Swiss J Geosci (2010) 103:101–119 DOI 10.1007/s00015-010-0008-2 Facies, depositional environment, and palaeoecology of the Middle Triassic Cassina beds (Meride Limestone, Monte San Giorgio, Switzerland) Rudolf Stockar Received: 18 June 2009 / Accepted: 20 January 2010 / Published online: 28 May 2010 Ó Swiss Geological Society 2010 Abstract The Ladinian Cassina beds belong to the fos- eruptions, respectively. Fossil nekton points to an envi- siliferous levels of the world-famous Middle Triassic ronment with only limited connection to the open sea. Monte San Giorgio Lagersta¨tte (UNESCO World Heritage Terrestrial macroflora remains document the presence of List, Canton Ticino, Southern Alps). Although they are a emerged areas covered with vegetation and probably rich archive for the depositional environment of an located relatively far away. Proliferation of benthic important thanatocoenosis, previous excavations focused microbial mats is inferred on the basis of microfabrics, on vertebrates and particularly on marine reptiles. In 2006, ecological considerations and taphonomic (both biostrati- the Museo Cantonale di Storia Naturale (Lugano) started nomic and diagenetic) features of the new vertebrate finds, a new research project focusing for the first time on whose excellent preservation is ascribed to sealing by microfacies, micropalaeontological, palaeoecological and biofilms. The occurrence of allochthonous elements allows taphonomic analyses. So far, the upper third of the an insight into the shallow-waters of the adjoining time- sequence has been excavated on a surface of around 40 m2, equivalent Salvatore platform. Finally, the available bio- and these new data complete those derived from new stratigraphic data are critically reviewed. vertebrate finds (mainly fishes belonging to Saurichthys, Archaeosemionotus, Eosemionotus and Peltopleurus), Keywords Cassina beds Á Middle Triassic Á allowing a better characterization of the basin. Background Monte San Giorgio Á Facies Á Palaeoecology Á sedimentation on an anoxic to episodically suboxic seafloor Depositional environment resulted in a finely laminated succession of black shales and limestones, bearing a quasi-anaerobic biofacies, which is characterized by a monotypic benthic foraminiferal Introduction and geological setting meiofauna and has been documented for the first time from the whole Monte San Giorgio sequence. Event deposition, The Middle Triassic sequence at Monte San Giorgio testified by turbidites and volcaniclastic layers, is related to (Figs. 1, 2), inscribed in 2003 on the UNESCO World sediment input from basin margins and to distant volcanic Heritage List because of its unique palaeontological value, consists of several distinct formations, overlying a pre-Permian metamorphic basement and a Permian vol- canic succession. The lowermost formation is the Bellano Editorial handling: Daniel Marty & Andreas Wetzel. Formation, a fluvio-deltaic sequence dated to the late Anisian (Illyrian) on the basis of its palynological content R. Stockar (&) Museo Cantonale di Storia Naturale, Viale Cattaneo 4, (Sommaruga et al. 1997). During the late Anisian a pro- 6900 Lugano, Switzerland gressive transgression of a shallow epicontinental sea e-mail: [email protected] from the east took place, in consequence of which car- bonate platforms began to grow (Furrer 2001a) north of R. Stockar Institut de Ge´ologie et Pale´ontologie, Universite´ de Lausanne, an emerged land buried today under the Po Plain (Brusca Anthropole, 1015 Lausanne, Switzerland et al. 1981; Picotti et al. 2007). Dolomitized microbial 102 R. Stockar Fig. 1 Map of the Monte San Giorgio area showing the Middle Triassic carbonate sequence and the location of the excavation site of the Cassina Switzerland beds near Cassina. Scale bar 1km Italy Lake Lugano Monte San Giorgio Serpiano Meride Cassina Limestone 45°54' N San Giorgio Dolomite Porto Ceresio Besano Formation Lower Salvatore Dolomite 50m Poncione d'Arzo Meride Besano Arzo E ' 7 5 ° Cassina beds / locality 8 limestones, characterized by stromatolitic laminations Meride Limestone (Frauenfelder 1916; Furrer 1995). The (Lower Salvatore Dolomite), were deposited in a shallow Besano Formation, the San Giorgio Dolomite and the subtidal to intertidal environment. During the latest Ani- Meride Limestone were deposited in the same basin, sian and Ladinian, a 30–100 m deep intra-platform basin whose east–west extension is estimated to have been with restricted circulation developed in the Monte San about 10 km or even 20 km if they were settled in the Giorgio area (Bernasconi 1994; Furrer 1995), whereas in same basin as the Perledo-Varenna Formation (Gianotti the north shallow-water sedimentation continued (Middle and Tannoia 1988; Bernasconi 1994). As the Meride