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Vengono descritti gli aspetti geologici, paleontologici, mineralogici e naturalistici dellìarea di Pramosio-Avostanis nelle Alpi Carniche

Guidebook of the field trip in the Carnic Alps

Conodont biostratigraphic and taxonomic analysis of the Malpasso section were presented by Perri & Spalletta (1991, 1998). The abundant ammonoid fauna was studied by Korn & House (1997) and Korn (1998). In Spalletta & Perri (2001) the section was proposed as possible candidate for the subdivision of the Famennian. The upper part of the section was re-sampled for conodonts and geochemical analysis by Kaiser (2005). The geochemical and conodont data have been reported also in Kaiser et al. (2008, 2009). According to Perri & Spalletta (1998) and Kaiser et al. (2009) the conodont data from the Malpasso section are perfectly correlatable with the ammonoid data of Korn (1998). ). Hartenfels & Becker (2009) on the base of the identification of a typical UD V-A1 ammonoid assemblage, including Endosiphonites sp., Kosmoclymenia lamellosa (Wedekin), Nanoclymenia sp., Nodosoclymenia sp., Rectoclymenia cf. disciforme (Schindewolf), Rect. lineare (Münster), and Procymaclymenia pudica (Czarnocki), and the opportunistic bivalve of eutrophic facies Buchiola in bed ML6k, inferred the position of the Dasberg Crisis Interval between bed ML6k and ML7. The Malpasso section is one of the reference sections of the Pal Grande Formation, showing the typical features of the formation constituted mainly of grey mudstone and wackestone here particularly rich in ammonoids (Spalletta et al., 2015).

The Pramosio area was studied by Michele Gortani since the beginning of the 20th century. The Pramosio Bassa section was briefly described by Perri et al. (1998) and Spalletta & Perri (2001). Mossoni (2014) studied magnetic susceptibility, magnetic hysteresis and major elements content of the section. The results of this study fit very well with the palaegeographic position of the Carnic microplate during the Late Devonian hypothesized by Schönlaub (1993) and von Raumer & Stampfli (2008). According to these Authors the Carnic microplate was located far away from land. Mossoni (2014) demonstrated that during the limestone deposition the detrital input was scarce, this is consistent also with the transgressive event considered as triggering cause of the Annulata radiation. All the studied parameters show values consistent with the identification of the Upper Annulata Event in correspondence of the level of sample PB5a (Mossoni 2014). In the Carnic basin a general transgressive tendency from the Lower marginifera Zone to the base of the Lower postera Zone (Palmatolepis marginifera marginifera–Polygnathus styriacus zones of Spalletta et al. 2017) was inferred by Perri & Spalletta (2000) based on conodont biofacies analysis. Hartenfels & Becker (2016) report that the level yielding a mass occurrence of Prionoceras (bed of sample PB5a) can be considered as an equivalent of the Wagnerbank of Thuringia. According to Hartenfels & Becker (2016) the goniatite-rich limestone of the Wagnerbank represents a regressive phase in the highest part of the annulata Zone (UD IV-A).

An updated stratigraphic scheme of the Pre-Variscan sequence of the Carnic Alps is proposed herein. Lithostratigraphic units have been carefully revised and homo- genized in each investigated time slice. Recognized units have been formalized accordingly. The data on the age of the boundaries have been improved as well. The succession spans the Middle Ordovician - early Pennsylvanian interval and has been formally divided in 36 formations, providing a sound reference for future geological studies in this part of the Carnic Alps and enabling a more global correlation with other paleogeographical domains

The Creta di Collina Formation consists of different interbedded facies. The stratotype section is composed of brachiopod-crinoidal calcarenite and calcilutite (mainly packstone, biopelsparite) with some breccias layers (rudite), brachiopod coquinite with calcilutite (packstone) intercalations, and brachiopod crinoidal calcilutite. At Porto di Cozzi the main units are dark gray calcarenite to calcilutite with scattered brachiopods and crinoid fragments (packstone), brachiopod coquinite, and brachiopod-crinoidal calcarenite (mainly packstone).

Bioclastic wackestone represents the major part of the formation. In the lower part of the formation also iron-coated bioclasts and micritic oncoids occur abundantly. The bedding is mostly obliterated by bioturbation (Schönlaub et al., 2004; Hüneke, 2006, 2007).

