Role of microbial mats and high sedimentation rates in the early burial and preservation of footprints in a siliciclastic tidal flat

Abstract

ABSTRACT Sedimentary processes in a microbial flat, developed in a progradational environment and trampled by vertebrates, were monitored under varying energy conditions. A vertebrate footprint made on the sedimentary surface was selected and was kept under observation for two years visited on nine field trips. Thus, this contribution provides a detailed analysis of the evolution of a microbial tidal flat with high-sediment-flux events and contributes to a better understanding of the sedimentary processes involved in the preservation of a true track. This study demonstrates that the formation of biolaminites (sequence of microbial mats interbedded with sand layers) in the coastal environment is caused by episodic pulses in the hydrodynamic regime of the area. Through the detailed inspection of a cross section of a sedimentary block containing the vertebrate footprint, the sedimentation history since the footprint creation is unravelled in relation to hydrodynamic records. Water energy was inferred using the measurements of a water-level sensor located on the tidal flat recording continuously every 10 minutes. The results indicate that the seawater enters into the zone by floods that occur during storm surges, reaching up to 70 cm height in the water column, and transporting abundant sediment, which produces the deposition of flat sand layers or sand ripples on the microbial mats. A sedimentation rate of 0.32 to 0.41 cm per year was calculated along the two-years monitoring. The study recognizes the plastic behavior of the microbial mat, one of their most important rheological properties, as a response to the registration of a vertebrate footprint. Petrographic analysis of microbial-mat layers reveals the precipitation of thin carbonate laminae during periods of seawater evaporation, which may enhance the preservation of sediments. Episodic sediment transport, in addition to the presence of a microbial-mat, creates the perfect conditions for formation, early burial, and preservation of the footprint in siliciclastic sediments.