~(40)Ar/~(39)Ar dating of the pre-evaporitic Messinian marine sequences of the Melilla basin (Morocco): a proposal for some biosedimentary events as isochrons around the Alboran Sea

摘要

In the Mediterranean region. The Melilla basin (NE Morocco) represents a key area that recorded biosedimentary events and environmental changes relative to the pre-evaporitic Messinian times. ~(40)Ar/~(39)Ar dating of volcanic tuffs interbedded all along the Melilla Messinian shelf carbonates and coeval basin deposits has been performed in order to date accurately three main pre-evaporitic biosedimentary events: the prograding bioclastic deposition and the oligospecific prograding Porites coral reef buildings with coeval Halimeda blooms, both within the platform, and the diatomite deposition basinward. The new age data allow a precise chronological framework to be established for the pre-evaporitic sequence of Melilla basin demonstrating that diatomitic deposits are coeval with both prograding bioclastic and reefal units. The prograding bioclastic carbonate unit related to boreal influences in coeval basinal diatomites, began at least at 6.73 ± 0.02 Ma and ended at 6.46 ± 0.03 Ma. The oligospecific prograding Porites coralreefs and Halimeda beds and coeval warm-water diatomites began at least at 6.46 ± 0.03 Ma. Previous ~(40)Ar/~(39)Ar ages indicate that they ended prior to 6.0 ± 0.1 Ma. Both biosedimentological similarities and chronological accordance within several platforms and adjacent basins all around the Alboran Sea show that these main bioevents, dated in Melilla, are synchronous over the Alboran realm. This accurate time scale for these pre-evaporitic biosedimentary events (6.9-6.0 Ma) is in accordance with the most recent work on the latter period corresponding to the Messinian Salinity Crisis.