The Hoher Trieb Formation consists of five well-bedded facies listed below: Facies A: medium dark gray very thick-bedded rudstone and floatstone; matrix consists mainly of grainstone; silicified corals are common among the clasts; Facies B: medium dark gray thin to thick-bedded pack-/grainstone locally showing evidence of tractive structures; Facies C: medium dark gray thin to medium-bedded wackestone; Facies D: black thin to medium-bedded laminated cherts; cherts are also locally present in nodules mainly within facies C; Facies E: black very thin to thin-bedded laminated shales.

The importance of the Fe- and the S-cycles in generating extreme (acidic) conditions of low pH and high concentration of heavy metals that can, however, sustain an high-level of microbial diversity, show how extremely robust life is, and how rapidly it can adapts to different conditions [1]. In addition, the acidophilic microorganisms are also representing a possible relic metabolism from early Earth’s life. Therefore, the presence of vast deposits of (c. 4 Ga old) Noachian sulfates and iron oxides on Mars, make the acidic, sulphate-rich environments interesting Mars terrestrial analogs. The Dallol hydrothermal field is a remote volcanic area of the northern Danakil Depression in Ethiopia. It hosts spectacular (active and inactive) mounds, terraces, metal crusts and concretions resulting from interactions between sulphuric hot springs, salty solutions, and recrystallization processes driven by hydrothermal waters, degassing, and rapid evaporation. Surrounded by a wide saline region, Dallol area is one of the hottest and most acidic places (pH < 1.0) of the globe that can potentially host life. We present preliminary field and analytical results obtained on samples collected during recent field campaigns. The geological, geochemical and mineralogical features of the Dallol and its location closed to regional basaltic volcanism of planetary-scale importance make it a suitable analogue to ancient Martian environments. It also provides contextual perspective for the interpretation of future Martian rover data. The geomicrobiological and astrobiological potential of the site will be also discussed.

The Pal Grande Formation consists mainly of gray mudstone and wackestone rich in fossils. The original bedding is often concealed by pressure solution and/or burrowing giving to the limestone a characteristic pseudo-nodular look. The reddish colour, when present, is usually limited to the lower part of the formation. At places, in the basal part of the formation, interlayered cm to dm calciruditic and calcarenitic (grainstone) intervals are present, as well as ruditic (intraclast breccia) levels. The ruditic beds can reach thickness of 1-2 m. Black shale, intepreted as equivalent to the Hangenberg Shale (Hangenberg Event) are present only at two localities (Kronhof Graben and Plan di Zermula) (Perri & Spalletta, 2001).

The Zollner Formation consists mainly of radiolarian chert with interbedded rare limestone and black siliceous shale. The limestones are represented by centimetric/decimetric lenses and rare layers of wackestone/mudstone; present along the whole unit. The black shales are more abundant in the lower part of the formation.

The Rauchkofel Formation mainly consists of dark gray to black laminated and well-bedded, partly platy limestones intercalated with black shales and marls of various thickness. Lateral variations are common. Thickness of beds varies from very thin to medium for limestones. Black graptolitic shales with graptolites are more abundant in the lower part of the unit, where they may constitute levels up to 15 cm thick. In the proximal parts of the basin thick to very thick calcarenitic beds and breccias occur more frequently in the upper part of the unit, but in places they may occur also in the lowermost part. A distinct horizon with loboliths is present in the lowermost part of the formation.

The Kellerwand Formation consists of four well-bedded facies listed in order of decreasing abundance. Facies A: medium dark gray, thin to medium-bedded, wackestone to packstone with moderate yellow thin laminae of silt; Facies B: medium dark gray, thin to thick bedded, often fining upward, moderately to well sorted and locally laminated grainstone (from very coarse up to granules to fine-grained) to packstone; Facies C: medium light gray, medium to thick bedded, moderately to well sorted crinoidal-bearing grainstone (biosparite); Facies D: medium dark gray, thick bedded, poorly sorted coral-bearing rudstone with clasts up to about 10 cm of diameter and poorly sorted very coarse to fine grainstone matrix.

