NUMERICAL MODELING OF OOID SIZE AND THE PROBLEM OF NEOPROTEROZOIC GIANT OOIDS

Temporal variation in ooid size reflects important changes in physical and chemical characteristics of depositional environments. Two numeri cal models are used to evaluate the effects of several processes influ encing ooid size. The first demonstrates that low supply of new ooid n uclei and high cortex growth rate each promote growth of large ooids. The second model demonstrates that high average water velocity and vel ocity gradient also enhance ooid growth. Several Neoproterozoic oolite s contain unusually large ooids, some reaching diameters of up to 16 m m. While lower nuclei supply and higher ooid growth rate may have prev ailed prior to the evolution of carbonate-secreting organisms, neither difference can explain the presence of giant ooids in Neoproterozoic deposits because Archean through Mesoproterozoic ooids rarely exceed 5 mm in diameter. In the presence of lower nuclei supply and higher gro wth rate, high average water velocity may have allowed growth of such large ooids. Higher average water velocity could have been due to a pr evalence of carbonate ramps over rimmed shelves during Neoproterozoic time.