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

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 anoxicsulphidic 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.