Palaeoenvironmental implications and genesis of lacustrine Lower Triassic giant ooids

This is the authors' abstract. We don't add key points for this paper.

The sedimentary environment and mechanisms of formation of giant ooids (> 2 mm) are not well understood. Here, we use petrographical, mineralogical and geochemical data of giant ooids from the Lower Triassic Buntsandstein Formation of central Germany to develop a coherent formation model. Structural preservation of calcite crystallites in ooid cortices in combination with comparatively low strontium concentrations (< 545 μg/g) and the absence of dolomite in the cortex suggest a primary low-Mg calcite composition. Shale-normalised rare earth elements plus yttrium (REE + Y) patterns lack a pronounced heavy REE over light REE enrichment and show a positive cerium anomaly (Ce/Ce* SN = 1.9–3.9). Together with a missing yttrium anomaly and low Y/Ho ratios (< 40), these data are largely consistent with ooid formation in a lacustrine environment. Positive cerium and lanthanum anomalies further point to formation and deposition in an alkaline to hypersaline playa-lake environment. Based on our findings, a five-stage formation model is proposed. We suggest nucleus formation by microbial spheres, followed by calcite crystal nucleation on biofilm templates at the sediment–water interface, and crystal/cortex growth inside the sediment pile. The spherical ooid form is achieved by wave-induced physical rolling motion after exhumation. Ooid growth ceases when the ooid reaches a critical size (here 4 mm), depending on local hydrodynamic properties. Inhomogeneous crystal growth and the inclusion of exogenous particles lead to structural and geochemical anomalies in the ooid cortex. Results presented here have far-reaching implications for the application of REE and other trace elements as environmental proxies in ancient carbonates.