Microbially induced cementation of carbonate sands: are micritic meniscus cements good indicators of vadose diagenesis?

Characteristic fabrics such as micrite envelopes, calcified filaments and m icritic grain-to-grain bridges are observed in a modern subtidal firmground (Wood Cay, Bahamas) and in a variety of firm- and hardgrounds of Lower Cre taceous and Upper Jurassic platform carbonates (Swiss and French Jura Mount ains). Their similarity to microbial fabrics described in grapestones and i n intertidal to continental vadose environments suggests that microbial act ivity played an important role in the initial stabilization and cementation of carbonate sands. 'Meniscus-type cements' (to distinguish them from vado se meniscus cements), which clearly formed in subtidal environments, are re lated to filament calcification, trapping of percolating micrite and microb ially induced carbonate formation. Such meniscus-type cements are commonly micritic, but meniscus-shaped precipitation of fibrous aragonite or spariti c calcite around organic filaments is also observed. Therefore, an interpre tation of vadose early diagenesis should not be based on meniscus cements a lone. Similarly, subtidally formed filamentous structures can strongly rese mble alveolar septal structures and be interpreted incorrectly as related t o subaerial exposure.