Ooidal ironstones and ferruginous microbialites: origin and station to sequence stratigraphy (Aalenian and Bajocian, wiss Jura mountains)

摘要

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 hydrodynarnic 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.