Ooids in Purbeck limestones (lowermost Cretaceous) of the Swiss and French Jura



ABSTRACT Ooids occurring in the shallow-water Purbeckian carbonate sediments of the Jura mountains can be grouped into six types. Gradations from one type to another and coexistence of the various types are common.

Type 1 ooids are small and well rounded. They display fine concentric micritic laminae. In many cases their cortices are dissolved and replaced by void-filling spar. Microsparitic neomorphic replacement occurs locally. Type 2 ooids are large and have irregular shapes. They show fine micritic laminae and occasional layers of fine-radial crystals. They commonly evolve into oncoids. Ooids of type 3 display many fine-radial cortical laminae and are patchily micritized. They are medium in size and mostly well rounded. This type of ooid may pass into large, irregularly shaped coated grains. Type 4 ooids have 1 to 4 cortical laminae with a fine-radial structure and patchy micritization. They are medium in size and well rounded. Type 5 ooids have only one lamina with a coarse-radial structure. They are small and well rounded. Associated are spherical grains containing bundles of elongate crystals. Ooids of type 6 show superpositions of two or more different, radial and or fine micritic laminae. The cortical structure may also change laterally in the same lamina.

The preferential dissolution of type 1 ooid cortices to form oomoulds indicates a primary composition of unstable carbonate. Sedimentological features and comparison with modern ooid occurrences point to formation on high-energy sandbars in normal-marine waters. Type 2 ooids grew in marine-lagoonal environments with quiet water and abundant cyanobacteria. The radially structured ooid cortices of types 3, 4 and 5 show no dissolution features. This implies that they were originally composed of stable carbonates, or that an unstable carbonate phase was transformed into a stable one at an early stage of diagenesis. Type 3 ooids occur together with marine faunas and indicate high water energy. Ooids of type 4 and type 5 originated probably from relatively quiet water of variable salinity.

Coexistence of different ooid types and mixed forms of type 6 implies gradual or rapid changes in hydrodynamic, geochemical and microbiological conditions which were a feature of the Purbeckian depositional environments.