The rise and fall of the giant stromatolites of the Lower Permian Irati Formation (Paran?a Basin, Brazil): A multi-proxy based paleoenvironmental reconstruction

Giant stromatolites are meter-scale laminated carbonate biosedimentary deposits formed by the action of benthic microbiota under very specific conditions. Although occurrences of giant stromatolites are relatively common in Precambrian deposits, the Phanerozoic record is still sparse. Here, we carried out an integrated analysis of the Lower Permian Santa Rosa de Viterbo giant stromatolite field, developed in a mixed carbonate-siliciclastic depositional system within a restricted intracontinental basin (Irati Formation, Paran ' a Basin, Brazil). Using available and new descriptions of stromatolite morphology and associated facies, we applied a multi-proxy approach based upon sedimentological, paleontological, geochemical, and isotopic data to develop a detailed paleoenvironmental model for this particular occurrence. The NE-SW elongated giant stromatolites - of >3 m in height, > 7 m in length, and > 1 m wide - have variable external shape and internal morphology, indicating changing growth strategy due to variations in the hydrodynamic conditions, bathymetry, and terrigenous input. Increasing 813C values towards the top of the succession are related to intense microbial activity, increased nutrient supply, and enhanced primary productivity, with the higher 813C values matching the global Permian seawater signal. Y/Ho and La/La* ratios indicate a less restricted setting towards the top of the succession, which is consistent with the increasing water depth recorded by the stromatolite morphology. The combination of smooth lamination, exclusively coccoidal microbial community, and fossil content, point to deposition under high (possibly hyper) salinity conditions for most of the succession. Our data suggests that the existence of stressful conditions (i.e., strong currents and high salinity) protected the benthic microbial communities from predation and favored EPS production, generating the ideal conditions for the growth of giant stromatolites, a very uncommon situation in the Phanerozoic.

Automated summary

The integrated analysis of the Lower Permian Santa Rosa de Viterbo giant stromatolite field in Brazil reveals a detailed paleoenvironmental model for this occurrence, demonstrating the influence of changing hydrodynamic conditions, water depth, and terrigenous input on the growth of the giant stromatolites. The increase in microbial activity, nutrient supply, and primary productivity towards the top of the succession is reflected in higher 813C values. The shift towards a less restricted environment is consistent with the morphological changes in the stromatolites, indicating increasing water depth. The deposition of the stromatolites under high salinity conditions is supported by the presence of smooth lamination, exclusively coccoidal microbial community, and fossil content.