomites and glauconititic limestones existing above, belong to the highest Latorpian - the horizon of Plesiomegalaspis estonica, and the lower part of the Volchov with the horizon of Megistaspis limbata included.

In the lower part of the limestone complex with the ferruginous oolites of the Llanvimian, the well marked horizon of Asaphus raniceps exists, in the upper part of which the specific fauna Illaenus sulcifrons Holm and I. planifrons Jaanusson was found, documenting the Aseri Stage. In this complex, the macrofauna showing the presence of the Lasnamagi Stage was not found.

The trilobite fauna (Pompecki) and Illaenus excellens Holm confirms the appurtence of the grey limestone complex to the highest stages of Lasnamagi and Uhaku.

In the Lower Caradoc, the organodetritic and marly limestone complex contains the fauna of the Kukruse Stage, such as Illaenus crassicauda (Wahlenberg), Ogmasaphus kiaeri Henningsmoen, and in some profiles contain the fauna of the Idavere Stage - Illaenus jevensis Holm and I. fallax Holm.

Higher in the profile exists the complex of grey-greenish marls. In the lower part of the marls the fauna Illaenus fallax Holm, Chasmops bucculenta (Sjogren) has been found, together with other fauna, showing the presence of the stages: ldavere, Johvi and Keila. In the central part of the marls, the plentiful trilobite fauna of the stages: Chasmops macrourus and Ch. wesenbergensis, belongs to the Oandu and Rakvere stages. In the upper part of the marls the fauna Chasmops eichwaldi Schmidt, Parillaenus roemeri and other have been found, belonging to the Nabala and Vormsi stages.

Above the sediments of the Caradoc in some profiles, the complex of light-grey and grey-pink limestones exists, which belongs to the Ashgillian. The Lower Ashgillian - Pirgu, and the Upper Ashgillian - Porkuni, is undoubtedly represented in some of the profiles.

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