The Ladinian San Giorgio Dolomite and Meride Limestone of Monte San Giorgio (Switzerland, WHL UNESCO)
Total Page:16
File Type:pdf, Size:1020Kb
Sedimentology (2013) 60, 239–269 doi: 10.1111/sed.12021 Palaeoenvironmental significance of organic facies and stable isotope signatures: the Ladinian San Giorgio Dolomite and Meride Limestone of Monte San Giorgio (Switzerland, WHL UNESCO) RUDOLF STOCKAR*†, THIERRY ADATTE*, PETER O. BAUMGARTNER* and € KARL B. FOLLMI* *Institut des sciences de la Terre, Universite de Lausanne, Batiment^ Geopolis, 1015, Lausanne, Switzerland †Museo Cantonale di Storia Naturale, Viale Cattaneo 4, 6900, Lugano, Switzerland (E-mail: rudolf. [email protected]) Associate Editor – Helmut Weissert ABSTRACT The over 600 m thick Ladinian carbonate section of Monte San Giorgio (World Heritage List, United Nations Educational, Scientific and Cultural Organization, Switzerland), including the San Giorgio Dolomite and the Meride Limestone, was analysed with respect to its sedimentology, organic- matter content (Rock-Eval and palynofacies) and stable carbon and oxygen- isotope composition. Application of geochemical proxies and optical data (transmitted light microscopy, epifluorescence, cathodoluminescence and scanning electron microscopy) allowed the assessment of the relative sea- level trend and the characterization of the organic-matter content. Three main organic-matter assemblages were defined according to their composition and stratigraphic position. Overall, results suggest immature organic matter, predominantly of marine bacterial origin with an upsection-increasing land plant-derived contribution. Forcing factors controlling organic-matter accu- mulation include changes in sea-level, productivity and runoff which, in turn, were probably promoted by periods of rainfall following explosive volcanic activity. Enhanced productivity during sea-level highstands is considered to have played a key role in black-shale formation under anoxic– sulphidic conditions (mainly in the Besano Formation). In contrast, sea-level lowstands, coupled with intensified runoff, resulted in increased basin restriction and in deposition of laminated limestone, mainly under lower dys- oxic to anoxic conditions (chiefly in the Lower Meride Limestone). Under the latter conditions, benthic microbial activity produced most of the hydrogen- rich organic matter, contributed to carbonate precipitation and also played a major role in taphonomic control on vertebrate fossil preservation. In more general terms, the Monte San Giorgio section proved to be an excellent testing ground, making it possible to compare diverse approaches with each other and, more specifically, to relate optical evidence to geochemical signatures. Keywords Ladinian, microbial carbonates, Monte San Giorgio, organic matter, oxygen and carbon stable isotopes. © 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists 239 240 R. Stockar et al. INTRODUCTION was situated at a northern intertropical latitude of about 15° to 18° (Muttoni et al., 2004) and The Middle Triassic carbonate succession of was strongly influenced by monsoonal circula- Monte San Giorgio (Southern Alps, Switzerland– tion (Preto et al., 2010). This passive continen- Italy; Fig. 1A and B) was inscribed in the tal margin open to the western Neo-Tethys was UNESCO World Heritage List (WHL) because of progressively submerged by a long-term trans- its unique palaeontological value. It is, in gression from the east. The marine ingression particular, world-famous for the exceptionally reached the eastern South-Alpine domain in the well-preserved fossil fishes and marine reptiles Late Permian and the westernmost (i.e. west of (Rieber, 1973a; Kuhn-Schnyder, 1974; Burgin€ Lake Como) South-Alpine domain in late et al., 1989; Sander, 1989; Furrer, 1995; Etter, Anisian times. The increasing differentiation of 2002; Stockar, 2010; Stockar & Renesto, 2011). depositional environments occurring from then The formations bearing the vertebrate fossils onward resulted in a rapidly changing pattern (Besano Formation and Meride Limestone) are of carbonate platforms, locally oxygen-depleted also regarded as the immature equivalents of the intraplatform and open-marine pelagic basins source rocks of one of the most important hydro- (Brack & Rieber, 1993). Coeval volcanic activity carbon petroleum systems of southern Europe produced high volumes of material stored in the (Bernasconi, 1994; Fantoni & Scotti, 2003). This basins as volcanic tuffs (‘Pietra verde’ type includes the Villafortuna-Trecate oil field in the layers). This marginal location of the Monte San western Po Plain, 60 km south of the Monte San Giorgio basin resulted in a peculiar sedimentary Giorgio at a depth of around 5000 m (Riva et al., succession and in at least temporarily dysoxic 