Ooids (,
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Formation
An ooid forms as a series of concentric layers around a nucleus. The layers contain crystals arranged radially, tangentially or randomly. The nucleus can be a shell fragment, quartz grain or any other small fragment. Most modern ooids are
aragonite, a polymorph of calcium carbonate; some are composed of high-magnesium
calcite, and some are bimineralic (layers of calcite and aragonite). Ancient ooids can be calcitic, either originally precipitated as calcite (as in
calcite seas), or formed by alteration (neomorphic replacement) of aragonitic ooids (or the aragonite layers in originally bimineralic ooids). Moldic ooids (or molds later filled in by calcite cement) occur in both young and ancient rocks, indicating the removal of a soluble polymorph (usually aragonite).
Variation
Whether ooids become calcitic or aragonitic can be linked to
strontium/
calcium substitution within the crystalline structure. This has been shown in some examples to be due to temperature fluctuations in marine environments, which affects
salinity levels, which in turn facilitate the substitution. Marine calcitic ooids were typically formed during
calcite sea intervals, especially during the
Ordovician and the
Jurassic Periods. The geochemistry of these seas was a function of seafloor spreading and fluctuating Mg/Ca ratios. Low Mg/Ca ratios favor the precipitation of low-magnesium calcite.
Growth mode
Ooids with radial crystals (such as the aragonitic ooids in the Great Salt Lake, Utah, US) grow by ions extending the lattices of the radial crystals. The mode of growth of ooids with tangential (usually minute needle-like) crystals is less clear. They may be accumulated in a "snowball" fashion from tiny crystals in the sediment or water, or they may crystallize in place on the ooid surface. A hypothesis of growth by accretion (like a snowball) from the polymineralic sediment of fine aragonite, high-magnesium calcite (HMC) and low-magnesium calcite (LMC), must explain how only aragonite needles are added to the ooid cortex. Both in tangential and in radial ooids, the cortex is composed of many very fine increments of growth. Some modern (and ancient) ooids partially or totally lack clear layering and have a micritic (very fine grained) texture. Examination of such micritic ooids by scanning electron microscopy often shows evidence of microbial borings later filled by fine cement.
Growth factors
There are several factors that affect ooid growth:
supersaturation of the water with respect to calcium carbonate, the availability of nuclei, agitation of the ooids, water depth, and the role of
microbes.
Ooimmuration
Sometimes
fossils are included in ooids, often forming the nuclei. This
taphonomy process is termed
ooimmuration (Wilson et al., 2021). The formation of the ooid cortex around the test or shell protects it from abrasion, fragmentation and bioerosion. Ooimmuration also retains fine organic remains that would ordinarily be winnowed away by currents.
Flügel, Erik (2010), Microfacies of Carbonate Rocks: Analysis, Interpretation and Application, 2nd ed., Springer, pp. 242–244. . Accessed 2014-06-23.
Wilson, M.A., Cooke, A.M., Judge, S.A. and Palmer, T.J. 2021. Ooimmuration: Enhanced fossil preservation by ooids, with examples from the Middle Jurassic of southwestern Utah, USA. Palaios 36: 326-329. https://doi.org/10.2110/palo.2021.036
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