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Abstract

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, the so-called “reefs" of the literature, are formed of massive carbonates, mostly crystalline dolomite (Sciliar Dolomite), but in many places the massive facies is replaced laterally by stratified carbonate rocks. 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) lies lateral to the core between the Serla Dolomite and the Sciliar Dolomite. Between the buildups this sequence is covered directly by volcanics and volcanoclastic rocks. Predominantly terrigenous sedimentation (Raibl Formation) ended Middle Triassic deposition, and these terrigenous rocks disconformably overlie the carbonate masses.

Stratigraphic relationships show that the buildups are of the same age as the Livinallongo Formation. At the end of their growth the reefs rose above the surrounding sea floor from a minimum of 700 to 800 m to a maximum of 1,500 m. This relief, a real measure of water depth, was produced gradually owing to different sedimentation rates in the two environments. Later, submarine volcanic eruptions filled the interreef depressions rather quickly, literally suffocating some buildups and preserving their primary morphology.

The buildups were mainly composed of pellctoidal micrites and skeletal sands. Organisms in the massive dolomite are relatively rare and poorly preserved (some corals, a few pelecypods and crinoidal plates) ; in the massive and bedded limestones green algae, crinoids, ammonites, mollusks, rare corals, foraminifers, and ostracods are present. Spongiostromata are almost everywhere present, both as grain coatings (oncolites) and as crinkled laminations and small heads (stromatolites).

The Triassic carbonate buildups of the Dolomites were topographically elevated entities, but they were not reefs in the ecological sense. They represent the indented edge of a broad, shallow-water carbonate platform which grew in cyclically repeated subtidal, intertidal, and supratidal conditions. Scattered coral patches probably settled at the shelf edge but were easily destroyed during subaerial exposure as shown by the organic boulders (Cipit Limestone) occurring in the interreef sediments. The controlling factor appears to have been tectonic uplift which localized the organic proliferation on topographic highs. Buildups were facilitated by the trapping action of algae; their binding was due to diagenesis during subaerial exposure as shown by the particulate and muddy texture of fragments of the talus breccia, by vadose silt and vadose pisolite horizons, and by various paleokarst features.

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