Biogeochemical and redox record of mid–late Triassic reef evolution in the Italian Dolomites

Abstract

Geochemical signatures and carbonate microfacies highlight contrasts between two distinctive mid–late Triassic reef communities in the Dolomite Alps, Italy. In the first community, sponges, bryozoans, calcified cyanobacteria and problematic organisms (Archaeolithoporella, Shamovella), together with a variety of micritic fabrics, formed compact reefs in high energy shallow-water at the margins of high-rise Ladinian–Carnian carbonate platforms. Debris from these margins created steep foreslopes, and some large blocks of the allochthonous material (Wengen–Cassian formations, Cipit Boulders) were buried in basinal shales that protected them from subsequent alteration and regional dolomitization. In the second and slightly younger community, small Carnian patch reefs (Heiligkreuz Formation, Alpe di Specie) developed in quieter shallow water, where they too were protected against alteration by enclosing shales. They were constructed mainly by scleractinian corals, sponges and red algae, and contain relatively large framework cavities with clotted-peloidal micrite. These early examples of coralgal reefs have broad similarities to present-day examples, whereas the community represented by the Cipit Boulders has more in common with Late Permian reefs.Both bioconstructions preserve primary microfabrics and biomarkers. The Cipit Boulder samples contain bacterial, mainly cyanobacterial biomarkers, lack specific molecules typical of sulfate-reducing bacteria (SRB), and have Rare Earth Element (REE) values indicative of oxic conditions. These signatures are consistent with their original high-energy platform margin location, compact structure, and presence of calcified cyanobacteria such as Cladogirvanella and Girvanella. In contrast, the coralgal patch reefs contain SRB biomarkers, lack specific molecules typical of cyanobacteria, and have REE values indicative of sub-oxic conditions. These signatures are consistent with their lower energy depositional conditions and well-developed skeletal framework that created protected low-oxygen micro-habitats. The SRB biomarkers can be linked to the associated clotted-peloidal fabrics which resemble those commonly present in younger coral-reef frameworks. These details of redox conditions and bacterial processes underscore the important biotic, structural and environmental changes that affected shallow marine reefs during the Triassic.