THE ORIGIN OF CHALKY ROCKS:
Messiniano’s salinity crisis
The chalky rocks of the “Chalky-Sulphurous Series” come from a suggestive and spectacular geological event: the Messiniano’s salinity crisis, that occurred about 6 million years ago.
In that time, the ancient Mediterranean Sea (called “Tetide”) was related to Atlantic Ocean trough two straits (Betico in the north and Rifan in the south).
Tectonic movements between Africa and Europe caused the progressive closure of the two straits, reducing gradually the communication between Atlantic Ocean and Mediterranean Sea. This last one was transformed in a marine space with very salty water, because of strong evaporation of the water that was not compensated with the fluvial and pluvial contribution.
Phase 1: Tetide Sea connected with Atlantic Ocean.
Phase 2: transformation in a big “evaporative tank”.
Phase 3: transformation in a brackish lake and partial desiccation.
Sea level decrease drastically: lots of areas of the Mediterranean appear and were born some closed basin in which, for the intense evaporation, the salinity increases a lot. The salts dissolved in the sea water lay down in the bottom, giving rise to big sediment thicknesses: carbonate, sulphate (gypsum) and chloride (rock salt and potassium salt). These storages, called “evaporite” for them genesis, are known, in geology, with the name of “Chalky-sulphurous series”.
About 5 million years ago, rocky threshold situated near the actual Strait of Gibilterra has suddenly given away, reestablishing the connection between Mediterranean Sea and Atlantic Ocean.
The water’s entrance, for some Authors, happened as a real waterfall, that quickly borrow the previous landscape, rapidly filling the existents depressions.
Were instituted typical condition of deep-sea environment, with deposition of “Trubi”, white rocks which have a calcareous and clay nature, mainly characterized by carbonate shells of some microorganism (Foraminiferi).
The gypsums keep bury for millions of years on the bottom of Mediterranean Sea, covered by other sediments. During the Pleistocene, because of a tectonic phase of raising, gypsums and sediments above definitively emerge; start process of erosion and modeling of the relief that contribute to define actual landscape.
A very common example of “Trubi” in Sicily is “Scala dei Turchi”, in the province of Agrigento.