Geological map
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Sample of dolomite belonging to the formation known as «San Salvatore Dolomite», from the name of the mountain near Lugano where it was initially featured. It composes the southern half of Monte Caslano. Photo: Museo cantonale di storia naturale |
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Distribution of the seas and lands emerged in the Middle Triassic and position of the formation of the rocks of Mount Caslano (starred). |
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The current Bahamian carbon platform could provide a similar environment to the formation of the San Salvatore Dolomite. Photo: NASA/Samantha Cristoforetti 2014 |
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During the Middle Triassic period (239-245 million years ago) the Thetis Ocean, advancing from the east, submerged the tropical region where the rocks of Mount Caslano were forming, covering it with warm, bright and shallow waters. In an environment rich in life and comparable to that of the present Bahamas, the dolomite, composed of calcium carbonate and magnesium, which constitutes the southern half of the mountain, accumulated. It is the same pale and massive rock that also forms Mount San Salvatore, from which it takes its name. At Monte Caslano it reaches a thickness of almost 500 metres.
The necessary material was provided by the countless organisms that populated the shelf. First of all, the calcareous algae of the Diplopora genus, very widespread in the bright waters of the new sea, still devoid of the modern colonial corals. The algae, green and photosynthetic, supported each other by covering themselves with an external calcareous skeleton in the shape of a perforated small tube. When they died, the skeletons accumulated on the seabed, generating fine whitish sand at the origin of the San Salvatore Dolomite.
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Fossil algae of the genus Diplopora on the surface of a sample of San Salvatore Dolomite. Photo: Museo cantonale di storia naturale |
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In addition to calcareous algae, the seabed was colonised by bivalves, gastropods, brachiopods and sea lilies, to which rare solitary corals and crustaceans were added. Cephalopod fishes and molluscs (ammonoids) populated the mass of water above.
A little further south, the sea deepened in the basin of Monte San Giorgio, the seabed instead privy of oxygen. These particular conditions guaranteed the exceptional conservation of the remains of the organisms that lived on its margins.
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Reconstruction of the formation environment of the San Salvatore Dolomite. Illustration: Vittorio Pieroni 2011 |
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