Lower Permian Palaeoichnology from the Orobic Basin (Northern Italy)

Geo.Alp, Vol. 2, S. 77–90, 2005 LOWER PERMIAN PALAEOICHNOLOGY FROM THE OROBIC BASIN (NORTHERN ITALY) Giuseppe Santi With 4 figures and 2 plates Dipartimento di Scienze della Terra, Via Ferrata 1, 27100 Pavia (Italia). e-mail: [email protected] Abstract The Lower Permian palaeoichnofauna of the South-Alpine region comes from the Collio Formation only, and mainly crops out in the Orobic and Trompia basins. It consists of traces of vertebrates (amphibians and reptiles) and invertebrates (insects, arthropods, burrowing, probable myriapods, gastropods, freshwater jelly- fishes, conchostraceans and freshwater bivalves). It is poor in taxa and similar to the coeval ichnoassociation of Central Europe, N. America and Argentina. Inside the trophic pyramid relevant to the biodiversity of the lower unit of the Collio Fm., the top carnivore is absent. This role is partially occupied by reptiles (e.g. araeos- celids) having features similar to true lizards. Shifting from the sediments of the lower part of the Collio For- mation to the upper part, impoverishment of the ichnocoenosis composition is linked either to a climatic shift (from more humid towards drier), or to tectonic activity that prevented the persistence of biotope formation. The Upper Permian uplift marks a strong ichnofaunistic change with the introduction of Triassic components. Introduction Brief stratigraphical framework In the South-Alpine region the continental For a long time it has been known that the Lower Permian is characterized by vertebrate and Palaeozoic of the Alps is very poor in vertebrate invertebrate ichnofossils and by rare floral remains remains, with the only exception being the ich- (macroplants, spores and pollen). They come nites, which have recently become a great strati- almost exclusively from the Collio Fm. cropping graphical tool (Avanzini et al., 2001). Recent out in the Orobic and Trompia (= Collio) basins, detailed research on the Permian of Europe (main- with the exception of rare fossiliferous remains ly in France and Germany) has enlarged our from the Tregiovo Basin. The features, problems knowledge, particularly of the invertebrates, and and hypotheses relating to the composition, distri- of the stratigraphical-chronological role played by bution and behavioural features of the trackmak- them (Gand et al., 2001 with references therein). ers and the ichnocoenosis variation are also dif- The South-Alpine region is characterized by a ferent, moving from the lower “Collio” to the series of basins created from W to E, as inherited upper unit of the same formation, and from this Hercynian structures have produced structural to the Upper Permian when the faunistic change highs of a metamorphic or igneous nature is profound. These aspects are very clear from (Cassinis & Perotti, 1994; Cassinis et al., 1999 with analysing the fossil beds of the Collio Fm. in the references therein; Perotti, 1999). The main basins Orobic Basin; this article will review the ichnofau- are the Orobic Basin and, to the east, the Trompia nistic assemblages from this basin and discuss Basin (Fig. 1), but other smaller basins are impor- their significance and the problems inherent to tant for their ichnofossil content (Tregiovo Basin, them. Tione Basin) (Conti et al., 1997). 77 Fig. 1: Schematic non-palinspastic section of the main Permian basin distribution of the South-Alpine region (Conti et al., 1997, mod.). 1 – Pre-Permian basement, 2 – Volcanic deposits, 3 – Clastic units of the first cycle of sedimentation (Basal Conglomerate, Collio Fm, Ponteranica Conglomerate), 4 – Verrucano Lombardo-Val Gardena Sandstone complex, 5 – Bellerophon Fm. Permian sediments occur in two tectono-sedi- Gardena Sandstone complex (Fig. 2). With their mentary cycles separated by an uncertain age gap deposition the Palaeozoic ended. (between 14 and 25 My, according to the most recent data in Cassinis et al., 2002a). The first cycle, The question of the use of a two- or three-fold of ?Upper Carboniferous–Lower Permian age, is subdivision of the Permian System for dating of composed of a continental succession of volcanic continental successions has been debated for a deposits (from intermediate to acid chemistry) and long time. A detailed discussion relating to dating by alluvial-to-lacustrine sediments that comprise of the Permian continental beds in the South- the Basal Conglomerate, the Collio Fm, the Alpine region was recently carried out by Cassinis Tregiovo Fm, the Ponteranica Conglomerate, the (2003), Cassinis and Ronchi (2001) and Cassinis et Dosso dei Galli Conglomerate and the Auccia al. (2002b). The traditionally adopted Permian sub- Volcanics. The second cycle is assigned to the division for research in the South-Alpine area is Upper Permian and is composed of the reddish “Lower Permian” (from about the Asselian to clastic deposits of the Verrucano Lombardo-Val Kungurian) and “Upper Permian” (from the 78 Geo.Alp, Vol. 2, 2005 Fig. 2: 1 - Chronostratigraphical sketch of the Permian of the Orobic Basin. 