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

The Cenomanian-Turonian Antruiles section, in the northeastern Dolomites, n orthern Italy, consists of a cyclical limestone-mart succession characteriz ed by several black intervals. Two formations have been recognized: the Sca glia Variegata and the Scaglia Rossa. Integrated calcareous nannofossil and planktonic foraminiferal biostratigraphic data enable us to assign the Sca glia Variegata Formation to the Cenomanian (from the nannofossil CC9c to th e CC10a Subzones p.p. and from the foraminiferal Rotalipora brotzeni to the Whiteinella archaeocretacea Zones p.p.), while the Scaglia Rossa Formation correlates with the Early-Middle Turonian (from the nannofossil CC10a Subz one to the CC12 Zone p.p., and from the foraminiferal W, archaeocretacea to the Helvetoglobotruncana helvetica Zones p.p.). The l-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 o f 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, contri buting to a better understanding of the extent of dysoxic versus anoxic con ditions. The TOC content reaches a high value only in the Bonarelli Level, which is devoid of both planktonic and benthonic faunas, while the other bl ack levels contain less than 1% TOC. Corresponding to these levels, both pl anktonic foraminiferal and nannofossil assemblages are generally well prese rved and diverse without any dominance of eutrophic indicators; on the cont rary, an increase of oligotrophic forms (rotaliporids and marginotrucanids) is recorded. Eutrophic radiolaria and calcispheres are very rare throughou t the section. These data suggest that bottom redox cycles probably prevail ed 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 ran ges from 0.5 to 1%, suggesting anoxic conditions on the sea floor. Where th e TOC is less than 0.5%, benthonic forms are present, indicating dysaerobic conditions of variable intensity. (C) 1999 Academic Press.