Geo-paleontological Excursion to Monte San Giorgio (CH) - part 1
- Kategorie: English articles
- Veröffentlicht am Freitag, 12. Oktober 2012 18:19
- Geschrieben von Luca Jaselli
- Zugriffe: 4948
Dieser Artikel ist auch in deutscher Sprache verfügbar. Jetzt zur deutschsprachigen Version wechseln.
Geo-paleontological Excursion to Monte San Giorgio (CH)
The “Fondazione Monte San Giorgio”, the Monte San Giorgio parish Committee and the municipality of Meride (CH) organized for the day of August 12, 2012, a geo-paleontological excursion on the Swiss side of Monte San Giorgio, with a guided tour of the excavations and fossiliferous sites. The trek, about 7 km long, with a vertical drop of 400 meters, had a guide: Prof. Rudolf Stockar, curator of the “Museo cantonale di storia naturale” in Lugano and in charge of the excavations. The event had a significant and unexpectedly large crowd, and 96 people were waiting at the appointed time at the parking lot at the entrance to Meride!
The study of fossil evidence found on Monte San Giorgio began in the mid 1800's with the first fossil fish being documented, followed by the discovery of large marine reptiles. These fauna of the Triassic period (Anisian/Ladinian), and better yet the sediments within which they remained buried for 240 million years, tell us about how at that time the area was occupied by a calm and shallow sea with a predominantly subtropical climate, therefore with environmental conditions ideal to life, as also witnessed by the richness of the fauna.
Observing the Monte San Giorgio from a stratigraphic point of view, we can see that this area has not always been subject to the conditions of a marine environment. Structurally it lies on the rocks of the crystalline basement in turn topped by Permian vulcanites which indicate that there was intense volcanic activity in this area. At the base of the Triassic and above the vulcanites, lies the sandstone of the Bellano formation, an object of strong erosion. Later the entire area was covered by water, and it created a marine environment which lasted until the early Carnian (230 MA), when the sea level began to recede, although only temporarily as evidenced by the composition of the Pizzella Marl.
During our excursion we'll look at rocks and sediments for a period of time stretching from the Permian to the Carnian when the conditions of the marine environment were established, and we will visit excavation areas that were, and still are, the subject of careful study and from which have been extracted fossils which are extremely important from a scientific point of view. After a short briefing on the geomorphological aspect of the area, the route and a brief description of the sites that we visit, a long queue of people slowly start to make its way along the narrow streets of Meride and then began to clamber up along the arduous path that leads to Cassina.
Stratigraphic scheme including lithologies and visit stops (not in scale)
On the way to Cassina.
STOP 1 – CASSINA
Cassina's site is at an altitude of 900 meters. The area has been studied by the paleontologist Bernhard Peyer (1885-1963) who first started the research in the entire area. The outcrop was discovered in 1933 and exposes about three meters of laminated rocks of the Calcare di Meride Inferiore (240 MA). The Calcare di Meride Inferiore (lower Meride limestone) is clearly visible rising from the village of Meride and it also paves the way to Cassina, though we were unaware that we were walking over it for quite some time before reaching the first stop. Among the lower layers of the sequence, we see the Cassina level: laminated, dark in color due to an anoxic event that witnesses the lack of oxygen at the bottom of the sea. The absence of oxygen, however, does not completely preclude the existence of life. Despite the anoxia, small-sized organisms of about 0.3 mm were found. Directly below the Cassina level is a reddish bank consisting of volcanic ash or tufites which indicates the presence at that time of volcanic activity. On this site during the first excavations, two marine reptiles around two meters long and a Ceresiosaur: Ceresiosaurus calgagnii over two meters in length were brought to light. In 1937 excavations still occurred in Cassina, but they were interrupted until the early 1970s. Researchers from the University of Zurich reopened excavations from 1971 to 1975 and found fish of the genus Saurichthys and the skull of a five-meter long reptile.
The Cassina site.
The excavations were then again stopped until 2006 when the Museum of Natural History of Lugano resumed research activities in Cassina by opening a new site 150 meters higher. In the wall exposed at the new site you can see a geological feature where the layers have been partly replaced by the presence of a glacial moraine.
In the Cassina beds, fossils are never found on the cleavage plane of the strata, but rather in the cross section. For this reason researchers proceed by analyzing individual plates in cross section under the microscope to identify fossils. If something is found, they look for other parts of the specimen in the adjacent slabs, numbering and assembling them almost as it were a puzzle. This is because the slabs often break, separating the different parts of the fossil which must then be reattached. Once the puzzle is complete, it is then possible to expose the complete fossil using an engraving instrument.
The Cassina dig site tells about a lagoon paleo environment frequented by fish (240.6 MA). The lagoon was separated from the open sea by the platform formed by the Dolomia of S. Salvatore and its waters had an anoxic seabed populated by bacteria. The richness in ichthyofauna is well demonstrated by the finds in Cassina, such as the fish Eosemionotus or Saurichthys of which have been found about 300 specimens, the largest had a length of around 42 cm. Saurichthys was a predator, like the barracuda. Some female specimens have been also found with embryos intact.
