Microbially-colonized sediments in a wind-driven hydrodynamic coastal sistem.

CUADRADO D. G.; PAN, JERONIMO; LUIS ARIEL RANIOLO

Geneva

Congreso; 19th International Sedimentological Congress; 2014

Universite de Geneve / IAS

Special attention has been given in recent years to the study of microbial mats in modern environments, in relation to the processes involved in the formation and deformation of sedimentary structures. These modern studies aid in the recognition of analogous structures in fossil records with the consequent inference in paleoenvironment research. However, the reconnaissance of the physical processes behind fossil mat structures still remains a challenge, since a number of physical processes such as those dominated by currents and wind may produce similar signatures in rocks. This research documents the wind action over structures and provides a basis for their identification in sediment profiles. The study area (Paso Seco, Argentina; 40°33′S; 62°14′W) comprises a blind tidal channel choked by a sandbar forming a wide sedimentary platform (~2.5 × 0.3 km) with the presence of small saline ponds (S = 60.5, pH = 8.8). With a maximum tidal amplitude of 0.27 m (measured at spring tide with a HOBO water level logger), tidal range is negligible. The closed basin, a sabkha-type evaporitic environment, is comprised mainly by siliciclastic sediments colonized by microbial mats. Although the area has a semi-arid climate (precipitation < 400 mm year-1) with strong local NNE winds (average maximum velocity 40 km h-1), the underlying sediments obtain moisture from precipitation and the ascending capillary movement of sub-surface water, stimulated by evaporation. Three types of mat structures, typical of evaporitic environments, were recognized in relation to their proximity to water. Type I structures, found along the shoreline of the tidal channel, are similar to those found in estuarine environments, such as desiccation cracks with upward curved edges, characterized by spring-tide flooding. Type II structures include cauliflower-like nodules encrusted with salt and knotty structures, which are typically colonized by cyanobacteria. Type III structures, located farthest from the tidal channel on a platform that gets inundated only by precipitation, exhibit the formation of mat-tears, flipped-over mats and wrinkles; and more resistant structures such as folds and roll-ups (with several involutions). Large portions of mats are detached from the underlying substrate, and strong winds sweep them starting along pre-existing desiccation cracks, thus forming these structures due to their flexible behavior. The axes of folds and crumples are perpendicular to the prevailing wind direction. A striking feature of these thick (>5 mm) mats is a highly coherent structure produced by interwoven filaments of the cyanobacteriumMicrocoleus chthonoplastes, giving it a remarkable leathery appearance. Pennate diatoms (e.g. Naviculaphyllepta, Gyrosigma spencerii, Cylindrotheca closterium) appear in smaller proportions. Consortia of EPSproducing prokaryotes are found on the top layer of the mat. The EPS and the microbial architecture of interwoven trichomes provide high cohesiveness and elasticity, conditions necessary for a torn mat to exhibit flexible deformation under wind-shear stress. Core sections show biolaminites up to 3 cm-thick, which reflect aeolian conditions with the presence of rounded medium-sand grains (0.25 to 0.5 mm). This research contributes novel evidence of wind-related mat deformation structures, which can be interpreted as signatures in geological records, provided microbial mats enhance the preservation of these structures.