Biofacies evolution in the Triassic platforms of the Dolomites, Italy

Abstract

The Triassic of the Dolomites includes many carbonate platform generations, ranging in age from the Anisian to the Norian-Rhaetian. After the Permian-Triassic biological crisis, “reef” communities reappeared during the Anisian time. These buildups were generally characterized by a limited relief, lacking any primary skeletal framework and evidence of syndepositional cementation. The microfacies are dominated by micrites, mainly allochthonous or detrital in origin. The sparse biota are generally binder and buffler organisms, such as dasycladacean algae, sphinctozoans and briozoans. The second generation of carbonate buildups (late Anisian–early Ladinian, Sciliar Fm) are dominated by syndepositional cements (e.g. Marmolada Platform). These cements represent the main component of margin and upper slope facies. They form more or less isolated or laterally linked bodies: the “evinosponges”. During the late Ladinian and Carnian p.p., the post-volcanic platforms developed (Cassian Dolomite). The microfacies of these platforms manly consist of micrites, cements and skeletons. The automicrites constitute more than 50% of the rock volume, the cements the 20%, and the skeletal organisms less than the 10%. The metazoan contribution is subordinated to that of skeletal cyanobacteria, like Cladogirvanella cipitensis and microproblematica, like Tubyphites. The primary marine cements provide evidence of a widespread early syndepositional lithification. Towards the top of Julian Substage (Carnian), at the base of the Heiligkreutz/Durrenstein Formation (i.e. Alpe di Specie), small calcareous bioconstructions, interpreted as patch-reefs, show a much more “modern” faunal association. For the first time in the Triassic, a primary skeletal framework developed, largely formed by calcified demosponges and scleractinians. Corals were still subordinated to sponges. Taxonomic diversity increases greatly and the skeletal component exceeds the 50% of the rock volume. These biofacies anticipate the “modernization” of the reef-building communities, occurring at a global scale between the Late Carnian and the Norian-Rhaetian.