Analiza zespołów mikrofauny Oksfordu kimerydu Polski niżowej w zależności od różnic facjalnych

Wanda Bielecka, Olga Styk




The present authors characterize the foraminifer and ostracod assemblages from the Oxfordian and Kimmemdgian deposit of the Lowland area of Poland, and illustrate in brief a dependence of distribution of these assemblage upon lithological changes in the individual substages (Figs. 1 and 2).
In the Oxfordian and Kimmeridgian deposits of siltstone-marly-sandy facies (Figs. 1–8) the microfauna is fairly poor. It differs only in its composition of species. Besides few representatives of the genera Lenticulina, Ophthalmidium, Eoguttulina, Epistomina, Spirllina and Trocholina, there are found Glomospira, Textularia, Haplophrogmoides, Ammobaculites and others. The considerably richer foraminifer assemblage has been found in deposits of siltstone-clayey-marly facies, where the amount of species and of individuals of these genera is largel.
In the Oxfordian deposits of carbonate facies (Figs. 3–6), a similar, but poorer foraminifer assemblage occurs. With the increase in CaCO2 in the deposits, such genera as Tolypammina, Nodobacularia, Saccorhiza, Paalzowella become more characteristic; in Rauracian – Nautiloculina; in Astartian – Pseudocyclammina and Quinqueloculina.
In the Kimmeridgian deposits of siltstone-clayey-marly facies and of carbonate facies represented are such genera as Lenticulina, Astacolus, Marginulina, Frondicularia, Vaginulina and Citharina. There are found here also certain species of the genera Epistomina, Spirillina, Trocholina,  Pseudocyclammina, Paleogaudryina, Conorboides, Pseudolamarckina and Ammobaculites.
Where character of carbonaceous deposits changes into more arenaceous, the foraminfer assemblage of both Oxfordian and Kimmeridgian distinctly becomes poor. Only few representatives have been ascertained in the deposits of calcareous-arenaceous facies, e.g. Lenticulina, Epistomina, Spirillina and Trocholina.
In the deposits of arenaceous, subordinately also of arenaceous-siltstone facies, foraminifer assemblage is still poorer, and appears only with Lenticulina, Spirillina and, at places, with Haplophragmoides. In the arenaceous, non-calcareous deposits, and in the dolomitic deposits, microfauna can hardly be encountered.
The foraminifer assemblages of Oxfordian age does not differ greatly in its genera from that of Kimmeridgian age, although considerable differences can be observed in the composition of species. At the close of the Rauracian, numerous species die out, and only few pass into Astartian. At the Astartian time, in turn, several nev species appear, a part of which passes into Kimmeridgian. At the outset of the Kimmeridgian, a rampant development of foraminifer fauna takes place, and the carbonate facies changes into shaly-marly, at places also into siltstone-marly faces. New species appear, distinctly affecting the composition of the assemblage.
An analysis of the foraminifer assemblages that occur in the Oxfordian and Kimmeridgian deposits allows us to state that certain species are restricted in their distribution space to one, or to several substages only. Their reaction to lithological changes also feebler, thus they seem to be of stratagraphical importance.
Depending upon the character of the sediments, the composition of species in the ostracod assemblage changes to a higher degree than it can be observed in the case of the foraminifer assemblage. The best conditions rare found in clay deposits. With the increase on sandy material, the ostracod assemblage becomes poorer. In arenaceous deposits there are found only insignificant amounts of ostracod, similarly as in calcareous, marly ones.
In general, the Oxfordian calcareous and calcareous-sandy deposits of the Lowland area of Poland reveal a very small amount of ostracods. For the most part, they occur in the deposits of siltstone clayey-marly facies. The following are forms characteristic of the Oxfordian: Lotphocythere multicostata, L. cruciata oxfordiana, Pontocyprella suprajurassica, Krausella? argoviensis, Paracypris acris; in the Upper Oxfordian there are found Vernoniella sequana, Macrodentina punctata and Cytherelloidea paraweberi at places.
In the Lower Kimmerdgian deposits of siltstone-cayey-marly facies are found Orthonotacythere interrupta, Rectocythere regularis, Cytherelloidea undulata, C. weberi, Amphicythere semisulcata, A. confundens, Protocythere furcata, P. rodewaldensis, Macrodentina proclivis, M. wicheri and M. steghausi. Typical species of the Lower and Upper Kimmeridgian are: Rectocythere iuglandiformis, Nodophthalmocythere vallata, Exophthalmocythere fuhrbergens is. As concerns the Upper Kimmeridgian, the characteristic species are represented by Protocythere sigmoidea, Macrodentina sculpata and M. klingleri.
The Kimmeridgian deposits of the Lowland area of  Poland contain more abundant ostracod assemblage that the Oxfordian ones. Generally speaking, the occurrence areas of the individual ostracod species of the Oxfordian and Kimmeridgian within the Polish Lowland are similar to those cited from these stages by H. J. Oertle (1959) for Swiss Jura, Paris basin and North Germany areas.

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