Tubestone microbialite association in the Ediacaran cap carbonates in the southern Paraguay Fold Belt (SW Brazil): Geobiological and stratigraphic implications for a Marinoan cap carbonate

Abstract

The restriction of tubestone structures tomicrobialitic laminites in cap carbonates associated with the Marinoan glacial event in North and South America, Namibia, Australia and Oman makes them an important stratigraphic marker for the base of the Ediacaran system. This association has been recognized in the Mirassol D'Oeste Formation (635 Ma) adjacent to the northern Paraguay Fold Belt, and are reported here in isolated outcrops at Morraria do Sul and Forte de Coimbra in the southern Paraguay Fold Belt, west-central Brazil. The tubestone-microbialite associations at all localities reveal very similar macro- and microstructures, mineralogy, textures and fabrics. The microbialites consist of microbial laminites made up of dolomicrite clustered in microclots with dolospar-filled fenestrae. Lamination is defined by alternation in the relative abundance of these two components, suggestive of simple oscillations within a relatively uniform depositional environment and paleoecological setting. In the two new localities the tubestone fillings consist mainly of massive dolomicrite, although subordinate portions with concave lamination defined by concentrated very fine siliciclastic grains also occur. The presence of both massive and laminated tube fillings indicates variation in the processes responsible for their formation. These results extend the occurrence of the post-Marinoan tubestone-microbialite association at least 600 km southward from Mirassol D'Oeste in the north and document minor variations among the localities, which is what one would expect over such a broad distribution of this feature. The results also indicate that the isolated dolostones at Morraria do Sul and Forte de Coimbra do not belong to the Bocaina Formation (Corumbá Group), with which they have previously been correlated.