Ichnological record of deep-sea palaeoenvironmental changes around the Oceanic Anoxic Event 2 (Cenomanian–Turonian boundary): An example from the Barnasiówka section, Polish Outer Carpathians

Abstract

Trace fossil assemblages including Planolites, Thalassinoides, Palaeophycus, Taenidium, Chondrites and Trichichnus have been determined in the Upper Cenomanian turbiditic deep-water succession, corresponding to the pre-OAE-2 interval in the Barnasiówka–Ostra Góra section, the Skole Nappe, Polish Outer Carpathians. These assemblages indicate that oxygenation within sediment fluctuated from oxic to dysoxic zone. The overlying uppermost Cenomanian organic-rich facies (Bonarelli-equivalent horizon), corresponding to the main interval of the OAE-2 contains black shale layers, devoid of trace fossils representing anoxic (euxinic) events, and green shales that include Chondrites, Planolites and Thalassinoides, recording dysoxic intervals. Three subintervals (phases) within the OAE-2 sediments have been distinguished in the Barnasiówka section, based on trace fossil diversity, ichnofabrics and chemical redox indices. These phases correspond to intervals with various intensity of sea floor oxygenation. Ichnofabrics within the OAE-2 succession was also controlled by consistency of sediment. Post-OAE-2 interval recorded by the earliest Turonian red and green siliceous–manganiferous shales and two Fe–Mn layers represent sediments deposited in well oxygenated conditions under extremely low sedimentation rate. Most of the shales are parallel-laminated, and they do not include any benthic microfossils and trace fossils, however some of the red and green layers display cryptic bioturbation. These features are related to food scarcity, cessation of gravity flows, unfavorable characteristics of bottom water, and a low consistency of substrate.