CONICET | Buscador de Institutos y Recursos Humanos

Microbialites are defined as ?organosedimentary? structures, resulting from the interaction between environmental and microbial factors. The two dominant fabrics leading to their formation are trapping and binding and/or precipitation of minerals linked to microbial processes. In the last years, extensive research has been carried out aiming to better understand the microbe-mineral interaction, the relationship between environmental conditions and macroscopic and microscopic structures of living microbialites and their preservation in the fossil record. Thus, studying these different aspects can provide key information on the prevailing environmental conditions during the formation of fossil microbialites. The Maquinchao Basin in northeastern Patagonia (Argentina) provides an outstanding research area encompassing different types of microbialites such as fossil stromatolites, carbonate laminated crusts and living microbial mats. Fossil stromatolites are found in continuous and extensive banks outcropping mostly along the Maquinchao River or as distinct buildups. Carbonate crusts and living microbial mats are found at the riverbed, in dry and active ponds of the modern Maquinchao River, respectively. Preliminary results comprise field observations including digital outcrop modeling using SfM-MVS photogrammetry and dGPS surveys-reconstruction using multi-images, light microscopy and SEM inspection of selected samples along with EDX and XRD analyses. The different outcrops are located at a constant elevation of 840 m, a higher altitude than the actual riverbed of the Maquinchao River. The field examination of fossil stromatolites reveals various types of structures linked to their stage of preservation/erosion. Domal columnar-like constructions are very well preserved whereas those specimens with only the outer part as well as a visible nucleus are less well preserved and appear to have suffered major erosion. These structures are often embedded within millimeter-sized particles resulting from the erosion of larger buildups. Mineralogical analyses on selected samples show a dominance of low Mg-calcite, as well as the presence of clay especially in the fossil microbialites. Three-dimensional microscopic examination of selected samples using multi-scaled X-ray CT-scanning techniques (X-ray tomography) reveals the presence of a very fine porosity in all three types of microbialites present in the Maquinchao Basin. The shape of this porosity is elongate/tubular with a diameter ranging between 5 and 7 micrometers. These preliminary results indicate very similar structures in both fossil and modern carbonate crusts pointing towards comparable formation processes. Moreover, the described small porosity is difficult to visualize without high-resolution techniques but fundamental for understanding the internal structure of these fossil microbialites as well as their degree and stage of erosion. Combining the field observations, image analyses, sedimentary petrography and 3D tomography with geochemical data will contribute to develop a model of formation and preservation of both fossil and modern microbialites that can be applied to similar build ups at different geographical and temporal scales.

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