Estructuras sedimentarias inducidas por actividad microbiana (ESIAM) en la planicie de marea de Puerto Rosales, estuario de Bahía Blanca.

Abstract

The study of microbial mats in modern siliciclastic coastal marine settings represents an important analogue for the analysis of fossil deposits in similar paleoenvironments. In the Bahía Blanca estuary, mat-covered tidal flats were studied from a geobiological perspective with the aims of identifying and describing microbially-induced sedimentary structures (MISS), and to distinguish features in modern systems that allow the recognition of such microbial communities in fossil deposits. Systematic biological and sedimentological samplings were carried out, and the identification of microorganisms and the recognition and characterization of typical textures and authigenic minerals were performed through light and electronic microscopy techniques. Microbial mats were dominated by cyanobateria, followed by diatoms; both groups of microorganisms are known to secrete extracellular polymeric substances (EPS) and biostabilize the sedimentary surface. Characteristic textures of epibenthic mats, such as oriented grains and mat-bound small grains, framboidal pyrite and zeolites were recognized. In the intertidal-supratidal zone, MISS termed gas domes, sponge pore fabric, and sieve-like surfaces were identified. Moreover, shrinkage cracks, flip-overs, mat chips, erosional remnants and pockets, folds, as well as multidirected ripple marks were observed. These structures are related to calm conditions/latencies, prolonged subaerial exposure (desiccation), and storm events with high hydrodynamic energy (erosive) acting over the tidal flat (e.g. waves and currents). In this sense, MISS constitute important indicators of specific environmental conditions and thus their recognition in the rock record allows a refined interpretation of the paleoenvironment in coastal marine successions.