Górny portland (wołg) i berias (riazań) na Niżu Polskim

Sylwester Marek, Wanda Bielecka, Janina Sztejn




As a result of the Young Crimmerian activity (Deister phase), the Jurassic sea withdrew, in the Polish Lowland area, from the sedimentary basin (Fig. 1). Regressive tendencies in the Lower Portlandian led to certain restriction of ammonite migration, already at the zone Zaraiskites scythicus (marginal area of the Świetokrzyskie Mountains) and, in the central part of the basin (Kujawy region) – at the zone Virgatites virgatus (J. Kutek, 1962; Z. Dąbrowska, 1962; J. Dembowska, 1966).
Thus, the sedimentation ended in the open sea, although marine conditions prevailed still in the central part of the basin. Hovewer, in the peripheral portions of the basin, the sedimentary environment became brackish (ostracod zone F, Figs. 2 and 3).
Later on, a Purbeckian lagoon basin developed, covering only the Kujawy, and Pomeranian parts of the Danish-Polish furrow. On the basis of the microfaunistic data elaborated by W. Bielecka and J. Sztejn (1966), who have distinguished 6 ostracod zones (from F to A, Fig. 3) in the Jurassic Cretaceous transition beds within the Polish Lowland area, we may suppose that, in the Kujawy part of the relict Jurassic basin, the Purbeckian facies governed already at the end of the sedimentary period of the middle series of “Münder-Mergel”, i.e. at the Portlandian-Purbeckian boundary, in the English sense. In genera1, three complexes of deposits may be distinguished in the formation of the Purbeckian facies: brackish and hyperhaline deposits, brackish-freshwater deposits, and brackish-marine deposits (Fig. 2).
In the first phase of the development of the Purbeckian lagoon basin, a considerable concentration of sulphides and carbonates took place, and huge masses of anhydrites and limestones of chemical origin precipitated. Periodically, also brackish and carbonate deposits were formed, disclosing oligohalite ostracods, and a few species of euryhaline ones. This is an ostracod assemblage characteristic of the upper part of the marls from Münder (ostracod zones E and D).
After this phase, the basin changed into a freshwater one, and clay-marly and Cyrena brackish-freshwater deposits were formed. Ample ostracod oligohaline fauna (ostracod zones C and B) is an evidence of a fact that the deposits were laid down during the formation of the Lower Serpulite in Germany.
In the central portion, at the basin, the uppermost part of the Purbeckian facies formation consists of brackish-marine deposits (Figs. 1-3). They are developed as marly siltstones and arenaceous limestones with Cyrena shell rocks. Beside the ostracods of the genus Cypridea, these deposits reveal also the first agglutinized forminifers. The Ostracod assemblage (ostracod zone A), evidences that the brackish-marine series of deposits is an age equivalent of the Upper part of the German Serpulite, and may be correlated with the Cinder bed of the English Purbeckian.
The marine Berriasian of the Polish Lowland area is characterized by the ammonite fauna, mainly of Mediterranean, subordinately also of Boreal nature. The fauna is represented by the following genera: Riasanites, Berriasella, Subthurmania, Himalayites, Neocosmoceras, Euthymiceras, Neocomites and probably Surites.
In the light of the ammonite fauna, and as a result of the general palaeogeographical evidences, a conclusion may be drawn that the beginning of the Berriasian transgression within the Polish Lowland area corresponds to the transgression of the Riazanian Berrasian in Russia, to the Middle Nodular Bed of the Spilsby Sandstones in North England, and to the transgression of the Swiss and the Caucasus-Crimean Berriasian. The Jurassic-Cretaceous boundary, defined in this way, falls, as it has already been suggested by R. Casey (1963), on the bottom part of the middle sub-zone of Berriasian with Subthurmannia boisieri, in south-east France.

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