Alteration of Organic Matter During Chertification of a Cenomanian Black Shale (Umbria-Marche Basin, Central Italy)

A number of black shales, generally comprising clayey/siliceous intercalations were deposited in many locations at (or near to) the Cenomaniard Turonian boundary. One of the best-known and most extensively studied Cenomanian/Turonian black shale deposits is the "Bonarelli" horizon spanning a large area in the Umbria-Marche Appenines of Central Italy. Recently, the occurrence of 40, relatively thin (from less than 1 cm to 15 cm), black levels was reported below the Bonarelli horizon (Beaudoin et aL, 1996). These black levels occur in the uppermost 20 m of the section and their deposition spanned about 2.5 million years. Moreover, their periodicity corresponds to multiples of 20 ky. Such black levels were observed and correlated over the entire Umbria-Marche Basin and considered as precursors of the Bonarelli horizon (Beaudoin et al., 1996). They are either homogeneous (laminated clayey layers, cherty layers) or heterogeneous (superimposition of both layers or laminated clayey layers comprising black chert nodules).