The shallow-water sediments of western Lake Geneva are composed mainly of sand-size coated grains ranging from oncoids to ooids. In surficial sediments the coatings consist of anhedral microcrystals of low-Mg calcite and often show a well developed intercrystalline porous network containing bacteria and residues of degraded mucus. Nucleation occurs in close association with organic films formed by mucus produced by filamentous cyanobacteria and diatoms. Similarities between oncoid and ooid cortex (crystal size, shape and arrangement) indicate that the processes controlling the building of these types of particle are identical. In subsurface sediments the microstructure of the coatings changes considerably: the intercrystalline porosity is reduced by aggrading neomorphism and the micritic porous and organic-rich coatings are transformed into a dense xenotopic mosaic with scattered molds of filamentous cyanobacteria. This occurrence of ooidal-oncoidal sands in Lake Geneva may help in interpreting ancient lacustrine formations and brings additional information on the processes of genesis and diagenesis of coated grains.

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