Paleoambiente e evolução dos estromatólitos gigantes da capa carbonática marinoana do Cráton Amazônico, Tangará da Serra-MT

Abstract

Giant domical stromatolites, associated with tubstone structures, are recorded here for the first time the sequence of the Puga cap carbonate the southeast of the Amazonian Craton region of Tangará da Serra, Brazil. The well-preserved giant mounds reach 12 m in diameter and up to 10 m in height, forming a 200 m laterally continuous biostrom, with individualized domes, which display an expressive synoptic, metric to decametric relief. This work aimed to de- scribe and interpret the first occurrence of giant stromatolites and attest to their biogenicity, considering their macro, meso and microscopic aspects. It also determined the stratigraphic biogenic facies relationship, formed by organomineralization processes, whether bioinduced or bioinfluenced together with the other inorganic facies, originated by precipitation and/or sedimentary (mechanical) processes of the Puga carbonate sequence and finally, proposed an evolutionary model for giant stromatolites with the other facies, providing new inferences for the hydrodynamics of the epicontinental sea, of the SE of the Amazonian craton, with pale- oenvironmental and bioevolutionary implications for the beginning of Ediacaran. The begin- ning of Ediacaran marked by epicontinental, shallow and stratified seas, formed by dense and hypersaline waters, contrasting with the melting waters, light and rich in nutrients. The origin of the giant stromatolites is the product of a sum of anomalous paleoenvironmental and biotic factors, which occurred after the Marinoana glaciation (635Ma). The diamicton substrate was colonized by extremophiles, halophilic microbial communities in hypersaline waters with little or no direct influence of hydrodynamic processes, forming stratiform stromatolites with gypsum pseudomorph cement. With the advancement of greenhouse conditions, there was a continuous generation of accommodation space, caused by rising sea levels, influenced by syn-deglacial transgression and by the glacio-isostatic adjustment (GIA) that caused the uplift of the coastal zone and the mixing of the waters. The mixture of the waters that occurred at the end of the glaciation was essential for the development of microbial mats, since the cool water was, rich in nutrients. The microbial communities developed initially in balance with the constant migration of the photic zone, caused by the gradual rise in sea level, with the end of the GIA's influence. The accommodation space is controlled only by post-glacial transgres- sion, resulting in a more significant impact on hydrodynamic processes, registered in the ex- pressive synoptic relief. The sum of these conditions would have propitiated the development of microbial communities that would become giant stromatolitic mounds, with metric to dec- ametric synoptic relief. The constant increase in energy would result in the reworking of mi- crobial mats, generating macropeloids, agglomerates of micropeloids. The record demon- 1 strates an alternation of macropeloids and micropeloids blades, which suggests that in times of stabilization, there was an attempt to colonize microbial communities, that were reworked continuously by the wave. The decline of giant stromatolites, in the southern of the Amazoni- an craton, would be related to the height of greenhouse conditions, with an sudden increase in the influence of post-glacial transgression, which would condition a massive siliciclastic en- try, promoting the burial of microbial communities. Thus, the stromatolitic strata were suc- ceeded by a limestone factory, induced mainly by inorganic processes, in a sea saturated with CaCO3. Giant stromatolites are considered here as an essential record for understanding post- glacial conditions, from the beginning of Ediacaran.