Stratygrafia i pozycja facjalna osadów górnojurajskich okolic Barcina i Piechcina na tle obszarów przyległych

Abstract

W związku z prowadzoną systematyzacją wydzieleń litostratygraficznych w górnej jurze konieczne stało się przedstawienie szczegółowej stratygrafii osadów górnojurajskich z rejonu Barcina i Piechcina na Kujawach. Jest to jedno z nielicznych miejsc występowania na powierzchni skał górnojurajskich w Polsce północnej i powinno być brane pod uwagę przy wyznaczaniu profilów typowych różnych jednostek litostratygraficznych. Profil osadów górnojurajskich Barcina i Piechcina porównano z profilami obszarów przyległych: profilem Kcyni na północnym zachodzie i profilem Lipna na wschodzie. Porównania te umożliwiły poruszenie niektórych ogólnych prawidłowości rozwoju sedymentacji w górnej jurze regionu kujawskiego.

THE UPPER JURASSIC ROCKS AT BARCIN AND PIECHCIN; THEIR STRATIGRAPHY AND FACIES AS CQMPARED WITH NEIGHBOURING AREAS

In the vicinites of Piechcin and Barcin, Kujawy region (Fig. 1), Upper Jurassic rocks form south-western limb of the Zalesie anticline. The rocks were encountered in numerous drillings and lower part of the Upper Jurassic section is also accessible for direct studies in three large quarries.

The accepted lithostratigraphic subdivision (Fig. 2) is consistent with that proposed for northern Poland by J. Dembowska (1979). The Callovian is here directly overlain by marly siltstones and claystones with sponge spicules and ammonites of the lowermost Middle Oxfordian. The rocks, assigned to the siltstone (Łyna) formation, are about one meter thick whereas the Lower Oxfordian is missing in the studied section. They are overlain by a thick (about 200 m thick) series of carbonates with numerous siliceous sponges, assigned to the spongy limestone formation, and a series of marly rocks of the limestone-marly formation, about 125 m thick. Ammonites indicate the Lower Kimmerdgian age of the major part of the latter formation. The youngest deposits described from that area belong to the shale-marly-siltstone (Pałuki) formation, characterized by predominance of marly clays and siltstone. Oolitic and organodetrital limestones appear twice in the section of that formation. The deposits are dated at the Upper Kimmeridgian - Lower Volgian and the boundary, between these stages is delineated below the base of the lower oolitic limestones (see Fig. 2).

Upper Jurassic rocks of the vicinities of Piechcin and Barcin were compared with coeval ones from the areas of Lipno and Kcynia (see Fig. 3). The development of carbonate megafacies was related to the Lipno area. Spongy limestone facies, related to a deeper part of the basin, appeared in that area in the Early Oxfordian, in the area of Barcin and Piechcin somewhat later than the beginning of the Middle Oxfordian but it did not reach the Kcynia area at all. The extent of shallow-water coral-oolite facies of the Lower Kimmeridgian was still smaller as it is found to comprise the Lipno area but not that of Piechcin and Barcin. Unification of facies took place in the Late Kimmeridgian but the relevant sections from the Kcynia area markedly differ in thickness from those from Piechcin and Barcin and Lipno.

Both the extent of facies and differences in thickness appear related to differences in tectonic activity in the above mentioned areas. The Lipno area belonged to the East-European Platform in the Late Jurassic and those of Piechcin and Barcin and Kcynia were situated within the Pomeranian-Kuiavian Trough, characterized by higher rates of subsidence than in the adjoining areas in the Permo-Mesozoic times. The trough, subdivided by several active transversal dislocation zones, was also separated from the East-European Platform by dislocation zones of the Teisseyre-Tornquist Line (R. Dadlez, 1980). Differentiation in thickness and types of Upper Jurassic deposits was also markedly influenced by activity of tectonic zones existing within the area of the Pomeranian-Kuiavian Trough whereas the zones separating the Trough and East-European Platform were less active.