2 - Chronostratigraphical sketch of the Permian of the Trompia Basin. Ufimian to Tatarian, according to the Cis- er referred to the Upper Permian) is evident can Uralian/Russian Standard Scale), and this last rarely the use of the marine stages be justified. For these includes the Middle Permian, corresponding reasons, and in agreement with Cassinis (2003), in approximately to the Guadalupian Series this study the continental Permian “Lower” and (Menning, 2001; Cassinis, 2003, Fig. 1). It is based “Upper” subdivisions are used. on the palaeontological data from macroplants, Therefore, it is Lucas’s opinion (pers. comm.) palynomorphs, tetrapod footprints, and the radio- that in this study the term “Upper Permian” should metric and palaeomagnetic investigations. include the “Middle Permian” (Ufimian and Therefore, the stratigraphical resolution is rather Kazanian), and only the Tatarian should really be poor compared with the marine equivalents; so the “Upper Permian”. As such, it may be better to uti- absence of detailed data and of the wider correla- lize the marine timescale terms (Roadian, Wordian, tions for the continental beds prevents the use of Capitanian, Wuchiapingian, etc.) and not the old the three-fold subdivision of the Permian System Russian terms. The utilized chronostratigraphy into “Lower”, “Middle” and “Upper”. Only in those (Cisuralian and Russian stages) for the Early places where the lateral transition between the Permian represents the international subdivision of continental and marine deposits (i.e. in the the Permian System, but in the dating of the con- Dolomite region between the Val Gardena tinental beds, to leave out the post-Kungurian Sandstone and the Bellerophon Formation, togeth- Russian terms that, in Lucas’s opinion (pers. Geo.Alp, Vol. 2, 2005 79 Fig. 3: Permian stratigraphy (SCPS = Sub-Commission of Permian stratigraphy) (Vachard & Argyriadis, 2002. mod.) comm.), are only the regional stages for the marine cracks, raindrop imprints, ripple marks and fossil timescale, is more difficult for the reasons plant remains, as well as vertebrate and inverte- advanced above. Fig. 3 shows the different scales brate ichnites. of the Permian stratigraphy. This formation is interfingered with the Ponteranica Conglomerate (Casati & Gnaccolini, In the classic succession of the Trompia Valley 1965, 1967). Utilising the fossils collected in the (Collio Basin) the COLLIO FORMATION was deposited on Trompia Basin, the Collio Fm. is referred to the volcaniclastic rocks (ignimbrites) which do not Lower Permian based on chronological data pro- crop out with continuity within the Orobic Basin, vided by macroflora (Geinitz, 1869; Jongmans, but are abundant in other areas (e.g. in the 1960; Remy & Remy, 1978; Kozur, 1981; Visscher et Acquaduro Valley –Introbio- and in the Cedrino al., 1999), pollen (Clement-Westerholf et al., 1974; Pass) (Sciunnach, 2001) and in the mainly “berga- Cassinis & Doubinger, 1991, 1992) and tetrapod mask” sector of the same basin (Jadoul et al., footprints (Ceoloni et al., 1987; Conti et al., 1991, 2000). Other subdivisions of lithofacies have been 1997), and also for its position below the angular proposed on a petrographical basis by Cassinis et unconformity ascribed to the main post-Saalian al. (1988), Cadel et al. (1996), Forcella et al. (2001) phase (Palatine) of the Hercynian orogenesis. and Sciunnach (2001). The Collio Fm. can be infor- mally subdivided into two units: the lower one is composed of grey-green and black sandstones and Vertebrate and invertebrate ichnocoenoses of the siltstones, while the upper unit is defined by main- Orobic Basin ly reddish sandstones and pelites of volcanic ele- ments with quartz, plagioclase and muscovite. It is In Italy, early knowledge of vertebrate foot- well stratified and locally contains some conglom- prints from the Collio Fm. in the Trompia Valley eratic beds. The typical arenaceous zones frequent- was advanced by Geinitz (1869) and Curioni ly contain fragments of black clay (clay chips) and (1870). Later, these fossils were studied by Gümbel display planar lamination, while in the pelitic (1880); the same ichnofauna from the Orobic Basin intervals there are different structures such as mud was analysed by Dozy (1935) and later re-exam- 80 Geo.Alp, Vol. 2, 2005 ined by Haubold (1971). The studies of Berruti enter in the so-called “red-bed ichnofacies” (1968), Haubold (1996, 2000), Haubold & Stapf (defined as a variety of fluvial, deltaic, lacustrine (1998), Casati & Gnaccolini (1967), Ceoloni

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