EN IT

New fossiliferous outcrops in the Lepontine Alps (Val Rezzo and Val Sanagra).

A review of the actual and possible paleontological content in the stratiform facies of the Main Dolomite (Zorzino Limestone). Brief mention of the discovery of a crustacean in the Riva di Solto Shale outcropping at Bene Lario.

Texts: Attilio Selva

La first report refers to a sector of the Rezzo Valley, located upstream of the village of Corrido, near the Gnino Mountains. The first discovery of fossil finds was in 1999, following an inspection carried out by the writer who was driving along the road connecting Corrido to Seghebbia. Having passed the chapel of the Gnino Mountains, where the hydroelectric power plant is under construction, facies stratiforms of the Main Dolomite are observed on the sides of the road, highlighting a different depositional environment. The discovery of the Gnino Mountains allows connections to be made with the previously mentioned finding of Paralepidotus ornatus.

La second discovery, very recent (July 2005), concerns the territory of Menaggio, in the hamlet of Croce, in theRiverbed of the stream that descends from the Castrolla Mountains and down to Paullo. In this valley, at the contact with the Main Dolomite and the lower limit of the Riva di Solto Argillite, outcrops a slab limestone layer blackish-gray in color, probably attributable to the same Argillite from Riva di Solto. Within these rocks the writer reports the Finding of 15 fish, 2 small crustaceans in poor condition and an undetermined vegetable. The fish belong to the genus Parapholidophorus, although further study must be awaited to identify the species. In addition to the fish, additional fossil traces have been recovered that are difficult to catalogue.

The third finding concerns a slope of the Sanagra valley, at the Gottro Mountains, where bituminous facies of the Zorzino Limestone outcrops. The finds consist of. traces of fish scales and teeth, very similar (in the case of the scales) to the remains from the Rezzo Valley. These fossils also need further careful study.

The whole area consists of Dolomite Main, a formation dating from the Upper Triassic (Noricum) that forms the backbone of all the carbonate reliefs between Val Sanagra and Val Rezzo itself. Before analyzing the paleontological content, it is necessary to conduct a review of geological knowledge about the fossiliferous formation studied.

Dolomia Principale and its facies (Zorzino Limestone) are sedimentary rocks dating from the Noricum, a term that identifies an Upper Triassic plateau dated between 225 and 218 million years old.In the Lepontine Alps these rocks outcrop between Val Sanagra and Valsolda, progressively replacing each other in a north-south direction.

The first studies in the Larian area concerning these formations date back to 1880 by Taramelli, who interpreted the Tn2s as Rhaetic deposits, i.e., distinct formations precipitated at a later time than the Tn1s.
A few years later, 1902, Repossi correctly concluded that the two deposits (Tn1 and Tn2) were coeval and definitely belonged to the Noricum. Repossi's study made it possible for the first time to point out the heteropia of the two facies, that is, that Tn1 and Tn2 had originated in the same time interval but in distinct depositional environments.

The different formation environment explains the heterogeneous morphological and stratigraphic structure, in fact usually a formation of local extent originated in the same time interval presents a homogeneous structure. The attribution of the Tn2 deposits to the Noricum was also confirmed in later times (Lehner, 1952; Conti, 1954; Gnaccolini, 1965).
While Tn1s retained the term "Main Dolomite" over time, Tn2s were given different terminologies in relation to the different areas investigated by various researchers, we recall:

Plattenkalk des Haupt Dolomites (Bistram, 1903)
Zoned dolomites (Desio & Venzo, 1954)
Zorzino limestone (Casati, 1964), Tn2 f. 17 (Chiavenna).
Infraretico (Old, 1950)
Araralta Group (Jadoul, 1985)

As already anticipated, this diversity of terms is attributable to the wide distribution of the above-mentioned outcrops (Tn2), characteristic of the Lombard basin, and present not only in the territory described but also in the Bergamo area, the Brescia area and the western shore of Lake Garda.

The study of Tn1s and Tn2s matured in concrete ways in later work (Gianotti, 1984 a; Lualdi & Tannoia, 1985; Cirilli & Tannoia, 1985) that not only provided more detailed descriptions but also elaborated a paleogeography reconstruction.

In order to understand the nature and origin of the heterotypic Main Dolomite (Tn1) and Zorzino Limestone/Main Dolomite (Tn2) formations described here, it is appropriate to make a paleogeographic framing aimed at reconstructing the paleoenvironments where the sediments described originated.

An important reconstruction of the depositional environments of the Italian Triassic is collected in a detailed paper (Stefani et all., 1992) that provides a comprehensive descriptive picture of the carbonate basins and platforms gravitating in the belt of the great prealpine lakes, from Lake Garda to Lake Lugano. The map attached in the research highlights how the Lepontine Alps area constitutes a limited portion of a large "mosaic" system of carbonate basins and platforms. The set of basins and platforms described were part of an even larger system: in fact, in the Upper Triassic (Noricum), the entire Italian territory was similar to a large gulf, considered a small western offshoot of a broad oceanic incisura called Tethys by geologists. According to completed reconstructions, the Tethys was a large oceanic strip that separated the ancient continental agglomeration Eurasia (present-day Europe+Asia) from Africa. The progressive counterclockwise rotation of Africa in the direction of Eurasia subsequently occluded this ancient ocean, causing the orogenesis of the Alpine and pre-Alpine ranges.