Two continuous carbonate sequences across the Frasnian–Famennian boundary in the Carnic Alps (Italy and Austria) have been studied intensively. According to conodont data, the sampled part of the Pramosio A section (Italy), ranges from Frasnian Zone 12 to Upper trachytera Zone. Around the F–F boundary this sequence, which is easily accessible for sampling, reflects a rather high sedimentation rate (centimetric scale per thousand years) in facies typical of an open shallow marine environment (shelf–shore) with a well ventilated bottom. The Freikofel T section (Austria–Italy border), ranging from Frasnian Zone 13a to Middle crepida Zone, is characterized by shallow-water to pelagic facies. Oxygen isotopes—measured on whole-rock and on conodont apatite—display conspicuous variations up-sequence conceivably corresponding to shortand medium-term climate changes.

The Spinotti Formation can be subdivided into three units: Unit 1, a lower massive stromatoporoid and crinoid-rich bioclastic limestone (from the onset of massive limestone below the ladder at 1880 m to elevation 1970 m at Sentiero Spinotti; elevation 1840-1860 m at Seekopf base), followed by Unit 2, a thick-bedded to massive light gray grainstone to rudstone. Beds are two to three meters thick with thin (25-30 cm) dolomitic interbeds. Higher up section the bed thickness decreases to 0.5-1 m and dolomite beds become more prevalent. Components are largely peloids and intraclasts comparable to the Eiskar Limestone described by Kreutzer (1990, 1992a). Unit 3 is characterized by bedded limestone with well-developed birdseye structures from elevation 2020 m to 2200 m. Dark limestones with Amphipora become more common in this upper part. Above the trail at 2120 m are bedded limestones with Stringocephalus exposed. This unit also contains many gastropods and amphipores. Bandel (1972: 30) observed stringocephalids in the area called “dolines” beyond Costone Stella at elevation 2180 m. These cannot be correlated with the ones at 2120 m and must belong to a higher stratigraphic unit, suggesting that the Eifelian-Givetian boundary is located approximately in the middle of the section.

The Findenig Formation consists of red to pink nodular centimetric thick beds of mudstone/wackestone, with interlayered millimetric to centimetric red marls, and, at places, some centimetric to decimetric calcarenitic (grainstone) intervals (allodapic layers) and breccia (calcirudite) beds in the upper part of the unit.

Massive gray limestone with abundant bioclasts and biogenes or reef-building organisms. Largest biogenes are stromatoporoid colonies and tabulate and colonial rugose corals. Amphipora and Stachyodes are common accessory reef builders.

The Carnic Alps correspond to the non- to low-metamorphic portion of the Variscan substratum of the Southern Alps, and display an Upper Ordovician to Upper Carboniferous succession that represents the best preserved example of Variscan succession within the Alps. The Pragian to Frasnian stratigraphic interval is characterized by the differentiation of the basin in shallow water, including carbonate buildups, and pelagic parts, and the so-called ‘transitional facies’ represent a sort of ‘connection’ between these two parts. We analyzed the depositional evolution of the basin through these facies because they 1. reflect well the depositional dynamics; 2. are datable using conodonts, unlike the shallow water units; 3. are less subject to tectonic elisions/repetitions than the basinal units. We performed geological mapping, measured stratigraphic sections of all the units and dated by conodonts all the lithostratigraphic transitions, in order to infer the lateral correlations throughout the basin, also supported by some marker beds/levels. The depositional environments of the different units have been recognized. The main aspects of the basin evolution can be summarized as follows. A transgression between the uppermost Lochkovian and the basal Pragian drowned the first patch reefs recorded in the Carnic Alps. The margin was ramp-type until the Emsian with patch reefs and shoreface deposits distally passing to tempestite and pelagic sediments. During the Emsian, a by pass slope margin separating the shallow water and the basin started to develop as shown by the increasing amount of gravitative driven deposits that persisted until the lower Famennian. In the lower Givetian the gravitative-driven deposits show their maximum progradation on top of the basinal deposits. From the Frasnian, a transgression, possibly enhanced by extensional or transtensional tectonic activity, led to progressively drowning of the carbonate buildups. The depositional evolution appears to follow the supra-regional eustatic trends until the Frasnian. From the Frasnian, the evolution differentiates from the eustatic supra-regional trends, suggesting the importance of a local tectonic control.

The Plotta Formation is mainly composed of white, grayish or blackish porous chert. Locally, at its base, a thin breccia layer is developed. It is composed by small subrounded limestone clasts, angular relict chert fragments and dark siliceous crust.