Lithostratigraphic analysis of Milankovitch cyclicity in pelagic Albian deposits of central Italy: implications for the duration of the stage and substages

Estimations of Albian Stage and substage durations are proposed on the basi s of a cyclostratigraphic study of a continuous pelagic section of the 'Mar ne a Fucoidi' formation of the Umbria-Marche Basin, central Italy. An analy sis of the vertical distribution of black shales, marls and limestones is d ocumented. The black shales are organized in bundles of 3-5 levels (similar to 100 kyr) and composite groups (similar to 400 kyr), which are the sedim entary expressions of the two eccentricity modes of the Earth's orbit. The specific distribution of this lithology coupled with the vertical stacking of the marl-limestone couplets, interpreted as an expression of the precess ion of the equinoxes, enables the breakdown of the Albian Stage and its sub stages into astroclimatic cycles and calibration of their duration. The str atigraphic boundaries are fixed by a combination of planktonic foraminifera , calcareous nannofossils and dinoflagellate cyst 'events', which have been correlated with European ammonite biozonations. The Albian Stage is estima ted to have lasted for 11.6 +/- 0.2 myr, and the Aptian/Albian boundary is proposed at 110.1 +/- 0.7 Ma. Substages are respectively estimated at 1 myr (Lower Albian), 3.7 myr (Middle Albian) and 3.4 myr (both Upper and upperm ost Albian, also called Vraconian). These durations have enabled the determ ination of a new calibration of the average durations of the Albian ammonit e zones, which are obtained by translation of the Italian micropaleontologi cal data on the international chart. The duration of the biozones is very v ariable and the stratigraphic resolution of each zone fluctuates between 10 0 kyr for the Lower Albian and 1.7 myr for the uppermost Albian. The applic ation of the new Albian durations in sequence stratigraphy has implications for the genetic causes of the unconformities, particularly for the Lower A lbian where the unconformities were produced by astronomical parameters of the Earth's orbit. (C) 2001 Academic Press.