Orbital control on pelagic clay sedimentology: the case of the late Albian "Amadeus Segment" (central Italy)

The Amadeus Segment which falls within the Oceanic Anoxic Event 1c, is a composite stratigraphic interval of late Albian age, composed by frequent alternation of black shales characterised by organic carbon content usually lower than 1 % and carbonate-richer sediments. Geochemical and clay mineral signatures indicate a coarser grain size for the black shales, and geochemical data from literature indicate a continental origin of the organic matter. Most of the geochemical variables analyzed show a record which follows an orbital cyclicity, with the exception of Mn and Ba, whose mobility after burial was identified by independent compositional criteria. The spectral analysis succeeded in identifying such compositional peculiarities, and also enabled to correlate the Amadeus Segment with the Segment-1 of the nearby Piobbico Core. The Milankovitch cyclicity (low eccentricity and obliquity) deeply affects the sedimentation of the Amadeus Segment, because orbital frequencies characterize the distribution of foraminiferal and compositional variables, which are both controlled by organic productivity and by detrital supply (dilution effect excluded). The high frequency changes from carbonate-rich layers to black shales are interpreted as changes in oceanographic regimes which in turn caused variations in the dispersal regime of continental inputs and in productivity.