Sedimentological, geochemical and microfaunal responses to environmental changes around the Cenomanian–Turonian boundary in the Outer Carpathian Basin; a record from the Subsilesian Nappe, Poland

Abstract

Sedimentological, micropalaeontological and geochemical data from the upper Cenomanian–lower Turonian Zasań section (Outer Carpathians, Subsilesian Nappe, Poland) provide new insights into the sedimentary environment and its changes in a basin at the northern margin of the Western Tethys. A global oceanic event (OAE2) has been identified on the basis of lithology and radiolarian stratigraphic data. This event is marked by a series of organic-rich (TOC up to 5%) non-calcareous shales, practically devoid of benthic microfossils, indicative of bottom water anoxia. The marine type of organic matter and the occurrence of radiolarian-rich layers, with abundant small opportunistic forms, suggest that coastal upwelling was probably the main driving force for the accumulation of organic matter during the latest Cenomanian. The early diagenetic ferromanganese nodules followed by a 30 cm-thick series of manganese-rich non-calcareous variegated shales and bentonites, directly overlie the black facies of OAE2. The Fe–Mn nodules formed in two stages reflecting changes in bottom water dynamics across the C–T boundary. During the first stage, the Fe–Mn carbonate nodules were precipitated in presence of anoxic pore and/or bottom water that could be episodically oxygenated. Later, an increase in deep-water circulation led to strong oxygenation of the bottom and pore water, causing diagenetic modification of the nodule carbonate into ferromanganese oxides and hydroxides. The manganiferous sediments recorded in the Zasań section represent an isochronous horizon within the Outer Carpathian Basin, probably concurrent with manganese enrichments in many other epicontinental seas and deep-water basins, coinciding with the end of the carbon isotope excursion close to the base of the Turonian. The manganese event may have been related to increased deep water circulation associated with the maximum flooding of the late Cenomanian–early Turonian transgression. The Mn event may have also been enhanced as a result of remobilization of Mn 2+ dissolved in anoxic bottom and pore waters in the organic-rich sediments directly underlying the manganese sediments. The lower Turonian succession records increasing, but not completed, oxygenation of bottom water. The oxygenation was interrupted by periods of anoxia, marked by a few intervals of dark laminated black shales, strongly enriched in organic matter (TOC up to 10%). Their marine origin and the occurrence of radiolarian-rich layers demonstrate enhanced primary productivity and a low clastic supply during the sea-level high-stand. Frequency of radiolarian-rich layers increases up section, reflecting an increase in upwelling circulation at the northern margin of the Carpathian Basin during the Early Turonian.