Flysch-type graywacke beds of the Gault Formation of 3 Alpine nappes (Falknis, Tasna, and Flysch Nappe of the East Alps) usually contain less than 5% (maximum 19%) of ooids whose aragonite concentric rims are mostly silicified. Other unstable carbonate components, such as echinoderm and bryozoan fragments (high-Mg calcite), are also preferentially silicified. Most of the other carbonate grains are little altered.

The ooids generally consist of a large nucleus (of quartz, feldspar, glauconite, or other grains) covered by a relatively thin oolithic coating. Silicification has nicely preserved the internal structure of the coatings--tiny inclusions of clay particles and iron-stained minerals delineate the original concentric structure.

This kind of selective silicification is a diagenetic process which probably took place after final redeposition of the sediment (in an assumed trenchlike environment). It is less likely to have occurred at an early stage of diagenesis immediately after formation of the ooids in a high-energy, near sea-level environment, or during transport on the shelf.

Epigenesis at a transitional stage to very low-grade metamorphism has enlarged the crystal size within the siliceous rims. In the unmetamorphosed Flysch Nappe, the rims are cryptocrystalline. In the Falknis and Tasna Nappes (lower chlorite-schist and stilpnomelane facies), many of the silicified coatings display distinct small quartz crystals.

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