Assessing the correlation between Ediacaran-Cambrian units of SW Gondwana: The Tagatiya Guazú (Itapucumi Group, Paraguay) and Tamengo (Corumbá Group, Brazil) formations

Abstract

The Tagatiya Guazú (PY) and Tamengo (BR) formations are South-American Ediacaran-Cambrian carbonate units that share striking similarities between their sedimentological, paleontological, and geochemical records. The Tagatiya Guazú Formation is mainly composed of grainstone and abundant microbial facies, bearing a diverse fossil content represented by autochthonous to parautochthonous remains of late Ediacaran tubular skeletal taxa, such as Cloudina, Corumbella, and Namacalathus, in addition to complex trace fossils indicative of the Ediacaran-Cambrian transition. The Tagatiya Guazú Formation is interpreted as deposited in a protected inner ramp setting. The Tamengo Formation is constituted by grainstone, packstone, wackestone, mudstone, marl, shale, and microbialite facies, also containing Cloudina and Corumbella as allochthonous and parautochthonous remains, respectively. The possible recognition of hummocky and swaley cross-stratification in the Brazilian unit, led to the interpretation of deposition in a storm-dominated middle to outer ramp setting in slightly deeper conditions than the Tagatiya Guazu Formation. Both units display the characteristic late Ediacaran Positive Carbon Isotope Plateau (EPIP) detected in terminal Ediacaran successions worldwide, commonly interpreted as a result of an intensely redox-stratified ocean, which is probably responsible for a decreasing δ13C trend from inner to outer ramp settings. The existence of this gradient can be inferred between the Tagatiya Guazú Formation and its lateral correlate, the Camba Jhopo Formation. On the other hand, the Tamengo Formation shows an unexpected opposite pattern (i.e., increasing δ13C values from inner to outer ramp settings). The limited size of the putative hummocky and swaley cross-stratifications, together with the problematic occurrence of parautochthonous skeletal fossils in deep (and probably anoxic) waters and the inverse δ13C isotopic gradient, suggest some inconsistencies in the Tamengo Formation depositional model. The reinterpretation of these deep-water facies as deposited in a shallower setting would solve (or reduce) the isotopic gradient problem and provide a better fit between the Tamengo Formation redox model and the ones interpreted for other units worldwide. Finally, multiple lines of evidence indicate that both the Tagatiya Guazú and Tamengo formations represent coeval successions (with the former being slightly younger) deposited in similar, shallow water depths in carbonate ramps opened to the Clymene Ocean.