Discussion on the article "Remarks on the Permian-Triassic transition in Central and Eastern Lombardy (Southern Alps, Italy)" by G. Cassinis, M. Durand and A. Ronchi.
In their paper "Remarks on the Permian-Triassic transition in Central and Eastern Lombardy (Southern Alps, Italy)", Cassinis et al. (2007) propose regional correlations in the Southalpine area (Northern Italy) around the P/Tr boundary. Their reconstructions are based on the integration of paleontological, stratigraphic, and sedimentological data from published works and original analyses on the studied succession, spanning from the Early Permian (Cisuralian) to the Olenekian. The authors attempt to integrate their observations in a coherent scenario, but their conclusions are largelly biased by the incorrect preconception that climatically-controlled sedimentary features should be necessarily synchronous, even if in contrast with biostratigraphic evidence.
Specifically, the occurrence of ventifacts (windworn pebbles, interpreted as markers of arid climate) is not only assigned an undemonstrated chronostratigraphic significance, but even claimed to merit predominance on the occurrence of the cosmopolitan marker Claraia (Valsassina, Orobic Alps, Posenato et al., 1996). The Claraia horizon of the Servino Formation, representing the Lower Triassic in the western Southern Alps (Sciunnach, 2005), is thus arbitrarily shifted upwards in the stratigraphic chart, to comply with preconceived ideas based on equivocal sedimentological evidence.
The Valsassina Claraia horizon is located in the middle Servino Formation about 7 m above the basal Prato Solaro Member (Posenato et al., 1996; Sciunnach et al., 1996, 1999), which contains the ventifacts of Cassinis et al. (2007). The fossiliferous horizon only yields the bivalve Claraia, an opportunistic crisis progenitor taxon (sensu Kauffman and Harries, 1996), which is characterised here, as everywhere, by a wide range of morphological variability. The following morpho-species have been recognized: Claraia intermedia (Bittner), C. radialis (Leonardi), C. tesidea (Leonardi), C. cf. bittneri (Ichikawa), C. cf. clarai (Emmrich) and C. cf. aurita (Hauer). But for the latter, all these taxa have been classified, applying a species-population concept, into C. intermedia (Posenato et al., 1996), a species belonging to the Claraia stachei group (Ichikawa, 1958), which is characterised by well developed radial ornamentation. Specimens of this group have been reported in the Claraia clarai subzone of Dolomites (Broglio Loriga et al., 1983, p. 559), while Claraia intermedia and C. aurita dominate the Claraia beds located between the C. clarai and C. aurita subzones of the Dolomites (Leonardi, 1960). Therefore the Claraia intermedia horizon of Valsassina was correlated to the transitional beds between the C. clarai and C. aurita subzone (sensu Broglio Loriga et al., 1983; 1990) of the Dolomites (Posenato et al., 1996), which range in age from late Griesbachian to Dienerian (Broglio Loriga et al, 1983, 1990).
Cassinis et al. (2007), adducing the high morphological variability of Claraia, disregard its chronostratigraphical value and cite some old papers where C. claraia and C. aurita were also quoted in Smithian units of the Southern Alps (e.g. Yin, 1985). Such younger position, however, has never been demonstrated, and all recent papers report a Claraia range in the western Tethys (Dolomites and Hungary) restricted to the Induan Mazzin and middle Siusi members of the Werfen Formation (Broglio Loriga et al, 1983, 1986, 1990; Neri, 2007) and lower-middle Aracs Marl and Alcsutdoboz formations of the Transdanubia (Hungary) (Kozur, 1987; Broglio Loriga et al., 1990). In the Dolomites, this dating is constrained by conodont biostratigraphy (e.g. Perri, 1991), magnetostratigraphy (Scholger et al., 2000), and chemostratigraphy (Horacek et al., 2007a). The Induan age of the Claraia beds has been recently also re-proposed in Iran (Horacek et al., 2007b) and South China (Tong and Yin, 2002; Zhao et al., 2007), where the genus appears in the late Changhsingian (He et al., 2007). Smithian occurrences of Claraia in the Tethys concern the Claraia decidens (Bittner) group, for which the new genus Crittendenia has been proposed (Newell and Boyd, 1995), or species with only dense, concentric lines (e.g. Claraia hubeiensis Chen: Tong et al., 2006, pl. 2, figs. 11-14).
Ventifact-bearing conglomerates of the French Buntsandstein have "pratically no element for direct dating" and the "correlation between NE France and northern Germany ... are well still debated", although the Volpriehausen Formation of the Germanic basin is "the best candidate" (Durand, 2006, p. 290) to record the arid phase which "may correlate to the late Dienerian to early Smithian interval" (Durand, 2006, p. 291). However, a younger dating for the Volpriehausen Formation is known in literature, e.g. a middle-late Smithian age has been proposed by Kozur and Bachmann (2005). In the Germanic basin, another, older arid episode is recorded in the uppermost Nordhausen Fm (now Calvorde Fm) which was correlated to the Andraz Horizon (Werfen Formation) of the Dolomites (Kozur, 1989), a prevailing evaporitic unit located between the Claraia wangi-griesbachi and C. clarai subzones (Broglio Loriga et al., 1990). The Prato Solaro Member may thus be correlated with the Andraz Horizon, consistently with the correlation between the Claraia beds of the Valsassina and the transitional Claria claraia--C. aurita beds (Siusi Member) of the Dolomites proposed by us (Posenato et al., 1996). This part of the Siusi Member records the MFS of the second Scythian sequence (Sc 2) of the Dolomites (Gianolla et al., 1998; Neri, 2007).
In summary, we consider the hypothesis that ventifacts should necessarily occur in a single interval during the Early Triassic, as proposed by Cassinis et al. (2007), as unjustifiable and biased by an unscientific approach. Incidentally, we note that because of incomplete fossil record of central Europe continental successions, and consequently uncertain chronostratigraphic framework, biostratigraphy, magnetostratigraphy, and sequence stratigraphy of continental Buntsandstein (e.g. Kozur, 1993; Kozur and Bachmann, 2005; Szurlies, 2007) were calibrated by marine Lower Triassic successions (e.g. Claraia beds of Western Tethys).
Received: 17/1/08 / Accepted: 23/01/08
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R. Posenato (1), D. Sciunnach (2), E. Garzanti (3)
(1) Dipartimento di Scienze della Terra, Universita di Ferrara, Via Saragat 1, 44100 Ferrara (Italy) firstname.lastname@example.org
(2) Regione Lombardia, Infrastruttura per l'Informazione Territoriale, Via Sassetti 32/2, 20124 Milano (Italy)
(3) Dipartimento di Scienze Geologiche e Geotecnologie, Universita di Milano-Bicocca, Piazza della Scienza 4, 20126 Milano (Italy)
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|Author:||Posenato, R.; Sciunnach, D.; Garzanti, E.|
|Publication:||Journal of Iberian Geology|
|Date:||Jan 1, 2009|
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