SIGNIFICANCE OF MENISCUS-TYPE OOID-TO-OOID BRIDGES PRECIPITATED IN A HIGHLY SUPERSATURATED LOW-ENERGY SHALLOW SUBTIDAL ENVIRONMENT

Whole, broken and recoated radial calcite ooids occur in the Upper Jurassic platform interior carbonates (Adriatic platform, Croatia). The oolitic units also contain “grapestone-like” aggregates, peloids and a restricted biota of gastropods and dasyclad algae. Ooids commonly have cerebroid outlines with radial structure, and lesser concentric micrite envelopes. The cortex contains numerous dark micron-sized inclusions of possible organic material that under SEM resemble microbial filaments and dissolved aragonite laths. Coatings show little evidence of abrasion of radial crystal terminations nor is there much rounding of broken ooids. Some units have superimposed fenestral fabric and many ooids are linked by micritic meniscus bridges. Dark color, platform interior setting, very restricted biotas and lack of high-energy sedimentary structures indicate that ooids were formed in hypersaline settings, similar to those in low-energy lakes, and marginal ponds and lagoons.

The fabrics have been considered to be subaerial vadose. However, most of oolitic units occur at bases of shallowing-upward parasequences overlying fenestral carbonates (commonly with erosional contacts) and are transgressive deposits. The micritic meniscus cements resemble those from Bahamian subtidal settings (Ginsburg and Wanless, pers. comm., 2006) and they are not necessarily subaerial vadose as they lack well-developed tepees, vadose pisolites and caliche. Lack of such subaerial vadose fabrics, and the arid depositional setting of modern radial, oversized ooids suggests that the ooids and micrite meniscus cements formed in arid, low-energy, very shallow subtidal to intertidal settings of the platform interior, under the influence of microbial films.