1986; Mattavelli & Novelli, 1990; Picotti et al., to anoxic bottom water conditions (Bernasconi, 2007). In order to provide a more comprehensive 1994; Furrer, 1995; Rohl€ et al., 2001; Etter, interpretation of the environmental conditions 2002; Stockar, 2010). The Middle Triassic and depositional processes that prevailed in this succession at Monte San Giorgio (Fig. 1B) starts basin during Ladinian times, the over 600 m with fluvio-deltaic deposits (Bellano Formation, thick Ladinian carbonate section of Monte San Illyrian; Sommaruga et al., 1997) unconformably Giorgio was logged and analysed with respect to overlying a Lower Permian volcanic basement. its sedimentology, organic-matter content and The upper Anisian sediments testify to the stable-isotope composition. The main objectives progressive transgression and to the initiation of of this study are to: carbonate platform growth (Lower Salvatore 1 test widely used different analytical meth- Dolomite/Esino Limestone; Zorn, 1971). While ods by comparing results from both the optical in the north and in the east shallow-water sedi- (petrographic and palynofacies) approach [trans- mentation continued during the latest Anisian mitted light, epifluorescence, cathodolumines- and Ladinian, in the Monte San Giorgio area cence (CL), scanning electron microscopy the formation of an intraplatform basin with (SEM)] and geochemical signatures (Rock-Eval restricted circulation resulted in the deposition parameters, d13Candd18O values); of the Besano Formation, the San Giorgio Dolo- 2 determine the composition of preserved mite and the Meride Limestone. The Besano organic matter (OM) throughout the section and Formation (‘Grenzbitumenzone’; Frauenfelder, to highlight the processes involved in its 1916) directly overlies the Lower Salvatore production and preservation; Dolomite and is composed of a 16 m thick alter- 3 characterize palaeoenvironmental changes nation of black shales and laminated dolostone. and to identify the main trends underlying the Its uppermost part includes the Anisian/Ladi- depositional history of the basin; and nian boundary (equivalent to the base of the E. 4 highlight the most significant implications curionii Ammonoid Zone; Brack & Rieber, 1993; for the taphonomic history of the vertebrate Brack et al., 2005). Most of the spectacular ver- fossils. tebrate fossils (reptiles and fishes), together with important index fossils including ammonoids and daonellid bivalves, come from this forma- GEOLOGICAL SETTING tion (e.g. Rieber, 1969, 1973b; Kuhn-Schnyder, 1974; Burgin€ et al., 1989). The Besano Forma- The Monte San Giorgio belongs to the western tion grades upwards into the San Giorgio termination of the Southern Alps (Fig. 1C). In Dolomite and the Meride Limestone, together Middle Triassic times, the South-Alpine domain constituting a 614 m thick sequence in total. © 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 Ladinian organic-rich carbonate rocks of Monte San Giorgio 241 A Carnian Pizzella Marls B E ' ? 7 Lake Lugano 5 ° 8 Kalkschieferzone Lake Lugano Monte San Giorgio 'Site D' Serpiano Upper 2 i m e s t o n M e r i d L 1 45°54' N Ladinian 'Dolomitband' Porto 3 Cassina beds Cava superiore beds Ceresio 4 Val Cava inferiore beds Lower Serrata 5 tuff Besano 6 San Giorgio Dolomite Meride r i a s c T M i d l e Besano Fm. Lower Salvatore Dolomite Switzerland Italy Bellano Fm. Anisian 1 km Rhyolites and associated volcaniclastics Meride Limestone Permian Reptiles Fishes San Giorgio Dolomite Switzerland Ticinosuchus Chondrichthyes 100 m Nothosauria Crossopterygii Besano Formation Protorosauria Actinopterygii Lower 0 m Placodontia (other than Saurichthys) Salvatore Dolomite Italy Ichthyosauria Saurichthys C Periadriatic lineament 50 km Seceda Dolomites Insubric lineament Perledo- Varenna Bagolino Giudicarie lineament Tretto Monte 'Buchenstein' San pelagic sediments Giorgio Recoaro Platform carbonates Po Plain Intra-platform Milano organic-rich sediments Fig. 1. (A) Simplified location map of the Monte San Giorgio showing the Middle Triassic carbonate succession. Numbered circles indicate the location of the partial sections: (1) Val Porina; (2) Valle della Cassina; (3) Val Sceltrich; (4) Val Serrata; (5) Fontana Fredda; (6) Val Mara. (B) Stratigraphic section of the Middle Triassic sediments in the Monte San Giorgio area with the classic fossil-vertebrate levels (modified and updated from Furrer, 1995). (C) Distribution of the lower Ladinian sediments in the Southern Alps (E. curionii Ammonoid Zone). From Brack & Rieber (1993), simplified. © 2012 The Authors. Journal compilation © 2012 International Association of Sedimentologists, Sedimentology, 60, 239–269 242 R. Stockar et al. Fig. 2. Measured composite stratigraphic log of the Ladinian succession at Monte San Giorgio, with