Abstract

Miocene palustrine ooids located in the Nullarbor Plain, southern Australia, illustrate the importance of microbes and soil processes in the formation of these spherically laminated grains in pedogenic settings. These particles are composed of dense minimicrite and include a wide assortment of microbially produced structures. Such biogenicity includes interpreted extracellular polymers (EPS), algal filaments, palygorskite and sepiolite nano-fiber mats, bacterial spores, and meniscate fabrics. Ooid nuclei are peloids that are interpreted to have formed via microbial binding of lacustrine sediment in a subsequent palustrine environment, both at the surface and inside underlying karst cavities. The peloids are encapsulated in multigenerational micrite laminations that form the spherical cortex. Individual cortical laminae are made up of degraded minimicrite crystals and complex encrusting mats of palygorskite and sepiolite nano-fibers. Cortex generation is interpreted to have been an annual process involving alternating soil hydration states. In this conceptual model, during the wet season, ooids were covered in bacterial mucus that bound ions from solution and produced hydrated Mg-Si gels that evenly surrounded the entire ooid. Gel dehydration during the following dry season resulted in precipitation of fibrous palygorskite and sepiolite. Succeeding seasonal variations in moisture produced numerous laminae that ultimately resulted in a multigenerational ooid cortex. This annual microbial process provides a new mechanism involving an autogenic formation of ooids that may be applied to a wide variety of pedogenic sediments. Unlike the more common marine ooids that require constant movement to achieve cortical precipitation, the process of grainification, combined with microbially mediated mineral precipitation under different soil hydration states, results in the in-place formation of terrestrial ooids.