Life in the aftermath of Marinoan glaciation: The giant stromatolite evolution in the Puga cap carbonate, southern Amazon Craton, Brazil

Abstract

Giant domal stromatolites associated with vertical tubestone are reported for the first time in the Marinoan (635 Ma) Puga cap dolostone on the southern Amazon Craton, Brazil. The well-preserved giant domal mounds reach 12 m in diameter and 10 m in height, forming laterally continuous biostromes of at least 200 m. The giant mounds occur mainly on the top cap carbonate succession overlying stratiform stromatolites directly underlying glacial diamictites marking the Cryogenian-Ediacaran boundary. We discuss the sedimentological and biological conditions during the greenhouse period that led to the development of these exceptional stromatolitic features. The microbial communities primordially colonized the diamictite substrate, forming stratiform bioconstructions in a shallow platform under hypersaline calm water conditions. The transition from stratiform to domal morphology reflects the progressive deepening of the platform, concomitant with increases in microbial activity. The high production and pre-lithification of carbonate muds, that formed a rigid load-resistant framework, allowed for the accumulation by vertical accretion, increasing the potential for preservation of giant mounds. The progressive generation of accommodation space related to sea-level rises and glacial isostatic adjustment (GIA) caused subsidence in the coastal zone, as proposed by the Snowball Earth hypothesis. The maximum flooding was associated with an increase in siliciclastic inflow, causing burial of microbial communities and the demise of giant stromatolites. The succession was succeeded by the deposition of long-term transgressive lime mud in a CaCO3-oversaturated sea. The giant stromatolites consist of a new element of the Marinoan Puga cap carbonate and provide essential insights to enlarge the discussion about the biological and sedimentological interaction that occurred on post-glacial carbonate platforms worldwide.