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Stromatolites form by the close interaction between the microbial activity and the environment in a great range of depositional settings resulting in heterogeneous growth morphologies and fabrics. The link between morphological diversity and internal structure of stromatolites to environments is not straightforward due to the dual influence at all scales of biological and physico-chemical factors. Specifically, in low energy settings biological controls are the dominant in influencing stromatolites, as suggested from modern analogues.In this study we examine geometries and fabrics of stromatolites from the mixed carbonate-siliciclastic marginal lacustrine succession of the Yacoraite Formation (Cretaceous-Paleogene) in Tres Cruces (Salta Basin). The outcrops that extend along a W-E transect of 10 kilometres, provide exceptional exposure allowing to analyse the geometries and lateral continuity of the stromatolite beds. Current stratigraphic research interprets the Yacoraite succession as a closed, saline lake with two evolutionary stages. The first stage, dominated by carbonate production, represents a shallow-water perennial lake with moderate wave energy. The second stage corresponds to a rapidly fluctuating, low-water energy ephemeral lake with abundant fine-grained siliciclastics and frequent subaerial events. This change is related to climatically driven lake-level fluctuations. Marked differences in the lateral continuity of the stromatolite beds and their associated facies have been observed along the Yacoraite succession. In the perennial lake, the stromatolite beds overlie oolitic facies and can be traced laterally for several kilometres whereas in the ephemeral lake the stromatolites grow on both oolitic and siliciclastic facies and form discontinuous levels along hundreds of meters that pass laterally into the oolitic facies. Stromatolite growth morphologies, however, show a more homogeneous distribution that does not clearly reflect the evolutionary changes of the Yacoraite paleolake. Generally, the stromatolites of the perennial lake exhibit planar morphologies that pass upwards into coalescent domes forming structures that range in height from few decimetres up to 1 m. The stromatolites of the ephemeral lake are mainly planar, wavy (dm-scale) or domes that are coalescent and form tabular decimetric structures. Morphological vertical zonation is rare. The change in stromatolite morphology can be tentatively attributed to decreased accommodation and water energy conditions. However, this interpretation needs to be taken carefully considering that all these types of stromatolite morphologies have been observed along the Yacoraite succession, regardless of the lake stage. Stromatolites show mainly well-developed internal lamination. Their microfabrics are either fine-grained (micritic, clotted and/or filamentous) or formed by combinations of fine-grained and sparry layers composed of fibrous calcite crusts, calcite spherulites and/or shrubs.Further work will intend to better understand the spatial and temporal distribution of the stromatolite geometries and fabrics along the Yacoraite Formation to shed light on the influence that environmental and biotic factors exert in stromatolite macro, meso and microscale in low-energy lacustrine settings. |
