Geology and Paleontology of Sardinia

Thanks to fossil evidence: the Sardinia you don’t expect

Sardinia in its more recent geological history has been home to fauna and flora that we may encounter today in seas far warmer than the Mediterranean Sea. We are talking about a period, the Miocene, that began about 23 million years ago and ended just over 5 million years ago. Imagining that we can go back in time, we will have seen a very different Sardinia than the one we know today. First, his position. We will not have seen the island in the middle of the Mediterranean, but in front of the coast of Provence, practically in France! What is even stranger is that not only Sardinia was in that area, but also Corsica, part of North Africa and part of Calabria. In particular, Sardinia and Corsica formed (and still form today) what is called the Sardinian-Corsican Bloc. We know from earthquakes that the Earth’s crust is always in motion; generally plate shifts of a few millimeters per year to a few centimeters are observed. Slow but ever-present movements. It is precisely because of these movements that slowly, at the beginning of the Miocene, the Sardinian-Corso block began to separate from the Provençal coast and rotate counterclockwise to its present position. This ended about 16 million years ago. The movement of the plates created large fractures with shifting blocks of Earth’s crust (faults). From the deeper ones, an intense magma rise occurred, giving rise to one of the strongest periods of volcanic activity ever seen in Sardinia. Large explosive-type volcanoes were formed, of which few traces remain today. We need to imagine scenes like those seen by the inhabitants of Pompeii. Burning clouds and avalanches of ash and lapilli travel dozens of kilometers before stopping, covering everything they encounter on the road. As millions of years passed, this volcanic material slowly welded together and became rock. Where can we observe these rocks (trachytes) in Sardinia? In the Sulcis, Marghine, Planargia, Sassarese, Anglona… These are those rocks that are commonly called “trachytes,” but actually from a geological formation point of view, it would be more correct to call them “ignimbrites,” trachytes being another type of rock, by the way quite rare in Sardinia. What is even more amazing is that even the seabed was in some ways not a quiet place. Here, too, the effects of early Miocene volcanic activity were felt! The results of this submarine volcanism, we can observe well in Marmilla (central Sardinia). Underwater lava formations have been preserved here, which are named Pillow Lava (or “pillow lavas”). These are nothing but spheroidal blocks with radiating structures starting from the center toward the outer edge. Often alteration erases part of this internal structure, but generally one can observe these blocks quite well preserved. Generally these pillow lavas are 1 to 1.5 meters in diameter, but in Sardinia we have an almost world record just on this strange type of submarine lava. In the municipal territory of Masullas (OR), one can admire one of these lava “pillows” of record dimensions of 12 meters long by 8 meters high, hence the name “Mega Pillow.” Because of this uniqueness, it has been declared a natural monument. Fortunately, every once in a while volcanic activity, whether underwater or on land, had lulls. At these quiet junctures, marine life could resume development, going on to colonize the new environments. The shallows were teeming with life. Coral reefs, expanses of sea urchins, crabs…and dugongs. Yes, because at this time the Sardinian coast was also populated by marine mammals quite similar to today’s dugongs. Today we can find these strange and placid animals in the tropical belt in the Red Sea and the western shores of the Pacific Ocean. A little further out to sea, however, the waters were far less certain; in fact, we may have encountered one of the largest predators that has ever existed in Earth’s seas since the disappearance of the giant marine reptiles of the Mesozoic. We are talking about the famous Megalodon(Carcharocles megalodon). The appearance was perhaps vaguely reminiscent of a white shark, but the size was closer to that of a whale! It is estimated to be about 18 meters long with teeth that could reach 20 cm in length. Today a white shark can reach a maximum of 7 to 8 meters. Since the open sea could have been a mortal danger, would it have been better to sunbathe on the beach? Mah… We may not have been safe there either because we might have had to deal with crocodiles. Alligators and Gavialoids (similar to those in Indonesia) warmed up on riverbanks or coastlines waiting to take to the waters again in search of food. What is left of all this today? The extensive limestone, sandstone and marl outcrops that formed the hills of Marmilla, Trexenta and Sarcidano for example, conceal within them the fossils of this extinct tropical marine fauna. The Duidduru Paleontological Geosite of Genoni offers a glimpse of this lost world–and it is a glimpse in the true sense of the term, as the geosite is a hillside now halved by aggregate quarrying activity. Cultivation of the quarry led to the discovery of dozens of square meters of rock surfaces with perfectly preserved accumulations of sea urchins. Not only that, the latest excavation work led to the discovery of a crocodile tooth displayed, along with other artifacts from this site, in the halls of the P.AR.C . One of the few artifacts on display, but from the Orroli area, is a Megalodon tooth, preserved in the museum’s halls because of the collaboration between municipal administrations and the Soprintendenza Archeologia, Belle Arti E Paesaggio for the Metropolitan City of Cagliari and the Provinces of Oristano and South Sardinia. Article by Geologist and PARC science director Luigi Sanciu