During the Late Triassic, a climate change known as the Carnian Pluvial Event (CPE) resulted in a major crisis for carbonate producers. In the western Tethys, the change in carbonate production led to a dramatic modification of depositional geometries. The steep clinoforms of the high relief pre-crisis carbonate platforms were replaced by low angle ramp geometries. A quantitative three dimensional modeling of the geometry of sedimentary bodies in the Cortina d'Ampezzo area (North-eastern Italy) formed before, during and after the Carnian crisis, coupled with facies analysis, was performed in order to investigate in detail how the changes in shallow water carbonate precipitation influenced the depositional geometries of carbonate platforms.
Facies and depositional geometries suggest that after the demise of high-relief, microbial dominated carbonate platforms, a phase of intermediate sedimentation took place, in which microbial carbonate mounds and loose carbonate and terrigenous sediments coexisted. The subsequent evolution to a ramp is characterized by the onset of a tide-dominated environment. In the study area, the sedimentary succession developed in a marine strait connecting two small basins. Both ebb and flood paleocurrents are documented and their directions are in agreement with the strait orientation, as inferred from the paleogeographic position of relict high-relief carbonate platforms. Facies and sedimentary structures related to variations in the hydraulic regime have been observed, including planar cross stratification, herringbone cross stratification, flaser to wavy to lenticular bedding and cyclical alternations of mainly oolitic-siliciclastic vs. mainly bioclastic-muddy laminae. The peculiarity of this tidal system is that, differently from all other known cases, it did not develop in a tectonically confined submarine graben or canyon, but in a marine passage between relict high-relief carbonate platforms.
Sedimentary facies analysis, coupled with geological three dimensional modeling, led also to constrain the sequence stratigraphy of this complex stratigraphic interval, in which the eustatic and climatic signals can be disentangled. The climate change predates the sea-level drop and caused the demise of the microbial-dominated high-relief carbonate platforms. A surface similar to a drowning unconformity was thus generated, even though a transgression was not taking place. Only small isolated microbial carbonate mounds survived the crisis of high-relief platforms. The complete disappearance of microbial carbonates (i.e. mounds) and the definitive switch of the shallow water carbonates to loose-sediment dominated ramps coincides with a subsequent sea-level drop. Thus the demise of the Upper Triassic microbial dominated high-relief platforms of the Dolomites can be interpreted as a two step process: at first a climatic event killed the km-scale high-relief platforms; later a drop in sea level led to the definitive disappearance of the microbial carbonates
