The role of microbial mats in the movement of stones on playa lake surfaces

• We shed light into the debated mechanisms which drive rocks across mudflats.
• Microbial mats play a crucial role in the destabilisation and motion of rocks.
• Rock tracks and microbial mat deformation structures are produced synchronically.
• The movement of rocks and mat fragments is caused by wind-generated water waves.
• The rock transport on mudflats is a geographically widespread phenomenon.

This paper sheds light into the debated mechanisms that move rocks across low gradient surfaces during storm episodes. Microbial mats are recognised to play a crucial role in sediment destabilisation and the subsequent motion of rocks in a playa lake from central Spain. Widespread countless scars are present on the playa lake sediment surface, each terminating at a stone, and/or a mound of overfolded fragments of microbial mats. All available objects, including cobble-sized stones, on the surface were transported for several metres leaving behind furrow-like tracks.The starting point of the tracks was found underwater on a surface veneered by microbial mats poorly attached to the substrate. The flotation and transportation of the microbial upper layer by wind-generated water currents are suggested to be a critical element promoting the destabilisation and subsequent transportation of the attached sediment, including the rocks. The sudden exposure of bubble-separated sediment is also envisaged as a source of buoyancy required to lift out the rocks at the starting point of the tracks and to reduce the friction on the base of the rock. At the end of the tracks the rocks and the sediment mounds often overlap, which reveals that rocks were embedded and transported by sediment rafts across the playa lake surface.The combination of the specific sedimentary and weather conditions involved in the transportation of rocks occurs periodically in mudflats. The implications are that this is not a local phenomenon, but rather, is a geographically widespread sedimentary process that may have occurred repeatedly throughout geological time. The effects of storm episodes can be inferred in ancient mudflat deposits by the assemblage of variably-sized stones and microbial-mat related structures.