The 'Tortonian salinity crisis' of the eastern Betics (Spain)

Abstract

The late Miocene depositional history of the Lorca and Fortuna basins, both occupying an internal position in the eastern Betics of Spain, is marked by a regressive sequence from open marine marls, via diatomites and evaporites, to continental sediments. Based on facies similarities, these evaporites have often been correlated to the well-known Mediterranean evaporites of the Messinian salinity crisis, although this correlation was never substantiated by reliable chronological data. In this paper, we present an integrated stratigraphy of this regressive sequence which shows that the evaporites of the Lorca and Fortuna basins are entirely of late Tortonian age and as such have no relation with the Messinian salinity crisis. The main phase of basin restriction, resulting in deposition of diatomites and evaporites, took place at 7.8 Ma, while the last marine deposits (massive evaporites of the Lorca basin) are dated at 7.6 Ma. Consequently, this `Tortonian salinity crisis' of the eastern Betics had a duration of approximately 200 kyr, while continental deposition prevailed throughout the entire Messinian as also revealed by the fossil mammal record. The `Tortonian salinity crisis' of the eastern Betics is obviously related to a local phase of basin restriction caused by uplift of the metamorphic complexes at the basin margins, probably in concert with strike-slip activity along SW-NE trending fault systems. The development of a submarine sill is of crucial importance for the increase in salinity because it allows marine waters to continuously enter the basin at the surface while it restricts or prevents the outflow of dense saline waters at depth. Furthermore, we show that evaporite and diatomite cyclicity in these restricted basins is predominantly related to precession controlled circum-Mediterranean climate changes and that glacio-eustatic sea level changes only play a minor role. It is remarkable that the lithological sequence of the Tortonian salinity crisis mimics in many aspects that of the Messinian salinity crisis. This suggests that the diatomaceous facies is an essential part of the lithological sequence associated with basin restriction.