The Jurassic–Cretaceous boundary in the Kurovice section (Southern Moravia, Czech Republic): trace fossils, stable isotopes, and magnetic susceptibility

Abstract

In the Kurovice Quarry (southern Moravia), the paleoenvironmental conditions around the Jurassic–Cretaceous boundary were deciphered using geochemical, magnetic, macrofaunal, microfaunal, and ichnologic evidence. In addition to weakly bioturbated horizons, four discernible ichnologic assemblages (IA1–IA4) were distinguished. Geochemical changes were recorded both between individual ichnologic assemblages and within them. IA1 is dominated by Chondrites, Thalassinoides, and Nereites, accompanied by Bergaueria and Phycodes, and occurring in pelagic sediments. IA1 also occurs in an impoverished form, in deposits characterized by a low ichnofabric index (i.e. impoverished IA1). IA2 consists of Trypanites in layers with microbially induced sedimentary structures; it is present in beds 20–21. IA3 is represented by subvertical shafts filled with pyrite and Trypanites isp., found in beds 30 and 63–72. IA4 consists of common Balanoglossites and rare Trypanites, Thalassinoides, and Planolites; it occurs in beds 102–104, 126–127, and 137–140. Four intervals with prominent Balanoglossites (IA4) and occasional Trypanites (IA2) mark probable hardgrounds and/or condensed deposits. These deposits are also characterized by slightly increasing magnetic susceptibility and enhanced biogenic reworking. Enhanced bioturbation/bioerosion commonly concords with higher susceptibility intervals. Geochemically, the beds hosting impoverished IA1 show a slight decrease in the δ13Ccarb followed by an increase in the δ13Ccarb values; this may reflect a general rise in productivity accompanied by organic matter burial. Similar trends in the δ13Ccarb values are also associated with the IA2 and IA3 characterized by the presence of vertical pyrite-filled tubes. Increases in δ18Ocarb values within IA2 and IA3 are interpreted to express cooling episodes suggesting a rise in oceanic productivity. Therefore, this primary isotopic signal reflects nutrient changes within an open ocean, which is not (in this case) influenced by bioturbation activity. IA4 (Balanoglossites) is characterized by an apparent increase in δ18Ocarb values (the trend is similar to that in IA2 and IA3) but δ13Ccarb values are stable.