Szkic paleogeograficzno-stratygraficzny kredy dolnej na Niżu Polskim

Sylwester Marek

Abstract


PALAEOGEOGRAPHICAL AND STRATIGRAPHICAL SKETCH OF THE LOWER CRETACEOUS IN THE POLISH LOWLAND AREA

Occurrence area of the Lower Cretaceous in northern Poland is practically restricted to the Kujawy-Pomeranian anticlinorium and the adjoining Łódź - Mogilno - Szczecin synclinorium on the one hand, as well as to the marginal synclinorium on the .other.

The center of the Lower Cretaceous basin, where sediments are best developed and reach their maximum thickness (as proved tm now -up to 600 m), stretches in the area of Mogilno synclinorium, Kujawy anticlinorium and middle part of marginal synclinorium. Thus, the direction of the zone of greatest thicknesses is almost W - E, disclosing only a slight deviation of the western sector to the north. As concerns palaeogeography of the Cretaceous basin, the deep zone running along the present Kujawy-Pomeranian anticlinorium is of considerable importance there.

The Lower Cretaceous transgression entered the Polish Lowland area already at the Infravalanginian time. In all probability, its main phase came from the boreal basin of the Russian platform, perhaps through the northern peripheries of the southern geosynclinal basin. A connection with this latter is undoubtedly proved by mediterranean forms characteristic of the Berriasian Subthurmannia boisieri zone.

At the Lower Valanginian time (Platylenticeras-Valanginian) the importance of communication with the Middle Russian boreal basin probably decreases, and an influence of the North-European basin begins, however, a connection with the southern area continues.

The Middle Valanginian (Polyptychites-Valanginian), almost completely unfossiliferous, is characteristic of a distinct shallowing of the sedimentary basin.

After the period of regression, the traces of which may be observed in the Middle Valanginian (Polyptychites-Valanginian) sediments in various regions of the West-European basin, a new Neocomian transgresion has begun at the Upper Valanginian time, connected with the simultaneous deepening and enlargement of the sedimentary basin. Maximum extension of the basin, although signalizing already its shallowing process, falls to the Lower Hauterivian time.

The ammonite assemblage of the Upper Valanginian points at the existing connection between both the West-European and the Middle Russian basins of the boreal province. Due to the regression which begun in the Central Russia at the decline ·of the Upper Valanginian, the connection with the basin of the Russian platform was broken up in the Lower Hauterivian.

The Upper Hauterivian deposits of the Polish Lowland area were determined in the present paper in a conventional way, since no adequate faunistic data were available. In the lower part of this stage a considerable shallowing of the sedimentary basin took place, in the upper part, however, a marked deepening of the basin followed.

Sediments covering formations referred to the Hauterirvian and resting below the Upper Albian, thus corresponding to the Barremian - Middle Albian period, undoubtedly constitute a new sedimentary cycle of the Cretaceous. These are arenaceous deposits lacking palaeontological evidences and this fact does not allow to reconstruct uniformly the palaeogeographical conditions of that time.

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