Deep-water facies succession around the Cenomanian–Turonian boundary in the Outer Carpathian basin: Sedimentary, biotic and chemical records in the Silesian Nappe, Poland

Abstract

Facies succession, organic carbon stable isotopes and chemical data, together with benthic foraminiferal records from the Barnasiówka–Trzemeśnia composite section (Silesian Nappe, Poland) reflect changing oceanographic conditions across the Cenomanian–Turonian transition in one of the largest basins of the Outer Carpathians, whose floor was situated beneath the calcium compensation depth. The sediments consist of non-calcareous radiolarian shales, turbidite intercalations rich in foraminifers and sponge spicules as well as siliciclastics, and black shales with marine type of organic matter (TOC up to 6.5 wt.%), corresponding to global Oceanic Anoxic Event 2. The base of OAE-2 has been identified on the basis of organic carbon isotope data, as a positive excursion from − 26.0‰ to the peak of − 23.2‰ in the latest Cenomanian. The top of the OAE-2 lies within a succession of oxidized siliceous–manganiferous sediments including a Fe–Mn layer with macronodules. The organic-rich sedimentation continued for about 400 kyr at a rate of ca. 5.5 mm/kyr, and finished before the start of the Turonian. Deep-water agglutinated foraminiferal diversification and extinction of several taxa occurred during the early part of the OAE-2, due to expanding bottom water anoxia. Chemical indices and benthic foraminiferal assemblages reveal periodic intervals of intensification and expansion of oxygen minimum zone at the basin floor before and after the OAE-2. Two early diagenetic ferromanganese layers with macronodules and siliceous–manganiferous red and green shales directly overlying the organic-rich facies, reflect changes in oxygenation of the bottom water, related to changes in their dynamics, which took place near the C–T boundary and in the earliest Turonian. This condensed Fe–Mn succession records the maximum flooding of the late Cenomanian–early Turonian transgression.