Ediacaran ramp depositional model of the Tamengo Formation, Brazil

Abstract

The Tamengo Formation corresponds to one of the most complete records of Ediacaran carbonate deposits in South America that contains Cloudina and Corumbella calcified exoskeletons macroscopic fossils. With the objective of characterizing its paleoenvironment, a total of ten sedimentary facies indicative of a carbonate platform were identified. In the outer ramp region there are massive mudstones, massive marl to claystones, and shales. In the mid-ramp there are facies that are storm-dominated with swaley/hummocky cross-stratified grainstone, tangential cross-bedding grainstone, massive bioclastic grainstone and massive marl to claystone. Lastly, in the inner ramp portion there are massive intraclastic grainstone, massive intraclastic rudstone, massive oolitic grainstone and laminate mudstone/shale rhythmite, and microbial textures are present at the base of the rhythmites, indicative of a back-ramp environment. Faciology data indicate that the Cloudinas inhabited a protected environment between oolitic bars in the inner ramp and back-ramp. These organisms were periodically transported and reworked by waves generated by storms in the mid-ramp and outer ramp zones. The positive values for δ13C in carbonates (1,5 to 5,4‰) and δ15N (between 3,5 and 4,5‰) of deposits with Cloudina indicate a marine paleoenvironment with shallow, hot, and oxygenated waters with a high productivity of organic matter. In this context, the facies analyses and stratigraphy, aided by C, O, and N isotopes from the Tamengo Formation in outcrops from the Corumbá regions in the southwest of Brazil allows for the reconstruction of an Ediacaran carbonate ramp and inferences about the habitat of Cloudina, thus increasing understanding of this important chapter of Earth history.