The carbonates of the Permian Lakes of North Massif central, France

During the Permian, large fluvio-lacustrine basins occurred within the West European Hercynian chain. The deposits were mainly detrital (conglomerates, sandstones and pelites). The two studied sites, Couy borehole and Aumance basin contain several dolomitic beds alternating with pelites containing Ostracods. The carbonate layers constitute various dolomitized facies with dolosparite, laminated and bituminous dolomites, oolites, oncolites and stromatolites, and pedological nodules. Microfacies are varied. The dolomite crystals can be either isolated rhombs scattered in pelites or rhombs and xenomorphic crystals cementing sandstones; in dolomicrites, dolomicrosparites and dolosparites, crysta…

The unusual hydrodynamical behaviour of freshwater oncolites

Algal concretions are frequent in calcareous environments, occuring as individual particles (oncolites) that are easily transported during floods. Several parameters have been measured and calculated for a population of 127 particles from French streams. These oncolites have a high porosity (mean 42.7%) and a low density (mean 1.65), in respect to quartz and calcite. Oncolites have a settling velocity 1.5 times lower than that of quartz grains with the same nominal diameter. The computation of multiple correlations results in a classification in terms of decreasing efficiency: shape coefficients (Corey, ψ, and SI of Sneed and Folk), nominal diameter, density, edge sharpness and roughness. A…

Discovery of Ca oxalate crystals associated with fungi in moss travertines (Bryoherms, freshwater heterogeneous stromatolites)

Buffered decalcification of live moss and liverwort (Hepaticae) travertines resulted in the release of a large number of organisms (bacteria, cyanobacteria, fungi, eukaryotic algae, and small animals), which constitute an “organic mat”; (also called an algal mat or biofilm). This mat is calcified and commonly has laminations, allowing moss travertines to be considered as stromatolitic structures. After decalcification of 300 samples of travertines (using dilute acetic acid), only 9 released Ca oxalate crystals in the form of needle bundles, spherulites, and tetragonal bipyramidal prisms. These crystalline forms are identical to those found in some phanerophytes and soils. Mycelian filaments…

Calcitic radial palisadic fabric in freshwater stromatolites: diagenetic and recrystallized feature or physicochemical sinter crust?

Abstract When favorable conditions occur for carbonate precipitation (physicochemical processes) in tufas and travertines, organisms intervene either directly (the framework created by organisms acts as a crystal support) or indirectly (organic products influencing the mineralogy, nucleation, growth and habit of primary crystals). In the literature, fluvial and travertine crystalline sparite crusts with palisadic or prismatic crystals are generally interpreted as the result of early recrystallization of micrite in algal buildups (mostly composed of Schizothrix). Therefore, biological mediation is essential for the construction of continental bioherms and biostromes, even if organic remains …

A classification of rhizogenic (root-formed) calcretes, with examples from the Upper Jurassic-Lower Cretaceous of Spain and Upper Cretaceous of southern France — Discussion

Freshwater organisms that build stromatolites: a synopsis of biocrystallization by prokaryotic and eukaryotic algae

In freshwater environments such as river and stream bottoms, rocks and submerged vegetation are covered with a biological felt (also called a periphyton, microbial mat, biofilm, etc.) that is susceptible to calcification. Compilation of an extensive bibliography and our own observations have allowed the identification of 44 species of Coccogonophyceae, 122 Hormogonophyceae, 2 Chrysophyceae, 35 Chlorophyceae, 3 Xanthophyceae, 2 diatoms, and 3 Rhodophyceae that grow on calcareous tufa and coat vegetation. Diverse genera include species that are also calcified but impossible to determine because they lack reproductive organs. Crystals have been described from 74 species in the literature and w…

Spherulites in Calcrete Laminar Crusts: Biogenic CACO3 Precipitation as a Major Contributor to Crust Formation

ABSTRACT Spherulites (calcitic fibro-radial spherulitic polycrystals) are a major component of calcite layers in Quaternary calcrete laminar crusts. To explain the formation of spherulites, petrographic and chemical studies were performed on Pleistocene calcrete laminar crusts, Holocene biological crusts, and laboratory (in vitro) cultures of cyanobacterial strains. Spherulites were found to be formed of acicular and radiating crystals, either smooth and regular-edged, or twisted and irregular-edged. Spherulites were composed of low-Mg calcite, the concentration of Mg in the spherulites increasing from nucleus to periphery. The shape and chemistry of the in vitro spherulites and laminar cru…

Palaeoecology of non marine algae and stromatolites: Permian of France and adjacent countries

Abstract Some Permian, continental basins from Algeria, Morocco, France, Italy, Germany and Poland reveal algal remains and stromatolites, mainly during Lower Permian, but also during Middle and Upper Permian. The algae belong to 8 morphogenera and 12 morphospecies. Two species are new. The taxonomic attributions are difficult, even if some species resemble living species. The algae make unorganized masses, or laminated builtups (stromatolites, oncolites, oolites). Algal masses and stromatolites are contained in fluviatile sediments (active or abandoned channels) and lacustrine deposits (playas, ephemeral lakes shorelines, lakes several meters or decameters deep). Considering the sedimentol…

Hydrogéomorphologie du site, paléoenvironnements et dynamique alluviale.

In Landuré C., Arcelin P., Arnaud-Fassetta G. (Eds.) Le village de La Capelière en Camargue (Arles, Bouches-du-Rhône) du début du Ve s. av. n. è. à la période du Haut Moyen Âge - 2017