BIODYNAMICS, SALINE GIANTS AND LATE MIOCENE CATASTROPHISM

The Messinian Salinity Crisis is several crises that fit the context of catastrophic modeling of the history of a Mediterranean "saline giant." Of the explanations available, we prefer the deep-basin/deep-water precipitation model for the early, deep-basin, massive gypsums and halites, contemporaneous with marginal, lagoonal gypsums; and the deep-basin/shallow-water desiccation theory to explain the later, shallow cyclic gypsums with associated lago mare faunas. The western basins before and after the Crisis were deep enough to maintain intermediate oceanic water-masses which is not possible today because of shallow thresholds. Early Pliocene mean paleodepths of the western basins were at least in the order of than 1500 m, which is the mean depth of the present Mediterranean. Tectonic changes in the threshold to the Atlantic were responsible for the isolation of the Paleo-Mediterranean during a eustatic high stand, followed by regression consequent to the evaporite precipitation phases. The context of Messinian stratigraphy has changed to include more events leading up to the Crisis. It began with evaporite deposition in the eastern basins during the Tortonian, and served to concentrate saline bottom-waters that later overflowed across the Pelagian Sea toward the west. In the western basins, it started at 6.4 Ma with a general water-budget deficit that altered the inflow over the threshold in the Rifian Corridor. A time of unusually high organic productivity and water-mass stratification followed, contemporaneous with evaporite deposition in some of the marginal seas and lagoons. Massive gypsum and halite deposition took place in the deep western basin in Chron 5 between 5.4 and 5.3 Ma. Our studies in Morocco and Spain indicate that passages to the Atlantic were closed during the cyclic gypsum and "lago mare" deposition, which took place during the long negative polarity reversal of Chron Gilbert from 5.3 to 4.9 Ma. We doubt that there were multiple marine inundations from the Atlantic into the Mediterranean after the first drawdown. The Rifian Corridor was closed by olistostromes at about 5.5 Ma. The last "leak" through the Gibraltar Barrier probably occurred in a passage through Oued Laou in northern Morocco at 5.4 Ma. The Oued Laou valley, which now forms a gorge through the Rif Mountains, could have furnished the influx of enough marine water in 27,000 years to furnish the nearly kilometer thick salt deposits of the deep Mediterranean basins. Gypsum deposits associated with shallow Paratethyan caspi-brackish (less than 8 parts per thousand salinity) ostracode and mollusk faunas, in the lower parts of both the Tyrrhenian and Algero-Provencal Basins, show that water depths in the deep basins changed from more than 1000 m to in the order of 100 m or less. However, faunal alternations typical of playa deposition are missing. The chemical composition of both gypsum and fossil shells indicate significant meteoric water, signaling a "fresh-water crisis." It is probable that phreatic, artesian, saline waters nourished a great sea-lake in the west. The Salinity Crisis is ended by the catastrophic flooding of all of the Mediterranean basins by deep-sea waters from the Atlantic.