Regardless of subsequent events, the analysis of the genesis of the sedimentary rocks (Tn1 and Tn2) of the Lepontine Alps cannot disregard the paleoenvironmental reconstruction of the ancient Tethys gulf in the neighboring sectors of the Lepontine Alps area.
An initial model (of the reconstruction) is based on a series of data collected in the Menaggio valley from work carried out between 1985 and 1991 (Cirilli & Tannoia, 1985; Lualdi & Tannoia, 1985; Gaetani et all, 1987; Stefani et all., 1991; Bertotti, 1991).

In the Norian, the Gulf of the Tethys consisted of a complex of carbonate platforms and islands, interspersed with very deep basins that communicated with the deep oceanic waters of the Tethys located to the east. The term "carbonate platforms" meant extensive marine shallows similar to those found today in the Bahamas, Australia or the Persian Gulf. The platform environment, between 15 and 20 m deep, is constantly subject to the tidal cycle, therefore emerging periodically (Nosotti, 1993). The high productivity of carbonate sediments of biogenic origin dictated that they precipitated in massive accumulations, the result of which culminated in the formation of the limestone-dolomitic rocks that make up the Main Dolomite (Tn1).

It should be mentioned that the earliest claims of a marine environment in the area under consideration are evident with the deposition of the Nobiallo Chalks, which occurred in pre-Noric (Carnic) times. The finding of evaporitic deposits (gypsum intercalations) in the area adjacent to the Sanagra valley (see Negri & Spreafico, 1869; Renevier, 1879 ;see also Bertotti, 1991) confirms that the earliest marine transgressions were characterized by shallow seas, in which significant level variations occur, causing evaporation cycles and consequent precipitation of Gypsum. This situation, although not as extreme, also persisted in the Noricum although deeper basins were beginning to open at this time in connection with tectonic activity (see below).

The tectonic activity present in the vicinity of the carbonate platforms allowed the formation of numerous narrow basins a hundred meters deep. These basins, being located within the platforms and in isolation with ocene waters and currents, created stagnant environments where water tended to become depleted of oxygen and accumulate organic matter from the upper layers. The absence of water exchange, the accumulation of the remains of organisms that inhabited the upper water layers, and the resulting oxygen depletion created the establishment of anoxic conditions. The stagnation of water and likewise the lower energy of water allowed the accumulation of fine-grained sedimentary deposits mixed with high organic load. The diagenesis of these accumulations led to the formation of the bituminous facies of the Zorzino Limestone (Tn2).

This first reconstruction of the depositional environments of the Main Dolomite and Zorzino Limestone is extremely simplified and does not correspond to the conditions actually present. To make this clear, it is sufficient to recall that the carbonate platforms themselves did not maintain a constant bathymetric profile; in fact, extensive shallow areas were interspersed with lagoons or periodically emerging sectors.

Another non-negligible factor emerged in the study of the Sasso San Martino platform, the Piano-Porlezza area (Cirilli & Tannoia, 1985; Lualdi & Tannoia, 1985) and the Sasso di Mont in Valsolda (Gaetani et all., 1987). In this body of work, a series of Sedimentological facies originating from the accumulation of deposits in the connecting zones between carbonate platforms and deep basins described. The studies, concentrated at Sasso San Martino and Sasso di Mont, showed lithofacies at Megabrecce, Pebbly mudstones, Slump deposits, Carbonate turbidites, Microtorbidites, and Blackish Mudstones. These lithofacies show an articulated morphology of transition between platforms and basins, which is more evident in the western sector (Valsolda) due to the greater basin depths, imputed to the tectonic activity of the Lugano fault (Gaetani et all, 1987).

Ultimately, geologic investigations of the Lepontine Alps show that the area was the site of a basin with a predominantly north-south trend and bounded by the platform areas currently represented by the Dolomia Principale of Sasso San Martino (to the east) and the Valsolda (to the west). The platforms, with their own active margin, sloped into an escarpment (Valsolda) that continued into a slope (Val Solda-middle Val Sanagra-Monti di Croce, Menaggio) at the base of which was the proximal basin (Valsolda-lower Val Sanagra) and the basin plain (Piano-Porlezza).

I findings of the Gnino Mountains consist of five specimens: 3 small fish and isolated fragments of scales.
One of the fish seems attributable to the genus Thoracopterus, which has already been identified in the Bergamo area (Cene, Val Seriana).

The findings from Val Sanagra, come from the Gottro Mountains and consist of some scales very similar to those found in Val Rezzo; associated with the scales are some isolated teeth.

Fossils from the Castrolla Mountains include. 7 fish belonging to the genus Parapholidophorus, while 1 specimen, lacking the cephalic region, is of doubtful attribution. In addition to the fishes, some finds of difficult identification are reported.

All of the artifacts mentioned (Val Rezzo, Castrolla Mountains, and Val Sanagra) are preserved at the Val Sanagra Naturalistic Ethnographic Museum.