Erosion-initiated stromatolite formation in a recent hypersaline sabkha setting (Abu Dhabi, United Arab Emirates)

Laminated microbial mats and microbialites are documented from a variety of coastal marine environments. These features form through: a) the combination of trapping and binding of allochthonous grains, and b) microbially-mediated or controlled precipitation of a variety of minerals, including high-magnesium calcite and dolomite. Intertidal pools and associated microbial features have been previously documented from the coastal sabkha of Abu Dhabi, but have not been studied in detail. This study therefore aims to provide the first detailed descriptions of thrombolite and stromatolite structures in the coastal sabkha complex of Abu Dhabi. These detailed descriptions will be utilised to develop a new model for their formation, and to consider the implications for the interpretation of similar features from the depositional record. It is proposed here that the development of intertidal pools within the laminated microbial mat zone is the result of localised erosion as a result of storm surges and tides. The formation of erosional scours, which continued to develop into the pools observed today, led to a switch in microbial communities from filamentous cyanobacterial mats into coccoid cyanobacterial mats as a result of reduced environmental stress under conditions of permanent flooding. In addition, the continuous circulation of seawater initiated the abiotic lithification of submerged microbial mat and carbonate rudstone by acicular aragonite cements. Simultaneously, (proto-)dolomite was formed in the stromatolites between individual laminae that were enclosed by a bacterial extra-polymeric substance. These structures therefore developed through the combined effects of erosion, abiotic early lithification and microbially-mediated processes, and may actively respond to changes in sea level. The thrombolites and stromatolites therefore represent a link between unlithified laminated microbial mats and domal stromatolites. This model of stromatolite formation has strong implications for the interpretation of similar fossil structures observed in ancient stratigraphic sequences.