Authors: Athina Tzevahirtzian (University of Palermo) & Beatriz Martínez-Rius (Sorbonne Université)
The Mediterranean seabed hides precious secrets that geologists are struggling to understand for years now. Their major obstacle? Technological and technical difficulties that convert the seafloor in an unreachable place… Unless you are in Sicily, where you can literally touch the rocks without getting “wet”.
The rocks that were buried in the seabed millions of years ago are now exposed in the Sicilian landscapes, and the outcrops formed during the Messinian Salinity Crisis can be seen while walking around the countryside. During the first Saltgiant fieldtrip, Prof. Antonio Caruso and our fellow Athina, both of them from the host University of Palermo, drove us through the main formations of the Messinian evaporite deposits, such as Gibliscemi, Falconara, Capodarso, Eraclea Minoa… Places that, further from being nice scenarios, can give us hints about the geological history of the Mediterranean basin, 5 million of years ago.
This article is born from two different perspectives: the one of Beatriz, a historian and newcomer in geology, and the one of Athina, the early stage researcher who is getting specialized in this area.
For me, Beatriz, visiting and studying the lithological sections challenged some of my previous assumptions about the “journey” of the rocks, how they are moved, destroyed, and generally displayed in the landscape; and, above all, it taught me how the geologists observe and interpret the outcrops.
The very first question that I asked myself was: How can we see the rocks that were deposited in the bottom of the sea, more than 5 million years ago? Athina replied that the answer can be found in the plate tectonics’ movement. The convergence of the African and the Eurasian plates induced an orogenic belt, that is to say, mountain belts formed by the compression of the material at the edge of the plates that are colliding, forming the Appenine-Maghrebian chain. Its frontal part corresponds to the “Gela thrust belt”, which crosses the continental and oceanic part of the southern Sicily, from west to east. This event had as consequence the deformation of very deep levels, pushing and uplifting the seafloor area, and moving it outside the seawater. All the these formations of the Caltanissetta Basin, located in the centre of Sicily and in the frontal part of the Gela thrust belt, started to be uplifted more than 5 millions years ago, during the terminal Messinian, and became continental 900.000 years ago (middle Pleistocene).
By observing these on-land formations, the consequences of the Mediterranean’s partial evaporation can be read in the rocks. The Licata Formation corresponds to normal marine conditions in which phenomenon of water mass stratification start to appear, inducing poor exchanges of oxygen in the water column. These conditions can be observed in the impressive and clear outcrop of Gibliscemi, where the pattern of sediments demonstrate how the conditions of sedimentation were repeated in cycles. More than 200 cycles of marls and clayey marls alternating with dark layers, known as “sapropels”, outcrop in Gibliscemi (images 1 and 2). There, the cyclicity is controlled by the precession of the equinoxes (21.000 years) and hence, correspond to 4 millions years (Athina has explained it more comprehensively here).
Image 1: The Saltgiant team at Gibliscemi.
We find next the Tripoli Formation, where changes in the minerals were triggered by precessional cycles, each one corresponding to 20 thousand years. Tirpoli represents the pre-evaporitic conditions and the gradual passage from normal marine conditions to hypersaline conditions: this marks the beginning of the Messinian Salinity Crisis. The importance of this outcrop relies on the fact that, by now, the pre-messinian conditions can only be understood by studying in-land outcrops, since the formations underneath the evaporites in the deep Mediterranean basin remain unreachable.
The increasing salinity during the Tripoli Formation culminated with the deposition of the Calcare di Base and the Lower and Upper Gypsum Formations (image 3). The latter are rich in gypsum, formed by the deposition of salt and minerals during the salinity crisis. Lastly, the return to normal marine conditions can be glimpsed in the Trubi formation, materialized in the touristic spot of the Scala dei Turchi (images 4 and 5).
Image 4 and 5. Scala dei Turchi.
Before embarking in Geology, I would have thought that those formations were visible one above the other, in a perfect vertical row, in the same outcrop. However, far from that, erosion and Earth movements that have affected those rocks during the last millions of years have dismantled this idealistic structure of a vertical row. Thus, in order to visit the different formations, sub-formations and local particularities, we spent the week climbing through mountains, crossing forests and walking along the seashore (and also having some fun in the beach) in the quest for the Messinian outcrops.
But not everything was geological mountaineering: we enjoyed the Sicilian cannoli and gelati on each occasion we had, we swam in the beautiful waters of Scala dei Turchi, we took some reinforcing small nap after our lunchtimes to continue our long walks to the outcrops; we devoted time to write down the soundtrack of our Saltgiant team during the endless bus excursions, and we even had time to visit the Valle dei Tempi, to enhance our historical knowledge (image 6). Since in every trip there is some unexpected incident, our bus decided to let us down for 3h. But we, positive geologists, biologists, physicists and historians, decided to improve our dancing and singing skills by sharing regional and traditional songs of each of our country.
The Sicilian formations constitute a must visit for all those who study the Messinian. But, although they have been intensively studied by geologists from the early 1950s, some answers remain a mystery. Was the MSC a unique event that affected the entire Mediterranean basin at the same time? Or did it happen during different phases? How was the Calcare di Base formed and why do we found different types of it? How the Mediterranean basin moved back from hypersaline stressed conditions to normal marine conditions again?
All those questions cannot be solved just by looking at the Sicilian rocks, but those formations are an essential piece to solve the Messinian jigsaw.