The Triassic carbonate platforms of the Southern Alpine Dolomites offer a good opportunity to investigate the relationship between carbonate production and the depositional geometry. In the region, the Anisian low relief platforms were followed by late Anisian and early Ladinian steep-sided, high relief buildups. This evolution records the increasing importance of the autochthonous micrite production, the syndepositional cementation, and, subordinately, the skeletal metazoans growth in the bioconstruction. Only in a few key outcrops in the Dolomites, less affected by the late diagenetic modifications, it is possible to perform micromorphological, mineralogical and geochemical investigations. One example is represented by the Sasso Bianco area, where an. upper Anisian buildup belonging to the Contrìn Formation is well exposed and preserved. The succession is characterized by micritic layers, alternated to calcarenites and less common fine-grained calcirudites. To characterize the microfacies of the succession, and to correlate them to the depositional geometry, optical and electron microscopy analyses were applied. Microfabric, epifluorescence and siliciclastic elements distribution were utilized to discriminate between autochthonous and allochthonous micrite. The lower part of the section is characterized by detrital calcareous facies, composed of bio-intraclastic packestones/wackestones. This facies is rich in bioclasts of echinoderms, gastropods, thick shelled pelecypods, brachiopods, sponges, etc., and benthic microforaminifers. Fragments of dasycladacean algae (Teutloporella) are sometimes abundant. Commonly the grains are engulfed into allochthonous micrite, rich in smaller bioclasts. Autochthonous micrite is rare in this lower portion, and, when present, it is subordinate to the detrital one. In the middle portion of the section the syndepositional cemented autochthonous micrite increases and alternates in dominance with the loose mud fraction. In the upper portion, the autochthonous micrite becomes dominant. The in situ deposition is testified by peloidal to clotted peloidal fabrics, engulfing Tubiphytes, encrusting foraminifers (e.g. Tolypammina gregaria) and sponges (e.g. Olangocoelia otti). The dasycladacean remains are absent and crinoids clasts are here rare. Agglutinated tube worms are on the contrary common within the clotted peloidal fabrics. Their tube are composed by small peloids and they are very similar to the terebellids recently recorded, in symbiosis with sulfate reducing bacteria, in cryptic cave environments (Guido et al., 2014). The increasing activity of bacterial communities and their role in the stabilization of the carbonate geometries is testified by the gradual increase of clotted peloidal micrite. The gradual change of the micrite type, from the loose detrital fraction, dominating the lower part of the section, to the syndepositional lithified fraction of the upper part, is associated with a gradual change in the depositional geometries of the carbonate body. A carbonate bank, initially developed in shallow water, evolved into an isolated platform with steep slopes, prograding onto a deeper water basin. We suggest that, during the sea level rise, suboxic/anoxic condition settled and permitted the proliferation of sulfate reducing bacteria. These communities probably developed on the available organic matter remains and induced micrite precipitation that cemented and stabilized the sediments. A clear correlation between the changing carbonate production and the evolution of the carbonate platform dynamics is therefore visible.

Autochthonous versus allocthonous micrite in the Anisian Sasso Bianco platform from the western Dolomites

STEFANI, Marco;
2014

Abstract

The Triassic carbonate platforms of the Southern Alpine Dolomites offer a good opportunity to investigate the relationship between carbonate production and the depositional geometry. In the region, the Anisian low relief platforms were followed by late Anisian and early Ladinian steep-sided, high relief buildups. This evolution records the increasing importance of the autochthonous micrite production, the syndepositional cementation, and, subordinately, the skeletal metazoans growth in the bioconstruction. Only in a few key outcrops in the Dolomites, less affected by the late diagenetic modifications, it is possible to perform micromorphological, mineralogical and geochemical investigations. One example is represented by the Sasso Bianco area, where an. upper Anisian buildup belonging to the Contrìn Formation is well exposed and preserved. The succession is characterized by micritic layers, alternated to calcarenites and less common fine-grained calcirudites. To characterize the microfacies of the succession, and to correlate them to the depositional geometry, optical and electron microscopy analyses were applied. Microfabric, epifluorescence and siliciclastic elements distribution were utilized to discriminate between autochthonous and allochthonous micrite. The lower part of the section is characterized by detrital calcareous facies, composed of bio-intraclastic packestones/wackestones. This facies is rich in bioclasts of echinoderms, gastropods, thick shelled pelecypods, brachiopods, sponges, etc., and benthic microforaminifers. Fragments of dasycladacean algae (Teutloporella) are sometimes abundant. Commonly the grains are engulfed into allochthonous micrite, rich in smaller bioclasts. Autochthonous micrite is rare in this lower portion, and, when present, it is subordinate to the detrital one. In the middle portion of the section the syndepositional cemented autochthonous micrite increases and alternates in dominance with the loose mud fraction. In the upper portion, the autochthonous micrite becomes dominant. The in situ deposition is testified by peloidal to clotted peloidal fabrics, engulfing Tubiphytes, encrusting foraminifers (e.g. Tolypammina gregaria) and sponges (e.g. Olangocoelia otti). The dasycladacean remains are absent and crinoids clasts are here rare. Agglutinated tube worms are on the contrary common within the clotted peloidal fabrics. Their tube are composed by small peloids and they are very similar to the terebellids recently recorded, in symbiosis with sulfate reducing bacteria, in cryptic cave environments (Guido et al., 2014). The increasing activity of bacterial communities and their role in the stabilization of the carbonate geometries is testified by the gradual increase of clotted peloidal micrite. The gradual change of the micrite type, from the loose detrital fraction, dominating the lower part of the section, to the syndepositional lithified fraction of the upper part, is associated with a gradual change in the depositional geometries of the carbonate body. A carbonate bank, initially developed in shallow water, evolved into an isolated platform with steep slopes, prograding onto a deeper water basin. We suggest that, during the sea level rise, suboxic/anoxic condition settled and permitted the proliferation of sulfate reducing bacteria. These communities probably developed on the available organic matter remains and induced micrite precipitation that cemented and stabilized the sediments. A clear correlation between the changing carbonate production and the evolution of the carbonate platform dynamics is therefore visible.
2014
Carbonate Platforms; Triassi; Automicrite
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/2289024
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