REE+Y AND CYCLICITY IN CRYOGENIAN GIANT OOIDS: EVIDENCE FOR RAPID OOID PRECIPITATION IN A CARBONATE SUPERSATURATED ANOXIC OCEAN?

There is an unusual abundance of giant ooids in the Neoproterozoic era. Growth rates, growth processes, and links between giant ooids and marine chemistry are not well understood. We present evidence for cyclicity in cortical laminae of iron-oxide-rich giant ooids of the late Cryogenian Trezona Formation, South Australia. Ooid cortical laminae form finely laminated iron-oxide-rich and iron-oxide-poor couplets that progressively thicken and thin with Fast Fourier Transform peaks at ~8–12.5 cycles per couplet – potentially consistent with tidal cyclicity. Systematic geochemical variability in iron — iron-oxide-rich versus iron-oxide-poor couplet intervals — are superimposed over the tidal rhythmite(?) cycle. Tidal rhythmites(?) in ooids are suggestive of rapid ooid growth over daily and neap-spring-neap tidal cycles. Within giant ooids, intervals of rapid growth are interrupted by erosional surfaces/unconformities, corresponding to intervals of ooid erosion, abrasion, and/or non-deposition. Observations provide evidence for rapid ooid precipitation balanced by intervals of non-deposition or erosion, consistent with recent modelling and experimental work. Rapid growth rates in giant ooids imply extremely carbonate supersaturated marine conditions in the late Cryogenian. Average rare earth element profiles lack a negative cerium anomaly, consistent with a largely anoxic late Cryogenian ocean. Extreme carbonate supersaturation and rapid giant ooid growth may be linked with late Cryogenian marine anoxia.