Nature and evolution of Middle Triassic carbonate buildups in the Dolomites (Italy)

During the Middle Triassic, an evolution from carbonate bank to buildup, highly elevated in respect of the adjacent basins, is observed in the Alps. The Dolomites in the Southern Alps, Italy, are here considered as an example. During the Anisian, subtidal banks of the Upper Serla Fm., with rare peritidal cycles show only a limited biologic density and diversity. With the Late Anisian (Contrin Fm.) a gradual development of binding and framework-building communities can be observed. The sinking of wide areas of the Contrin carbonate bank into anoxic conditions (Plattenkalke Fm.) allowed the birth of isolated buildups. Their increased carbonate production, due to a greater diversity and density of organisms, was able to keep up with the rate of subsidence. The Latemar buildup — with additional data for the slope taken from Mt. Coldai (Civetta) and Sass da Putia — is used to illustrate the main character of a Lower Ladinian buildup. The restricted platform sediments display peritidal cycles: prevalent in the subtidal portion is Dasycladacean packstone/grainstone, associated with gastropods, ammonoids and blue-green algae; in the supratidal portion, desiccation and tepee structures are present. Scleractinian and sphinctozoan frameworks are rare on the upper part of the slope which is bound predominantly by encrusting organisms, mainly blue-green algae, porifers and Tubiphytes. The prevailing sediments on the upper part of the slope are bindstones, whilst packstones and rudstones are in the lower part of the slope. The latter carbonates were cemented early under submarine conditions, mostly by isopachous-palisade cement. Polyphasic internal sediments with phreatic-vadose influence are present, and can in the middle-lower part of the slope replace most of the original sediment.