and Upper Salvatore Dolomites; Zorn 1971). Extensional Limestone is considered to be the source rock of the oil tectonics have been suggested as a controlling factor for of Trecate, Villa Fortuna and Gaggiano fields (Italy; the intra-platform basin (e.g. Zorn 1971; Furrer 1995)but Picotti et al. 2007) the intra-platform basin where it was conclusive evidences are missing in the Monte San deposited extended southwards for over 60 km. The Giorgio/Monte San Salvatore area, where no Triassic Besano Formation and the dasycladalean-rich Middle tectonic structures are recognizable (Bernasconi 1994). In Salvatore Dolomite are roughly coeval, representing a the Monte San Giorgio area, the Besano Formation was basinal and a shallow-water facies, respectively. The San deposited on top of the Lower Salvatore Dolomite Giorgio Dolomite and the Meride Limestone are time- (‘‘Grenzbitumenzone’’; Frauenfelder 1916), and consists equivalent to the Upper Salvatore Dolomite, which shows of an up to 16 m thick alternation of black shales and the same intertidal to shallow subtidal facies as the dolomites, including in its uppermost part the Anisian– Middle Salvatore Dolomite from which it is distinguished Ladinian boundary (Ro¨hl et al. 2001). A volcanic ash by a marker horizon bearing small bivalves (Zorn 1971). layer lying a few metres below this boundary yielded an The Ladinian Lower Meride Limestone, 90 m (Wirz U–Pb minimum age of 241.2 ± 0.8 Ma (Mundil et al. 1945) to 150 m thick (Furrer 1995) consists of well- 1996; Brack et al. 2007). Most of the spectacular verte- bedded limestones and marlstones with very rare biotur- brate fossils (reptiles and fishes) together with important bation structures. As a whole, the Lower Meride index fossils, including ammonoids and daonellid Limestone is interpreted as a sequence of lime mud bivalves, come from this formation (Rieber 1969, 1973; turbidites deposited under anoxic to dysoxic conditions Kuhn-Schnyder 1974;Bu¨rgin 1992). The Besano Forma- (Bernasconi 1994). Three levels (Cava inferiore beds, tion grades upwards into the approximately 60 m thick Cava superiore beds and Cassina beds), consisting of San Giorgio Dolomite followed by the 400–600 m thick finely laminated limestones and black shales with Cassina beds of the Monte San Giorgio 103 Fig. 2 Stratigraphic section of the Triassic sediments in the Monte San Giorgio area Liassic Moltrasio Fm. (modified and updated after Macchia Vecchia / Broccatello Furrer 1995). Vertebrate- Rhaetian Tremona Series bearing beds and index fossils of the Meride Limestone are indicated c i s s Norian a i r T Dolomia Principale e t a L Carnian Pizzella Marls Kalkschieferzone c i s s "Dolomitband" Ladinian a i r Meride Limestone T Cassina beds e l d Cava superiore beds d i Cava inferiore beds M San Giorgio Dolomite Besano Fm. * "Grenzbitumenzone" Lower Salvatore Dolomite Bellano Fm. Anisian n a 100 m i Rhyolithe and associated volcaniclastics m r e P 0 m Age diagnostic fossils Reptiles Fishes Echinitosporites iliacoides (Scheuring 1978) Ticinosuchus Chondrichthyes Nothosauria Crossopterygii Arpadites arpadis (Wirz 1945; Furrer 2001a; Protorosauria Furrer et al. 2008) Actinopterygii Placodontia (other than Saurichthys) Daonella moussoni (Wirz 1945; Schatz 2001, 2005b) Ichthyosauria Saurichthys * 241.2 +/- 0.8 Ma (U-Pb age; Mundil et al. 1996) Scattered/poorly documented occurrences intercalated volcanic ash layers, are present in the upper Upper Meride Limestone is a sequence of alternating part and yield different vertebrate assemblages. Organic well-bedded limestones and marlstones with an increasing carbon content averages around 1% TOC (Picotti et al. clay content towards the top. The uppermost part is the 2007) but it reaches up to 20% in thin black shale lay- 120 m thick ‘‘Kalkschieferzone’’, made up of thin-bedded, ers (Bernasconi 1994). The top of the Lower Meride laminated marlstones and claystones with peculiar fish Limestone is defined by a dolomite marker bed faunas, crustaceans and arthropods. It represents the late (‘‘Dolomitband’’; Frauenfelder 1916), reaching a thickness evolution of the intra-platform basin, recording strong of about 30 m (Wirz 1945; Furrer 1995). The overlying seasonal variations of salinity and water level and 104 R. Stockar progressively buried by an increasing input of siliciclastic Tanystropheus longobardicus by Nosotti (2007). The material (Furrer 1995). associated fish fauna turned out to be dominated by On the Swiss side of Monte San Giorgio, from 1994 to the basal actinopterygian Saurichthys (S. curionii and 2005 a team from the Palaeontological Institute and S. macrocephalus; Rieppel 1985). In addition, smaller Museum of the University of Zurich (PIMUZ), led by actinopterygians ascribed to three different genera were