Chronicle of a carbonate platform demise: facies and geometry changes and the roles of climate and sea-level

During the Late Triassic the “Carnian Pluvial Event”, a global episode of climate change that involved increased rainfall and possibly global warming, a major crisis for carbonate producers is observed. In the western Tethys, the change in carbonate production led to a significant modification of depositional geometries of shallow water carbonate bodies. Sedimentary facies analysis coupled with quantitative 3D georeferenced field data acquisition of selected outcrops in the Cortina d’Ampezzo area (Dolomites, North-eastern Italy) reveal that the climatic event began before a drop of sea-level and caused the demise of km-scale microbial-dominated high-relief carbonate platforms that dominated the Dolomites region, and was followed by a period of coexistence of m-scale microbial carbonate mounds interlayered and laterally onlapped by arenaceous skeletal–oolitic grainstones. A subsequent sea-level fall brought to the definitive disappearance of microbialites, and shallow water carbonates switched to ramps dominated by oolitic–bioclastic grainstones. The crisis of high-relief shallow water carbonate platforms generated a geological surface similar to a drowning unconformity, although no transgression occurred. As the high-relief microbial-dominated carbonate platforms switched to gently inclined oolitic–skeletal–siliciclastic ramps, the neighboring basins were rapidly filled. The carbonate depositional systems turnover was associated with an increase in siliciclastic sediment input, in turn triggered by the onset of the Carnian Pluvial Event and only later to a drop of the sea-level. Results show that the evolution of the described carbonate systems of the Dolomites cannot be interpreted in the light of sea level changes only, pointing out that climate changes, and consequent ecological changes in the main carbonate producing biotas, can induce significant modifications in depositional geometries. The climatic change to more humid conditions precedes a sea-level drop in this Carnian case study.