The largest known ooids and their implications for sedimentology

ABSTRACT Marine ooids exhibit dynamic size changes that can potentially serve as proxies for ocean chemistry and environmental evolution. This study addresses a longstanding sedimentological question: what is the maximum size ooids can attain? This study documents the discovery of giant ooids from the middle Cambrian on the North China Platform, with maximum diameters exceeding 35 mm, providing new insight into their formation. These pebble‐sized ooids, the largest documented to date, are primarily concentrated within a ~1.5‐m‐thick oolitic sequence. Sedimentary features, including gutter casts, hummocky cross‐stratification, variable grain size and meteoric cements, indicate formation in a storm‐influenced coastal environment. Petrographic, mineralogical and elemental spatial imaging analyses at various scales reveal smooth, uniform, concentrically banded cortical fabrics composed of alternating micritic (<4 μm) and microsparitic (4 to 16 μm) laminae, which are respectively characterised by high and low contents of organic matter, extracellular polymeric substances and phosphorus. Both lamination types trap fine‐grained detritus, including clay‐sized particles and heavy minerals, whose abundance varies cyclically within the ooid cortices. This detritus is more concentrated in the microsparitic laminae, often at levels exceeding the intergranular matrix, implying periodic changes in the ooid growth environment and a contrast between growth and preservation settings. The ooid growth mechanism—characterised by surface microbial mats inhibiting external cement precipitation while internal laminae promote carbonate nucleation through organomineralisation—enables prolonged active growth states, resisting premature cementation during the resting stage. Furthermore, a unique combination of middle Cambrian physical, chemical and biological conditions (e.g. extreme levels of high temperatures, carbonate saturation and P influxes, as well as prevalent storms), likely driven by intense Pan‐African orogenesis, facilitated the widespread development of these exceptionally large ooids.