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

In the Mediterranean region, the Melilla basin (NE Morocco) represents a ke y area that recorded biosedimentary events and environmental changes relati ve to the pre-evaporitic Messinian times. Ar-40/Ar-39 dating of volcanic tu ffs interbedded all along the Melilla Messinian shelf carbonates and coeval basin deposits has been performed in order to date accurately three main p re-evaporitic biosedimentary events: the prograding bioclastic deposition a nd the oligospecific prograding Porites coral reef buildings with coeval Ha limeda blooms, both within the platform, and the diatomite deposition basin ward. The new age data allow a precise chronological framework to be establ ished for the pre-evaporitic sequence of Melilla basin demonstrating that d iatomitic deposits are coeval with both prograding bioclastic and reefal un its. 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 coral-reefs and H alimeda beds and coeval warm-water diatomites began at least at 6.46 +/- 0. 03 Ma. Previous "OAri"gAr ages indicate that they ended prior to 6.0+/-0.1 Ma. Both biosedimentological similarities and chronological accordance with in several platforms and adjacent basins all around the Alboran Sea show th at these main bioevents, dated in Melilla, are synchronous over the Alboran realm. This accurate time scale for these pre-evaporitic biosedimentary ev ents (6.9-6.0 Ma) is in accordance with the most recent work on the latter period corresponding to the Messinian Salinity Crisis, (C) 2000 Elsevier Sc ience B.V. All rights reserved.