The new Cassina site.
In these pictures: A Saurichthys ready for preparation. A specimen at the end of the preparation phase. A comparison between a Saurichthys and a reconstruction in scale of this fish.
Some fish from Cassina. The biggest one in these pictures is a fish from genera Eosemionotus.
Marine reptiles are a rarity at the Cassina site, only one being found during excavation operations. These animals are often found in the lower layers of the Besano Formation. Instead there have been finds of plants that bear witness to the presence of land very close to the lagoon. These are finds such as coniferous tree Elatocladus cassinae, a new species discovered in this place, but also ferns of the species Ptilozamites sandbergeri.
Elatocladus cassinae und Ptilozamites sandbergeri.
STOP 2 - POINT 902
Leaving the site of Cassina, the trail descends down through a thick forest until you reach a fork called Point 902. This name derives from the surveying of the area located precisely at 902 meters above sea level. The rock outcrops amongst the vegetation show substantial differences compared to the previous lithologies. We have moved on only a few tens of meters and the Meride limestone has given way to the Dolomia of S.Giorgio (241 MA). This stone (dolomite) is very rich in radiolarians, plankton and it lies over Besano Formation (reptiles).
Queue of hikers heading toward the second stop.
STOP 3 - QUOTA 902
A few dozen meters away from Stop 2 there is some shale debris visible amongst the dense vegetation. On this site, outcrops of the Besano Formation and excavations which have been made by the University of Zurich from 1950 to 1968 exist. The bituminous shale layers alternate with dolomite, with a total thickness of about 16 meters, but they are no longer visible, as well as any other excavation work because the whole thing was covered with a dump of debris left over from the end of the research activities, which are then covered by vegetation. By removing the dry foliage, however, it is nevertheless possible to observe some debris consisting of dolomite and bituminous shale. The importance of this site stems from the fact that it was the first excavation to be opened only for paleontological purposes and not tied to oil-shale extraction or other geological exploration research. The excavation was considerable and had an area of 250 square meters. This led to the discovery of numerous reptiles, including 62 Mixosaurus. In addition to reptiles there were also found about 80 specimens of fishes of the genus Saurichthys that were preyed upon by the Mixosaurs, as is also witnessed by the presence of coprolites. The fossils at this site were not only found only in the oil shale, but also in the dolomite. Here it was mostly ammonites which proved very important for Stratigraphic correlation that has led to the definition of the anisic-ladinian limit.
This is what remains of the site called Quota 902.
STOP 4 - THREE FOUNTAINS ZONE
Continuing to descend along the path, you arrive at the Three Fountains zone. Here lays a mine dug into the Dolomia of S. Salvatore where there are also some intercalations of shale. The mine was opened in 1907 for the industrial purpose of extracting the oil shale for the production of saurolo, an ointment of mineral origin used in the medical and pharmaceutical industries. As a consequence of this, only the shale was used and the dolomite extracted was discarded and piled in stacks of debris, while the shale was carried downstream for processing and the preparation of the ointment. In 1919 Bernhard Peyer went to visit the locations where the saurolo was produced and casually, in a landfill of material lying around, found an Ichthyosaurus paddle. Peyer understood the fossiliferous potential of this area and in 1924 he started the search for fossils in the same tunnels where the shales were being mined. Here he found a Paranothosaurus amsleri about 4 meters long. But he soon realized that it was quite difficult to extract large reptiles intact in the tunnel due to the mode of excavation with explosives that provided mostly only fragmented material. For this reason in 1927 he shifted his search to an open-air area situated alongside the mining excavations. Here he found a two and half meter long Ichthyosaurus as well as various Saurichthys. In 1933 there was then another great discovery, a Tanystropheus longobardicus of over four meters, also known as giraffesaur due to its disproportionately long neck compared to the rest of the body.
Today all that remains of this mine is just a little hole where you can still see part of the entrance Gallery. Everything else is engulfed in vegetation and one barely notices the piles of debris that make up the dump, the remains of a hopper and a few rails that emerge from beneath the foliage and on which the shale wagons for the downstream transport ran.
After this stop, we follow the path down towards the valley through dense vegetation where occasionally very scenic and picturesque views over the lake below open up. On the way down, below the dolomite, the vulcanites appeared, reddish in colour and you realize this also because of the marked change in vegetation, which is much thicker due to the fertile soil.
A scenic view of the Lugano Lake through the vegetation.
The mine entrance illuminated by Prof. Stockar.
STOP 5 – ACQUA DEL GHIFFO
In 1924 the paleontologist Bernhard Peyer bought in Meride from Mr. Gaetano Fossati a specimen of a Neusticosaurus. It's a big reptile that lit up the mind of the researcher: from then on he started to search at Monte San Giorgio for some potential sites that may contain other similar specimens and after he visited several locations, he arrived at the site now called Acqua del Ghiffo. The name is due to the presence of a ferruginous spring. In 1927 Peyer started to dig here next to the entrance of a cave, now closed by a gate to preserve the concretions. Lithologically we are in the lower Meride limestone. Stratigraphically we are just 28 meters below the excavation of Cassina. Here Peyer had the luck to find a number of Neusticosaurus and later also a Ceresiosaurus.
