FORMATION MECHANISM OF COATED GRAINS IN SPRING LAKES

MONFERRAN, MATEO D.; CABALERI, NORA G.; ARMELLA, CLAUDIA; GALLEGO, OSCAR F.; ALONSO-ZARZA, ANA MARÍA; HUERTA, PEDRO; MARTINEZ GARCIA, BLANCA; SABATER, L.; UBALDON, MARÍA CECILIA

Paraná

Congreso; VIII Congreso Latinoamericano de Sedimentología; 2021

This abstract aims to describe and interpret the growth of coated grains associated with lacustrine bioconstructions and their lateral detritic deposits, of the Carinao Formation (Oligocene-Miocene), which outcrops on the eastern flank of the Tecka range (Chubut). The structures are hemispherical, have a flat or concave base, and a convex up top. They are grey to white in colour, with an average size of 0.5 to 1.5cm in diameter and 0.2 to 0.6cm in height. Internally, they are constituted by overlapping concentric micrometric calcite layers converging towards the base. In some cases, the layers are separated by voids parallel to them (fenestras). In most oncoids the nuclei are either a convex ostracod valve or a whole ostracod. We recognized different types based on their nuclei and internal morphology, Type 1: Sets of convex-up ostracod carapaces and laminae of radial-fibrous calcite/dolomite or reticulate calcite. Each set may contain up to 4 valves and rarely the lower valve is preserved. The valves are of equigranular sparite/dolomite, fibrous calcite or silica, covered by a micrite film. Between the sets there are fenestras. The external cover is composed by radial calcite and may have micropeloid adhesions. Type 2: Composed of fibrous radial calcite and opal laminae, which are thinner towards the edges and thicker in the central areas. Nuclei are ostracod shells with moldic porosity. Between the laminae there is an important irregular porosity. Micropeloids may be adhered to the external fibrous laminae. Type 3: Set of irregular grumous micrite laminae with high fenestral porosity, micropeloids and ghosts of the fibrous calcite, radial fibrous calcite laminae with micrite cover and silica laminae. The outer layer is micritic, inlaid with silica. Nuclei are composed of articulated valves with calcite infill. Type 4: Poorly defined internal structure. Nuclei are consistent of ostracod carapaces, composed of radial sparite, with development of a thick cover of radial calcite fans, amalgamated crystals of sparite, peloids and fenestras. The upper lamina is of homogeneous radial calcite, and the outer cover is micritic/microsparitic. Porosity is fenestral. Type 5: Mixed oncoids. Nuclei are sets of laminar microstructure of syntaxial calcite, radial calcite/silica, with dolomite, silica and fenestral porosity. The accretional zone is formed by dolomitic laminae, covered by micritic calcite. The top of the structure is composed by silica (opal) laminae and sets of sparite and grumous peloidal micrite. Different types of oncoids were found associated with ponds in underground water springs. Extreme alkalinity conditions limited the diversity of fauna and only ostracods were recognized as colonizing groups: Candona sp., Eucypris sp., Heterocypris sp. (euryhaline organisms), Limnocythere sp. and Chlamydotheca sp. (usually found in stagnant water bodies), together with microalgae and cyanobacteria. The mechanism of in situ formation results from the calcium carbonate precipitation probably associated with biofilms developed initially on ostracod shell surfaces, and later on the calcite laminae. The agglutinations were probably done by thin microbial mats attached successively on the valves that in a way favored the trapping and fixation of the valves and so the growth of the grains. The fenestral porosity could record non mineralized parts of the biofilm, unstable mineral phases that have been dissolved, in the presence of organic matter necrosis. The concentric structure in the convex upside of the oncoids is the result of the growth of carbonate laminae, mainly on one side. This could indicate a stable position of these particles during most of their growth, although their presence in clastic deposits suggests that could have been transported. The accretion is produced by the stacking of valves and/or molts and the carbonate crystals superposition, which grows within the micro-organic matrix of biofilms, due to chemical alterations in cell surfaces. The precipitation of calcite and dolomite in these saturated waters produces the adsorption of metal ions, which attracted carbonate ions, in the presence of extracellular polymeric substances (EPS) of microorganisms. In this way, the precipitated minerals caused the calcification of the microbes. These processes are favored in ponds of low energy environments, surrounded with carbonate bioherms boundstone pinnacles, with a complex texture composed of: fibrous calcite crystals, feathers, inequigranular calcite mosaics and micrite. The different characteristics of the coatings reflect the varied mostly biotic processes that interplayed in their formation: ostracod shells agglutination by thin microbial mats; microbial and inorganic micrite precipitation; microbial induced precipitates with physico-chemical processes in high turbulent waters, originated by two or more processes.