Realmonte Salt Mine, Sicily: © Rob Butler
Regionally extensive salt deposits are rather rare in the geological record as they require very large, land-locked marine basins from which sea-water can evaporate with only sporadic periods of recharge. Along with the ancient Zechstein Sea, one of the best studied of these so-called “saline giants” is found in the region now containing the Mediterranean Sea. It formed much more recently than the Zechstein – during the Messinian (the last stage of the Miocene epoch) between 8 and 5.5 million years ago. As the African, European and Arabia continents jostled together, forming the great mountain ranges of North Africa and Europe, what was left of the Tethys ocean that once separated these landmasses became progressively restricted and eventually landlocked. The remaining portions of this old ocean basin are preserved as the bed of the Eastern Mediterranean. Below more recent sediments are deposits of Messinian salt many hundreds of metres thick, left behind as the remnants of the Tethys ocean evaporated.
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Schematic of the Mediterranean Sea during the Messinian
salinity crisis: © Paubahi
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But these rocks are still buried and submerged. To see the best examples of Messinian salt and its related strata at the Earth’s surface, go to Sicily. Since the Messinian, the island has gradually emerged from below sea-level to expose strata that accumulated on the edge of the Tethys ocean – and these include spectacular Messinian sedimentary rocks. The salt can reach thicknesses in excess of 800m, preserved in the subsurface in the cores of downward-curving folds (synclines) formed as the collision between Africa and Europe progressed. These folds created basins, akin to ‘watch glass’ evaporating dishes like those used for school chemistry experiments on evaporation and precipitation – but many km across. And just as in these experiments, different deposits precipitated on the flanks and in the middle of these basins. Carbonates formed on the high flanks, where the water was shallowest, then gypsum, halite and potash salts formed progressively towards the basin centre. Today the deposits in the synclines of Sicily are mined for the resources they contain. Much of the salt (halite) from Realmonte, on Sicilly’s south coast, is shipped to northern Europe – to de-ice roads in winter.
At the very end of the Miocene, the Mediterranean refilled and normal marine conditions (i.e. not hypersaline) were restored. Although there is continuing debate over exactly how this happened, it seems like that water flooded in rapidly from the Altantic through the Strait of Gibraltar, perhaps largely refilling the Mediterranean in just a few months or a couple of years. Today, despite the narrowness of the strait, sea water is effectively exchanged between the Mediterranean and the open Atlantic ocean – so no longer is salt accumulating on the floor of the Mediterranean. Saline giants require just the right arrangement of tectonics and continents to form.
Further reading:
Butler, R.W.H., Maniscalco, R., Sturiale, G. & Grasso, M. 2015. Stratigraphic variations control deformation patterns in evaporite basins: Messinian examples, onshore and offshore Sicily (Italy). Journal of the Geological Society, London, 172, 113-124..