W sprawie terminologii stratygraficznej najwyższego malmu w Polsce

Zofia Dąbrowska

Abstract


STRATIGRAPHICAL TERMINOLOGY OF THE UPPERMOST MALM IN POLAND

The author suggests that in Poland a term :Portlandian should be accepted to the beds younger than the Kimmeridgian and older than the Lower Cretaceous·ones. These formations should be regarded as deposits of a portion of the vast Portlandian basin.

According to the recommendations of the Colloquium held in Luxemburg, the term Purbeckian may be recognizedas a facial one. It represents a facies developed at the Upper portlandian time.

The individual parts of the Portlandian basin differ in their faunal composition in quantity of some genera, in appearance of different species, a.o. The author discusses here some genera, i.e. GravesiaSubplanites and Pavlovia.

The Danish-Polish Portlandian basin was situated out of the way, and had a hard connexion with the open sea, being also narrow and elongated. In this basin a regression of the sea and a development of the Purbeckian facies took place considerably earlier than in·other Portlandian basins. Probably, somewhere in the area of the present North Sea, there must have existed some obstacle (threshold) impeding the circulation of fauna. Probably, there had also existed some local unfavourable palaeobiological factors which made impossible the development of several ammonite genera.

In the Polish Lowland area, we may distinguish, within the Portland deposits, merely three ammonite zones, i.e. Subplanites sp., Zaraiskites scythicus and Virgatites virgatus. Higher up, marine deposits developed at a short period of time, containing no ammonite fauna, and then a lagoon formed and the Purbeckian facies orginated. This took place at that time, when in the other Portlandian basins marine deposits containing ammonite fauna were still laid down, and in the central part of the basin evaporates of considerable thickness sedimented. At the boundary with the Cretaceous a brackish facies predominated in the whole of the Polish Lowland area. The boundary between Jurassic and Cretaceous runs, at·present, in the area of the Polish Lowland, within deposits developed in the brackish facies.

The facial character of the basin is undoubtedly connected with the Portlandian basin. Within the whole area covered by the Portlandian sea, the Jurassic .ends in a strong marine regression leading to a formation of the Purbeckian facies. At present, an opinion predominates that the term "Bononian" should not longer be accepted in Poland. There are three theoretical ,possibilities to replace this term: (1) to give the term "Volgian" to the deposits younger than the Kimmeridgian and older than the Lower Cretaceous ones. This proposal, because palaeobiological, palaeofacial and palaeogeographical respects, calls, however, in question; and (2) to distinguish, within the Polish Lowland, the deposits younger than the Kimmeridgian and older than the Lower Cretaceous ones, as a new stage. This is, however, unfounded since no endemic fauna exists in the area of the Polish Lowland, and it is not necessary to distinguish a new stage here. The third proposal - discussed in this paper - concerns the introduction of the term Portlandian with reference to the area of Poland, the term being proved by geological factors according to the Colloquium in Luxemburg, the purpose of which was to uniform the stratochronological nomenclature.


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