Neogene Mediterranean desiccation and mega-flooding

Around 6 m.y. ago in the Neogene, the Mediterranean basin underwent two unique drastic geologic events; a Late Miocene desiccation of over 1500 meter deep seawater, during which the Mediterranean became a partially dry deep basin, and was followed by an Early Pliocene rapid flooding of seawater through the Gibraltar Strait from the Atlantic Ocean, and the Mediterranean returned to a normal deepwater basin as today.
Geological puzzlements
Geologists who studied the Tertiary geology around the Mediterranean basin were puzzled by some of their observations on outcrops, which could not be explained by the then known ordinary geologic processes. With the early 1970s deep sea drilling results, these extraordinary geological features were finally put together to form a coherent but atypical geologic history close to the Miocene and Pliocene boundary. These observations are as follows:
Exceptional deep incised valleys
Based on polar ice cap volume, the global sealevel variations are in the range of 205 meters (50 meters rise and 125 meters drop). The depth magnitudes of the Quaternary incised valleys on the continental shelves are within this range. Similar conclusion on the magnitude of sealevel drop has been reached by the investigation of the last glacial maximum.
The incised valleys around the Mediterranean have been observed with depths far exceeding this Quaternary magnitude. For example, the buried Afiq Canyon in Eastern Mediterranean, Israel, down cuts the underlying gypsum formation and is filled with Late Miocene fluvial deposits, indicating the canyon was exposed to subaerial fluvial erosion during a sealevel drop event. Based on the canyon depth the sealevel drop has been estimated to be over 800 meter.
In northern Libya, a Late Miocene incised valley has a dimension of 150 km long and 700 meters deep. The valley down cuts the underlying middle Miocene limestone and is filled with Pliocene fluvial sediments. Incised valleys of similar age have also been found in eastern Libya and western Egypt. This seems to suggest that a Late Miocene sealevel drop in the Mediterranean region could have reached over 700 meters.
Globally the Late Miocene to Early Pliocene eustatic changes do not exhibit such prominent sealevel drop. For example, in the Gulf of Mexico, the supersequence of this period during the sealevel fall only shows size-reduced shelf edge delta, and no significant incised valleys have been reported. An extensive sequence correlation between the Mediterranean and North Atlantic regions, prompted Hondell et al.(2001) to conclude that such a magnitude of sealevel drop in the Mediterranean requires a different cause(s) other than glacioeustatic changes.
Drastic depositional environment changes from subaerial to deep marine
In Cyprus, the Miocene and Pliocene boundary is represented by a sudden change from soil layer to deep marine sediments, without transitional shallow marine sediments, a clear indication of rapid sealevel rise.
In the eastern Mediterranean continental margin, Early Pliocene deep water Gilbert-type delta, overlie directly the Late Miocene evaporate, again suggesting a rapid sealevel rise from terrestrial salt basin to deep water continental margin setting.
In fact, this rapid change of depositional environments appears widespread, even extending to the bottom of the Mediterranean Sea. Deep sea drilling results demonstrate a thick middle Miocene evaporate section interbedded with pelagic sediments with normal marine fossils. This implies that the terrestrial salt lake environments, as represented by the evaporates, were frequently submerged in deep marine environments (over 1000 meters deep), as represented by the pelagic sediments, without noticeable transitional environments in the sedimentary records.
A coherent geological history
Integrating extensive outcrop information with marine seismic and drilling results, a rational geological history has been established to comprehend the above-mentioned extraordinary geological features. A Late Miocene marine regression caused the subaerial exposure of the Gibraltar sill, which blocked the Atlantic seawater from entering the Mediterranean. Evaporation of the Mediterranean seawater led to the deposition of over 1500 meters thick evaporites when the basin became a partially dry deep terrestrial salt basin. An Early Pliocene marine transgression or tectonic subsidence submerged the Gibraltar sill, and caused the Atlantic seawater flooding the Mediterranean basin.
Supporting geologic evidence for deep terrestrial basin in the Late Miocene
Region-wide spread of evaporites
Onshore outcrops: A Late Miocene marine regression has been identified and is represented by the regional distribution of evaporites in the Mediterranean. Onshore occurrence of evaporites has been found in Spain, Italy, Turkey, Israel, Salt diapirs of similar age are also known in the western Mediterranean Baleraric Basin and in Cecily.
Offshore occurrence: Earlier marine seismic surveys have recognized a regional seismic reflector named “M”, which was subsequently penetrated by deep sea drilling and found to represent the top of a Late Miocene evaporite sequence. The sequence can be correLated to the type section of the Late Miocene evaporites in Sicily.
The occurrence of the Late Miocene evaporites, both onshore and offshore, suggests a basin wide deposition. The onshore outcrops of the evaporites are uplifted by Later tectonic movements, whereas the time equivalent evaporites in deep water are in situ.
Deep basin desiccation
Close to the end of Late Miocene marine regression, the entire Mediterranean basin was almost dried out by evaporation. The major evidence to support this inference comes from a basin wide erosional surface, the Messinian erosional surface.
Supporting geologic evidence for mega flooding in the Early Pliocene
Strait of Gibraltar: An erosional channel with about 390 kilometers in length and few hundreds meters in depth, from the Gulf of Cádiz through Gubraktar to the Algerian Basin in the western Mediterranean has been documented to evidence the subaerial erosion by the fluvial processes from the Atlantic to the Mediterranean in the Early Pliocene. Base on hydrodynamic modeling, it has been estimated that 90% of the Mediterranean water is filled in few months to two years.
Ionian Basin: In the eastern Mediterranean, seismic data reveal a section of chaotic deposits which overlies the Late Miocene evaporites and underlies the Early Pliocene normal marine sediments in the eastern continental margin of Sicily. These chaotic deposits are believed to represent a catastrophic flooding from the western to eastern Mediterranean Sea.
Dispute
There have been three models of deposition suggested for the widespread Late Miocene evaporites in the Mediterranean Basin:<ref name="Hsu"/>
Shallow basin with shallow water: The basin started as the present day Dead Sea environments with terrestrial salt lakes, and subsequently sank and submerged in 2000 meter deep marine water. This scenario has so far no evidence to support such a magnitude tectonic movement.
Deep basin with deep water: The basin remained as a deep marine basin and the super saline water from the surrounding coastal areas, because of high density, sank to the bottom and was deposited as evaporites. This hypothesis is theoretically possible but has no modern example. Furthermore, drilling results show mud cracks, a desiccation feature, in evaporites, contradicting to persistent deep marine environments.
Deep basin with shallow water: A widely accepted origin of evaporites advocates that the basin continued as a deep marine basin and became a region of subaerial salt lakes and playas after evaporation of more than 2000 meters of seawater duo to the blockage of inflow of seawater from the Atlantic at Gibraltar Strait.

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