Role of microbial mats in the genesis of soft‐sediment deformation structures in a siliciclastic environment

ABSTRACT This study addresses gaps in existing knowledge about what triggers deformation structures in soft sediments and the siliciclastic rocks that form from them. These structures are extremely diverse due to the variety of processes and agents that cause them, although several studies have been carried out to disentangle the triggering mechanisms of their formation. This study considers the role of microbial mats (with characteristic millimetre to centimetre‐heterolithic lamination) in the formation of soft sediment deformation structures, and proposes what processes are involved in their formation. It is hypothesized that wet microbial mats exhibit extremely flexible and plastic deformation that may be analogous to some structures found in sedimentary rocks. The present study documents convoluted sedimentary structures characterized by deformation, which were identified at a depth of 10 to 15 cm in a coastal sedimentary supratidal flat subject to flooding during storm surges and studied using morphological and petrographic analysis. The research was conducted at a modern coastal site characterized by sediments colonized by thick microbial mats. Currents during storm surges create deformed microbially‐induced sedimentary structures, such as metre‐scale microbial roll‐ups, in which the laminated microbial mats exhibit multiple folds. Additionally, microbial folds and gas domes, characterized by a convex‐upward microbial mat, have a cavity when they form that is filled with sand due to liquefaction and/or fluidization during subsequent flooding by storm surges. These processes are related to the incoming seawater and the height of the water column (up to 70 cm) above the microbial surface, which is also influenced by wave action. The results may help to provide new insights into the genesis of some soft‐sediment deformation structures in the coastal environment, documenting a non‐seismic mechanism of formation in siliciclastic deposits.