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Petrology of carbonate beds from the stratotype of the Carnian (Stuores Wiesen section, Dolomites, Italy): the contribution of platform-derived microbialites

Geo.Alp 9, 2012, p. 12–29

The modal composition of carbonate grains in para-autochthonous carbonate beds from the Ladinian-Carnian stratigraphic succession of Stuores Wiesen in the Dolomites was evaluated by point-counting. The stratigraphic series of Stuores, which includes the GSSP of the Carnian stage, is mostly composed of shales and marls deposited in a rather deep-water setting, at the foot of high-relief carbonate platforms. Numerous cm- to dm-scale calciturbidite beds, ranging from carbonate mudstone-siltstone to rudstone, are intercalated within the fine siliciclastics. In the Ladinian - lowermost Carnian portion of the succession, many of these calciturbidites are mixed carbonate-siliciclastic. Most of the carbonate grains in the granular facies of Stuores are fragments of microbialite with clotted peloidal fabric, Tubiphytes, calcimicrobes, peloids, ooids, mollusks, echinoderms and other types of skeletal grains. The major constituents of the carbonate grain associations are microbialitic allochems of various types, accounting for 64% of the volume of carbonate on average, with peaks of ca. 90%. This is in agreement with estimates on in-situ microbial-metazoan boundstones of the Dolomites with approximately the same age, and suggests that not only boundstone facies, but also basinal calciturbidites faithfully register the modal composition of microbial carbonate platforms. It is presently impossible to reconstruct the provenance of these microbialites within the depositional profile of the carbonate platform, because the formation of microbialites is not restricted to the photic zone: microbial carbonates may form at water depths of 0 to 250 m and below. Ooids are commonly associated with coarse terrigenous sand, and often grew on siliciclastic nuclei, demonstrating that siliciclastics were encroaching the submerged shelf on adjacent platforms. Overall, terrigenous supply did not hamper the development of high relief carbonate platforms during the Late Ladinian - Early Carnian at Stuores, but seems to have had an influence on carbonate grain associations. Fine carbonates contain calcareous nannofossils, which however contribute minimally to the sediment budget at Stuores.

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