INTRODUCTION

     Ever since the term “Messinian salinity crisis” was coined by Selli (1960) and, particularly, after the discovery of evaporitesin the subsurface of the abyssal plains of the Mediterranean during the Leg 13 of the DSDP (Friedman, 1973; Hsu et al., 1973a; Nesteroff, 1973), giving rise to the hypothesis of the deep desiccated basin (Hsu et al., 1973b), the salinity crisis has been widely considered to be one of the most dramatic episodes of oceanic change in the last 20 million years (Krijgsman et al., 1999). The great interest sparked in the scientific community by this event is due possibly to the attraction of visualizing the Mediterranean as a desert at more than 1500 m below global sea level (Hsu, 1983), a scenario which has no precedents in the history of the Earth and which has no comparable cases at present. The importance of the salinity crisis of the centre of the Mediterranean, especially assuming the hypothesis of the deep desiccated basin, is rooted in the following aspects: 1) it affects an area of more than 2.5 million km2 (Hsu et al., 1977); 2) it gave rise to the accumulation of more than 1 million km3 of evaporites (Ryan 1976); 3) it resulted from a sea-level fall of 2000 m (Hsu et al., 1973b); 4) it reduced global oceanic salinity by c. 6% (Hsu et al., 1977); and 5) it produced a subaerial erosive surface with deep incised valleys (Ryan and Cita, 1978; Clauzon, 1982). The Messinian salinity crisis was not exclusive to the centre of the Mediterranean, but rather has been registered also in the peripheral basins or marginal sectors, currently at more than 2000 m of elevation with respect to the central or abyssal basins. In fact, manifestations of the crisis have been documented from Cyprus in the eastern Mediterranean to the Betic and Rif Cordilleras (Gibraltar region) in the western Mediterranean.

 

 

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In the marginal sectors, the crisis is expressed by two fundamental events. One is the precipitation of evaporites, which accumulated in marine basins periodically isolated from the main body of Mediterranean water; and the other is the formation of a subaerial erosive surface (Messinian unconformity) related to the sea-level fall that originated the central evaporites. In addition, in these basins, other sedimentary records are linked to the crisis. One is the called Lago Mare episode of the end of the Messinian, which occurred after the evaporitic phase of the centre of the Mediterranean as a consequence of a general inundation; this Lago Mare episode is characterized by the development of hypohaline or brackish water environments, though cyclic sea-level oscillations which caused stages of gypsum precipitation (Upper Evaporites). The other is the complete reflooding of the Mediterranean at the beginning of the Pliocene (Pliocene transgression), which is the event that definitively marked the end of the salinity crisis.

 Our study centres on a marginal basin of the western Mediterranean: the Bajo Segura Basin. In this basin is presents one of the most complete Messinian and Pliocene records of the Mediterranean margins, both from a temporal standpoint as well as considering the variety of depositional environments.

     The aim of the AMG is to study the sedimentary features, the palaeoenvironmental significance and the chronology of the pre-evaporitic, evaporitic, and post-evaporitic stages, emphasizing the palaeogeographic meaning of the events that mark the limits between these stages. In addition, the record of the basin is correlated with other basins of the Mediterranean domain, where previous works have illustrated the manifestations of the salinity crisis. In synthesis, the Bajo Segura Basin is presented as a singular scenario to explain how and when the salinity crisis took place in the marginal basins of the western Mediterranean.

     Please, for cited references see Soria et al. (2008), Sedimentary Geology, 203, 267-288