The Permo-Triassic Passive Margin
When, toward the end of the Middle Permian, megashear movements vanished, most of the present Italian territory was in a rather simple paleogeographic setting. The emergent remnants of the Variscan Orogen were lying roughly to the north and west, while a wide, passive margin was opening toward the SE facing a Tethyan seaway. To name this seaway is a matter of discussion, but I think the best choice is to call it Palaeo-Tethys. At the foot of mountains and hills, still under erosion, the landscape setting was with low gradients. There was a passage from the alluvial braided river plain, gradually interfingering with coastal marine, often evaporitic sediments, to more open marine, but never in very deep situations. This general panorama lasted for at least 30 My, from the latest Middle Permian to the beginning of the Middle Triassic. Minor tectonic movements affected this picture locally. Different settings, in different geodynamic scenarios, developed in western Sicily and in the Lagonegro area in Basilicata. They will be discussed separately.
The wide, passive margin
The preserved outcrops are in the Southern Alps where remnants of the passive margin extend over several hundreds of km (Fig. 7). Elsewhere, the Austroalpine nappes or the Northern or Central Apennines were still sites of erosion during this timelag and usually were reached later by sedimentation. Only locally thin veneers of sediments are preserved, as in Tuscany or Sardinia.
Figure 7. Wordian map.
Palaeogeographic map of the Wordian (Middle Permian) (from Dercourt et al., 2000, reduced and modified).
Toward the very end of the Middle Permian and at the beginning of the Late Permian, erosion of the Variscan mountains provided clastic material feeding the wide fan of the Verrucano Lombardo (deposed mostly in a braided river setting) vs. Arenarie di Val Gardena (deposed mostly as meander alluvial plain). The original reconstruction of Assereto et al. (1973) is still valid with minor improvements (Cassinis et al., 2008) (Figs. 8, 9). The climate was monsoon-like, with a wet season allowing river drainage and a dry season with marked oxic conditions; hence, the typical reddish colour of sediments. This caused sediments barren of fossils, but there are a few places where for short periods, marine-life ingression is mixed with the alluvial plain sediments. Tetrapod tracks and footprints may be locally common (Avanzini et al., 2008).
Figure 8. The Early Triassic transgression.
Paleogeologic map of the basal surface on which the Lower Triassic sediments transgressed westwards (top). The progressive encroachment of the marine sediments on the wide continental margin from the latest Middle Permian to Early Triassic (bottom). (From Assereto et al., 1973, redrawn and modified).
Figure 9. W-E transect for Permian to Lower Triassic.
Updated interpretation of the structural pattern accommodating the Permian to Lower Triassic sediments and volcanics along the same alignment of the Assereto et al. (1973) original scheme, here reported in Fig. 8. (From Perotti, 2010).
During the passive margin stage, several sedimentary sequences are superposed with the common feature that the next sequence aggrades inland more than the previous, usually for some tens of km. Therefore, while the passage from Arenarie di Val Gardena to Verrucano is roughly at the Adige Valley, the evaporitic to marginal sediments of the Formazione a Bellerophon arrive to the west of it, for at least 20 km. And eventually the Werfen/Servino cycles may arrive with marine facies until central Lombardy.
The marine strata are mostly represented by the Bellerophon Fm. It has been classically subdivided into two parts (Massari et al, 1988; Massari and Neri, 1997). In the lower part, “the encroachment of the transgression resulted in fluvial to terminal-fan settings, still dominating in the westernmost areas, graded first into coastal sabkhas and then into a hyperhaline lagoon in which a subtidal evaporitic complex was laid down” (Massari and Neri, 1997).
In the upper part, a low-gradient, homoclinal carbonate ramp replaced the previous evaporitic basin.
Deposition took place in a low-energy, low-gradient ramp without large-scale circulation and hence, without open marine biota. For an exhaustive sedimentological analysis of the couple Val Gardena Sandstone/Bellerophon Fm., refer to Massari and Neri (1997).
The topmost part of the Bellerophon Fm., along with its passage to the basal layers of the overlying Werfen Fm., has been actively studied in the last 25 years because it contains the Permian/Triassic boundary. The last few meters of the Bellerophon Fm. contains a fully marine fauna and flora, even if still in proximal shelfal conditions. The sections of Tesero and Bulla have been considered as auxiliary sections for the global definition of the boundary. Sedimentology (Farabegoli et al., 2007), sequence stratigraphy (Neri and Stefani, 1998), chemiostratigraphy (Kearsey et al., 2009), magnetostratigaphy (Scholger et al., 2000), marine biota (Posenato, 2009) including foraminifera (Groves et al. 2007), conodonts (Farabegoli et al., 2007), ostracods (Crasquin et al, 2008), and brachiopods (Posenato, 1998, 2001; Chen, 2006) were studied in detail (Fig. 10).
Figure 10. The Permo/Triassic boundary interval.
The Permo/Triassic boundary interval has been studied in much detail in the western Dolomites, where auxiliary section for the GSSP has been described (from Farabegoli et al., 2007, modified).
