The Bonarelli Level and other black shales in the Cenomanian-Turonian of the northeastern Dolomites (Italy): calcareous nannofossil and foraminiferal data

Abstract

The Cenomanian-Turonian Antruiles section, in the northeastern Dolomites, northern Italy, consists of a cyclical limestone-marl succession characterized by several black intervals. Two formations have been recognized: the Scaglia Variegata and the Scaglia Rossa. Integrated calcareous nannofossil and planktonic foraminiferal biostratigraphic data enable us to assign the Scaglia Variegata Formation to the Cenomanian (from the nannofossil CC9c to the CC10a Subzonesp.p. and from the foraminiferalRotalipora brotzenito theWhiteinella archaeocretaceaZonesp.p.), while the Scaglia Rossa Formation correlates with the Early-Middle Turonian (from the nannofossil CC10a Subzone to the CC12 Zonep.p., and from the foraminiferalW. archaeocretaceato theHelvetoglobotruncana helveticaZonesp.p.). The 1-m-thick black shale separating the Scaglia Variegata from the overlying Scaglia Rossa has been identified as the Bonarelli Level, and is considered to be the expression of the global OAE2 (Oceanic Anoxic Event). In the section described in this paper, more anoxic/dysoxic episodes, predating and postdating the Bonarelli Level, have been documented. The Total Organic Carbon (TOC) values and the microfossil distribution patterns are discussed for these horizons, contributing to a better understanding of the extent of dysoxic versus anoxic conditions. The TOC content reaches a high value only in the Bonarelli Level, which is devoid of both planktonic and benthonic faunas, while the other black levels contain less than 1% TOC. Corresponding to these levels, both planktonic foraminiferal and nannofossil assemblages are generally well preserved and diverse without any dominance of eutrophic indicators; on the contrary, an increase of oligotrophic forms (rotaliporids and marginotrucanids) is recorded. Eutrophic radiolaria and calcispheres are very rare throughout the section. These data suggest that bottom redox cycles probably prevailed over productivity cycles, among the mechanisms leading to the formation of the Cenomanian-Turonian black shales recognized in the Antruiles section.Benthonic foraminiferal fauna is absent from the deposits where the TOC ranges from 0.5 to 1%, suggesting anoxic conditions on the sea floor. Where the TOC is less than 0.5%, benthonic forms are present, indicating dysaerobic conditions of variable intensity.