Événements biostratigraphiques et environnementaux enregistrés dans le corridor sud rifain (Maroc septentrional) au Miocène supérieur avant la crise de salinité messinienne

The Riffian southern corridor (Northern Morocco) is one of the two principal seaways which allowed the exchanges of water between the North Atlantic and the Mediterranean Sea in the late Miocene. Along this corridor, late Miocene sedimentary sequences recorded various biostratigraphic events preceding the messinian salinity crisis. Here we propose a bio-chronologic framework for the marine sediments during late Miocene based on planktonic Foraminifera and dinokystes. The evolution of the marine environment during this period is analiyzed and some events which lead to the messinian salinity crisis are pointed. Two drillings of late Miocene belonging to two basins (Saïs and Guercif basins) of southern Riffian corridor were studied. The detailed biostratigraphic analysis based on planktonic foraminifera allowed to recognize Tortonian and Messinian successive bio-events which can be correlated with other bioevents registred in Western and Eastern Mediterranean basins and calibrated with astronomical and magnetostratigraphical scales. The succession of these events permitted to recognize probable middle Tortonian, late Tortonian and lower Messinian. It also leads to recognize and better characterize the Tortonian-Messinian boundary. The study of the dinocystes provided information on the marine environment of southern Riffian corridor during late Miocene. The indices of distality and temperature allow to appreciate the relative variations of the marine level and to estimate the surface water temperatures during Tortonian and lower Messinian. Our data allow to precise the timing of the progressive closure of the South Riffian corridor leading to the end of the Atlantic-Mediterranean connections and the starting of the so-called messinian salinity crisis. From Tortonien to lower Messinian, planktonic foraminifera and dinocystes assemblages reflect warm conditions of surface water.