Growth and distribution of the Proterozoic stromatolites of the Atar group in the light of the Cambrian Llano River and modern Australian stromatolites

The development and the growth of stromatolites/thrombolites and the relationships with their environments are keys to understand much of the carbonate systems, particularly those of Proterozoic age, encountered in Mauritania, Brazil, Namibia or Siberia.The architecture of the stromatolite types developed in the Members Point Peak and San Saba of the Cambrian Wilbern Formation along the Llano River (Texas) is comparable to those encountered in the field in the Atar Group (Mauritania). Similarities in morphology (size, shape) and structure (pillars and walls), and in microbialite distribution indicate a complex sedimentary system. Regional studies and studies of sedimentary environments in the Llano river area show that detrital input and the ramp setting influenced distribution and spatial evolution of microbialites in the Atar Group. These results are further confirmed by the analysis of outcrops of the Vazante Group (Sao Francisco, Brazil). There, stromatolites of the Conophyton type (C. Metula) show deformation or breaks related to strong hydrodynamism, similar to observations in Shark Bay (Western Australia). The main axes of the stromatolitic domes are parallel to the wave directions or locally parallel to the main current directions.Microbialite distribution and dynamic controls are key parameters to evaluate the potential reservoirs in those specific environments.