The Record of Environmental and Microbial Signatures in Ancient Microbialites: The Terminal Carbonate Complex from the Neogene Basins of Southeastern Spain

The Messinian microbialites of the Terminal Carbonate Complex (TCC) from the Neogene basins of southeastern Spain show both diversified morphologies and an excellent preservation of primary microbial microstructures. Their stratigraphic architecture, fabric (micro-, meso-, and macro-fabric), and mineralogical composition were investigated in eight localities from three sedimentary basins of southeastern Spain: The Sorbas and Bajo Segura basins and the Agua Amarga depression. Two recurrent microbialite associations were distinguished. Laterally linked low relief stromatolites predominated in Microbialite Association 1 (MA1), which probably formed in low energy lagoons or lakes with fluctuating normal marine to hypersaline water. The microfabrics of MA1 reflected the predominance of microbially induced/influenced precipitation of carbonates and locally (Ca)-Mg-Al silicates. Microbialite Association 2 (MA2) developed in high energy wave and tidal influenced foreshore to shoreface, in normal marine to hypersaline water. High-relief buildups surrounded by mobile sediment (e.g., ooids or pellets) dominated in this environment. MA2 microbialites showed a significant proportion of thrombolitic mesofabric. Grain-rich microfabrics indicated that trapping and binding played a significant role in their accretion, together with microbially induced/influenced carbonate precipitation. The stratigraphic distribution of MA1 and MA2 was strongly influenced by water level changes, the morphology and nature of the substratum, and exposure to waves. MA1 favorably developed in protected areas during third to fourth order early transgression and regression phases. MA2 mostly formed during the late transgressions and early regressions in high energy coastal areas, often corresponding to fossil coral reefs. Platform scale syn-sedimentary gypsum deformation and dissolution enhanced microbial carbonate production, microbialites being thicker and more extended in zones of maximum deformation/dissolution. Microbial microstructures (e.g., microbial peloids) and microfossils were preserved in the microbialites. Dolomite microspheres and filaments showed many morphological similarities with some of the cyanobacteria observed in modern open marine and hypersaline microbialites. Dolomite potentially replaced a metastable carbonate phase during early diagenesis, possibly in close relationship with extracellular polymeric substances (EPS) degradation. Double-layered microspheres locally showed an inner coating made of (Ca)-Mg-Al silicates and carbonates. This mineral coating could have formed around coccoid cyanobacteria and indicated an elevated pH in the upper part of the microbial mats and a potential dissolution of diatoms as a source of silica. Massive primary dolomite production in TCC microbialites may have resulted from enhanced sulfate reduction possibly linked to the dissolving gypsum that would have provided large amounts of sulfate-rich brines to microbial mats. Our results open new perspectives for the interpretation of ancient microbialites associated with major evaporite deposits, from microbe to carbonate platform scales.

The Messinian stromatolites of the Sierra del Colmenar (Western Mediterranean): facies characterization and sedimentological interpretation

A representative outcrop of the Messinian stromatolites belonging to the Terminal Carbonate Complex unit, from the northern sector of the Bajo Segura basin (Caja de Ahorros del Mediterraneosection, Sierra del Colmenar, SE Spain) has been studied. Here, we present a detailed analysis of the architecture, external morphology, and internal morphology in order to reconstruct the environmental and palaeoecological conditions for their growth. The stromatolites macrostructure consists of a continuously doming type morphology (build up and sheets areas). These developed close to the coast and acted as a palaeogeographic barrier, reducing physical stress, channeling the erosive effect of water and favoring restricted conditions. This stromatolitic macrostructure exhibits variations in its internal morphology, giving rise to seven subfacies, which are a product of the environmental changes experienced during the growth of the microbial mats. Although broadly suggesting a coastal environment, restricted and shallow during formation, the variation in internal morphology (mesostructure and microstructure) is evidence of minor changes in the physical environment that indicate a progressive shallowing.

The Messinian stromatolites of the Sierra del Colmenar (Western Mediterranean): facies characterization and sedimentological interpretation

The Messinian stromatolites belonging to the Terminal Carbonate Complex unit, from the northern sector of the Bajo Segura Basin (CAM section, Sierra del Colmenar, SE Spain) have been studied. To understand the direct relationship between the morphologies of the stromatolites and their deposition context in order to reconstruct the environmental conditions for their growth, a detailed study of their architecture, external morphology and internal morphology was carried out (macrofabric and microfabric). The stromatolites are made up of domic bodies laterally linked to each other, generating a macrostructure (bioherms) with lateral continuity. This stromatolitic macrostructure presents variations in its internal morphology, giving rise to seven subfacies product of the environmental changes experienced during the growth of the microbial bushes. The stromatolites are arranged parallel of coastline acting as paleogeographic barriers to reduce the physical stress of the environment. Although in general lines suggests a coastal environment, restricted and shallow for the formation of the whole level, the variation in internal morphology is evidence of minor changes in the physical environment.

The Messinian stromatolites of the Sierra del Colmenar (Western Mediterranean): facies characterization and sedimentological interpretation

The Messinian stromatolites belonging to the Terminal Carbonate Complex unit, from the northern sector of the Bajo Segura Basin (CAM section, Sierra del Colmenar, SE Spain) have been studied. To understand the direct relationship between the morphologies of the stromatolites and their deposition context in order to reconstruct the environmental conditions for their growth, a detailed study of their architecture, external morphology and internal morphology was carried out (macrofabric and microfabric). The stromatolites are made up of domic bodies laterally linked to each other, generating a macrostructure (bioherms) with lateral continuity. This stromatolitic macrostructure presents variations in its internal morphology, giving rise to seven subfacies product of the environmental changes experienced during the growth of the microbial bushes. The stromatolites are arranged parallel of coastline acting as paleogeographic barriers to reduce the physical stress of the environment. Although in general lines suggests a coastal environment, restricted and shallow for the formation of the whole level, the variation in internal morphology is evidence of minor changes in the physical environment.