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 deformationstructures in soft sediments and the siliciclastic rocks that form fromthem. These structures are extremely diverse due to the variety of processesand agents that cause them, although several studies have been carried outto disentangle the triggering mechanisms of their formation. This study considersthe role of microbial mats (with characteristic millimetre tocentimetre-heterolithic lamination) in the formation of soft sediment deformationstructures, and proposes what processes are involved in their formation.It is hypothesized that wet microbial mats exhibit extremely flexibleand plastic deformation that may be analogous to some structures found insedimentary rocks. The present study documents convoluted sedimentarystructures characterized by deformation, which were identified at a depth of10 to 15 cm in a coastal sedimentary supratidal flat subject to flooding duringstorm surges and studied using morphological and petrographic analysis.The research was conducted at a modern coastal site characterized by sedimentscolonized by thick microbial mats. Currents during storm surges createdeformed microbially-induced sedimentary structures, such asmetre-scale microbial roll-ups, in which the laminated microbial matsexhibit multiple folds. Additionally, microbial folds and gas domes, characterizedby a convex-upward microbial mat, have a cavity when they formthat is filled with sand due to liquefaction and/or fluidization during subsequentflooding by storm surges. These processes are related to the incomingseawater and the height of the water column (up to 70 cm) above the microbialsurface, which is also influenced by wave action. The results may helpto provide new insights into the genesis of some soft-sediment deformationstructures in the coastal environment, documenting a non-seismic mechanismof formation in siliciclastic deposits.