Podstawy stratygrafii dolnego i środkowego oksfordu północnej i północno-zachodniej Polski

Abstract

STRATIGRAPHICAL BASIS OF THE LOWER AND MIDDLE OXFORDIAN IN THE NORTH AND NORTH WEST POLAND

Summary

The Lower and Middle Oxfordian deposits developed in marly-mudstone and arenaceous-mudstone facies have been encountered when drilling in the northern and northwestern areas of Poland (Szczecin-Poznań-Gołdap - Fig. l). The deposits are exactly dated by means of faunistic evidences. Stratigraphy has mainly been based on ammonite fauna. Lower Oxfordian (Divesian + Neuvizyan). Shales, claystones, mudstones and arenaceous marls are main rock type characteristic of the Divesian in the area considered. The deposits are frequently dolomitized. The greatest thickness of the Divesian deposits has been observed in the bore hole Osowa (21,60 m), the smallest one in the bore hole Bartoszyce (? 10 cm). It results from the data so far obtained that three zones may be distinguished within the Divesian deposits, i.e. lower zone Quenstedticeras flexicostatum, middle zone Quenstedticeras lamberti and upper zone Quenstedticeras mariae. A faunistic documentation of the Divesian deposits occurring in the area under study shown on Table 1. Above the Divesian deposits may be distinguished the Neuvizyan ones among which are mainly mudstones, marly mudstones and sandstones. The maximum thickness of these deposits has been determined in the bore hole Lulewiczki to be approximately 75 m. It results from the Table 1 that the Neuvizyan deposits may, due to the cardioceras fauna, be correlated with the zones distinguished by the present author, i.e. with the lower zone Cardioceras bukowskii and the upper zone Cardioceras excavatum (L. Malinowska, 1963, and in the print). Middle Oxfordian (Argovian). The Middle Oxfordian deposits found in the bore holes made in the northern and northwestern areas of Poland are developed in two facies: mudstone-clay-marly facies and arenaceous-mudstone facies (Fig. 1). The detailed subdivision of the Middle Oxfordian has been made by the present author by means of Cardioceratidae (Fig. 2). Above the palaeontologically evidenced Lower Oxfordian deposits, the zone Cardioceras tenuiserratum has been distinguished on the basis of a continous occurrence of the species Cardiocerars tenuiserratum(Opp.). Thickness of this zone ranges from 0,5 m to 6,0 m. Higher up, the author has stated the presence of the so-called cardioceras gap characterized by the lack of forms of the family Cardioceratidae (Fig. 2). The extent of this gap ranges from 2 m in the bore hole Gołdap to about 50 m in the bore hole Kcynia IV. At the time of the cardioceras gap the conditions caused that ammonites were badly preserved and fragments of lamellibranch shells, numerous individuals of Serpula sp., and charred plant fossils are frequently found. These changes, which can be observed only due to the "sensibility" of the forms of the family Cardioceratidae, are no doubt connected with some dynamical processes of the sea bottom. Above the deposits of the cardioceras gap the author has also found deposits containing fauna of the family Amoeboceras. The zone named Amoeboceras alternans has been distinguished on the basis of numerous fragments of the species Amoeboceras alternans (Buch) and on its wide distribution. The zone Amoeboceras alternans correspond to the upper part of the Middle Oxfordian. Fauna characteristic of this zone has been shown on Table 3.