OOIDAL IRONSTONES AND FERRUGINOUS MICROBIALITES - ORIGIN AND RELATION TO SEQUENCE STRATIGRAPHY (AALENIAN AND BAJOCIAN, SWISS JURA MOUNTAINS)

Aalenian and lower Bajocian rocks in the central and northern Swiss Jura mountains comprise a series of parasequences that mainly reflect a shallowing-upward trend in a shallow, mixed carbonate/siliciclastic depositional environment. Within a parasequence, ooidal ironstones may occur at three specific types of horizons. These are: regressional discontinuities and transgressional discontinuities formed by sediment bypassing, and omissional discontinuities formed by starvation. Ooidal ironstones, which principally are autochthonous, accumulated during both sea-level rises and falls in a relatively broad bathymetric and hydrodynamic spectrum. The key physical factor for ferruginous ooid genesis is non-deposition. Ferruginous ooids and microbialites consist of goethite, chamosite and mixtures thereof, with subordinate amounts of apatite and silica.Ferruginous ooids grew stepwise on the sediment surface in an oxygenated marine environment. Ferruginous microbialites, being the product of benthic microbial communities, grew - partly in cavities - in aerated moderate- to high-energy environments. Thus, chamosite evolved from a precursor substance stable under oxidizing conditions. The close mineralogical and micromorphological resemblance of ferruginous microbialites and ooids suggests a common biogenic origin. Structural rearrangement of a biologically accreted gel-like precursor substance consisting of various amorphous hydroxides is considered a probable mode of mineral genesis in both ferruginous ooids and microbialites.