# Why do ooids not grow larger than 0.5 mm?

I know ooids form in high energy water, by rolling and building layers around a nucleation site, but I don't understand what limits their size.

What keeps these such a regular, small size?

• Where did you get the idea that there's a size limit? – kwinkunks Feb 5 '16 at 14:47
• @kwinkunks sure, OCR Geology AS & A2 Student Book ISBN 0435692119 – Tim Feb 5 '16 at 19:13
• I have edited the book title into your question, but it would be even better if you quote the actual statement that says so. Please edit the question. – Jan Doggen Feb 6 '16 at 16:33

I was unaware of a 0.5 mm maximum rule with ooid size. The Sam Boggs Jr. book on Principals of Sedimentology and Stratagraphy indicates that ooids can grow up to 2 mm. Grains larger than this are pisoids. The size limitation may depend on how you define an ooid. To me, an ooid carbonate grain with a nucleus and concentric layers of calcite or aragonite.

The size is likely a function of the availability of calcium and carbonate ions in the water (calcite or aragonite saturation). I think that microbial action also contributes to large grain size, but if microbes are involved, then you might call it an oncoid instead.

The similarity of ooid size is a function of similarities in the environment of deposition.

Microfacies of Carbonate Rocks by Erik Flugel is a great book that discusses these features in detail.

• Good answer. Because geology sometimes descends into epic jargon wars, I wanted to add the general remark: don't take tables of grain sizes too literally... Like most jargon in geology, the word is just there to help describe things, not to define it in any strict way, least of all in terms of size. Some geologists just use other words for large ooidy things. – kwinkunks Feb 5 '16 at 14:49

As already noted by Inkenbrandts answer the size limitation of ooids to 2 mm seems rather arbitrary, from Richter (1983):

Though most ooids are smaller than 2 mm, it is not meaningful to postulate a grain size boundary between "ooids" and "pisoids" at a diameter of 2 mm as suggested by Choquette (1978) and Donahue (1978), based on older usage, because the ooids of several occurrences exceed this limit without structural and mineralogical changes.

Most Phanerozoic ooids are smaller than 2 mm, but there are some Neoproterozoic ooids which are considerably larger, up to a diameter of 16 mm. This might be because the conditions on carbonate platforms were different. Controlling factors for the growth of ooids are:

Nuclei supply: The ooids stop growing as soon as they are buried, when there is a large influx of new nuclei, small ooids get buried before they are 'fully grown'

Growth rate: This is dependent on chemical conditions, also note that the growth rate is dependent on the surface of the oid:

$$\frac{\Delta m_{growth}}{t} \propto 4 \pi r^2$$

Agitation: For ooids to grow, they need to be mobile. When the velocity of the water is not sufficient to keep them moving, they stop growing.

Abrasion: When the growing ooids impact each other in high enery conditions, the mass loss during each impact is dependent on their mass: $$\frac{\Delta m_{abrasion}}{t} \propto r^3$$ the larger the ooids, the quicker they loose mass when colliding with others. If there was only growth rate and abrasion on could simply calculate the radius of ooids were they stop growing, but the other factors have also have to be considered.

The authors from whom I compiled the list of factors (Sumner and Grotzinger (1993) argue argue that for the growth of large ooids there must be a low nuclei supply, so that ooids have time to grow before getting buried, and the environment has to be energetic enough that large ooids still move. One reason for low nuclei supply might be that common sources for nucleation like skeletal fragments and fecal pellets were not available in proterozoic time. The second factor could be that the large ooids grew on carbonate ramps on rimmed carbonate platforms where the high energy environment is given.

Richter, D. K. (1983). Calcareous ooids: a synopsis". In: Coated grains. Springer,pp. 71-99.

Sumner, D. Y. and J. P. Grotzinger (1993). Numerical modeling of ooid size and the problem of Neoproterozoic giant ooids". In: Journal of Sedimentary Research 63.5.