The Werfen/Servino lithosomes form a complex pattern of mixed silicoclastic/carbonate sediments, deposed on the structural platform with a fairly high subsidence rate because especially the Werfen Fm. may reach 100m/My of non-decompacted sediments. Since 1960, a number of subunits have been proposed, now reaching a total of nine (Broglio Loriga et al., 1982; De Zanche et al., 1993) (Fig. 11). They depict a complex pattern of shallow-water carbonate and fine siliciclastics. Noteworthy is the abundance of oolitic grainstone, linked to the prevailing inorganic precipitation of carbonates when the biotic crisis at the P/T boundary reduced the organic ability to fix the carbonates.
Figure 11. Internal subdivisions of the Werfen Formation.
The subdivisions of the Werfen Formation in Dolomites and Carnia according to De Zanche et al. (1993), with sequential interpretation.
The environment interpretation identifies a wide tidal flat on which a shallow-water shelf ingresses with hybrid sedimentation, carbonatic and distal silicoclastic. Locally and prevailing in the Tesero and Gastropod Oolite mbs, there are shoals with oolitic bars.
The good exposures of the Dolomites, and in minor extent in Carnia, are not replicated in Lombardy. Here, the rocks of the Lower Triassic are often disrupted at the base of the thrust sheets and their thickness is much reduced and internal gaps are more frequent. Sciunnach et al. (1999) (Fig. 12) made an attempt to extend the subdivisions of the Dolomites to overcome the traditional subdivision of the Servino Fm., which was separating a lower part, mostly silicoclastic, from an upper part, mostly carbonatic. The Servino is a traditional name originating from the mining terminology (Brocchi, 1808; Sciunnach in Cita et al., 2007). Due to its content rich in barite and siderite it has been mined for iron since Roman times.
Figure 12. Subdivision of the Servino Fm. in Lombardian Alps.
Subdivision of the Servino Fm. in Lombardian Alps with correlation to the Werfen subdivision in Dolomites and Carnia (from Sciunnach et al., 1999).
The Permian of Sicily
For more than a century, carbonate blocks have been known in the valley of the River Sosio in Western Sicily, containing a very rich and well-preserved macro and microfauna of Permian age, mostly Middle Permian (Gemmellaro, 1887, 1888 a,b, 1889, 1890). More recent research has dealt with the palaeontology of conodonts, radiolarians, ostracods, palynomorphs, and stratigraphic and sedimentological interpretations (Mascle, 1979; Catalano et al., 1991, 1992; Flügel et al., 1991; Kozur, 1991a,b; Gullo, 1993; Kozur et al., 1996; Jenny-Deshusses et al., 2000; and Carcione, 2007; among others). Outcrops are highly disrupted, and superficial sliding is frequent in the softest units. The depositional scenario that resulted from these studies is still debated.
The shallow-water carbonate blocks like “Pietra di Salomone” or “Pietra del Passo di Burgio” no doubt contain fossils of Permian age and are considered as resedimented blocks. Similar rocks are known in outcrops at Jebel Tebaga in S. Tunisia (Khessibi, 1985; Angiolini et al., 2008; and ref. therein) and are penetrated by commercial boreholes in Tunisia. The fine siliclastics and the red and grey shales were thought to be autochtonous and thus still Permian, including evidence from the Early, Middle and Late Permian ages. The same should hold true for marls, red shales and cherty, nodular limestone of Lower and Middle Triassic age. A tentative succession from Permian to the base of the Upper Triassic was proposed by Gullo (1993), identifying seven litho units. This interpretation was shared and stressed by Di Stefano and Gullo (1997).
In contrast, there were others who claimed that most of the stuff is to be included in the Lercara Formation of Triassic age (Cirilli et al., 1990). Both olistholits and fine clastics should be reworked and embedded in this unit. Recently, Carcione (2007) in her Ph.D. thesis reached the conclusion that all the rocks with evidence from Permian to Middle Triassic ages are resedimented and should be included in the Lercara Formation, which was deposited during the Middle Triassic/early Carnian. Therefore, in this interpretation, even if there is paleontological evidence for Permian or earliest Triassic deep-water organisms, they all are reworked in the Lercara Fm. However, this interpretation casts doubt. For instance, if some ostracod assemblages should be the product of reworking, how can their very fine ornamentation be preserved in a grain-by-grain reworking (Crasquin et al., 2008)?.
For palaeogeographic reconstructions, autochthonous or reworked have a minor importance. As a matter of fact, since the Middle Permian a fairly deep trench bordered by a shallow-water carbonate platform was open in that area. Because it includes deep-water fauna of Middle and Late Permian age with some Tethyan affinity, the interpretation was advanced that a deep-water branch of the Tethys was already established in that area during the Permian (Catalano et al., 1991).
Similar meaning may have the Abriola shallow-water blocks embedded in the Monte Facito Fm. (Basilicata) (Donzelli & Crescenti, 1970; Ciarapica et al., 1986; Panzanelli Fratoni, 1991). Recently, Passeri and Ciarapica (2010) wrote that “the calcirudites with fusulinids are known only as scattered boulders and pebbles in tectonic mélange and debris. These boulders are interpreted as tectonic shavings coming from mélanges and reworked in the postorogenic debris flow. They are the only witness of the Permian in the Southern Apennines.”