Konodonty ordowickie obniżenia podlaskiego

Maria Nehring-Lefeld

Abstract


Przedstawiono wyniki badań konodontów występujących w osadach arenigu, lanwirnu, landeila i karadoku, napotkanych w 9 otworach wiertniczych we wschodniej części obniżenia podlaskiego. Stratygraficzno-paleontologiczna analiza zespołu konodontów pozwoliła na udokumentowanie obecności ekwiwalentów niektórych biostratygraficznych poziomów konodontowych, które zostały ustalone dla osadów ordowiku rozwiniętych na obszarze paleozoogeograficznej prowincji północnoatlantyckiej.

ORDOVICIAN CONODONTS FROM THE PODLASIE DEPRESSION

Results of studies of conodonts from the Arenigian, Llanvirnian, Llandeilian and Caradocian sediments found. in 9 boreholes in the western part of the Podlasie Depression (NE Poland) are presented. These studies allowed to document equivalents of several conodont biostratigraphic horizons within the Ordovician sediments in that region. The conodont assemblage is very similar to those ones known from the coeval sediments of Sweden, Norway and Soviet Baltic countries. It is typical for the North Atlantic palaeogeographic province. The paleontological material making the basis of this paper is quantitatively poor and usually badly preserved.

A relatively abundant conodont assemblage occurs in the Arenigian sediments. Typically Tremadocian species occur in the glauconitite correlated with Latorpian together with species known from Latorpian and Lower Volkhovian and with those ones which appear above the bottom of the Volk-hovian stage. The specific composition of the assemblage suggests a possibility to correlate the glauconitite with the Paraoistodus proteus horizon. Conodonts are also common in the dolomitic limestones and glauconitic dolomites that rest over the glauconitite and are correlated with the Volkhovian. This assemblage is similar to the coeval ones known from Estonia and Scandinavian countries. The occurrence of Microzarkodina flabellum Lindstrom, Drepanoistodus forceps (Lindstrom) and also Baltoniodus navis (Lindstrom) allows to correlate these sediments with the Lower Volkhovian. Llanvirnian is represented by cherry-brown organodetrital limestones with ferruginous oolites. Also these sediments contain abundant conodonts known from Llanvirnian and Volkhovian. The occurrence of Microzarkodina ozarkodella and Amorphognathus variabils points to a possibility to correlate those sediments with the Lower Llanvirnian. Lllandeilian is represented by gray, recrystallized organodetrital limestones which are poor in organic remains. The data about conodont occurrence in the Llandeilian sediments are scanty so far. Among stratigraphically important species the presence of Prioniodus variabilis Bergstrom has been stated - a species known from the Upper Llandeilian and Lower Caradocian. Conodonts are more abundant in the undivided Llandeilian-Caradocian sediments in the Podlasie Depression. Those are gray organodetrital limestones passing upward into marly claystones and marls. Prioniodus variabilis occurs there as well together with some species typical for the Caradocian.

Among the studied boreholes the Caradocian sediments have been distinguished only in the Mielnik IG 1 borehole. Those are gray organodetrital limestones passing upwards into marly rocks. Among the conodonts there are Amorphognathus tvaerensis Bergstrom which is characteristic species in the Lower Caradocian and indicative for A. tvaerensis horizon. In higher beds at Mielnik IG 1 there are many fragments of Amorphognathus and Ambalodus which points to a possibility to look for counter parts of the A. superbus horizon which is correlated with the highest Caradocian.

Descriptions of selected conodonts arc given in the paleontological part of this paper. Natural taxonomy was applied as much as possible using definitions of multielement species (M. Lindstrom, 1971; S. Bergstrom, 1971; W.A. Van Warnel, 1974). Because of scarcity of material some species could be determined only as formal ones hence their names are marked with a shortening s.f. = sensu formae.


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