Acqua del Ghiffo actually has two distinct excavation sites separated from each other by a few meters, the higher one being Peyer’s excavation. After Peyer finished his work there was a long pause until 1984 when the task was taken up again for a short duration at the lower site by the University of Zurich. In 1995 the site was reopened and excavations were resumed again following the discovery of a Ceresiosaurus by the Lugano Natural History Museum. They then continued on from 1997 to 2005, finding a good number of fish and also a Neusticosaurus of the same species found also in Cassina. The originals of these finds were recomposed and are now on display at the museums in Lugano and Zurich.
After this stop, we continue on downhill towards Spinirolo, and then begin to mount again and head into the Sceltrich valley.
Upper excavation site in Acqua del Ghiffo. Gate at the cave entrance.
STOP 6 – SCELTRICH
The Sceltrich site is a brand new excavation, opened in 2012. The site is approximately 700 meters above sea level and exposes rocks of limestone of the Upper Meride Formation (240 MA) previously little studied because it was deemed lacking in fossils. The fossil discovered by Prof. Stockar during the 2010 dig campaign was followed up and allowed for the discovery of a dozen fish including Saurichthys. In addition to ichthyofaunal specimens were also found certain plants such as conifers like Voltzia. Lithologically it features very thin slabs, dark in color due to Anoxia that show once again that the paleo environment was a closed basin. The maximum thickness of the layer is 30 cm. In some of the following pictures you can see a sample of Saurichthys discovered the on same morning of our excursion, still partly encased in the rock.
At the end of this stop, we leave Sceltrich Valley heading towards Meride then divert along the road to Serpiano skirting the Gaggiolo River.
In these pictures you can see the Sceltrich site during the 2012 excavation campaign. You can also see the recently found Saurichthys and Prof. Stockar showing a fish extracted in 2010.
Here is the upper Sceltrich dig site. You can also see the Saurichthys found this same morning, still in the rock.
STOP 7 - VAL MARA
Along the Gaggiolo River are outcrops of the summit of the upper Meride limestone formation, known as the Kalkschiefer zone (239.5 MA). The name Kalkschiefer does not derive as much from intercalations of limestone as it might suggest, but from the thickness of the layers. The thickness is so fine that it is not possible to find fossils in section but only on the strata. This level is usually poor in fossils, with only 0.1% organic matter present. However some freshwater crustaceans have been found, symptomatic of a lagoon close to the mainland.
The Kalkschiefer outcrops in Val Mara along the walls eroded by the Gaggiolo River. Site Vecchi Mulini.
Here was opened a first excavation from 1996 to 2003 which was completed by the University of Milan at the site called Vecchi Mulini (Stop 7.1). In 1994 the University of Zurich had already made further excavations where there were extracted some small fishes. Then in 2010 the Museum of Natural History of Lugano opened a new excavation site further downstream along the River, at the summit of the Kalkschiefer zone (Stop 7.2) at the top of the formation. Here they found an extinction event of fishes, but something much more important, they also found the specimen of a small insect of the genus Dasyleptus previously thought to have gone extinct at the end of the Permian. With this new discovery it was possible to realise that it survived the great mass extinction occurring about 251.4 million years ago, which marks the boundary between the Permian and the Triassic. This discovery was published: The first Mesozoic record of the extinct apterygote insect genus Dasyleptus (Insecta: Archaeognatha: Monura: Dasyleptidae) from the Triassic of Monte San Giorgio (Switzerland) - GÜNTER BECHLY & RUDOLF STOCKAR, Palaeodiversity 4: 23–37; Stuttgart 30 December 2011.
For further details you can directly consult this publication for free, following this link:
Just beyond Stop 7.2 along the Gaggiolo River, appears the Pizzella Marl indicating the end of the Kalkschiefer zone and the Ladinian too. The meeting with marls of Carnian age also marks the end of our excursion.
The rocks of the Meride limestone at this point are disappearing from our sight, but nevertheless they continue to be present underground. In fact, this rock formation continues with a slope of about 30 degrees along the Italian territory below the Pianura Padana. About sixty miles further toward to south, in the surroundings of Trecate, the Agip company extracts oil through drilling the same rocks situated in a depth of over four and a half kilometers. Pressure and temperature conditions have in fact permitted here the development of hydrocarbon, unlike Val Mara where the strata are exposed to the open air.
After nine hours of walking, the queue made by curious paleontophiles has been reduced to less than a third. The last diehards, albeit tired, head satisfied and scientifically richer to the car park where their adventure had begun. We would like to thank Prof. Rudolf Stockar for his politeness, helpfulness and patience during this long, fruitful and interesting excursion day.