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In the Middle Triassic of the Dolomites of northern Italy thick carbonate bodies (1,000 m) of relatively restricted extension are surrounded, and in places covered, by volcanic rocks and clastic sediments. These bodies, characterized at the periphery by inclined beds dipping basinward (15-35°), are formed by massive carbonates, mostly crystalline dolomite (Schlern Dolomite); in many places the massive facies is replaced laterally by stratified carbonate rocks. Organisms in the massive dolomite are uncommon (some corals, a few pelecypods and crinoidal plates) and poorly preserved; in the massive limestones and in the bedded carbonate rocks, calcareous algae, crinoids, mollusks, rare corals, foraminifers, and ostracods are present. The central part of the buildups lies directly on a biostromal blanket dolomite (Serla Dolomite) which extends throughout the region. A sequence of cherty micritic limestone (Livinallongo Formation), in places nodular or bituminous,
lies lateral to the core between the Serla Dolomite and the Schlern Dolomite; between the buildups this sequence is covered directly by volcanics (prevalently latites) and other volcanoclastic rocks. Prevalently terrigenous sediments (Raibl Formation) ended Middle Triassic sedimentation and disconformably overlie the carbonate masses.
In the literature, from the last century until the most recent papers, the carbonate buildups have been interpreted as coral reefs with internal lagoons (the bedded parts); the volcanics and the associated sediments filling the interreef basins have been regarded as contemporary with the reefs. The Dolomites became famous as a classic region for paleoreefs and spectacular lateral facies changes (heteropic contacts).
The following data are from a new detailed field examination and sedimentologic analysis. The massive part of the buildups is prevalently mud-supported grainstone (pelletoid and skeletal grains) and is characterized by the presence of "reef tufa" (fibrous carbonate in cavities with irregular, in many places lobate, walls). the stratified part consists of thick beds of mud-supported grainstone (pelletoids). The grainstone is commonly burrowed, has crinoids and algae (Diplopora), and has thin interbeds of laminated micrites, common pisolites, intraclastic breccias, sheet cracks, and "tepee" structures. The boundaries between carbonate bodies and surrounding volcanoclastic rocks are abrupt, without transitional facies; the bodies interfinger with the underlying Livinallongo Formation whi h, in the transition zone, contains abundant, commonly graded breccias. In places the upper part of the buildups interfingers with muddy carbonate sediments that end the basin sequence. These data suggest a different interpretation from the classic one. On the higher parts of the basal blanket dolomite, banks of shallow-water carbonate sediments developed and grew. These carbonate banks were exposed periodically, keeping pace with subsidence, and reached thicknesses of about 1,000 m.
A starved-basin sequence was deposited in progressively deeper waters to a maximum depth of about 800 m. Later the products of the submarine volcanic eruptions filled interbank depressions rather quickly, literally "suffocating" some buildups and fossilizing their primary morphology. At the end of the Middle Triassic the almost complete topographic leveling caused a renewal of the carbonate sedimentation which led to the development of slightly raised banks and, in places, small coral reefs.
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