Ooid
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Ooids (from
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
Variation
Whether ooids become calcitic or aragonitic can be linked to
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:
Ooimmuration
Sometimes
References
Flügel, Erik (2010), Microfacies of Carbonate Rocks: Analysis, Interpretation and Application, 2nd ed., Springer, pp. 242–244.
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
References
- ISBN 978-0-7112-5684-2.
External links
- Ooid Formation at the Wayback Machine (archived 20 June 2013)