Geochemical, delta C-13 and Eustatic curves as means for reconstructing and characterising deep marine carbonate stratigraphic sequences (Upper Cretaceous, Tunisia)

摘要

A geochemical study of a thick chalk Tunisian onshore section of the Campanian-early Maastrichtian Abiod Formation was undertaken in order to assess the previously proposed linkages between chemostratigraphy, sequence stratigraphy and eustatic variations. An up-to-date correlation of the Abiod Formation delta C-13 curve at Le Kef to other Tethyan, Boreal and NWPacific Ocean sections adds important data to improve and revise previous correlations. Fourteen correlation sequences including earlier published tie points are proposed, two of which are considered to be major isotopic events. The CaCO3, Al2O3, MnO, SiO2, TiO2 and Zr contents of Le Kef section were determined in order to assess the vertical elemental variations of such elements in pelagic sediments in response to eustatic fluctuations. For that, a correlation of delta C-13, CaCO3, Mn, Ti/Al, Zr/Al and Si/Al of the Campanian- early Maastrichtian sediments with published regional and global sea-level curves is proposed and overall infers that: (1) delta C-13 can be used for estimating eustatic sea-level cycles and have subsequently permitted to allocate the Abiod Formation, at Le Kef, to five third order cycles (UZA) corresponding to five stratigraphic sequences; (2) sequence boundaries are mainly characterised by minima in Mn-CaCO3 and a low Ti/Al ratio (or Si/Al, Zr/Al); (3) transgressive system tracts are characterised by increasing delta C-13, Mn, CaCO3 and Ti/Al ratio; (4) maximum flooding surfaces correspond to a rise in carbonate, manganese and Ti/Al or (Si/Al, Zr/Al); (5) and highstand system tracts show a Ti/Al ratio (or Si/Al, Zr/Al) decrease, a carbonate increase, a manganese rise and a delta C-13 reaching high levels. These conclusions fairly agree with the idealised relationship between chemostratigraphy, depositional sequences and sea-level curves proposed by Jarvis et al. (J Geol Soc Lond 158:685-696, 2001). Furthermore, the Abiod Formation is a proven reservoir in many oil fields in Tunisia, and we believe that the present sequence stratigraphic characterization, via geochemical signatures, can be of great future usefulness to better identify and correlate similar pelagic and hemi-pelagic horizons.