Granica kreda-trzeciorzęd w przełomie Wisły
Abstract
Analiza paleontologiczna i sedymentologiczna osadów górnego mastrychtu i danu z okolic Kazimierza Dolnego pozwala na odtworzenie historii sedymentacji tych utworów i rozstrzygnięcie kwestii położenia granicy kreda-trzeciorzęd (K-T) w przełomie środkowej Wisły. Granica ta była dotąd stawiana w spągu, w połowie, bądź w stropie warstwy piaskowca glaukonitowego, zawierającego wymieszane mastrychtskie i dańskie skamieniałości. Analiza stanu zachowania makrofauny z piaskowca glaukonitowego pozwala stwierdzić, że w warstwie tej brak form, które można jednoznacznie uznać za mastrychtskie in situ. Granica K-T winna być zatem stawiana w spągu piaskowca glaukonitowego. Ławica twardego wapienia stanowiąca strop niżejległych opok z Kazimierza nie jest twardym dnem, jak dotychczas przyjmowano.
THE CRETACEOUS-TERTIARY BOUNDARY IN CENTRAL POLAND
Summary
The upper Maastrichtian-Danian sequence as exposed near the town of Kazimierz Dolny (middle Vistula river section, central Poland) comprises three lithologic units (Fig. 1): 1) upper Maastrichtian siliceous chalk (Kazimierz Opoka) with burrowed limestone layer at the top; 2) glauconitic sandstone horizon (Greensand) containing a mixture of Maastrichtian and Danian fossils; 3) Danian gaizes and limestones (Siwak). A detailed analysis of the K-T boundary interval and of its fossil content, including trace fossils and driftwood remains (Figs 2-6), allows for the reconstruction of the depositional history at the K-T transition (Fig. 7) and a resolution of the question of the position of the K-T boundary. This boundary was placed at the base, in the centre or at the top of the Greensand by previous investigators (Fig. 2). There are no in situ Maastrichtian fossils in the Greensand horizon, however, and this unit should be entirely assigned to the Danian. As a consequence, the K-T boundary should be placed at the boundary between the Kazimierz Opoka and the Greensand (Fig. 2). Cross-cutting of successive sandstone-filled burrows (Fig. 5) in the limestone at the top of the Kazimierz Opoka indicates that this is not a hardground horizon, as regarded by previous authors. The J-shaped vertical burrows previously reported from the hard limestone and attributed to crabs related to the Recent intertidal ghost crabs, most probably represent fragments of spiral Gyrolithes-burrows (Fig. 4). There is no indication of the tidal fiat environment in the highest portion of the Kazimierz Opoka.
THE CRETACEOUS-TERTIARY BOUNDARY IN CENTRAL POLAND
Summary
The upper Maastrichtian-Danian sequence as exposed near the town of Kazimierz Dolny (middle Vistula river section, central Poland) comprises three lithologic units (Fig. 1): 1) upper Maastrichtian siliceous chalk (Kazimierz Opoka) with burrowed limestone layer at the top; 2) glauconitic sandstone horizon (Greensand) containing a mixture of Maastrichtian and Danian fossils; 3) Danian gaizes and limestones (Siwak). A detailed analysis of the K-T boundary interval and of its fossil content, including trace fossils and driftwood remains (Figs 2-6), allows for the reconstruction of the depositional history at the K-T transition (Fig. 7) and a resolution of the question of the position of the K-T boundary. This boundary was placed at the base, in the centre or at the top of the Greensand by previous investigators (Fig. 2). There are no in situ Maastrichtian fossils in the Greensand horizon, however, and this unit should be entirely assigned to the Danian. As a consequence, the K-T boundary should be placed at the boundary between the Kazimierz Opoka and the Greensand (Fig. 2). Cross-cutting of successive sandstone-filled burrows (Fig. 5) in the limestone at the top of the Kazimierz Opoka indicates that this is not a hardground horizon, as regarded by previous authors. The J-shaped vertical burrows previously reported from the hard limestone and attributed to crabs related to the Recent intertidal ghost crabs, most probably represent fragments of spiral Gyrolithes-burrows (Fig. 4). There is no indication of the tidal fiat environment in the highest portion of the Kazimierz Opoka.