Palaeoenvironmental significance of organic facies and stable isotope signatures: the Ladinian San Giorgio Dolomite and Meride Limestone of Monte San Giorgio (Switzerland, WHL UNESCO)

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 accumulation 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 dysoxic 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.

New holostean fishes (Actinopterygii: Neopterygii) from the Middle Triassic of the Monte San Giorgio (Canton Ticino, Switzerland)

“…In particular, biotas of latest Anisian—early Ladinian age are not known in the otherwise comparably rich Triassic successions of China ( Benton et al , 2013 ; Tintori & Felber , 2015 ; Sun et al , 2016 ). Furthermore, the detailed geological information available for the Middle Triassic sequence of the Monte San Giorgio allows the placement of these fishes into a precise stratigraphic and palaeoenvironmental context ( Rieber , 1973a ; Bernasconi , 1994 ; Furrer , 1995 ; Röhl et al , 2001 ; Brack et al , 2005 ; Stockar , 2010 ; Stockar, Baumgartner & Condon , 2012 ; Stockar et al , 2013 ). Therefore, neither the Chinese nor the Alpine Triassic faunas overcome the value of the others (as awkwardly stated by Tintori & Felber , 2015 ), but they complement very efficiently.…”

“…Along with sauropterygian remains ( Stockar & Renesto , 2011 ), land plant remains ( Stockar & Kustatscher , 2010 ) and quasi-anaerobic foraminiferal faunas ( Stockar , 2010 ) the new excavations brought to light many complete and well-preserved specimens of several actinopterygian fishes, including the taxa Saurichthys , Besania , Eosemionotus , Peltopleurus , Eoeugnathus ( Renesto & Stockar , 2009 ; Stockar , 2010 ; R Stockar, 2014, unpublished data), and the new basal neopterygian genus described herein. Coupled with widespread oxygen-depletion excluding benthic scavengers, rapid coating of skeletons by benthic bacterial mats and high sedimentation rate played the key role in protecting the carcasses from decay and in holding skeletal elements together ( Stockar et al , 2013 ; Beardmore & Furrer , 2016a ; Beardmore & Furrer , 2016b ). The top of the lower Meride Limestone is defined by a very discontinuous dolostone horizon (“Dolomitband”; Frauenfelder , 1916 ) resulting from late diagenetic dolomitization cutting across the stratification of the Meride Limestone ( Stockar et al , 2013 ).…”

“…Coupled with widespread oxygen-depletion excluding benthic scavengers, rapid coating of skeletons by benthic bacterial mats and high sedimentation rate played the key role in protecting the carcasses from decay and in holding skeletal elements together ( Stockar et al , 2013 ; Beardmore & Furrer , 2016a ; Beardmore & Furrer , 2016b ). The top of the lower Meride Limestone is defined by a very discontinuous dolostone horizon (“Dolomitband”; Frauenfelder , 1916 ) resulting from late diagenetic dolomitization cutting across the stratification of the Meride Limestone ( Stockar et al , 2013 ).…”

“…The overlying upper Meride Limestone is a sequence of alternating well-bedded micritic limestone and marlstone. Its lowermost part includes a newly discovered fossil vertebrate level, the Sceltrich beds ( Stockar et al , 2013 ; Stockar & Garassino , 2013 ). After a first exploration in 2010 yielding the first fossils from this horizon, two small bed by bed excavations on a surface of around 6 and 10 square meters, respectively, were started in 2012 by the Museo Cantonale di Storia Naturale (Lugano).…”

Alpine sedimentology: An introduction and tribute to Albert Matter and Daniel Bernoulli

“…In this issue, Stockar et al . () describe Ladinian hemipelagic carbonates preserved in the region of Monte San Giorgio (south Switzerland) with regard to their organic‐matter content, sedimentology, and carbon and oxygen isotopic records. These authors suggest relations between sea‐level change, runoff, productivity, microbial activity, carbonate deposition and organic‐matter preservation.…”