A cancelled field excursion: Upper Permian to Middle Triassic brachiopod beds of the Dolomites (Italy)

The Upper Permian marine succession of the Dolomites is represented by the Bellerophon Fm, an overall transgressive sequence, punctuated by some transgressive-regressive cycles. It consists of a lower sulphate-evaporite unit, deposited in a barren basin, and an upper shallow-marine carbonate unit, deposited along a low-energy ramp setting. The older brachiopod occurrence is represented by large sized lingulid shells of Trentingula prinothi Posenato, which has been found in a clayey fine-grained sandstone bed from the upper part of the Val Gardena Sandstone tongue (lower part the Lo 4 sequence; Posenato et al., 2014; Posenato, 2016). The older rhynchonelliform brachiopod-bearing beds (lower Comelicania beds) of the Bellerophon Fm, about 1.5 m thick, contain only the athyridid Comelicania. They are located at about 50 m (maximum flooding surface of the sequence Lo 4) below the top of the Bellerophon Fm and have been correlated to the Changhsingian Clarkina changxingensis Zone. In the Gardena Valley, the lower Comelicania beds contain a rich nautiloid fauna characterized by Tirolonautilus crux (Stache). Comelicania is represented by C. doriphora Merla and C. haueri (Stache), characterized by mid sized shells (about 50 mm wide; Posenato and Prinoth, 2004; Posenato, 2010). The middle Comelicania beds (about 2 m thick) occur at about 15 m below the top of the Bellerophon Fm (maximum flooding surface of Lo 5 sequence). The brachiopods (C. haueri, about 10-12 cm in width) are contained in black bioclastic wackestone with abundant calcareous algae and diversified foraminifer assemblage, which suggest more stable and fully marine conditions with respect to the lower Comelicania beds. The upper Comelicania beds (from 0.4 cm to 1.5 m thick) are contained within the black bioclastic wackestone/grainstone of the Bulla Member (upper Bellerophon Fm), which contains the most diversified foraminifer assemblage of the Upper Permian marine succession. The brachiopod fauna is again dominated by the athyridid Comelicania, which is represented by very large sized shells (up to 15 cm in width) of C. haueri (Stache), C. megalotis (Stache), and C. merlai (Posenato). The latter species, characterized by short wings, is frequent in the uppermost packstone/grainstone (max 10-15 cm thick) of the Bulla Member, where many other brachiopod species first appear (Janiceps peracuta (Stache), J. cadorica (Stache), J. papilio (Stache), J. bipartita (Stache), Comelicothyris recticardinis (Merla), C. laterosulcata Posenato, Ombonia tirolensis (Stache) and Orthothetina ladina (Stache) (Broglio Loriga et al., 1988; Posenato, 1988, 2001, 2010, 2011). The upper Comelicania beds have been referred to the lower H. praeparvus Zone (Farabegoli et al., 2007). The Bulla Mb (Bellerophon Fm) is overlain by the Tesero Mb of Werfen Fm, a thick mixed siliciclastic-carbonate succession, predominantly Lower Triassic in age (approximately 200 - 600 m thick), which records the survival and early recovery phases connected with the end-Permian mass extinction. The Bulla and Tesero members are separated by a barely perceptible erosional surface, which occurrence and interpretation have been deeply discussed in the literature (subaerial erosion and microkastification, leaching by acid rain or acid marine water; Farabegoli et al., 2007; Posenato, 2009; Farabegoli and Perri, 2012). The basal beds, 5 – 20 cm thick, of the Tesero Mb consist of crystalline ooid grainstones, which record the end Permian mass extinction and thermal peaks in the Dolomites (e.g., Farabegoli et al., 2007; Posenato, 2010, Brand et al., 2012). This unit (Ombonia and Orthothetina beds) contains a brachiopod assemblage dominated by the orthotetid Ombonia and Orthothetina. The athyridid Janiceps and Comelicothyris (reworked?) are still present, while Comelicania seems to be disappear (Posenato 2010, 2011). The last occurrence of the rhynchonelliform brachiopods is recorded within marlstone lenses occurring between microbial limestone and stromatolitic beds of the Tesero Mb (H. changhsingensis Zone and lower H. parvus Zone). The brachiopods are common only in the Tesero section, where they disappear at about 2.5 m above the formational base. The brachiopod assemblage is dominated by the ambocoelid Orbicoelia dolomitensis Chen followed, in order of abundance, by the orthotetid Teserina nerii (Posenato) and Ombonia tirolensis (Stache). The athyridids are represented by mm-sized individuals of ?Spirigerella teseroi Chen. Spinomarginifera and Prelissorhynchia are also present, but very rare (e.g., Broglio Loriga et al., 1988; Chen et al., 2006; Posenato 2009, 2010). The brachiopods occurring in the overlying members of Werfen Fm are only represented by lingulids, a disaster taxon recording the early Triassic aftermath of the end-Permian mass extinction. They appear in the Mazzin Mb, few meters above the base of Werfen Fm (8 m at Tesero section, H. parvus Zone). These lingulids (Trentingula mazzinensis Posenato) are characterized by small-sized shells (4-5 mm in length) and occur in great abundance, generally as storm accumulations, throughout the Mazzin Mb. These brachiopods record the Lilliput effect, a survival strategy of skeletal miniaturization related to the end-Permian environmental crisis (e.g., Twitchett, 2007; Posenato et al., 2014, Posenato, 2016). Lingulids are also occasionally present in the overlying members, where they are characterized by larger shells (Trentingula lorigae Posenato), generally double in size with respect to those of the Mazzin Member. The Triassic recovery of the rhynchonelliform brachiopods occurs in the Pelsonian (Anisian) Recoaro / Dont Fms. The most common brachiopod species belong to the following genera: Angustothyris, Coenothyris, Decurtella, Koeveskallina, Mentzelia, Punctospiriferella, and Tetractinella (e.g., Posenato, 2008). Brand, U., Posenato, R., Came, R., Affek, H., Angiolini, L., Azmy, K., Farabegoli, E., 2012. The end - Permian mass extinction: A rapid volcanic CO2 and CH4 – climatic catastrophe. Chem. Geol., 322-323, 121–144 Broglio Loriga C., Neri C., Pasini M., Posenato R., 1988. Marine fossil assemblages from Upper Permian to lowermost Triassic in the western Dolomites (Italy). Mem. Soc. Geol. It., v. 34 (1986), 5–44. Chen Z.-Q., Kaiho K., George A.D., Tong J., 2006. Survival brachiopod faunas of the end-Permian mass extinction from the Southern Alps (Italy) and South China. Geological Magazine 143, 301–327. Farabegoli, E., and Perri M.C, 2012. 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