Microbial mat-generated sedimentary structures in an evaporitic coastal environment

CUADRADO, D.G.; PAN, J; GOMEZ, E.A.; BOURNOD, C.N.; RANIOLO, A.

PUNTA DEL ESTE

Congreso; XV CONGRESO LATINOAMERICANO DE CIENCIAS DEL MAR; 2013

COLACMAR

The recognition of microbial mats in modern environments has gathered special relevance in recent years, in relation to their role in processes involving the formation and deformation of sedimentary structures. In that sense, the recognition of associations with similar structures in the fossil record allows the inference of the paleoenvironment in which they were formed. It is common to observe structures influenced by coastal physical processes such as waves and tidal currents; however wind-related structures are not frequently found. The study area (40°33′S; 62°14′W) comprises a tidal channel currently choked by a sandbar, and corresponds to a sabkha-type evaporitic environment. In sabkha environments, sediments characteristically retain moisture even when inundation is infrequent, due to the ascending capillary movement of sub-surface water, stimulated by evaporation. Several microbially-induced sedimentary structures, characteristic of evaporitic environments, were recognized in the field, such as desiccation cracks with bordering ingrowths, nodules with salt crusts, and knotted structures. All of them correspond to a colonization of salt-flat sediments by coccoid cyanobacteria. On the other hand, there were other structures related to strong winds, such as mat-tears, folds and wrinkles, among the weaker ones; and roll-ups and flippedover edges, among the more resistant ones. These structures evidenced decreasing levels of water saturation, together with increasing levels of wind energy. The most important factors governing the formation of these structures are inundation frequency, duration of atmospheric exposure/water cover, and the associated mineral precipitation. These factors exert a strong influence over the species assemblage that makes up the microbial mat, and consequently over the characteristic morphologies they generate.